E. V. Karpunina, V. P. Kryakovkin and N. P. in (72) Inventors (71) Applicant (54) MASS SPECTROMETRIC LEAK DETECTOR

FOR LEAK TEST

PROBE METHOD

The invention relates to testing products for tightness and can be used to test any products operating under low temperatures.

According to the main.aut. St. Yu 5302 13 is a mass spectrometric leak detector for leak testing using the probe method, containing an analyzer, a pump system for creating working pressure in it, a probe with suction and scattering nozzles, a cooled nitrogen trap connected in line between the analyzer and the suction nozzle of the probe, and completed

15 in the form of a closed reservoir with an outlet to a scattering nozzle, while an adsorption column is located in the nitrogen trap. The leak detector works as follows: the surface of the product filled with test gas is examined with the probe of the leak detector; vapors of nitrogen from a cooled nitrogen trap enter through the outlet to the diffuser nozzle of the probe and create a protective environment around the suction nozzle from background flows of test gas and flows from leaks located outside the zone being examined at the moment; the test gas flowing out of the leak enters the mass spectrometric analyzer through the suction nozzle and is recorded by it.

The disadvantage of the leak detector is the unreliability of testing products operating at low temperatures, since in these cases, due to temperature deformations of the product, leaks that are absent in the product at normal temperature may occur.

The purpose of the invention is to increase the reliability of testing products operating at low temperatures, and by providing a leak detector with the possibility of modeling in the leak zone on a low temperature product.

This goal is achieved by the fact that a mass spectrometric leak detector for checking the tightness of ie "spruce by the probe method, containing an analyzer, a pumping system for creating a working pressure in it, a probe with suction and scattering nozzles, a cooled nitrogen trap included in the line between the analyzer and suction nozzle of the probe and made 10 in the form of a closed tank with an adsorption column and a branch to a scattering nozzle, equipped with a tube installed in the tank. With the possibility of immersing one end of it in the refrigerant and connecting the second end.: to the outlet, and the tank is connected to a pressure source. At the same time, the surface of the probe and the tap are covered with a layer of thermal insulation.

The drawing shows the mass spectrum "rometric leak detector, longitudinal section.

The mass spectrometric leak detector contains an analyzer 1 connected to a pumping system, including mechanical 2 and steam jet 3 pumps. Probe 4 with dissipating 5 and suction 6 nozzles and a needle 7 that regulates the conductivity of the nozzle 6 through the 30 cooled nitrogen trap 8, made in the form of a reservoir with an adsorption column 9 with an adsorbent 10, a pipeline 11 and an inlet valve 12 are connected to the analyzer 1. The reservoir of the trap 8 is filled with refrigerant (liquid nitrogen) and closed with cover 13, on which valve 14 is installed, which regulates the pressure in the tank, which is connected through branch 15 to diffuse nozzle 5. end into the refrigerant and connections to

Branch 15, while the surface of the sound

4 f pa 4 and outlet 15 is covered with a layer of thermal insulation 17. A pressure source 18 is connected to the tank of the nitrogen trap 8.

The leak detector works as follows.

The surface of the product filled with test gas is examined with probe 4 of the leak detector. In the tank of the cooled nitrogen trap 8, pressure is increased by means of a pressure source 18, and the refrigerant from the tank through the tube 16 and the outlet 15 enters through the diffuser nozzle 5 to the surface of the product.

In this case, the surface is cooled and the operating conditions of the product at low temperatures are simulated.

The test GAE through the suction nozzle 6 enters the analyzer t.

Zone cooling. leaks on the product with liquid refrigerant while creating a protective environment around the suction nozzle of the probe allows you to increase the reliability of testing products operating at low temperatures, without cooling the entire product.

Claim

Mass spectrometric leak detector for tightness testing by the probe method according to ed. St. U 5302 13, characterized in that, in order to increase the reliability of testing products operating at low temperatures, it is equipped with a tube installed in the tank with the possibility of immersing one end of it in the refrigerant and connecting the other end to the outlet, and the tank is connected to a pressure source.

Sources of information taken into account in the examination

I 530213, class. G 01 M 3/00, 1975..926544

Compiled by A. Korvina

Editor S. Yusko Techred I. Reives Proofreader Y. Makarenko

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Lecture plan. Scope, test and control substances. Physical foundations: viscosity of liquids and gases, types of flows and the passage of substances through leaks. The choice of the tightness control method according to its sensitivity. Hydraulic, gas analytical methods, test method for welded joints with kerosene.

Leak test (= leak detection), refers to the type of NDT product quality penetrating substances (GOST 18353 - 79). Leak detection is a type of test based on the registration of substances penetrating through leaks (GOST 26790 - 85).

tightness- this is the property of structures to prevent the penetration of substances (gas, liquid or steam-gas) through them.

Flow- a channel or a porous area in a structure that violates its tightness. When checking for tightness, the presence of leaks is judged by the amount of gas or liquid flowing through them per unit time.

Absolute tightness cannot be ensured and controlled. Based on this, controlled structures are considered tight if the flow of gas and liquid through the walls and joints does not lead to a disruption in the normal functioning of the control object during its service life or to a deterioration in its characteristics during storage.

Degree of tightness- a quantitative characteristic of tightness, which is characterized by the total flow of a substance through leaks. Gas quantity Q defined as the product of gas pressure R for the volume V:

(13.1) .

Gas flow is its quantity flowing through the channel-leak. This is one of the basic concepts used in leak detection. Change in the amount of gas at a constant volume occupied

If this change occurs in time t, then

where J is the gas flow required to change the pressure on dP in a container with a volume V. With constant pressure changes over time, gas flow (m 3 × Pa / s \u003d W)

where ∆ R- pressure change over time interval Δ t.

The physical meaning of the fact that the flow is measured in units of power is that the product of pressure and volume is the energy stored in the gas, and the change in energy over time is the power. However, in practice, the dimension of the gas flow in m 3 × Pa / s is more often used.

leakage- the penetration of a substance from the outside into the sealed object under the action of a difference in total or partial pressure.

A leak- outflow of a substance from a sealed object. Leakage and leakage are estimated by the gas flow and have its dimensions.

For unambiguous characterization of the leak and the possibility of comparing the degree of leakage of products tested and operating in various conditions, the concept is introduced normalized leak. This is a stream of air flowing through a leak from the atmosphere into a vacuum at room temperature.

In the process of leak testing, test, ballast and indicator substances are used. The main initiating functions are performed by a test substance, the penetration of which through a leak is detected during the control process. As test substances, as a rule, gases with a low molecular weight are used, with a low content of them in the atmosphere, inert gases that do not interact with the OC material and the substance: inside them. Table 13.1 provides information on some of the test substances used. In some cases, the role of the test substance is performed by the working substance that fills the sealed object during operation or storage, for example, freon in refrigeration units. The working substance in combination with the test substance can sometimes enhance the indication effect. In other cases, the technical conditions for products do not allow contact of the working substance with the test substance, then the process of testing such products for tightness becomes more complicated.

Table 13.1. - Gases used as test substances

To create a large pressure drop, increase the sensitivity of tests at low concentrations of test substances, a ballast substance is used, for example, air at an increased overpressure. This is done when the task of saving a test substance, for example, helium, arises during multi-cycle tests or when testing large volumes.

When testing equipment by the chemical method, an indicator substance is often used, which, as a result of interaction with the test substance, contributes to the formation of a signal about the presence of a leak.

Tightness rate is characterized by the total consumption of the substance through the leaks of the sealed product, at which its operable state is maintained. As a rule, the highest total consumption of a substance is determined by calculation and established by regulatory and technical documentation. Usually, the tightness rate is set (calculated) by the designer.

Technological criterion of tightness these are consumer requirements in the form of a condition under which the product or process equipment can be operated.

Leak test methods. Tightness control methods are divided into three groups depending on the type of test substances used:

a) gas, gas (helium, argon, air, etc.) is used as a test substance;

b) gas-hydraulic, gas (air) is used as a test substance, and the liquid plays the role of an auxiliary medium in determining the fact and location of a gas leak;

c) hydraulic, liquid (water, oil) is used as a test substance.

PNAEG-7-019-89. Tightness control. Gas and liquid methods.Hydraulic control method consists in the fact that water pressure is created in the controlled product. The location of the defect is established visually by the appearance of jets, drops and streams of water. The test pressure and the duration of the product under pressure are established by the design documentation and indicated in the drawings.

Fluorescent-hydraulic method consists in the fact that an excess pressure of an aqueous solution of a phosphor of a certain concentration is created in the controlled product for a specified time. The location of the defect is established after wetting the controlled surface by the glow of the phosphor in the rays of ultraviolet light. After sealing, the controlled product is pressed with a luminescent aqueous solution of disodium and ammonium salts of fluorescein with a concentration of 0.09-0.1% (1-0.9 g/l) to the pressure required by the drawing or the corresponding technical documentation. The pressure during the control should not exceed the value regulated by PNAEG-7-008-89.

When conducting hydraulic control with luminescent indicator coating on the outer surface An indicator coating is applied to the controlled product, the product is pressed with water, maintained at a test pressure for a specified time, and the controlled surface is examined in the rays of ultraviolet light. In the presence of a leak, water penetrates to the outer surface of the product and a glow appears on the indicator coating at the defect site.

A method of controlling the flow of water without pressure. Water is poured into the product to the height specified in the design (design) documentation. The locations of defects are established visually by the appearance of jets, streaks and drops of water on the controlled surface. The duration of the presence of water in the controlled product is indicated in the design (design) documentation, taking into account the time required to inspect the entire controlled surface.

Method of control by luminescent penetrating liquids consists in the fact that a penetrating liquid based on kerosene is applied to the surface of the product, and an absorbent coating is applied to the opposite surface. After exposure for a predetermined time with periodic (after 15 - 20 min) application of an additional amount of penetrating liquid, the surface is inspected in the rays of ultraviolet light. In places of leaks, the luminescent liquid penetrating through the wall of the product gives a glow in the rays of ultraviolet light. The exposure time of the controlled surface in contact with kerosene is determined depending on the thickness of the welded metal or the calculated height of the fillet weld and the position of the weld in space.

Down position:

Up to 6 mm - 40 min

6 - 24 mm - 60 min

Over 24 mm - 90 min

Vertical, horizontal and overhead positions:

The thickness of the metal or leg of the seam:

Up to 6 mm - 60 min

6 - 24 mm - 90 min

Over 24 mm - 120 min

Choice of control method leak detection depends on the tightness class of the product, set by the designer and the sensitivity of the method. AT nuclear power depending on the operating conditions and repair possibilities, all equipment is divided into 5 tightness classes (Table 13.1). Each of the tightness classes corresponds to certain test methods, depending on their sensitivity. Class I, for example, includes steam generators, pipelines of the 1st circuit and other critical products, the reliability of which must be very high due to specific features their operation.

Table 13.1. - Classes of tightness of products in nuclear power engineering.

The halogen method arose during the period of widespread industrial development of refrigerators using freons as a refrigerant. But soon the method began to develop rapidly and be applied in various industries industry. At present, it is one of the most common instrumental methods of leak detection, second only to mass spectrometric methods. The method is widely used in aviation, shipbuilding, instrumentation and rocket building, energy, and other industries. The method is preferred when testing the tightness of large volumes or systems with branched communications, gas-filled cables and pipelines, sealed systems that cannot be evacuated. Especially effective is the use of the halogen method in the control of products in which halogen-containing substances are used as workers (aerosol packages, refrigerators, air conditioners).

A halogen method of tightness control based on halogen leak detectors is being implemented. The action of these devices is based on the property of platinum heated to 800 ... 900 ° C to sharply increase the emission of positive ions in the presence of halogen-containing substances. This effect, discovered by Rice in 1910, is realized in a two-electrode system consisting of a collector and an incandescent emitter, between which an electric field is created. The effect is observed both at atmospheric pressure and in vacuum. With a potential difference between the electrodes of 200 ... 250 V, the emitted ions are transferred to the collector, forming electricity in an external circuit, registered by an indicator.

The background and activated currents during the halogen effect are due to alkali metal ions formed as a result of ionization on the surface of platinum of alkali metal atoms that diffuse from the depth of platinum or come to its surface as a result of evaporation from the heated ceramic base of the emitter. When halogens reach the surface of the emitter, the latter react with alkali metal ions, and the surface, to a greater or lesser extent, is freed from adsorbed ions. The work function of the emitter increases, respectively, the ionization efficiency increases and the ion current increases. When the supply of halogens stops, the emitter surface is again covered with a layer of alkali ions, the work function of the emitter decreases and the ion current decreases to the background value.

Degree of surface ionization, i.e. ion ratio N+ to the number of neutral molecules N about, leaving the surface in 1 s, is expressed by the Langmuir-Saha formula:

N + / N 0 = β exp [(-eV+ F) / kT],

where β - constant depending on the type of gas and metal; F- the work function of the electron from the metal; e is the electron charge; V- ionization potential of gas molecules; k- Boltzmann's constant; T- absolute temperature emitter.

Ion current value:

J=eN +=e N 0 β exp [(- eV+ F) / kT],

The stock of alkaline impurities in platinum is small, and the stability of the effect is maintained mainly by the supply of neutral alkali metal atoms to the platinum surface from the ceramic base in contact with the emitter.

When more halogens enter the emitter, the phenomenon of "poisoning" is observed - the partial or complete disappearance of the halogen effect, which is restored when the emitter operates in an atmosphere of clean air.

Since its inception, halogen leak detectors have been constantly improved in order to stabilize the background signal and reduce the likelihood of emitter poisoning.

Much attention is paid to the technology of ceramics preparation and its composition. In particular, it is possible to use ceramics based on β-A1 2 O 3, allowing the use of the sensor when low temperatures(300 ... 600 instead of 800°C in case of using ceramics from steatite). This stabilizes the background current, reducing the risk of poisoning. By changing the design of the sensor, carry out preliminary training samples for stabilization temperature regime sensor, achieving selectivity of the latter in relation to various types of freons, reducing the risk of poisoning. The ionization efficiency of the sensor is increased with the help of a gas flow shaper to its emitter.

Leak detection methods are very diverse and differ significantly in sensitivity, selective reaction to a test substance, principles for detecting a leak of this substance, in the type of test substances used in the implementation of the method, etc.

Classification of methods. Tightness control methods are divided into three groups depending on the type of test substances used:

a) gas, when any gas (helium, argon, air, etc.) is used as a test substance;

b) gas-hydraulic, when gas (for example, air) is used as a test substance, and the liquid plays the role of an auxiliary medium in determining the fact and location of a gas leak;

c) hydraulic, when a liquid (for example, water, oil) is used as a test substance.

In table. 10.2 provides a brief description of the main methods of tightness control.

Analysis of the table. 10.2 shows that there is a wide range of leak control methods used in practice that allow leak control over a wide range. At the same time, the above table is only a guideline when choosing a specific control method. In the future, the most common methods for monitoring the tightness of products, their advantages and disadvantages are considered in sufficient detail. On fig. 10.1 for clarity shows the areas of application of the most common control methods for the Range of controlled leakage of the test substance. The dotted lines characterize the limits of the flow indication only under certain conditions, for example, when using additional substances and materials that are not typical for use in the classical interpretation of the corresponding method.

Mass spectrometric method. The method was first used in nuclear physics and electronics. It finds wide application in the practice of industrial testing. This is primarily due to its high sensitivity in all types of vacuum and atmospheric tests. The wide distribution of the method is largely facilitated by the serial production of mass spectrometric leak detectors, long experience in their operation, wide variability of their use, including in the automation mode. Unlike other leak detection methods, the mass spectrometric method allows one to assess the leak not only qualitatively, but also to perform quantitative measurements of the flow through it with an accuracy of 10%.

The method is based on creating an increased partial pressure of the test gas on one side of the OC surface and sampling the test substance on the other side for mass spectrometric analysis for the presence of sample gas molecules.

Table 10.2

Basic leak detection methods

Continuation of the table. 10.2

Figure 10.1 Areas of application of the main methods of tightness control

The partial pressure of a gas is the pressure that a gas that is part of a gas mixture would have if it alone occupied a volume equal to the volume of the mixture at the same temperature.

During testing, the flow of test gas flowing through the through defect along the way to the mass spectrometric chamber is ionized by the flow of electrons generated by the ionizer. This process is shown in Fig. 10.2. The mass spectrometer contains the following main components: an ion source, where sample gas molecules are converted into ions (with mass m, charge e) and an ion beam is created with constant energy; analyzer, where the ion beam is divided into components by value m/e; a collector with which these components are recorded and their peak values ​​​​are measured. The ion source consists of chamber 2, into which the test gas enters. From the hot cathode 1 into the chamber with a positive voltage relative to the cathode, an electron beam goes, which ionizes the gas. To focus the electrons along the direction of their movement, a magnetic field H 1 is created along the lines of which the electrons propagate in a spiral. Two diaphragms 3 and 4 form a directed ion beam and accelerate it due to the potential difference U 0 . Ions are accelerated to the same energy, which is determined by the formula

(10.4)

where V is the speed of the ions. Due to the difference in ion masses, this speed is different for ions of different elements. Next, the ions enter the analyzer, which consists of a mass spectrometric chamber and a system of collectors. A vacuum of the order of 1.33 10 -3 Pa is created in the chamber using vacuum pumps. A magnetic field R is created perpendicular to the movement of ions. Under the action of the Lorentz force eVH ions move along trajectories in the form of circles of radius R. From Newton's second law mV 2 /R=eVH substituting V, find the radius of the trajectory

Thus, the radius of the trajectory depends on the ratio m/e. In the analyzer, the ions are deflected by 180°. In this case, a focusing effect arises: the ions emerging from the source in the form of a beam diverging at a certain angle, deviated by 180°, are again collected in a band. In front of the collector 6 (see Fig. 10.2) there is a diaphragm 5 with an entrance slit at the focus of the ion beam with a given value of the mass number corresponding to singly charged ions of the test gas. The collector ion current is further amplified and recorded by an output meter. The appearance of a test gas in the gas mixture supplied to chamber 2 sharply increases the ion current.

Rice. 10.2. The principle of operation of a mass spectrometric leak detector

Helium is usually used as a test gas in the implementation of the mass spectrometric method. It has a number of advantages. By size m/e helium is very different (by 25%) from the nearest ions of other gases. This allows the use of a wide slit in diaphragm 5. Small value m/e for helium, it helps to reduce the radius of the trajectory, and, consequently, the size of the entire leak detector. Helium has a low molecular weight and therefore penetrates well through small leaks. There is little helium in the air (10 -4%), so the background effects of leak detectors based on the mass spectrometric method are relatively small. Helium is inexpensive and chemically inert.

Mass spectrometric leak detectors consist of units and systems that provide the processes of detecting a test gas leak, converting and processing information.

The sensitive element of the leak detector is, as a rule, a 180-degree magnetic analyzer 3 (Fig. 10.3), which converts the leak into an electrical analog signal, amplified by the amplifier. Due to the fact that the process of separating the ions of the test substance occurs at a high vacuum, all mass spectrometric leak detectors have a vacuum system 4, consisting of a fore-vacuum and high-vacuum pumps, vacuum communication, valves and a nitrogen trap.

To control solenoid valves, assemblies vacuum system and other elements of the leak detectors are equipped with a control system 1, a vacuum and leak recorder 2. Leak detectors of the latest models have built-in microprocessor units or microcomputers 5 for processing information from the leak detector, optimizing its operation and diagnosing the main systems.

Consider the principle of operation and design of a mass spectrometric leak detector. The mass spectrometric leak detector is a highly sensitive magnetic mass spectrometer tuned to detect a test substance. It consists of two main parts: the vacuum system and the electronic unit. The vacuum system (Fig. 10.4) includes a mass spectrometric chamber with a permanent magnet, a steam-oil pump 11, a mechanical pump 1, a calibrated helium leak 14, a nitrogen trap 8, a fore-vacuum cylinder 5. vacuum sensor 7, thermocouple pressure transducer 2, shut-off valves 4, 6, 10, 13, inlet valve 3, exhaust throttling valve 9, and inlet valve 12.

The mass spectrometric camera performs the main functions of a leak detector. It includes an ion source and an ion receiver. The operating pressure (0.710 -2 Pa) in the mass spectrometric chamber is provided by an exhaust system consisting of a mechanical (for example, NVR-0.5 D) and steam-oil (for example, N-0.025-2) pumps. A mechanical (forevacuum) pump provides a vacuum in the leak detector system of 0.1...1 Pa. The steam-oil pump increases the vacuum to 10 -4 ... 10 -5 Pa. The nitrogen trap helps to protect the mass spectrometric chamber from oiling and stabilizes the vacuum in it. To control the sensitivity of the leak detector, a calibrated helium leak of the “Helit” type is used, which provides a given gas flow due to helium diffusion through a quartz membrane. New helium leaks instead of a quartz membrane (Fig. 10.5). The test gas fills the capillary 1 through the opening ends 2 of the hollow loop-shaped fiber passing through the baffle 3, in the housing 4, and then diffuses through the walls of the fiber, creating a flow directed further into the test cavity. The advantages of such leaks include increased operational reliability and a wider range of test substances with which such a leak can work.

The electronic part of the leak detector is made in the form of a control panel 1 and separate units: ion current measurement 3 with an external electrometric cascade 2, pressure measurement 4, vacuum valve power supply 5, chamber power supply 6. Interrelation of the listed units among themselves, mass spectrometric chamber 7 and vacuum system 8 is shown in fig. 10.6.

The leak detector is adjusted using a calibrated leak. First of all, the fluctuation amplitude of the background signal is determined as the difference between the maximum and minimum a f max values ​​of the background signal:

(10.6)

Then the minimum helium flow is determined by the formula

(10.7)

where J t - helium leak flow (according to the marking on the leak body), m 3 Pa / s; a t - signal from a leak J t, in scale divisions. The value of division of the pointer device of the block for measuring the ion current of the leak detector is found from the formula

(10.8)

Leakage flow J g in m 3 Pa / s when working with pure helium is estimated by the formula

(10.9)

where a d - reading on the pointer device, due to the leakage of helium into the test volume. If a mixture of helium with air is used instead of pure helium, then the factor 1/ j, where j is the concentration of helium in the mixture.

A general view of one of the domestic leak detectors is shown in fig. 10.7. It has a threshold of sensitivity to the sample gas flow of 7 10 -13 m 3 Pa/s, provides semi-automatic access to the high-vacuum evacuation mode of the analyzer after pressing the "Start" button and semi-automatic shutdown of the leak detector after pressing the "Stop" button, allows continuous operation during the day at maintaining their technical characteristics. The leak detector is equipped with various systems that protect it from adverse situations. When the pressure in the analyzer rises to a level of approximately 2 10 -2 ... 3 10 -2 Pa, the heater of the analyzer's ion source cathode is automatically turned off. In the event of an emergency shutdown of the mains voltage, the PMN valve is automatically closed (evacuation of the oil-steam pump) and the “Inlet” valve (atmosphere inlet) is opened. The leak detector consists of two main blocks: SV-14 (vacuum system) and UR-14 (recording device).

The leak detector device is shown in fig. 10.8.

The main unit is a mass spectrometric analyzer 6, which enters through valves 4 and 7 with electromagnetic drives; nitrogen trap 2 and manually operated valve 3 are supplied with a flow of test substance. The ion collector of the analyzer is connected to the input of electrometric amplifier 5, the signal from which is fed to the amplifier direct current 21. At the same time, the signal of the leak detector is monitored using device 9. The output of this amplifier includes a pointer device, acoustic and light indicators. To control the sensitivity of the leak detector, a helium leak 12 is used. The working pressure in the mass spectrometric analyzer is provided by an exhaust system consisting of a rotary vane pump of the 3NVR-1D 20 type and a steam-oil pump of the H-0.25-2 type 13. Inlet pressure control with on the side of the OK and in the preliminary vacuum line is carried out by manometric transducers 11 and 16 of the PMT-6-3 type, and the pressure in the high-vacuum volume of the leak detector is controlled by a magnetic electric discharge manometric transducer 8. The vacuum system of the leak detector when it is turned on, off and in operation is controlled using electromagnetic valves 4, 7, 14, 15. Valves 1, 3, 10 with manual actuators.

The solenoid valves are controlled from the control unit 17. The program for semi-automatic control of the process of turning the leak detector on and off is set by the vacuum automation device 22. The manual controls are located on the control panel 18. The state of the vacuum system is reflected by single indicator display devices 19. The recording device UR-14 also contains emission stabilizer 23, display elements 24 and power supply 25.

The diversity of objects in terms of volume and performance results in a variety of ways to implement a mass spectrometric test method. The choice of test methods is significantly influenced by the operating conditions of objects and the requirements for the degree of their tightness.

Figure 10.7, Mass spectrometric leak detector type TI 1-14

Rice. 10.8. Block diagram of the leak detector TI 1-14

The general methodology for testing objects for tightness is as follows. As a rule, at the first stage of testing, the overall tightness of the test object is assessed. In the future, if there is a need for this, leaks are searched and the location of leaky areas is clarified. After elimination of the identified leaks, the initial stage of testing is repeated in order to establish the degree of tightness of the OK. In this case, the best results are achieved under conditions when the entire gas flow is pumped through the leak detector. Therefore, it is recommended to test objects, the gas separation flow of which does not exceed the allowable working flow of the leak detector, to be carried out with the auxiliary pumping means turned off and to pass the entire gas flow through the leak detector. For example, for the leak detector TI1-14, the maximum allowable working flow is J\u003d 2 10 -4 m 3 Pa / s.

Rice. 10.9. Typical test schemes

In the practice of testing, the method of helium chambers and covers, the method of a vacuum chamber (pressure chamber), the method of vacuum suction cups, the method of accumulating test gas in the chamber, the probe method, etc. are used. Let us consider typical test schemes that implement specific control methods. On fig. 10.9, a shows the scheme used to test individual elements or parts of objects, the total gas flow of which exceeds the maximum allowable flow of the leak detector. On this diagram, as well as on all subsequent ones, the leak detector is indicated by a dash-dotted line. Here, the pumping group (foreline and diffusion pumps) and analyzer 9, helium leak 6, manual valve 7 for connecting the helium leak, solenoid valve 5 for inlet protection, pressure transducer 4 for vacuum control, valve 8 are used for throttling the leak detector inlet. Auxiliary foreline pump 3 is connected to object 1 through valve 2. This pump is switched off immediately after receiving the forevacuum (0.1 ... 1 Pa) in objects and in connecting lines, if the total gas flow does not exceed the maximum allowable flow of the leaker. If the total gas flow exceeds the allowable one, then the tests are carried out with a constantly running mechanical pump. The tested object according to this scheme is connected directly to the inlet flange of the leak detector.

Unlike the previous scheme shown in Fig. 10.9, b, is used when testing objects or their parts with a large gas fission and leakage, as well as in the case of connecting a leak detector to a high-vacuum object. According to this scheme, the test object is connected through the valve 2 to the high vacuum pump 10, which in turn is connected to the fore pump 3.

Rice. 10.10. Typical test schemes with leak localization

When it becomes necessary to ensure maximum gas extraction into the leak detector and a short signal settling time, and thereby provide an indication of low flows, the scheme shown in Fig. 10.9, c. Especially often such a scheme is used when testing strongly outgassing or strongly leaking objects of large volume.

The use of a high-vacuum (for example, steam-oil) pump for auxiliary pumping often makes it possible, even with a large gas separation or leakage of the test volume, to obtain a low total pressure in it, not exceeding the maximum operating pressure in the mass spectrometric chamber of the leak detector. This makes it possible to carry out tests with the inlet valve of the leak detector fully open.

The probe test method (Fig. 10.10, a) is used to detect leaks in gas-filled objects. The probe 1 is a suction device, the conductivity of which: ensures the passage of a flow through it 2 10 -3 ... 5 10 -3 m 3 Pa / s. All designations in the leak detector block (circled with a dash-dotted line) in fig. 10.10 are identical to the designations in. leak detector blocks in fig. 10.9. The probe is moved along the surface of the test object filled with helium. To control the thermality of sheet blanks, open, as well as gas-filled objects and their parts, the method of vacuum suction cups is used, the implementation of which can be performed according to the scheme in: fig. 10.10, b. During these tests, the vacuum suction cup 1 is installed on the tested area of ​​the surface, from the opposite side of which helium is supplied.

In the process of testing small-sized products that are checked in the high-performance control cycle, a scheme is used; shown in fig. 10.11. The scheme includes OK 2 placed in chamber 1. Excessive gas pressure is created inside the object. To create a vacuum of 0.7 ... 10 -2 Pa in the chamber, a fore pump 17 and a high vacuum pump 19 are used. Vacuum meters 26 and 25 are used to control low and high vacuum, respectively. To control the leakage of helium from OK 2 into the chamber, a leak-detecting mass-spectrometric device (leak detector) is included in the circuit, including a mass-spectrometric chamber 23, fore-vacuum 18 and high-vacuum 20 pumps, a nitrogen trap 21, a control leak "Helit" 22, vacuum gauges 27 and 28 and other auxiliary elements. In the process of checking the tightness of the object, the necessary vacuum is first created in the chamber, then, after appropriate preparation, the mass spectrometric chamber 23 is connected, which is a converter of leakage into an electrical signal. Circuit elements are connected through valves 3...15.

Recently, in the implementation of mass spectrometric control, turbomolecular pumps (TMPs) have been increasingly used. The interest shown in TMN is not accidental. These pumps have a number of advantages, such as a short test preparation time (3...5 min), no need to use liquid nitrogen in the control process, hydrocarbon vapors are largely absent in the spectrum of the TMP residual gas, the mass spectrometric chamber is protected from air penetration. In addition, they have a much lower degree of compression of light gases than heavier ones.

Rice. 10.13. Structural diagram of countercurrent mass spectrometric control

Turbomolecular pumps remove gas from a vacuum system using moving parts. This way of pumping is called molecular pumping. In practice, TMPs with mutually perpendicular movement of the working surfaces and the flow (indicated by arrows) of the pumped gas have become more widely used (Fig. 10.12). In the housing 2 fixed stator wheels 4 are installed, between which the wheels 3 rotate, fixed on the rotor 1. The rotor wheels are made in the form of slotted disks. The stator wheels have mirror slots of the same shape. The pumping speed of TMP weakly depends on the type of gas. Limit pressure 10 -7 ... 10 -9 Pa. Based on TMP, it turned out to be possible to create a countercurrent method of mass spectrometric control (Fig. 10.13). Product 1 is connected to the foreline pump 4 and to the pre-evacuation line of the turbomolecular pump 3. When the object is blown with helium and in the presence of through defects, helium, as a test substance, penetrates through the TMP in the direction opposite to the pumping direction into the chamber of the mass spectrometric leak detector 2 as a result of diffusion .

On the basis of the considered scheme, leak detection installations and automated tightness control systems have been created and are being created. We also note that under conditions of large gas loads, the counterflow method provides an increase in sensitivity by about 6–8 times. Given the above advantages of mass spectrometric schemes with TMP, developers are increasingly turning to their practical implementation.

halogen method. The method is widely used in leak detection technology and successfully competes with other methods. The method is used when testing products of large volumes or systems with highly branched pipelines. It is given preference when monitoring the tightness of objects in which halogen-containing substances are used as technological ones (aerosol packages, air conditioners, refrigerators, etc.).

Halogens (from the Greek halos and genes - giving birth) - chemical elements fluorine, bromine, iodine, chlorine, which make up the main subgroup of group VII of the periodic system.

The halogen method is based on using the effect of increasing thermionic emission from the surface of heated platinum in the presence of halogen-containing substances (freons, carbon tetrachloride, etc.). This effect was first discovered in 1944 by Rice. The author of this discovery and other specialists who subsequently studied this effect found that the phenomenon is observed both at atmospheric pressure and in vacuum, but in any case, the presence of a certain amount of oxygen or air is necessary. Halogen devices based on this effect have a characteristic dependence of the current increment on the concentration of the test substance, which has a maximum in current, then decreases, despite an increase in the concentration of halogens.

Based on the analysis of subsequent works, it was proved that the halogen method is based on a catalytic chemical reaction. It occurs in several stages: thermal dissociation of the initial molecule of the test substance, the formation of halogen oxides on the platinum surface, and their decomposition. The emission current density is proportional to the rate of this basic reaction. In parallel, the deactivation reaction of the sensitive element proceeds due to the action of carbon formed during the thermal decomposition of halogens.

Freons (freons), such as freon-12, freon-22, are used as trial halogen-containing substances. The characteristics of these freons are given in table. 10.3.

Table 10.3

Freons are chemically inert and low-toxic substances. Dehydrated freons in the liquid and vapor state are completely inert to all metals. However, being good solvents for many organic substances, they cause swelling of gaskets. Therefore, when freon is used as a test substance, freon-resistant rubber is used. For freon-22, polytetrafluoroethylene gaskets are recommended.

The halogen method, as well as the mass spectrometric method, makes it possible to control tightness according to various schemes, including on its basis to carry out tests in an automated mode.

The wide industrial application of the method in the country and abroad is facilitated by the serial production of halogen leak detectors - devices that are simple and reliable in operation and at the same time have a sufficiently high sensitivity.

Most often, the halogen method is used according to the probe method, in which a halogen-containing test substance is introduced inside, and a probe connected to a recording device (leak detector) is moved outside along the alleged leak sites. In order not to pollute the room with halogens, before testing with a halogen leak detector, it is necessary to test with less sensitive methods, for example, manometric. Testing with a halogen leak detector can only be started after gross leaks have been eliminated or it has been established that they are absent. It is important to keep this rule in mind whenever any highly sensitive leak test method is used, or when a test substance is used in the test process, the loss of which is undesirable for economic or environmental reasons.

Tests can be performed with pure freon or a mixture of freon with air. As a rule, tests with pure freon are carried out with small volumes of OK in accordance with the scheme shown in Fig. 10.14. Previously, with the help of a vacuum pump 3, air is pumped out through valves 2 and 4 OK 5, creating a small vacuum. Then, through valve 1, the OK is filled with freon, the pressure of which is limited by the elasticity of the freon vapor at the test temperature. So, for example, at a temperature of 20 ° C, the freon vapor pressure is 0.573 10 -5 Pa = 5.78 kgf / cm 2. After filling the OK with freon, an examination is carried out using the probe of a halogen leak detector. After testing, freon is sent for regeneration with a view to its subsequent use in further tests.

When testing with a mixture of freon with air, the scheme shown in Fig. 10.15. In this case, a certain amount of gaseous freon is first admitted into OK 5 under pressure, and then compressed air is supplied into OK through valve 6 to create the necessary pressure of the mixture of freon and air (other designations are as in Fig. 10.14). This ensures the necessary sensitivity of tests at a low concentration of freon as a test substance. After testing, the mixture is removed from the OK using a regeneration system. The sensitivity of pipe tests with a halogen leak detector is determined by the formula

(10.10)

where FROM- the concentration of freon in the mixture, R With- pressure of the mixture of gases; R a- Atmosphere pressure; η with - the viscosity of the mixture of gases, η in - the viscosity of the air.

By changing the pressure of the mixture or the concentration of freon, it is possible to change the sensitivity of the tests over a wide range.

Rice. 10.16. Sensing element of a halogen leak detector

Halogen leak detectors are based on using the property of heated platinum to dramatically increase the emission of positive ions in the presence of substances containing halogens.

The sensitive element of the leak detector, fixed on base 4, is a platinum diode with a direct-heated anode wound on a ceramic tube (Fig. 10.16). The alkali metals evaporated from the ceramic hollow element 3 are ionized on the heated surface of the platinum emitter 1. The ions from it enter the second electrode - the platinum collector 2, connected to the input of the DC amplifier. The pointer device at the output of the amplifier registers an increase in the ion current when a leak is detected. The signal is duplicated by a sound indicator.

The halogen converter is designed as a pistol type probe. In front of it is a sensitive element. The ventilation device is located behind the sensitive element and provides a continuous flow of the gas-air mixture through it.

In addition to the atmospheric converter, the set of the serial halogen leak detector GTI-6 also includes a vacuum converter. It is mounted on a flange and contains, in addition to the sensitive element, an oxygen injector heated by its own heat from the operating converter. The injector releases oxygen as a result of thermal decomposition of potassium permanganate (KMnO) 4 . The use of an oxygen injector contributes to maintaining the high sensitivity of the transducer operating in high vacuum conditions.

Halogen leak detectors are supplied with a calibrated "Galot" leak, the operation of which is based on the equilibrium outflow of a sublimating vapor of a solid substance (hexachloroethane) through a constantly open small hole. This simulates the flow of freon-12 in the range from 0.9 10 -7 to 1.3 10 -6 m 3 Pa/s.

To test objects (products) in the field or, if it is necessary to ensure autonomy of power supply, battery leak detectors of the BGTI-7 type are used, which have a registration unit with a sensitive element and a battery pack.

Since 1988, serial production of TI2-8 halogen leak detectors has begun, the sensitivity threshold of which corresponds to the sensitivity threshold of the GTI-6 leak detector. However, the TI2-8 leak detector is made on a new element base, is more compact and easy to use. It is designed to control the tightness of various systems and volumes that allow pumping out the internal cavity, as well as those filled with freon and a mixture of gases containing halogens. The time constant of the leak detector is not more than 1.5 s. Structurally, it is made in the form of a remote probe and a recording device. In addition, it is equipped with a vacuum sensor and a blower. Sensitivity threshold 1 10 -7 m 3 Pa/s. On its basis, tests can be implemented both in atmospheric conditions and in vacuum.

In recent years, new types of halogen leak detectors have begun to appear, the difference of which from serial models is that spatial separation occurs in the sensitive element ceramic material and an emitter with a collector. In this case, the possibility of poisoning the sensitive element is reduced and its overall performance is increased.

It should be noted that the area of ​​application of halogen leak detectors will narrow in the future, which is explained by the gradual withdrawal from the use of freon, which destroys the Earth's ozone layer, in testing. Apparently, in the future, halogen leak detectors will be most often used for monitoring halogen trace systems, in research laboratories and in special cases of testing objects.

The catharometric method of tightness control is based on using the dependence of the thermal conductivity of a gas mixture on the concentration of one of its components (test substance), the thermal conductivity of which differs significantly from the thermal conductivity of the other components.

To present the possibilities of the method, we present data on the thermal conductivity of certain gases λ g (Table 10.4).

A comparison of the thermal conductivities of individual gases and air shows that the use of the catharometric method is preferable in cases where helium or hydrogen is taken as test gases, or when chlorine is located inside the OK.

Table 10.4

Thermal conductivity of some gases and vapors at 0°C and 98.1 kPa

For practical application, the dependence of the thermal conductivity of a gas mixture on composition is described by an equation that is additive with respect to the thermal conductivities of the individual components of the mixture:

where FROM 1 ,FROM 2 ,..., C n- concentration of components in fractions of a unit; λ 1 , λ 2 ,…, λ n- thermal conductivity of the components.

The catharometric method is non-selective; it can be used to control the leakage of binary or quasi-binary test gases, for which relation (10.11) can be reduced to the form

where FROM n - volume fraction of test gas; λ cf - average thermal conductivity of the sum of undetectable components (for example, in air). In this case, λ g >>λ cf.

As follows from equation (10.12), for a binary gas mixture, its thermal conductivity is an unambiguous criterion for the flow of test gas.

To measure the thermal conductivity of a gas mixture, a current-heated conductor is used, placed in a chamber filled with the analyzed mixture. If heat transfer from the conductor to the chamber walls is mainly carried out as a result of heat conduction, then the following relationship takes place:

where Q m - the amount of heat given off by the conductor per second; l, d- length and diameter of the conductor; D- chamber diameter; λ cm - thermal conductivity of a mixture of gases; t P, t c is the temperature of the conductor and chamber walls.

At a constant heat given off by the conductor Q t and chamber wall temperature t c , depending on the ambient temperature, the thermal conductivity of the gas mixture will uniquely determine the temperature of the conductor, and hence its resistance, which is included in the circuit of the bridge measuring circuit. Based on this dependence, catharometric leak detectors and devices are made.

Rice. 10.17. Scheme of the sensitive element of the catharometric leak detector (a), bridge circuit of the leak detector (b)

The leak detector sensor consists of a body 1 with two parallel draw channels (Fig. 10.17, c) in which two thin platinum or platinum-rhodium filaments 2 are mounted, which act as electrical resistances. On fig. 10.17, b shows the resistance R 1 and R 2 included in the circuit of the bridge measuring circuit. The sensor is designed in the form of a remote probe, which is used for the process of probe testing of controlled objects. The leak detector kit includes several tips of different configurations for easy access to hard-to-reach controlled surfaces.

On the example of a leak detector type TP 7101M, the design and circuit features of katharometric leak detectors and possible directions for their improvement are considered. This leak detector is made portable, which makes it possible to test large-sized and extended objects by one or several operators, delimiting their control areas. The probe-transducer of the leak detector is connected to the measuring unit with a flexible hose. In the massive copper case of the converter there are working and comparative cells. The outlet openings of the cells are connected to a common gas flow source located in the measuring unit. To indicate a leak, the measuring unit is equipped with a pointer device and an audible signaling device. Evaluation of the dynamics of the catharometric leak detector showed that the time to reach the maximum signal is about 1 s. This is due to the delay in moving the test gas to the sensitive elements. The signal decay time is even longer and is approximately 5 s. Sensitivity threshold for helium 2.3 10 -6 m 3 Pa / s. Weight 4 kg.

As you can see, the sensitivity of the leak detector is low. However, the versatility of the leak detector is its great advantage, since the same device is more or less suitable for detecting leaks when products are pressure tested with various gases. It is promising to use such a leak detector to check gas pipelines with combustible gases (natural gas, propane, butane, etc.). The scope of katharometric leak detectors also extends to cases where it is necessary to detect gross leaks before highly sensitive tests, i.e. carry out preliminary control of objects.

The electron-capture method is based on the ability of the molecules of certain gases to capture electrons, while turning into electricity. negative ions. This property of substances is called electron affinity. It is characterized by the energy released during the formation of a negatively charged ion. For example, the electron affinity of oxygen atoms is 1.46 eV.

Schematically, this process can be considered on the basis of the relationship below. Under the influence of radioactive radiation of β-tritium, ionization of gas molecules occurs in the detector chamber N 2 and slow electrons are formed e m:

(10.14)

Under the influence of the applied voltage, these electrons move to the anode, as a result of which a current appears in the circuit. When a gas containing molecules with electron affinity enters the chamber of the sensitive element, negative ions appear. They have a much greater ability than electrons to recombine with positive nitrogen ions, which ultimately leads to a decrease in the number of electrons reaching the anode and, accordingly, to a decrease in the ionization (background) current. The decrease in this current as the sample gas passes through the sensing element serves as a measure of its amount.

Since different gases have different ability to capture electrons, the sensitive elements of such leak detectors are characterized by selectivity, for example, to halogen-containing organic compounds. The sensitivity of electron-capture sensing elements to various test gases depends on the degree of electronegativity or electron affinity of these gases. However, the electron affinity of a test gas varies with the energy of free electrons. The average value of the electron energy in the ionization chamber is determined by electric field and the nature of the carrier gas. The average energy of free electrons at a certain electric field strength is greater for monatomic gases (for example, argon) and less for polyatomic gases, for example, carbon dioxide. With an appropriate selection of the carrier gas and the potential applied to the chamber, electrons with any average energy can be obtained, as a result of which electron-capture leak detectors can be made selectively sensitive to various test gases.

There are several types of electronic capture leak detectors. All of them are characterized by leak indication using electronegative gases and vapors as test substances. To detect leaks in vacuum systems, the VTI-1 vacuum gauge-leak detector is convenient, which consists of a magnetron manometric transducer and a simple measuring unit. The converter is connected to a vacuum system. When searching for leaks using VTI-1, freon-12 and sulfur hexafluoride (SF6) are used. It is most expedient to use VTI-1 to check the tightness of oil-free vacuum systems.

Rice. 10.18. Scheme of an electronic capture leak detector

The area of ​​application of universal electronic capture leak detectors, which do not require evacuation of the tested objects, is much wider. First of all, this refers to the leak detector, which was called the electron-capture detector (after the name of the electron-capture detector widely used in chromatography). The leak detector is a two-electrode ionization chamber with a radioisotope (tritium) source of ionizing β-radiation. Converter I The leak detector consists of a detector 3, an ejector 2 and a throttle 4 for regulating the selection of a mixture of gases (Fig. 10.18). The ejector, creating a vacuum, provides the supply of test gas or air to the sensitive element. The transducer is connected to the cannula probe 1. Measuring unit II includes auxiliary pneumatic throttles 5 and 7 for adjusting the flow rate of the carrier gas, a filter 8 for cleaning the carrier gas from oil particles and other impurities. of these systems and blocks, the measuring part of the leak detector also includes an audible leak alarm generator, a comparator and other elements not shown in the diagram. The leak detector can be connected to external devices, such as a signal recording system, a device for automatically rejecting leaky products, etc.

Rice. 10.19. Diagram of a plasma leak detector

The use of the electronic capture leak detector under consideration is very effective when searching for leaks in high-voltage electrical devices with SF6 filling. It can compete with a manometric device by controlling air leakage in a chamber purged with nitrogen. In this case, a sensitivity threshold of 110 -5 m 3 Pa/s is reached.

Plasma leak detector TP2, which also registers leaks of electronegative test substances, consists of a discharge tube-leakage 1, capacitor electrodes 2, a measuring unit 3 and a leak indication unit 4 (Fig. 10.19). The leak detector is based on the use of the properties of a glow discharge, which, by shunting a high-frequency resonant circuit, causes a breakdown of high-frequency generation. When an electronegative gas appears in the discharge tube, the frequency of lasing interruptions increases due to an increase in the rate of ion recombination. The measuring unit provides the generation of signals proportional to the frequency of disruptions of high-frequency oscillations and the concentration of electronegative impurities in the air pumped through the tube.

The leak detector is portable, easy to use, sufficiently sensitive to test gases, has a small mass (2 kg), and is mainly used to search for leaks using a probe method. The sensitivity to the flow of SF6 gas is 0.7 10 -9 m 3 Pa/s, to the flow of freon-22 - 1 10 -8 m 3 Pa/s. Leak detector time constant - no more than 1 s.

Chemical method. In the control of objects operated with the use of special gases and gas mixtures, as well as in all other cases when the known methods of tightness control turn out to be of little use, the chemical method is the most acceptable. Several modifications of this method are known: application of an indicator mass on objects; the use of indicator tapes; use of indicator paint.

Common to all modifications is the use of the appropriate test gas, the creation of excess pressure of this gas in the object and visual observation of the effect of the interaction of the test gas with the chemical composition applied in one way or another to the alleged leaks. Most often, a process gas or mixture of gases is used as a test gas.

Various combinations of chemicals can be used as indicator masses. The main requirements for indicator masses are as follows: high sensitivity to test gas; preservation of technological properties during the time required to inspect the object; the indicator mass should not be aggressive with respect to the OK material.

As a test gas, carbon dioxide of various concentrations and some other gases are used. In the presence of leaks, the test gas, interacting with the indicator mass, causes spots to appear. various colors(yellow, blue, etc.). The persistence of stains after the termination of contact of the indicator mass with the test gas is up to 50 minutes. The properties of the applied indicator mass are preserved for tens of hours.

The principle of monitoring the tightness of equipment using indicator tapes is to stick the latter on the alleged leaks and observe the formation of spots when the indicator, which is impregnated with the test gas, interacts with the indicator. Indicator tapes are usually made from cotton fabrics. Their impregnation is carried out in a special solution until a uniform color is obtained. The composition of one of the recommended solutions with which the tapes are impregnated is 100 ml of ethyl alcohol, 15 ... 20 ml of glycerin, 1 ... 2 g of bromine-phenol blue and 20% ammonium sulphate solution. In addition to this solution, phenolphthalein and other compounds are also used. In order to exclude false colors of indicator tapes in gassed rooms, sometimes one of the tape surfaces is covered with a transparent gas-tight film, which has a sticky surface for connection with the indicator tape and the container under test. The presence of a transparent film contributes to the accumulation of gas escaping from the container under the film and the coloring of the indicator tape, as well as increases the sensitivity of the control and creates protection against coloring by the gases contained in the room.

Most often, an air-ammonia mixture with an ammonia concentration of up to 1 ... 3% is used as a test gas. The determination of tightness is reduced to a visual inspection of the alleged leaks, on which an indicator tape is applied, and to the fixation of spots on it corresponding to the leaks. The sensitivity of the method of indicator tapes is from 110 -7 to 710 -7 m 3 Pa/s.

Method of indicator paint finds application for the control of those Objects that are filled with a working medium during the manufacturing process, painted and dried, and then sent to the customer. In this case, the tightness control is carried out during drying. A special indicator, such as bromophenol blue, is added to the paint that serves as a paint coating, which reacts to the working environment. In places of leaks, the working medium enters into chemical reaction with indicator. As a result, blue spots form on the paint, indicating the location of the leak. One way to prepare indicator paint is to create a mixture of non-troglyphthalic gray paint with bromophenol blue indicator. The indicator paint retains its reactive properties for a long time, since it reacts to the leakage of the working medium even after it has dried. The sensitivity of control by the method of indicator paint reaches 1 10 -6 ... 10 -7 m 3 Pa/s.

Gauge method often used in practice, as it is one of the most accessible methods in implementation. It is based on registering a change in the total pressure in the OK or in the auxiliary chamber in which the OK is placed.

In recent years, in connection with the development of technology for monitoring small changes in pressure and temperature, the possibilities of the method have expanded. In practice, the drop (increase) in pressure is usually controlled for a certain time. The permissible change in the pressure of the gaseous medium in the object is set on the basis of the tightness standards determined by the designer.

The pressure change control method (manometric) is used mainly in preliminary testing of objects in order to identify relatively large through defects. Independently, this method is used for tightness control, when the requirements for the sensitivity threshold do not exceed 1 10 -5 m 3 Pa / s. When monitoring objects of small volume (Vl 10 -4 m 3 ), a sensitivity threshold of 5 10 -6 m 3 Pa/s can be achieved. /

Depending on the requirements for the degree of tightness of products, their dimensions, configuration and control purposes, tubeless or chamber (Fig. 10.20) methods of manometric control are used.

The mathematical model of the non-stationary process of pressure change in a manometric interconnected system has the form

(10.15)

where A 2 is a constant coefficient, depends on the parameters of the medium and the defect. In plane P, t the dynamic characteristics obtained on the basis of (10.15) have the form of parabolas (Fig. 10.21). The larger the defect, the faster the pressure equalizes in the product. R and and in the chamber R to at time t*.

In the figure, various curves marked with the corresponding signs (□, Δ, etc.) characterize the change in pressure in the object and in the chamber in the presence of a defect of a certain diameter in the wall of the object (for example, 50, 100 µm, etc.). For a tubeless control scheme, when
, passing to the limit, one obtains a mathematical model of such a system in the form

(10.16)

The second equation of this system shows that R k is a constant value, i.e. R k = P k 0 = R And where R a is atmospheric pressure.

Substituting this value R to the first equation (10.16), we obtain the differential equation

(10.17)

from which we find by integration

(10.18)

Graphs of the transient process for the considered control conditions are shown in fig. 10.22. The steepness of these characteristics is largely determined by the size of the defect.

With the tubeless version (see Fig. 10.20, a) in OK. create excess pressure P and 0 by supplying pressure P 0 to the inlet of the test system. Then valve 3 is closed. If there is a leak in OK 1, the leak sensor 2 registers the pressure drop P and, in accordance with the dynamic characteristics shown in Fig. 10.22.

For a chamber control circuit, the solutions of differential equations (10.15) have the form

(10.19)

(10.20)

Each of equations (10.19) and (10.20) defines in coordinates P, t parabola. The axes of these parabolas are parallel to the y-axis R and directed in opposite directions. They intersect at a point whose coordinates are determined by solving the equation

R and ( t) = Р to ( t)

Despite the apparent simplicity of the method, its use is often hindered due to the relatively low sensitivity of the method, and in some cases, the long duration of the measurement cycle. With the improvement of the method, the elimination of the influence of temperature on the results of control plays a leading role.

The gas-hydraulic method (bubble method) is based on the observation of test gas bubbles 4 (Fig. 10.23) released from the leak 3 during gas pressure testing of the control object 2 immersed in liquid.

The advantages of the bubble method are its simplicity: it does not require instrumentation and special test gases, it has a high sensitivity, the operations of detecting and localizing leaks are combined.

Its disadvantage is the need to immerse the product in the tank, which is impossible for large products. Coating the surface with a liquid film is a labor-intensive operation, there is a risk of surface corrosion as a result of prolonged exposure to liquid (water) residues. The sensitivity of the method is sometimes insufficient. The results of the audit to a large extent depend on the conscientiousness of the controller.

Using the bubble method as an example, it is convenient to trace the influence of the sensitivity threshold of the leak detection tool and test conditions on the sensitivity threshold of the leak detection method as a whole. The test gas bubbles are actually the means of detecting leaks. Let us consider the process of bubble formation to estimate the threshold of sensitivity. Under the influence of the pressing pressure created in the test object, a bubble is formed at the mouth of the leak. The amount of gas in it is determined by the product of the volume of the bubble V n on the pressure inside it R n. This pressure is less R ODA due to pressure drop on leaks. Let us determine Рп from the condition that it is equal to the sum of external pressures acting on the bubble: atmospheric pressure on the liquid surface R atm, hydrostatic fluid pressure R g and surface tension R n.

Value P r = gρ h, where ρ is the density of the liquid, a h is the height of the liquid column above the bubble. The pressure caused by surface tension forces R n = (2F burn cosθ)/r=4F burn /D. Here F zhg is the force of surface tension liquid - gas, per unit length on the surface of the liquid. For the case under consideration, D = 2r is the bubble diameter, θ = 0. Thus,

(10.21)

where t is the bubble formation time.

The flow of gas through the leak increases the diameter of the bubble up to the moment of its detachment. This moment occurs when the Archimedean force acting on the bubble gρV n becomes equal to, and then exceeds the forces of adhesion of the bubble to the surface, equal to the force of surface tension liquid - gas, multiplied by the perimeter of the leak: F zhg = π d, where d- leak diameter. Thus, the separation condition

Here D 0 is the bubble diameter at the moment of separation. The formula shows that the larger the leak diameter, the larger the bubbles. However, since from the leak diameter ( d) and quantities characterizing the properties of the liquid ( F zhg and ρ), the cubic root is extracted, the diameter of the detached bubble changes little with a change in the named values. Usually, the diameter of the emerging bubble is assumed to be 0.5...1 mm. Bubbles smaller than 0.5 mm are difficult to see. From here you can find the minimum diameter of the leak d min =2.8 µm.

The minimum gas flow recorded by the bubble house method can be found from the assumption that the time t 0 from the beginning of bubble formation to its detachment is 30 s. If this time is longer, then too infrequently formed bubbles are difficult to notice.

Typically, the hydrostatic pressure is much less than atmospheric pressure, it even tends to zero as the distance from the leak to the surface decreases. h. The pressure of surface tension forces is also significantly less than atmospheric pressure. As a result, from (10.31) we determine the minimum recorded gas flow using the bubble method:

(10.22)

At D 0 =0.5 mm, t 0 = 30 s, R atm = 101325 Pa we get J min \u003d (3.14 0.5 3 10 -9 101325) / (6 30) \u003d 2.2 10 -7 W. This value defines the sensitivity threshold of the bubble method as a means of leak detection. Now let's consider the sensitivity (lower limit of indication) of the entire system of leak detection by the bubble method.

Using equations for leakage through a channel - flow for viscous flow J c = π d 4 R 2 atm /256η in l, we determine the sensitivity of the entire leak detection system AT m i n reduced to standard conditions:

The sensitivity of the method to leaks can be increased not only by increasing R ODA, but also the use of gases with a viscosity lower than that of air. For example, if hydrogen is used instead of air, then η / η in \u003d 0.5 and P def / P atm \u003d 10, hence B min = 1.1 10 -9 W. This should be understood in such a way that with the help of hydrogen and a pressing pressure of 10 atm, the sensitivity threshold of the control system is removed and leaks are detected, which, during vacuum tests under standard conditions, will leak about 1 10 -9 W.

Consider some options for the bubble method. As noted earlier, instead of immersing the test object in the tank, it is covered with a liquid film (soaping method), in which the formation of bubbles is observed. The liquid should be viscous, flowing slowly, with low surface tension. It is prepared from an aqueous solution of soap, glycerin and gelatin (soap film) or from an aqueous solution of dextrin, glycerin, alcohol and other additives (polymer film). The viscosity ensures slow flow, and the reduction in surface tension facilitates the formation of bubbles.

The film is applied to the surface of the product with a soft brush or spray. Observation of the formation of bubbles begins 2-3 minutes after application soap film. When using a polymer film, the detection of large defects is observed immediately after the application of the film, and small defects - after 20 minutes. Bubbles in such a film do not burst, but remain in the form of "cocoons" during the day. The sensitivity is determined by the approximate formula (10.22).

The highest sensitivity of the bubble method can be achieved using the method of soaping and observation in a local vacuum chamber with a pressure of about 10 4 Pa. Such a chamber (Fig. 10.24) is "sucked" to the surface of the test object under the action of atmospheric pressure. Observation of the appearance of bubbles, cocoons or breaks in the film is carried out through the viewing window. In this case, the atmospheric and hydrostatic pressures are equal to zero, and formula (10.22), taking into account the double contact surface of the film with the gas, takes the form

Rice. 10.24. Local vacuum chamber:

1 - body. 2 - glass, 3 - pumping fitting, 4 - seal, 5 - wall of the test object, 6 - pressure gauge fitting.

Taking the previous test conditions and the value of surface tension for water 0.075 N/m, we get J m i n \u003d l,3 10 -9 W, i.e. the threshold of the sensitive method as a means of leak detection is reduced by a factor of 170 compared to a test in an atmospheric pressure tank. At the same time, the possibility noted above of increasing the sensitivity of the control method as a whole by increasing the pressing pressure and using hydrogen as a test gas instead of air remains. As a result, the bubble method will make it possible to detect leaks, which, during vacuum tests under standard conditions, will correspond to leakage of about 10 -11 W.

The bubble method is also used to test closed test objects containing gas under atmospheric pressure. Excess gas pressure inside the control object is created by immersing the object in a hot liquid. In this case, the change in pressure is determined from Charles's law

where R- pressure; T-absolute temperature; indexes "1" and "2" refer to cold and heated object.

Let us take normal conditions as initial conditions. Heating temperature T 2 is limited to the fact that bubbles begin to form in the liquid. For water it is 80°C. From here it is easy to find that

Substituting this value into (10.23), we find that the sensitivity of the method, reduced to standard conditions, is equal to 33 10 -6 W.

Possibilities for increasing the sensitivity lie in the use of liquids with a high boiling point. For example, vacuum oil has a bubble point of 150°C. This makes it possible to increase R OPD / R atm up to 1.55. In addition, the tests are carried out in a vacuum chamber with a viewing window. As a result, leak detection with a threshold sensitivity of about 10 -8 W is provided.

hydraulic methods. The process of hydrotesting, which is subjected to many products, can be used as a method of leak detection. The control for the detection of large leaks is called the tightness test. Such tests are subjected to ship hulls, hydraulic tanks.

Tests are carried out either at the static pressure of a water column 0.5 ... 2.5 m high with a holding time of at least 1 hour, or with a jet of water under pressure. Less responsible objects are controlled, with water without pressure or scattered, with a stream of water. The results are considered satisfactory if no jets, streams, continuously flowing drops of water are observed.

Vessels, housings, pipe systems and other objects that must withstand significant pressures are subjected to hydrotesting by pressure testing at a pressure significantly higher than the working one. This process is also used to find leaks, where a leak may be evidence of a leak on the wall of the object.

To facilitate the search for leaks and lower the sensitivity threshold of the method, the test liquid is made contrast, for example, it is given the property of luminescence. The most widely used luminescent-hydraulic method. It consists in the fact that a concentrated solution of the disodium salt of fluorescein (uranin) is introduced in the proportion of 0.1% (1 l / g) into the water intended for crimping. The composition is thoroughly mixed. The duration of holding under pressure is from 15 minutes to 1 hour (depending on the thickness of the walls of the test object).

Then, each controlled area, the surface of the OK is subjected to inspection in the rays of ultraviolet light of a mercury-quartz lamp. First, large leaks are detected, through which the water from the fluorescein solution does not completely evaporate and provides sufficient luminescence. Then the surface is moistened with a moisture spray and again inspected. Fluorescein, which has passed through small leaks, dissolves in this water and begins to glow. In ultraviolet rays, through defects are detected as luminous green dots (pores), stripes (cracks). Illumination of the room with visible light should be no more than 20 lux.

The sensitivity threshold of the luminescent-hydraulic method, as for all liquid methods, is determined empirically by comparison with the results of control by gas methods. At an excess pressure of at least 2 10 7 Pa, the luminescent-hydraulic method detects defects, which, when controlled by gas methods, correspond to leakage of 10 -10 ... 10 -9 W under standard conditions. When the pressure drops to 2 10 5 Pa, leaks of 10 -5 ... 10 -4 W are detected.

If hydropressure of the product is not provided for by the technology or the creation of a pressure difference is impossible due to the low strength of the product walls, a capillary (usually luminescent) method is used to detect leaks. It differs from that discussed in Chap. 2 by the fact that the penetrant and the developer are applied to different sides partition surface. The penetrating liquid (Noriol with kerosene) is applied with a brush in a generous layer and a certain amount of penetrant is added every 20 minutes. The developer (alcohol suspension of kaolin) is applied in a thin layer on the opposite surface. The search for defects by inspection under ultraviolet light begins no earlier than 10 minutes after applying the penetrant and developer. The total exposure time depends on the wall thickness of the product and the requirements for the product in terms of tightness, it can reach 14 hours. Long exposure time is the main drawback of the capillary leak detection method.

Less responsible objects are controlled by the kerosene test method. On the one hand, kerosene (penetrant) is applied to the surface of the partition, and on the other hand, a developing coating in the form of a solution of chalk in water. Exposure ranges from 40 to 120 minutes, depending on the thickness of the partition and its location. Leaks are identified by the appearance of dark spots of kerosene on the chalk coating.

Means and devices that provide the process of leak detection. The following tools are required to perform leak detection testing: a test substance, devices for creating and measuring pressure differences, means for detecting a test substance or measuring its amount, as well as tools and technology for preparing an object for testing. The effectiveness of leak detection control depends on the entire control system, i.e. combinations of a certain method, means, control mode and method of preparing an object for control. The threshold sensitivity of the control system is determined by the value of the minimum leakage under standard conditions that can be detected by this system.

The higher the sensitivity of the control system, the lower the sensitivity threshold.

Test substances should penetrate well through leaks and be well detected by means of leak detection. They should be inexpensive, not have a harmful effect on people and the object of control.

Gases (more often) and liquids are used as test substances. The lower the viscosity and molecular weight of the gas, the better it penetrates through leaks. The main requirement for test gases (as well as for all test substances) is the existence of highly sensitive methods for their detection. The most common test gases are listed in Table. 10.2.

In some cases, volatile liquids are used as test substances: alcohol, acetone, gasoline, ether. Usually, indicators capture the vapors of these liquids, and methods for monitoring such liquids are classified as gas.

Liquid test substances include water used in hydrotesting (hydropressure), water with luminescent additives that facilitate the indication of leaks, wetting liquids - penetrates.

Means for creating a pressure difference include liquid or gas (compressors), pumps, vacuum pumps, cylinders with test gas or liquid, pipelines, fittings (valves, fittings, nozzles), pressure gauges, etc.

During vacuum tests, the residual air pressure is 0.1 ... 1 Pa. This pressure is achieved using a mechanical foreline pump. Deeper vacuum (10 -4 ... 10 -5 Pa) is achieved with the help of steam-oil pumps. However, these pumps cannot pump air into the atmosphere. For them, the maximum outlet pressure is 10 ... 500 Pa, which is provided by a foreline pump. To prevent oil from steam pumps from entering the vacuum system, reflectors and traps are placed between them, cooled by water or liquid air, filled with sorbing substances. In this case, a vacuum of 10 -6 ... 10 -7 Pa is reached.

An important characteristic of the pump is the speed of action: the volume of pumped gas at a certain pressure at the inlet pipe of the pump. The concept of effective pumping speed S e is often used. It determines the volume of gas pumped out by the pump, taking into account the limited conductivity of the nozzles and valves connecting the pump to the pumped volume.

When pressure testing with gas, the pressure must be lower than the allowable calculated value for this object. Typically, pressure testing is used no more than 2 10 5 Pa (about 1 atm) and only in some cases up to 5 10 6 Pa. The limitation is associated with catastrophic consequences from the rupture of the control object, pressurized by gas.

With hydropressure, the rupture of an object is much less dangerous, since the liquids are practically incompressible. In this case, it is possible to use significantly: high pressures. For example, hydrotests for the strength of the control object are usually carried out at pressures that are 25 ... 50% higher than the calculated one. If the steam boiler is designed to operate at a pressure of 3 10 7 Pa (300 atm), then the pressure during hydrotesting is adjusted to 3.75 10 7 Pa and at the same pressure the control is carried out by the luminescent-hydraulic method.

When hydropressing, it is important that there are no "air cushions". Therefore, the control object before filling with liquid is pumped out or compressible air is released through the valve, which is located in the upper part of the object.

Manometers are used to measure pressure. Pressure above 10 4 Pa ​​is measured using mechanical deformation, piezoelectric and other types of pressure gauges. Lower pressures are measured using thermoelectric, ionization and other vacuum gauges (vacuum gauges). These gauges are calibrated using a liquid and compression gauge. Each type of pressure gauge has a measurement limit determined by the principle of its operation. For example, a pre-vacuum is measured with a thermal pressure gauge, and a high vacuum is measured with an ionization pressure gauge.

Leak detection tools. To detect leaks, special devices are used - leak detectors and non-instrument methods of leak detection. The most important characteristic of a leak detector is the sensitivity threshold. This is the smallest flow of gaseous or liquid test substance registered by the leak detector. Through experiments and calculations, it is converted to leakage under standard conditions. Leak detection tools are also characterized by the pressure range at which they operate, the time of preparation for work and testing, the possibility of quantitative readings, weight, etc.

In table. 10.2 lists the various methods for detecting leaks using the leak detection tool used, and indicates the principle on which they are based. The methods are arranged as the sensitivity threshold increases, i.e. deterioration of the ability to detect small leaks. The approximate sensitivity threshold of the air flow control system under standard conditions is indicated, which depends not only on the leak detection tool, but also on the method of using this tool. For example, the application of mass spectrometric method with accumulation gives the lowest threshold of sensitivity, and in dynamic mode it is 100 times higher.

Preparation of objects for control. The main task of preparing for control is to release leaks from the substances that cover them, oils, emulsions, condensed moisture from the surrounding air. High overpressure pressure tests dislodge plugging agents from leaks, so surface preparation is not required. In the control of wetting liquids, the surface preparation on both sides of the product is the same as in the capillary method. Surface preparation is most important when testing with a gas method with a small pressure difference, for example, when vacuum testing.

Protective surface coatings (painting) interfere with control, so the tightness is checked before they are applied. Oil, emulsion is removed by wiping with solvents. To open leaks (as well as outgassing), heat treatment of the surface is carried out, which is divided into several classes.

For complete opening of leaks (first class), the test object is heated in a vacuum. The optimum is heating to a temperature of 400°C at a vacuum of 0.1 Pa with exposure from 5 min to 3 h, depending on the object of control. Heating to a high temperature is necessary because the boiling of liquid in capillaries occurs at a higher temperature than in normal conditions. For example, water boils at a temperature of 300...400°C. If heating to such a high temperature is impossible, then the product can be heated in air to a temperature of 250 ... 300 ° C with an exposure of at least 30 minutes.

The second class of preparation is heating in air up to 150...200°C with a holding time of at least 10 minutes or in a vacuum (10 Pa) - up to 100...200°C with a holding time of at least 1 hour.

The third class Preparations - the same heating in air or in vacuum up to 80 ° C with an exposure of at least 2 hours. Finally, the fourth class provides only surface drying.

Promising methods. An analysis of the trends in the development of methods and methods for monitoring tightness has revealed promising areas in the technology of leak detection, which are currently developing.

First of all, the prospects for leak detection are associated with the expansion of hardware implementation of control methods. Thus, progress in the absorption spectroscopy of gases using monochromatic radiation to detect microimpurities in ambient air in combination with the optoacoustic effect has made it possible to take a new approach to solving the problem of increasing the reliability and efficiency of monitoring the tightness of thin-walled closed volumes. On this basis, the first samples of optical-absorption leak-detecting equipment were created using nitrous oxide as a test substance.

Promising physicochemical methods of tightness control based on the effect of the interaction of the test gas with the surface of the defect or a special composition, and promoting an increase in the conductivity of the defect, are widely developed. Based on the same methods, new types of sensitive leak detectors are created, for example, piezo-weighted ones, which use a special coating on the surface of quartz that interacts with the test gas.

In addition to the leak detection devices discussed above, which are mass-produced by instrument-making enterprises, a number of devices have been created that are used at individual enterprises to test specific types of products. These include manometric, acoustic, infrared, laser and other leak detection devices and systems.

Manometric leak detectors are usually made on the basis of serial membrane elements and blocks. Most often, such devices are based on highly sensitive membrane or bellows differential pressure gauges. The main search in the direction of strengthening the capabilities of manometric devices for monitoring tightness is associated with the selection of a membrane, the creation of temperature compensators and computerization of the process of manometric tests.

Acoustic leak detectors based on the registration of ultrasonic vibrations of a gas jet flowing through a through defect have not received the expected distribution due to their low sensitivity and the effect of extraneous noise on test reproducibility. As a rule, acoustic leak detectors (for example, type TUZ) allow you to find leaks with a nominal diameter of 0.1 ... 0.15 mm at an overpressure inside the products of 0.04 ... 0.05 MPa. The scope at the current level of their development will be limited simple terms their operation, low requirements for the degree of tightness of industrial products.

The search for new test substances and progress in the development of the optical absorption gas analytical method allowed aviation industry specialists to create new type leak detectors IGT-4. This is an optical absorption leak detector based on the indication of environmentally friendly test gas - nitrous oxide.

Its threshold of sensitivity to the flow of nitrous oxide is 6.5 10 -7 m 3 Pa/s. The IGT-4 type leak detector is simple and reliable in operation, it operates in automatic mode, which is carried out using a built-in microprocessor.

The development of science and technology in recent years leads to the emergence of new ideas for gas analysis, including leak detection equipment. This primarily applies to solid-state semiconductor technology for measuring the parameters of gas flows and gas traces. Apparently, in the coming years, the development of this direction will lead to the creation of new types of leak detection equipment.

USSR State Committee for Supervision

for the safe conduct of work in the nuclear power industry

RULES AND REGULATIONS IN NUCLEAR POWER

UNIFIED METHOD FOR CONTROL OF BASIC MATERIALS (SEMI-FINISHED PRODUCTS), WELDED JOINTS AND SURFACE OF NPP EQUIPMENT AND PIPELINES

Tightness control.
gas methods.
PNAE G-7-019-89

1. GENERAL PROVISIONS

1.1. The tightness control of structures and their components is carried out in order to detect leaks due to the presence of through cracks, lack of fusion, burns, etc. in welded joints and metallic materials.
1.2. Tightness control is based on the use of test substances and registration of their penetration through leaks in structures using various devices - leak detectors and other means of recording a test substance.
1.3. Depending on the properties of the test substance and the principle of its registration, control is carried out by gas or liquid methods, each of which includes a number of methods that differ in the technology for implementing this principle of registration of the test substance. At the same time, depending on the method used, the location of the leak or the total leakage (leakage degree) is determined during the tightness control. The list of applied methods and methods of control is given in Table 1
1.4. The magnitude of a leak or total leakage is estimated by the flow of air through the leak or all leaks present in the product, under normal conditions, from the atmosphere to vacuum. The ratios of flow units are given in the reference Appendix 1.
1.5. A control system is a combination certain ways and modes of control and method of preparing the product for control.
1.6. The threshold sensitivity of the control system is characterized by the value of the minimum detected leaks or total leakage.

2. CLASSIFICATION AND SELECTION OF LEAK CONTROL SYSTEMS

2.1. All control systems are divided by sensitivity into five tightness classes given in Table. 2.
2.2. The tightness class is set by the design (engineering) organization in accordance with the requirements current Rules control depending on the purpose, operating conditions of the product and the feasibility of the methods of control and preparation assigned to this class, and is indicated in the design documentation.
2.3. The choice of a specific control system is determined by the assigned tightness class, structural and technological features products, as well as technical and economic indicators of control.
2.4. In accordance with the assigned class of tightness, control is carried out according to the technology of control flow charts, which indicate specific methods of control and preparation of the product for control. In case of deviations from the requirements of this methodology, the documents must be agreed with the leading industry materials science organization.

3. EQUIPMENT AND MATERIALS

3.1. When testing the tightness, equipment, instruments and materials must be selected in accordance with reference appendices 2 and 3. It is allowed to use domestic and imported equipment, instruments and materials that do not meet the requirements of this document and are not specified in the annexes.
3.2. The parameters and technical characteristics of the equipment, instruments and materials used for leak testing must comply with the passport values, state standards and technical conditions.
3.3. Metrological verification devices are subjected, in the passports of which the volume and nature of verifications are indicated. Verifications are carried out by Gosstandart bodies at the respective enterprises. The frequency of verifications is carried out in accordance with the requirements of the passport for the device.
3.4. Leak detectors, regardless of the selected control method, must be set to optimal sensitivity in accordance with the instructions in the technical description and instructions for their operation.

4. GAS METHODS OF TIGHTNESS CONTROL

4.1. Requirements for the preparation of the surface of structures subject to control of tightness by gas methods

4.1.1. If a protective coating is applied to the surface of the product or assembly unit, the tightness control should be carried out before the specified operation.
Note . In case of technical impossibility, it is allowed to carry out a tightness test after application protective coatings, which should be specified in the production and technical documentation (PTD).
4.1.2. The surface of products, assembly units, welded joints of products to be checked for tightness should not have traces of rust, oil, emulsion and other contaminants.
4.1.3. Organic contaminants from accessible areas of the surface of the product should be removed by washing with organic solvents, followed by tilting the product or bubbling the poured solvent. The volume of solvent to be poured must be at least 100% of the free volume of the product.
4.1.4. Alcohol, acetone, white spirit, gasoline, freon-113 or other organic solvents should be used as cleaning liquids, providing high-quality removal of organic contaminants.
4.1.5. After cleaning, the solvent should be drained and the product cavity should be blown dry clean air until the odor of the solvent is completely removed.
4.1.6. The quality of cleaning should be controlled by wiping the controlled surface with a clean white lint-free cloth, followed by its inspection. The absence of dirt on the fabric indicates a quality cleaning of the surface.
4.1.7. With appropriate indication in technical process the quality of cleaning should be controlled by examining the surface area of ​​the product or welded joint in the rays of ultraviolet light, and if the surface is unacceptable for inspection in the rays of ultraviolet light, a piece of coarse calico after wiping the surface with it. The absence of luminous spots on the controlled surface or a piece of coarse calico when illuminated ultraviolet light indicates a good surface finish.
4.1.8. The final preparation operation - drying the surface of products and cavities of possible through defects from moisture and other liquid media - should be carried out immediately before the tightness test. After drying, in order to maintain the purity of the products, work should be carried out in clean overalls (robe or overalls) and gloves made of linen fabric.
4.1.9. Electric furnaces, inductors, heaters, installations, steaming stands, etc. should be used as heating means. For heating, you can use the method of electrical resistance using alternating or direct current.
4.1.10. When drying without evacuation, the holding time at the required temperature must be at least 5 minutes. The temperature is determined by the given tightness class.
4.1.11. If it is impossible to check the tightness of products immediately after drying, it is allowed to store the dried product for no more than 5 days. under the following conditions:

• controlled areas must be protected from contamination and liquid media by protective materials;
• atmospheric air moisture must not condense on the surface of the controlled product. To prevent the phenomenon of moisture condensation (for example, when products are brought into a room where the air temperature is higher than the surface temperature of the product, the air temperature in the room decreases, when the product is cooled when test gas is supplied from a cylinder), it is necessary to take measures, guided by reference tables of temperature ratios ambient air, relative and absolute humidity. For example, when relative humidity air 80% and a temperature of 20°C, the surface temperature of the product should not be less than 17°C;
• humidity in the room for storing dried products should not exceed 80%.

4.1.12. If it is necessary to transport products, the possibility of contamination and condensation of moisture on the surface of the product should be excluded.

4.2. Leak testing with helium leak detectors

4.2.1. Threshold sensitivity of helium leak detectors and control methods. Working scale.

4.2.1.1. The threshold sensitivity of leak detectors is characterized by the minimum flow of the test substance that the leak detector can register. The threshold sensitivity of helium leak detectors must be at least 1.3.10-10 m3* Pa/s (1.10-6 l×µm Hg/s). The threshold sensitivity of the control method is characterized by the minimum flow or amount of the test substance, which is fixed in the control scheme.
4.2.1.2. The threshold sensitivity of helium leak detectors is determined at the beginning of each shift according to the method given in Appendix 4.
4.2.1.3. The threshold sensitivity of the control method is determined after testing the product, a batch of similar products or a simulator, the design of which is consistent with the HOMO according to the method given in Appendix 5.
4.2.1.4. The threshold sensitivity of the vacuum (helium) chamber and thermal vacuum methods should be at least 6.7.10-10 m3 × Pa / s (5.10-6 l × μm Hg / s), helium blowing methods and helium probe - at least 6, 7.10-9 m3×Pa/s (5.10-5 L×µmHg).
4.2.1.5. If the threshold sensitivity of the control method is below the values ​​specified in clause 4.2.1.4, then the product or batch of products must be re-inspected.
4.2.1.6. A sign of the presence of a through defect is an increase in the instrument readings above the average background readings by an amount equal to the difference between the maximum and minimum values background in the test scheme. This value must not exceed 50 mV for all control methods (except for the probe method) and 100 mV for the probe method.

Notes :
1. Average background readings before starting the test by any method should not be more than 2/3 of the working scale.
2. If background readings exceed the specified value, a background compensation circuit should be used.

4.2.2. Helium (vacuum chamber) method.

4.2.2.1. The essence of the helium or vacuum chamber method lies in the fact that the controlled product is placed in a sealed metal chamber. A leak detector is connected to the chamber or product through an auxiliary pumping system, after which helium is supplied under pressure to the chamber (helium chamber method) or to the product (vacuum chamber method). In the presence of a leak, helium, as a result of a pressure drop, enters the evacuated volume connected to the leak detector. The control scheme by the vacuum chamber method is shown in Fig.1.

Rice. 1. Scheme of installation for control by the vacuum chamber method
1 - helium leak detector,
2 - leak,
3 - cylinder with argon,
4 - camera,
5 - product,
6 - manovacuummeter,
7 - gearbox,
8 - helium balloon,
9 - vacuum pump,
10 - vacuum valve,
11 - calibrated leak
4.2.2.2. When designing and manufacturing a helium (vacuum) chamber, the following requirements must be taken into account:

• to speed up pumping, the shape of the chamber is recommended to be cylindrical (it is allowed to manufacture the chamber according to the design configuration);
• the tightness of flange connections should be provided, as well as the tightness of the outlet from the structure itself or the technological adapter from the structure to the helium cylinder;
• the controlled structure must not come into contact with the inner surface of the chamber.

4.2.2.3. Control procedure:

• the controlled product is prepared in accordance with the requirements of subsection. 4.1;
• the product is placed in a metal chamber, inner surface which is pre-cleaned and dried;
• after sealing the chamber cover and installing a pressure gauge, the cavity of the chamber (product) is pumped out to a residual pressure of 7 - 8 Pa [(5-6) .10 -2 mm Hg. Art.;
• before filling the controlled product (chamber) with helium, its cavity is preliminarily pumped out to a pressure not higher than 700-1400 Pa (5-10 mm Hg);
• after reaching the required residual pressure in the chamber (product), the inlet valve of the leak detector opens and the auxiliary pumping system is turned off;
• in the case of a gradual decrease in pressure in the mass spectrometer chamber, it is necessary to supply dry nitrogen to the mass spectrometer chamber using regulating leaks;
• in the event of an increase in pressure in the chamber of the mass spectrometer, it is necessary to partially open the valve of the auxiliary pumping system or close the inlet valve of the leak detector;
• helium or an air-helium mixture is supplied into the cavity of the product (chamber) in the proportions established by the technological map for control;
• holding the product (chamber) under pressure.

4.2.2.4. The duration of exposure of the product (chamber) under pressure should be at least 5 minutes at a vacuumized volume of up to 0.1 m3, from 0.1 to 0.5 m3 - at least 10 minutes, more than 0.5 to 1.5 m3 - not less than 15 minutes, over 1.5 to 3.5 m3 at least 20 minutes, over 3.5 - 40 minutes.
4.2.2.6. Helium should be removed by blowing the cavity of the product (chamber) with dry compressed air or its pumping.
It is allowed to collect the removed helium for use in the subsequent control.
4.2.2.5. If it is necessary to control a section of a product or a separate welded joint, it is allowed to install a local camera on the controlled section or a welded joint.
The control procedure is similar to that specified in clause 4.2.2.3.
The duration of exposure under pressure is set depending on the pumped volume in accordance with clause 4.2.2.4.
4.2.2.7. When checking the closing weld of the product, the product is evacuated and helium is supplied into the cavity of the product, followed by welding of the closing seam in a helium flow. After welding, it is necessary to test the closing seam using the local vacuum chamber method. The duration of the control is determined by the volume of the chamber in accordance with clause 4.2.2.4.
4.2.2.8. Quantification of the total flow of the test substance through leaks in the product should be carried out according to the method described in Appendix 6 (reference).

4.2.3. A method for pressurizing closed shells with helium.

4.2.3.1. The control method of pressing closed shells consists in the fact that the product or the closing seam is placed in a special chamber in which helium pressure is created. If there is a leak in the seam, helium penetrates into the closed volume of the product. Next, the product is controlled by the accumulation of helium in a vacuum chamber in which the product is placed.
4.2.3.2. It is recommended to check the tightness of the closing weld by pressure testing for products with small volumes (up to 10 l).
4.2.3.3. Control should be carried out in the following sequence:

• the product is placed in a compression chamber and kept under helium pressure for a certain time;
• after pressure testing, the product is removed from the chamber, the outer surface of the product is blown with compressed air or nitrogen to remove helium and kept in air for 1–2 hours;
• before installing the product, the internal cavity of the chamber attached to the leak detector is pumped out with an auxiliary pump. The background readings of the outlet device of the leak detector are recorded at a pressure in the chamber of 1 - 7 Pa [(1 - 5) .10 -2 mm Hg. Art.] with the auxiliary pump turned off;
• the product pressed with helium is placed in a vacuum chamber and the chamber with the product is pumped out to a pressure of not more than 1-7 Pa, the auxiliary pump is turned off and helium is accumulated in the chamber for at least 1 hour, after which the inlet valve of the leak detector is opened and the leak detector readings are recorded.
• Exceeding the signal of the output device of the leak detector by 1 V or more above the background readings is a sign of a leak in the closing seam of the product.

Note . In order to exclude an increased helium background during the testing process, it is forbidden to use the chamber in which the product was pressed with helium.
4.2.3.4. The duration of pressure testing of the product with helium should be at least 120 hours at a pressure of 1.106 Pa (10 kgf/cm2), at least 50 hours at 2.106 Pa (20 kgf/cm2), at least 13 hours at 5.105 Pa (50 kgf/cm2).

4.2.4. The method of thermal vacuum testing.

4.2.4.1. The essence of the tests lies in the fact that the product to be controlled is heated in a vacuum chamber to a temperature of 380 - 400 ° C at a pressure inside and outside the product not higher than 0.1 Pa (10 -3 mm Hg), and then it is controlled when helium is supplied into the heated article or into the chamber in which it is placed.
4.2.4.2. Control procedure:

• the product is prepared for control in accordance with paragraphs 4.1.1 - 4.1.7;
• the product is placed in a metal chamber;
• the chamber and the internal cavity of the product are evacuated to a pressure not higher than 0.1 Pa (10 -3 mm Hg);
• the product is heated in furnaces or heating devices to a temperature of 380 - 400 ° C and maintained at this temperature for 3 - 5 minutes. The heating rate is determined by maintaining a constant pressure in the chamber and the product not higher than 0.1 Pa (10 -3 mm Hg) and the design of the product;
• the inlet valve of the leak detector opens when the pumping group of the chamber (or product) is turned off at the same time.
• Steady background readings of the leak detector are fixed;
• helium is supplied to the controlled product (or chamber) up to the required pressure;
• the product (chamber) is maintained under pressure, while the readings of the leak detector are recorded. The duration of exposure is selected in accordance with clause 4.2.3.4;
• after cooling to a temperature not exceeding 50°C, the chamber opens.

4.2.5. Helium probe method.

4.2.5.1. The essence of the method lies in the fact that the product is filled with helium or a helium-air mixture to a pressure above atmospheric, after which the outer surface of the product is controlled by a special probe connected by a metal or vacuum rubber hose to a leak detector. As a result of the pressure difference, helium penetrates through the existing through defect and enters the chamber of the leak detector mass spectrometer through the probe and hose. A certain design of the tip of the probe, made in accordance with the profile of the controlled surface, allows you to determine the location of the through defect in the product. The tip of the probe must cover the area to be checked in width by at least 5 mm on each side. If the width of the nozzle is smaller, then the control should be carried out in several passes.
The control scheme by the helium probe method is shown in fig. 2


Rice. 2. Scheme of installation for control by means of a probe
1 - helium leak detector,
2 - thermocouple lamp,
3 - vacuum hose,
4 - vacuum pump,
5 - (Note from Webmaster: nothing for 5)
6 - product,
7 - probe,
8 - manovacuummeter,
9 - helium balloon
4.2.5.2. When checking by the probe method, adjustable probes-catchers with a conical nozzle with a volume of not more than 1 mm3 and a distance of an adjustable locking needle from the controlled surface of not more than 5 mm are used. One of options design execution is a probe-catcher according to hell. 358-00-00 and 358-01-00.
4.2.5.3. The following requirements apply to a helium probe test facility:

• all connections of the installation must be checked with the probe in the closed position by blowing;
• the part of the installation intended for supplying helium to the controlled product must be tested by the helium probe method at a helium pressure of at least 1.5 P, where P is the helium pressure during control;
• in the case of using a hose made of vacuum rubber to connect the probe to the leak detector, the hose must be flushed to reduce gas separation with an alkali solution (15%), clean running water, distilled water and dried with rectified alcohol. The outer surface of the hose is rubbed castor oil;
• the length of the line connecting the probe to the leak detector should be minimal. possible. Maximum length the highway is determined by clause 4.2.1.4 when assessing the sensitivity of the method according to Appendix 5.

4.2.5.4. Control should be carried out in the following sequence:

• with probe 7 closed (see Fig. 2), hose 3 is evacuated by vacuum pump 5 for 15–20 minutes;
• The probe is adjusted so that joint work auxiliary vacuum pump and leak detector pumps, the residual pressure measured by thermocouple lamp 2 installed at the leak detector flange was 25 - 30 Pa [(1.8-2.2) .10-1 mm Hg. st.]. Setting the working pressure in the hose connecting the probe to the leak detector must be carried out simultaneously by adjusting the probe and the leak detector inlet valve;
• a pump with a pumping speed of 1 - 3 l / s should be used as an auxiliary pump. If a pump with a higher pumping speed is used, valve 4 should be closed, ensuring the appropriate pumping speed;
• the product prepared for testing, after plugging the holes and flange outlets, is pumped out to a pressure not higher than 700 - 1400 Pa (5-10 mm Hg);
• helium and a helium-air mixture (not less than 50% helium) are supplied to the product to the excess pressure required during testing.

You can see an illustration of the method in the video:

Notes:
1. If it is impossible to pre-pump pipelines or products chamber type it is allowed to blow the cavity with helium until it appears at the outlet of the pipeline or product. The appearance of helium is fixed with a probe by increasing the readings of the device above the background by 100 mV and above.
2. To obtain a helium concentration of at least 60% under a pressure of 0.1 MPa (1 kgf/cm2), after purging the cavity with helium, helium is supplied to the product or pipeline to a pressure of 0.1 MPa (1 kgf/cm2). To obtain a helium concentration of at least 75%, the pressure is reduced to atmospheric pressure and helium is again supplied to a pressure of 0.1 MPa.
3. For products with dead-end cavities, which exclude the possibility of purging and vacuuming, the holding time to achieve the required helium concentration is determined experimentally in each specific case on a simulator.
4.2.5.5. The control is carried out by moving the probe along the surface of the product at a constant speed equal to 0.10 - 0.15 m/min:

• when moving, the probe must be in direct contact with the controlled surface. Removing the probe from the controlled surface by 5 mm reduces the detection of defects by 10 - 15 times;
• control should begin with the lower parts of the product with a gradual transition to the upper.

4.2.6. Helium blowing method.

4.2.6.1. The essence of the method lies in the fact that the product being tested is connected to a leak detector, evacuated to a pressure that allows the inlet valve of the leak detector to be fully opened, after which the outer surface of the product is blown with a helium jet.
If there is a leak in the product, helium enters its cavity and is fixed by a leak detector.
The control scheme by the blowing method is shown in fig. 3.


Rice. 3. Diagram of installation for controlling the blowing method
1 - helium leak detector,
2 - leak,
3 - helium leak,
4 - vacuum pump,
5 - cylinder with argon,
6 - vacuum valve,
7 - product,
8 - blower,
9 - chamber with helium
4.2.6.2. Control should be carried out in the following sequence:

• prepared in accordance with the requirements of subsection. 4.1 the product is evacuated to a pressure of 7 - 8 MPa [(5 - 6) .10 -2 mm Hg. Art.];
• when the inlet valve of the leak detector is open to the product, the auxiliary pumping system is turned off and the outer surface of the product is blown with helium. If it is impossible to maintain the required pressure in the mass spectrometer chamber with the auxiliary pumping system turned off, it is allowed to carry out control with the valve of the auxiliary pumping system not completely closed or open, while determining the sensitivity according to Appendix 5 should be at the same position of the valve;
• airflow should be started from the points of connection of the auxiliary pumping system to the leak detector; then the product itself is blown, starting from its upper sections with a gradual transition to the lower ones;
• at the first stage of testing, it is recommended to install a strong helium jet, covering immediately large area. If a leak is detected, reduce the helium jet so that it is slightly felt when bringing the blower gun to the lips, and accurately determine the location of the through defect. The speed of movement of the blower on the controlled surface is 0.10-0.15 m/min; when checking products of large volume and length, it is necessary, taking into account the signal delay time, to reduce the blowing speed;
• in the presence of large through defects and the impossibility of achieving the required vacuum in the product to fully open the inlet valve of the leak detector with the auxiliary pumping system turned off, search for through defects with the auxiliary pumping system turned on. After the detection of large through defects and their elimination, a repeated control is carried out in order to find defects with a small amount of leakage.

4.2.6.3. In order to control the entire surface of the product or part of it in some cases, the controlled surface is closed soft case. Helium is supplied under the cover in an amount approximately equal to the volume of space under the cover.
The duration of exposure of the product under the cover is 5-6 minutes.
4.2.6.4. The blowing method can be used to control open structural elements. For its implementation, vacuum suction cups should be used, superimposed or fixed on the controlled surface from the side opposite to the blown one. One of the chamber designs is shown in Fig. 4. Test modes are specified in 4.2.6.2.

Rice. 4. Construction of the suction chamber
1- cover,
2- building,
3- rubber seals,
4- design,
5- pipeline,
6- welded connection

4.3. Leak testing with halogen leak detectors. Halide atmospheric probe method

4.3.1. Adjustment of leak detectors, determination and testing of the threshold sensitivity of halide leak detectors should be carried out using calibrated halide leaks in accordance with the technical description and operating instructions of the manufacturer's instrument.
4.3.2. The essence of the halide probe method lies in the fact that the tested product, previously evacuated, is filled with freon or a mixture of freon with air to a pressure above atmospheric. As a result of the pressure drop, freon penetrates through the existing leak and is captured by the leak detector probe connected electric cable with the measuring block of the leak detector.
4.3.3. The scheme of installation for control by the halogen probe method is shown in fig. 5.


Rice. 5. Scheme of installation for control by the method of halogen probe:
1 - cylinder with freon;
2 - reducer;
3 - vacuum pump;
4 - manovacuummeter;
5 - valve;
6 - product;
7 - measuring block of the leak detector;
8 - remote probe of the leak detector
The installation for injecting freon into the controlled product must be checked for tightness with a halogen leak detector at a pressure of saturated halon vapor at the test temperature.
4.3.4. Control procedure:

• after plugging the holes and flange outlets with through and blind plugs, the product is pumped out to a residual pressure of not more than 700 - 1400 Pa (5 - 10 mm Hg);
• by closing the valve, the vacuum pump is turned off and freon is supplied to the product to the excess pressure required during testing;
• in case of impossibility of preliminary evacuation of pipelines, it is allowed to displace air with freon with fixation of the presence of freon at the remote end of the pipeline. Next, freon is injected into the pipeline to ensure the freon concentration in the pipeline is at least 50%;
• for chamber-type products, freon injection is allowed without pumping out the product, provided that the freon concentration in the product is at least 50%;
• control is carried out by moving the remote probe along the surface of the product at a constant speed;
• when moving, the probe should be at the minimum possible distance from the surface. Removing the probe from the controlled surface by 5 mm reduces the detection of defects by 10 - 15 times;
• control should begin with the upper sections of the product with a gradual transition to the lower ones.

4.3.5. Control modes by halogen leak detectors:
the speed of movement of the probe along the surface of the product should not exceed 0.10 - 0.15 m/min;
the pressure of freon-12 or freon-22 must comply with the instructions of the working drawings or flow sheet for control. Freon pressure in the product must be lower than its saturated vapor pressure.
Note . The pressure of saturated vapors of freon-12 and freon-22, depending on the temperature, is given in reference Appendix 7.
4.3.6. After the control, freon must be removed from the structure outside the working room by pumping to a residual pressure of 130 - 650 Pa (1 - 5 mm Hg). After that, air must be admitted into the controlled product and re-pumped to the same pressure.
Note . Double evacuation of the controlled product to a residual pressure of 130 - 650 Pa guarantees a residual content of freon-12 no more than 0.01 mg/l, and freon-22 - no more than 0.006 mg/l.

4.4. Bubble leak test

4.4.1. Pneumatic method by air inflating.

4.4.1.1. The essence of the method lies in the fact that the controlled product is filled with test gas under excess pressure. A foaming composition is applied to the outer surface of the product. Test gas at leaks causes bubbles to form in the foam formulation (bubbles or breaks in the soap film when using soap emulsion; foam cocoons or breaks in the film when using a polymer formulation).
4.4.1.2. Control procedure:

• in the controlled product, the required overpressure of the test gas is created;
• With a soft hair brush or paint sprayer, a foaming composition is applied to the controlled surface of the product and visual observation is carried out.

Note . The components of foam formulations are given in Annex 8 (informative).
4.4.1.3. The time of monitoring the state of the surface when applying the soap emulsion is no more than 2 - 3 minutes after its application to the surface.
4.4.1.4. When applying a polymer composition to detect large defects (more than 1.10 -4 m 3 Pa / s), the inspection should be carried out immediately after applying the polymer composition. To detect small defects, the inspection time should be at least 20 minutes from the moment the composition was applied. Foamy cocoons are stored during the day.

4.4.2. Pneumohydraulic aquarium method.

4.4.2.1. The essence of the method lies in the fact that the product, which is filled with gas under pressure, is immersed in a liquid. Gas escaping at leaks from the product causes bubbles to form in the liquid.
4.4.2.2. Control is carried out in the following sequence:

• the controlled product is placed in a container;
• a test pressure of test gas is created in the product;
• liquid is poured into the container to a level of at least 100 - 150 mm above the controlled surface of the product.

4.4.2.3. A sign of a leak in the product is the formation of air bubbles floating up to the liquid surface, periodically forming on a certain area of ​​the product surface, or a line of bubbles.

4.4.3. bubble vacuum method.

4.4.3.1. The essence of the method lies in the fact that before installing the vacuum chamber, the controlled area of ​​the structure is wetted with a foaming composition, a vacuum is created in the chamber. In places of leaks bubbles, cocoons or film breaks are formed, visible through the transparent top of the chamber.
4.4.3.2. To ensure complete control of the entire welded joint, the vacuum chamber is installed so that it overlaps the previous controlled section of the weld by at least 100 mm.
The vacuum chamber can have a different shape depending on the design of the controlled product and the type of welded joint. For butt welded joints sheet structures flat chambers are made, for fillet welds - fillet chambers, for the control of circumferential seams of pipelines, annular chambers can be made. One of the possible options for the design of the vacuum chamber is shown in Fig. 6.


Rice. 6. Scheme of a vacuum chamber for tightness control:
1 - rubber seals;
2 - camera body;
3 - window;
4 - vacuum valve;
5 - leak in the welded joint
6 - rubber seals
4.4.3.3. Control is carried out in the following sequence:

• a foaming composition is applied to the controlled area of ​​the open structure;
• a vacuum chamber is installed on the controlled area;
• a pressure of 2.5 - 3.10 4 Pa ​​(180 - 200 mm Hg) is created in the vacuum chamber;
• the time from the moment of applying the composition to the moment of inspection should not exceed 10 minutes;
• visual inspection the controlled area is carried out through the transparent top of the chamber.

Note . In the case of application in the control of the polymer composition, the pattern of defects is preserved for a day.

4.5. Tightness control by manometric method (by pressure drop)

4.5.1. To carry out control by the manometric method, the product is filled with test gas at a pressure above atmospheric and kept for a certain time.
4.5.2. Pressing pressure and pressing time are set specifications on the product or design (project) documentation.
4.5.3. The product is considered sealed if the pressure drop of the test gas during holding under pressure does not exceed the norms established by the technical specifications or design (project) documentation.
4.5.4. Gas pressure is measured by pressure gauges of accuracy class 1.5 - 2.5 with a measurement limit of 1/3 more than the pressure of pressure testing. A shut-off valve must be installed on the supply pipe to regulate the gas supply.
4.5.5. A quantitative assessment of the overall leakage is carried out according to the formula

where
V- internal volume of the product and elements of the test system, m3;
DR- change in test gas pressure during pressure testing, Pa;
t- pressing time, s.