Ensuring the safety of gas-fired heating equipment is one of the most important tasks facing the designers and maintenance personnel of boiler houses.
The solution to this problem in practice is complicated by the deterioration of the equipment, its physical and moral aging, malfunction of individual elements of automation equipment, as well as an insufficiently high level of qualifications and low technological discipline of the maintenance personnel, which can lead to serious accidents accompanied by human casualties.
Investigation of emergency situations, especially those related to safety devices, is often difficult due to the lack of objective information about the reasons that led to their occurrence.
One of the most important elements, the state of which largely determines the level of safety of gas boiler houses, is the purge valve of the gas manifold.
Leakage of the purge valve gate is one of the reasons for gas leakage (losses) through the purge gas pipeline into the atmosphere, and in the presence of a malfunction of other elements of the gas shut-off valves, it creates dangerous prerequisites for unauthorized gas ingress into production facilities and boiler furnaces.
Existing design solutions for the automation system do not provide for the ability to continuously monitor the tightness of the purge valve.
We witnessed the accidental discovery of a leak in the valve of the gas manifold purge valve, when, at the stage of commissioning, during the check of the automatic ignition system of the reserve boiler with the solenoid valve After the spark was applied, stable combustion of the igniter torch occurred. The boiler room service personnel did not have information to detect this malfunction in a timely manner and take the necessary measures to eliminate it.
In order to prevent such situations, it is proposed to install a glass water seal filled with
glycerin. The control circuit consists of a pipeline of a gas manifold, a gas valve 1, a purge valve 2, a hydraulic seal 3, a filler neck 5.
Gas valve 1 is required if the purge valve is missed during boiler operation, as well as when revising or replacing the valve. Gas leakage is determined by bubbles in the water seal during purging and operation of the boiler.
If the first solenoid valve is leaking, the gas leak can be seen as bubbles that rise in the liquid when the burner is at rest.
If the purge valve is leaking during burner operation.
The device is designed in such a way that when the gas pressure drops, glycerin does not penetrate into the pipeline.
Another advantage of this device is that the section of the pipeline between the valves is not filled with air during long periods of inactivity.
The proposed technical solution contains well-known elements and can be implemented on the basis of typical industrial devices. The costs for the implementation of the proposed technical solution are insignificant and incommensurate with those losses that may arise as a result of an emergency situation caused by a leakage of the purge valve of the gas manifold.
Head of the laboratory of non-destructive testing of LLC "Contact" Ktitrov Konstantin Borisovich
Head of the Department for EPB ZiS LLC "Contact" Melnikov Lev Mikhailovich
Engineer 1st category LLC "Contact" Katrenko Vadim Fedorovich
Engineer-expert of LLC "Contact" Keleberda Alexander Ivanovich
Expert LLC "Contact" Kuznetsov Viktor Borisovich
Introduction
Chapter 1 Analysis of the state of the problem of automation of tightness control and formulation of the research problem 9
1.1 Key terms and definitions used in this study 9
1.2 Features of gas valve tightness control 11
1.3 Classification of gas test methods and analysis of the possibility of their application to control the tightness of gas fittings 15
1.4 Review and analysis of automatic pressure control devices according to the manometric method 24
1.4.1 Primary converters and sensors for automatic leakage control systems 24
1.4.2 Automated systems and leakage control devices 30
Purpose and objectives of the study 39
Chapter 2 Theoretical Study of the Gauge Leak Test Method 40
2.1 Determination of gas flow regimes in test objects ... 40
2.2 Investigation of the compression method of testing for tightness 42
2.2.1 Investigation of time dependences during the control of tightness by the compression method 43
2.2.2 Investigation of the sensitivity of the tightness control by the compression method with a cut-off 45
2.3 Investigation of the comparison method with continuous test pressure 51
2.3.1 Tightness control scheme according to the comparison method with a continuous supply of test pressure 52
2.3.2 Investigation of time dependencies during tightness control according to the comparison method 54
2.3.3 Investigation of the sensitivity of the tightness control by the comparison method with a continuous supply of test pressure 65
2.3.4 Comparative evaluation of the sensitivity of the tightness control by the compression method with cut-off and the comparison method 68
Water to Chapter 2 72
Chapter 3 Experimental Study of the Parameters of Leakage Control Circuits Based on the Comparison Method 75
3.1 Experimental setup and research technique 75
3.1.1 Description of the experimental setup 75
3.1.2 Test procedure for leakage control circuits 78
3.2 Experimental study of the tightness control circuit based on the comparison method 81
3.2.1 Determination of the characteristic p = f (t) of the lines of the tightness control circuit 81
3.2.2 Investigations of the temporal characteristics of the lines of the tightness control circuit according to the comparison method 86
3.2.3 Study of the static characteristics of the measuring line of the leakage control circuit 91
3.3. Experimental study of a device for tightness control, carried out on the basis of comparison method 97
3.3.1 Investigation of a model of a device for leakage control with a differential pressure sensor 97
3.3.2 Evaluation of the accuracy characteristics of devices for tightness control, made according to the comparison scheme 100
3.4 Probabilistic assessment of the reliability of sorting products during tightness control according to the comparison method 105
3.4.1 Experimental study of the distribution of the pressure value equivalent to the leakage of the test gas in a batch of products 105
3.4.2 Statistical processing of the results of the experiment to assess the reliability of sorting 108
4.3 Development of leakage sensors with improved performance 126
4.3.1 Leakage sensor design 127
4.3.2 Mathematical model and calculation algorithm of the leakage sensor 130
4.4 Development of an automated stand for tightness control. 133
4.4.1 The design of an automated multi-position stand 133
4.4.2 Selection of parameters for leakage control circuits 142
4.4.2.1 Method for calculating the parameters of the tightness control circuit using the compression method with a cut-off 142
4.4.2.2 Methodology for calculating the parameters of the tightness control circuit using the comparison method 144
4.4.3 Determination of the performance of an automated stand for tightness control 146
4.4.4 Determination of the parameters of the sealing seals for the automated stand 149
4.4.4.1 Method for calculating a sealing device with a cylindrical collar 149
4.4.4.2 Calculation method for mechanical seal 154
General conclusions and results 157
References 159
Appendix 168
Introduction to work
An important problem in a number of industries is the increased requirements for the quality and reliability of products. This raises an urgent need to improve existing, create and implement new methods and means of control, including tightness control, which refers to flaw detection - one of the types of quality control of systems and products.
In the industrial production of shut-off and distribution valves, in which the working medium is compressed air or other gas, existing standards and specifications for its acceptance regulate, as a rule, one hundred percent control of the "tightness" parameter. The main unit (working element) of such an armature is a moving plunger-body pair or a rotary valve element, which operate in a wide range of pressures. Various sealing elements and lubricants (sealants) are used to seal the gas fittings. During the operation of a number of gas valve structures, a certain leakage of the working medium is allowed. Exceeding the permissible leakage due to poor-quality gas fittings can lead to incorrect (false) operation of the production equipment on which it is installed, which can cause a serious accident. In household gas stoves, increased leakage of natural gas can cause fire or poisoning of people. Therefore, the excess of the permissible leakage of the indicator medium with the appropriate acceptance control of the gas fittings is considered to be a leak, that is, a product defect, and the elimination of defects increases the reliability, safety and environmental friendliness of the entire unit, device or device in which the gas fittings are used.
Gas valve tightness control is a laborious, time-consuming and complex process. For example, in the production of pneumatic mini-apparatus, it takes 25-30% of the total labor intensity and up to 100-120% of the time.
assembly. This problem in large-scale and mass production of gas fittings can be solved by using automated methods and means of control, which must ensure the required accuracy and productivity. In real production conditions, the solution to this problem is often complicated by the use of control methods that provide the required accuracy, but are difficult to automate due to the complexity of the method or the specifics of the test equipment.
About ten methods have been developed for testing the tightness of products using only a gaseous test medium, for the implementation of which more than a hundred have been created. different ways and controls. The development of the modern theory and practice of tightness control is devoted to the studies of A. S Zazhigin, A. I. Zapunny, V. A. Lanis, L. E. Levina, V. B. Lembersky, V. F. Rogal, S. G. Sazhina, Tru-shchenko A.A., Fadeeva M.A., Feldman L.S.
However, there are a number of problems and limitations in the development and implementation of tightness control devices. So most of the high-precision methods can and should be applied only to large-sized products, in which complete tightness is ensured. In addition, restrictions of an economic, constructive nature, environmental factors, safety requirements for service personnel are imposed. In serial and large-scale production, for example, pneumatic automation devices, gas fittings for household appliances, in which, during acceptance tests, a certain leakage of the indicator medium is allowed and, therefore, the requirements for the control accuracy are reduced, the first place when choosing a method of monitoring the tightness is put forward the possibility of its automation and, on this basis, the high productivity of the corresponding control and sorting equipment, which is necessary with one hundred percent control of product quality.
Analysis of the features of the equipment and the main characteristics of the gas tightness test methods most used in industry made it possible to conclude that it is promising for automating the control of hermetic
the nature of gas fittings using the comparison method and the compression method, which implement the manometric method. In the scientific and technical literature, little attention is paid to these test methods due to their relatively low sensitivity, but it is noted that they are most easily automated. At the same time, there are no recommendations for the selection and calculation of the parameters of the tightness control devices, made according to the comparison scheme with a continuous supply of test pressure. Therefore, research in the field of gas dynamics of dead and flowing tanks as elements of control schemes, as well as gas pressure measurement techniques as a basis for creating new types of converters, sensors, devices and systems for automatic control of the tightness of products that are promising for use in the production of gas fittings.
During the development and implementation of automated leak control devices, an important question arises of the reliability of the control and sorting operation. In this regard, a corresponding study was carried out in the dissertation, on the basis of which recommendations were developed that allow, with automatic sorting by the "tightness" parameter, to exclude the ingress of defective products into suitable ones. Another important issue is to ensure the specified performance of automated equipment. The dissertation provides recommendations for calculating the operating parameters of an automated stand for tightness control, depending on the required performance.
The work consists of an introduction, four chapters, general conclusions, a bibliography and an appendix.
The first chapter discusses the features of monitoring the tightness of gas fittings, which allow a certain leak during operation. An overview of the methods gas tests for tightness, classification and analysis of the possibility of their application to automate the control of gas fittings, which made it possible to choose the most promising - the manometric method. Devices and systems providing automation of tightness control are considered. The goals and objectives of the study are formulated.
In the second chapter, two methods of tightness control are theoretically investigated, realizing the manometric method: the compression method with pressure cut-off and the comparison method with a continuous supply of test pressure. Mathematical models of the investigated methods were determined, on the basis of which studies of their temporal characteristics and sensitivity were carried out at different modes of gas flow, different line capacities and pressure ratios, which made it possible to reveal the advantages of the comparison method. Recommendations are given on the choice of parameters for tightness control circuits.
In the third chapter, the static and temporal characteristics of the lines of the tightness control circuit are experimentally investigated by the comparison method at various values of leakage, line capacitance and test pressure, their convergence with similar theoretical dependencies is shown. The operability was experimentally checked and the accuracy characteristics of the device for leakage control performed according to the comparison scheme were evaluated. The results of assessing the reliability of sorting products by the "tightness" parameter and recommendations for setting up the corresponding automated control and sorting devices are presented.
The fourth chapter describes typical schemes automation of the gauge test method and recommendations for the design of automated equipment for tightness control. The original designs of the leakage sensor and the automated multi-position stand for leakage control are presented. Methods for calculating tightness control devices and their elements are proposed, presented in the form of algorithms, as well as recommendations for calculating the operating parameters of a control and sorting stand, depending on the required performance.
The Appendix presents the characteristics of gas leak testing methods and time dependences for possible sequences of changes in the gas flow regimes in the flow tank.
The developments and studies presented in the dissertation are related to gas fittings, during the manufacture of which the existing standards and technical conditions regulate one hundred percent control of the "tightness" parameter and a certain leakage of the working medium is allowed. The gas fittings considered in this work are understood as devices intended for use in different systems, in which the working medium is a gas or a mixture of gases under pressure (for example, natural gas, air, etc.), for the implementation of the functions of cut-off, distribution, etc. Gas fittings include: valves, distributors, valves and other means of industrial pneumatic automation high (up to 1.0 MPa) and medium pressure (up to 0.2 ... 0.25 MPa), shut-off valves for household gas stoves operating at low pressure (up to 3000 Pa). Both finished products and their constituent elements, individual units, etc. are subjected to tightness testing. Depending on the purpose of the products, the conditions in which they are operated and design features, various requirements are imposed on them with respect to their tightness.
The tightness of a gas valve is understood to mean its ability not to pass a working medium supplied under excessive pressure through walls, joints and seals. In this case, a certain amount of leakage is allowed, the excess of which corresponds to the leakage of the product. The presence of a leak is explained by the fact that the main unit - the working element of such devices is a mobile, difficult to seal pair: spool-body, nozzle-damper, ball, cone or saddle valves, etc. In addition, the design of the device, as a rule, contains fixed sealing elements: rings, cuffs, oil seals, lubricants, defects of which can also cause leakage. Leakage of gas fittings, that is, the presence of a leakage of the working medium exceeding the permissible level, can lead to serious accidents, breakdowns and other negative results in the operation of the equipment in which it is used. The shut-off valve (Fig. 1.1) is an important component of household gas stoves. It is designed to regulate the supply of natural gas to the burners of the stove and cut it off at the end of work. Structurally, the valve is a device with a rotary valve element 1, mounted in a split body 2, in which there are channels for gas passage. The mating points of the valve parts need to be sealed to ensure the maximum possible tightness. Sealing is carried out with a special graphite grease - sealant, manufactured in accordance with TU 301-04-003-9. Poor sealing leads to natural gas leakage during operation of the stove, which in a limited space household premises explosion and fire hazard, in addition, the ecology (human habitat) is violated.
In accordance with the established the following requirements for testing the tightness of the shut-off valve. The tests are carried out with compressed air at a pressure of (15000 ± 20) Pa, since higher pressures can destroy the sealing lubrication. Air leakage should not exceed 70 cm3 / h. The permissible volume of switching channels and capacities of the monitoring device is not more than (1 ± 0.1) dm3. Control time 120 s.
Compressed air leaks in laboratory conditions in accordance with the recommended control using a volumetric device (Fig. 1.2). The device consists of a measuring burette 1, to which air under pressure is supplied through channel 2, a reserve vessel 3, a vessel 4 to maintain the required level and the connection point of the test valve 5. It is allowed to control with the help of other devices, the fashion of which does not exceed the fashion of the volumetric device ± 10 cm3 / h. Leakage control is carried out by measuring the displaced volume of water.
To gas fittings of medium and high pressure that must be tested for tightness include pneumatic valves, switches, adjustable throttles and other pneumatic devices, typical designs of which are shown in Fig. 1.3 and 1.4. In fig. 1.3 shows a pneumatic valve with a cylindrical valve type P-ROZP1-S, a valve pneumatic valve with a flat valve type B71-33
channel 1 for the control signal, cylindrical valve 2, body 3, cover with channel 4 connecting to the atmosphere, working channel 5 and O-ring 6. In fig. 1.4 shows a valve pneumatic valve with a flat valve type B71-33, consisting of a body 1, a cover 2, a flat rotary valve 3, a handle 4, a shaft 5, working channels 6, 7, 8, 9, a channel 10 connecting to the atmosphere and a channel for supply of compressed air 11. The presence of a regulated leak in the pneumatic equipment is explained by the fact that its designs contain flat spools, cylindrical spools with a sealing gap, valve and valve devices, which involve the flow of compressed air from one cavity to another or leaks into the atmosphere through gaps and leaks ... The amount of permissible leakage of a particular pneumatic device is established by the developer on the basis of GOST and is indicated in its technical characteristics. The values of permissible leakage for various types of pneumatic devices at the nominal compressed air pressure set for this device are shown in Table 1.1. Pneumatic equipment is used in control systems for various industrial equipment, therefore, increased leaks of the working medium and, as a result, a drop in pressure can lead to device failure or cause false triggering, i.e. lead to an emergency, equipment breakdown.
When testing for tightness of pneumatic equipment, difficulties arise due to the variety of designs, a wide range of permissible leakage of the indicator medium (0.0001 ... 0.004) m3 / min; with different values of test pressure (0.16 ... 1.0) MPa and control time (from tens of seconds or more). In addition, the contamination of the indicator medium (compressed air) should not exceed class 1 in accordance with GOST 17433-91, the ambient temperature is 20 ± 5C. The error of the measuring and control devices, which determine the leakage value, should not exceed ± 5%. To control the tightness of pneumatic equipment, pressure sensors (signaling devices) and specially designed equipment are used. An analysis of these devices is given in section 1.4.
Leak test sensitivity is the smallest sample gas leak that can be measured during product testing. Let us investigate the dependence of the sensitivity of the tightness control of the compress throttle, i.e. with corresponding gas leaks through the leaks of the test object. Let us express the gas leak Y through the mass flow rate G Suppose that irrespective of the gas outflow mode at a conductivity f, the leak is equal to Vd, and at conductivity / the leak is equal to W. For the turbulent supercritical mode, after substituting formula (2.5) into (2.15), we obtain With the same test duration /, - (as a result of the transformation (2.19) and (2.20), we obtain the relation (2.21) Substituting (2.21) in (2.18), we obtain the relation Since in (2.23) the LA will have the same absolute value regardless of the relations Ud Y or Ud U, then to simplify the calculations we assume that Ud U. Then (2.23) can be represented as an expression - the pressure response pA to a change in the leakage AU. , then we obtain a formula for determining the smallest change in leakage Uch, which can be recorded when monitoring the tightness by the investigated method. on Y, is the sensitivity of the tightness control by the compression method with a cut-off in the turbulent supercritical mode
The transformation (2.25) with respect to p0 allows one to obtain an expression for determining the test pressure depending on the sensitivity Uch of the tightness control in a turbulent supercritical regime Substituting in dependence (2.35) instead of D /? On the sensitivity threshold pn of the manometric measuring device, we obtain the formula for determining the sensitivity UH of the tightness control compression method with a cut-off in turbulent subcritical mode Transformation (2.36) with respect to p0 allows one to obtain an expression for determining the test pressure depending on the sensitivity Uch of the tightness control in the turbulent subcritical mode 2.41) and (2.42) we obtain the relation
Investigation of the comparison method with a continuous supply of test pressure The general provisions and the scheme of the leak test by the method of comparison with the cut-off of the sample gas source are discussed in section 1.3.2. However, as the analysis has shown, a method of comparison with a continuous supply of test pressure is promising for further research. This is due to the fact that shut-off, distribution and switching gas valves in real conditions operate under constant operating pressure and technical specifications allows a certain amount of leakage. Therefore, to test for tightness of this class of devices, it is advisable to use the control scheme with a continuous supply of test pressure, as the most appropriate to the real conditions of their operation. In addition, the need to cut off the pressure source at each test is eliminated, which greatly simplifies the design of the monitor and facilitates the automation of the test process. 2.3.1 Leakage control scheme according to the comparison method with continuous test pressure supply. The circuit consists of a measuring line IL and an EL line of the reference pressure, the inputs of which are connected to a common source of test pressure pQ, and the outputs are connected to the atmosphere. The reference pressure line contains an input pneumatic resistance (choke) with conductivity / J, a capacitance with an adjustable Ge volume and an output pneumatic resistance with an adjustable conductivity / 2, which are designed to adjust the circuit. The measuring line contains the input pneumatic resistance with conductivity / t, and the test object OI, which can be represented as a container with a volume of Ki, which has a flow equivalent to the pneumatic resistance with conductivity f4. The measuring and reference lines form a pneumatic measuring bridge. Comparison of the pressures in the lines of the circuit is carried out by means of the differential manometric measuring device of the DUT included in the diagonal of the pneumatic bridge. In this scheme, the measuring device has a conductivity / = 0, therefore, the pressures / g and ph in the lines do not depend on each other. Each line in the circuit represents a flow container. When checking the tightness according to the scheme shown in Fig. 2.2, leakage is understood as the volumetric gas flow rate through all through leaks of the test object at a steady state of the test gas flow in the circuit lines. This mode corresponds to the same mass flow rate of gas through the input and output resistance in the line.
The experimental study was carried out using serial industrial samples of shut-off valves of household gas stoves (at low test pressure), shut-off and distribution equipment of pneumatic automation (at medium and high test pressure), as well as models of leaks. In this case, the following technique was used: 1. Length of the pneumatic line from the outlet of the air preparation unit to the stabilizer w Fig. 3.3 Special equipment for experimental research: a - variable capacitance; b - choke with a diameter of 0.1 mm; c - control leaks: 1 - cylinder; 2 - cover; 3 - piston; 4 - volume holder; 5-inlet fitting; 6 - outlet fitting; 7 - collet clamp; 8 - replaceable tube (inner diameter 0.1 mm) of pressure at the inlet of the experimental setup was no more than 1.5 m. 2. During the tests, stabilization of the test gas (compressed air) from fluctuations in the network pressure was ensured. 3. Contamination of the test gas did not exceed the requirements of class 1 in accordance with GOST 17433-80. 4. The setting of the value of the test pressure supplied to the models of circuits and the leakage control device was carried out by the adjusting screw of the pressure stabilizer of the experimental setup. 5. Measurement of the test pressure at the inlet of the models of circuits and the tightness control device was carried out with exemplary pressure gauges of class 0.4 with measurement limits of 0 ... 1; 0 ... 1.6; 0 ... 4 kgf / cm. 6. Measurement of pressure in the reference and measuring lines of the models of circuits and the leakage control device was carried out with exemplary pressure gauges of class 0.4 with measurement limits of 0 ... 1; 0 ... 1.6; 0 ... 4 kgf / cm and a liquid micromanometer with a relative measurement error of 2%. 7. In studies with an average (up to 1.5 kgf / cm "0.15 MPa) and high test pressure (up to 4.0 kgf / cm" 0.4 MPa), the required leakage was set by means of adjustable throttles, previously calibrated by a rotameter with a relative measurement error of 2.5%. 8. In studies at low test pressure (up to 0.3 kgf / cm "" ZOkPa), the required leakage was set by means of control leaks made in the form of metal slotted capillaries made of L63 brass (Fig. 3.3, c). Capillaries were obtained by drilling holes 1 mm in diameter and subsequent flattening of the end section 20 mm long. The control leaks were calibrated with air at a pressure of 15 kPa using a volumetric device with a relative error of 2%. setting equal capacities in lines - by means of variable (adjustable) capacities. 10. The measurement of the pressure drop between the lines in the control device model was carried out by a differential pressure gauge with a relative measurement error of 2% and measurement limits of 0 ... 25 kPa and 0 ... 40 kPa. 11. When taking the time characteristics, the time was counted using an electronic stopwatch with a relative measurement error of 0.5%. 12. Measurements of the corresponding parameters (pi, Ap, I) for each investigated characteristic or parameter of the model of the circuit or leakage control device were carried out with repetition of readings at least 5 times. 13. The processing of the results of each experiment was carried out by finding the average values of the parameters for each experiment. Based on the data obtained, the corresponding characteristics were constructed. Descriptions of the points of the research methodology for individual characteristics are given in the relevant sections of this chapter. Study of the characteristic p = / (/) of the lines of the tightness control circuit. mathematical model(2.48) and the operability of the tightness control circuit, made on the basis of the comparison method with a continuous supply of test pressure, an experiment was carried out to determine the characteristic p = f (J) - the change in pressure in its lines during the control time at high and low test pressure, which are used for control tightness in various gas fittings. In section 2.3.1 it was shown that this control scheme contains two lines, each of which can be represented as a flow tank. The study used the experimental setup shown in Fig. 3.2, as well as the recommendations of Chapter 2 that all parameters of the measuring and reference lines of the circuit should be equal, therefore the experiment was carried out only with the measuring line. For this, the valves 15 connecting the reference line to the source of the test pressure and the measuring line to the differential manometric device 14 were closed.
To determine the characteristic p = / (/) of the flowing capacity of the line at a high test pressure, an exemplary manometer 8 with an upper measurement limit of 4.0 kgf / cm (400 kPa) class 0.4 and an electronic stopwatch were used. The experiment was set the following parameters: test pressure /? about = 400 kPa; the amount of air leakage Y = 1.16-10-5 m3 / s; the total volume of the flow tank and pneumatic channels V "0.5 dm3. The amount of air leakage Y was set by a variable throttle 10 of the P2D.1M type calibrated with a rotameter, while the control leak 9 was blocked by valve 15. In the interval of an intensive increase in pressure, the readings of the manometer 8 were taken after 10 s. To construct the experimental characteristic p = / (/), the arithmetic mean values from five experiments were taken as the pressure change values.
Let's consider the main stages of technical design of automated equipment for tightness control. At the first stage, a technological analysis of the range and volume of a batch of products is carried out. At the same time, it should be borne in mind that the number of products in a batch should be large enough (if possible, correspond to medium and large-scale production) to ensure the necessary loading of the designed control equipment without changing it. If the production is diversified, and the batch volume is small, then it is recommended to combine products of various production batches and types into groups according to general technical conditions for tightness control, which makes it possible to use a single control scheme and instrumentation, as well as group them according to similar designs of product bodies and their input channels, which allows the use of common sealing elements, loading and fixing devices in the design. Here it is necessary to analyze the suitability of product designs and the requirements of technical conditions for their leak testing for the automation of this operation. Rational grouping of products allows you to design equipment with maximum productivity and minimum readjustment to control various types of products. For example, high-pressure pneumatic automation means can be grouped according to the same specifications for compressed air leakage control (according to the test pressure of 0.63 MPa and 1.0 MPa, as well as the same permissible leakage), according to a similar design of the inlet pneumatic channel, which makes it possible to use it in the equipment under development. in the first case, a common control unit, and in the second - the same sealing device (end or internal lip). This stage ends with the determination of the performance of the designed equipment, an example of the calculation of which is discussed in the section
At the second stage of design, the need for readjustment of the designed device is determined, which should provide for: the ability of the control system to function taking into account different times of testing under pressure; readjustment of the measuring and control unit to different allowable sample gas leakage values, as well as to different test pressure levels. Then the choice of the control method and the means of its implementation should be made. Preliminary technical conditions for tightness control should be considered when analyzing the terms of reference. Here, as a rule, preference should be given to typical, wide-range control and measuring devices. But in some cases, it is recommended to develop a special control unit, which fully meets the requirements of the designed machine or semiautomatic machine, for example, according to the requirement for readjustment of devices, the test pressure range. Examples of calculation and application of control equipment are discussed in Sections 4.3 and 4.4.
In the third design phase, the level of automation and adaptability of the entire device is selected. Leak testing machines include devices that carry out the entire process of tightness control, including sorting, as well as loading and unloading products without the participation of an operator. Automated devices (semiautomatic devices) for tightness control include devices in which the operator participates. He can carry out, for example, loading - unloading the test item, sorting into "Good" and "Rejects" according to the information of the control and measuring unit equipped with an automatic recording element. In this case, the general control of the device, including the drive of the transport device, clamping - unclamping (fixing), product sealing, control time delay and other functions are carried out automatically. Prospective schemes for automating tightness control using the manometric method are discussed in Section 4.2.
After assessing the level of automation, the next important task is to select and analyze the layout diagram to be drawn to scale. It allows you to rationally arrange all the devices of the designed equipment. Here, special attention should be paid to the choice of the position of loading - unloading the product, the trajectory of movement of the loading equipment. The problems are related to the fact that loaded products (test objects), as a rule, have a complex spatial configuration, therefore it is difficult to orientate, grip and hold. Because of this, the creation of special orienting and loading and unloading equipment is required, which is not always acceptable for economic reasons, so manual loading may be a rational solution. As an adequate solution to the issue, it is recommended to consider the use of industrial manipulators and robots. Examples of selection and calculation of parameters of some auxiliary equipment are given in section
The next important design stage is the choice of the control system and the synthesis of the control scheme. Here you should adhere to the recommendations and methods for developing control systems for technological equipment given in the literature. The choice of the air preparation scheme is quite simple, as it is well developed technically and covered in the literature. But underestimating the importance of this issue can lead to increased contamination of the compressed air (mechanical impurities, water or oil) used as a test gas, which will seriously affect the accuracy of control and the reliability of the equipment as a whole. Requirements for the air used in pneumatic control and measuring devices are set out in GOST 11662-80 "Air for powering pneumatic devices and automation equipment. 1 In this case, the pollution class must not be lower than the second in accordance with GOST 17433-80.
When choosing a test pressure supply scheme, one should take into account its mandatory stabilization with high accuracy, the need to connect to a rotary clock table or other moving equipment, as well as the simultaneous power supply of a large number of control units. These issues are considered on the example of an automated stand for tightness control in section 4.4.
At the final stage, an expert assessment of the project of an automated device for tightness control is carried out. Here it is advisable to assess the project collectively, according to certain criteria, with the involvement of specialists from the department where the implementation of the device being developed is supposed to be. Then carried out economic assessment project. Based on the conclusions made, final decisions are made on further development working documentation, creation and implementation of an automatic or automated device for tightness control for this project.
Kavalerov, Boris Vladimirovich
Izvestia VolgGTU 65 UDC 620.165.29 G. P. Barabanov, V. G. Barabanov, I. I. Lupushor AUTOMATION OF TIGHTNESS CONTROL OF GAS PIPELINE VALVES Volgograd State Technical University E-mail: [email protected] Methods of automation of control of tightness of gas pipeline shut-off and switching valves are considered. Constructive diagrams of devices are presented, which make it possible to implement in practice methods of automating the control of the tightness of various gas fittings. Key words: tightness control, gas fittings, test pressure. Automation methods of hermeticity control of gas pipelining laking and shifting fittings are considered. Structural schemes of devices, that allow to realize on practice hermeticity control of different gas fittings automation methods are given. Keywords: hermeticity control, gas fittings, test pressure. In the manufacture of gas pipeline fittings for industrial and household appliances, the final stage of its production is the control of the "tightness" parameter, which consists in detecting unacceptable gas leaks during the operation of these devices. Gas pipeline fittings include valves, gates, gas stove taps, etc. The elimination of gas leaks during the operation of pipeline fittings increases the reliability, efficiency, safety and environmental friendliness of both industrial and household gas appliances. However, the control of the tightness of low-pressure pipeline valves is caused by a number of problems associated with both the laboriousness of the control process and design features these products. So, when checking the tightness of the taps of a household gas stove, the value of the test pressure is limited to 0.015 MPa. This control condition is explained by the fact that at a higher test pressure, the viscous graphite seal separating the working cavities of the valve is destroyed. Checking the tightness by known means at such a low test pressure does not guarantee the required accuracy and performance. The solution of these problems in the conditions of large-scale production of gas pipeline valves is possible by choosing a rational method for monitoring the tightness and automating the control process. Analysis of the features of the control of the tightness of low-pressure pipeline valves, for example, for household gas appliances in terms of accuracy and the possibility of automation of tests, made it possible to single out two promising schemes that implement the manometric method of control. This method consists in creating the value of the test pressure in the cavity of the controlled product, determined by the requirements of the control, with the subsequent comparison of the value of the pressure at the beginning and at the end of the tests. An indicator of a product leakage is a change in the test pressure by a certain amount during a period of time established by the control conditions. As studies have shown, it is advisable to use this method when monitoring the tightness of products with working volumes of no more than 0.5 liters, since with an increase in the volume of the test chamber, the control time increases significantly. One of the schematic diagrams of the leakage control device by the drop in test pressure is shown in Fig. 1. Air from a pressure source through filter 1 and stabilizer 2, by means of which the required inlet pressure 0.14 MPa, fed to the inlet of the pneumatic switch 4. From the outlet of the pneumatic switch 4, air simultaneously enters the measuring line of the device and the membrane chamber 15 of the clamping device 11. The measuring line of the device is built on the principle of an equilibrium bridge with a reference and measuring circuits. The reference circuit consists of a series-connected unregulated pneumatic resistance 7 and an adjustable pneumatic resistance 8, which form a throttle divider (shown by a dotted line). The measuring circuit is formed by an unregulated pneumatic resistance 9 and a controlled valve 13. Compressed air enters the reference and measuring circuits 66 Izvestia VolgGTU under a test pressure of 0.015 MPa, which is set by a setpoint 5. A comparison element 6 is included in the diagonal of the measuring bridge, the output of which is connected to a pneumatic indicator 14. Comparison element 6 is powered by compressed air at a pressure of 0.14 MPa. With the help of an adjustable pneumatic resistance 8 and a reference circuit, the permissible leakage value is set. The pressure from the throttle divider is supplied to the lower blind chamber of the comparison element 6. The upper blind chamber of this element is connected to the channel between the pneumatic resistance 9 and the controlled valve 13. After installing the controlled valve 13 and clamping it in the device 11, a pressure proportional to the amount of air leakage will be established in the measuring circuit through a controlled tap 13. Fig. 1. Diagram of a leakage control device based on a drop in test pressure If the leakage value is less than the allowable value, then the pressure will be higher than the reference pressure, and the signal at the output of the comparison element 6 will be absent, i.e. the test valve 13 is considered to be sealed. In the event that the leakage value exceeds the allowable value, the pressure will become less than the reference one, which will cause the comparison element 6 to switch and a high pressure will appear at its output, which will be signaled by the pneumatic indicator 14. In this case, the test valve 13 is considered to be leaking. To install and seal the valve 13 in the control device, a clamping device 11 is used, which contains a hollow rod 10 fixed to the membrane of the chamber 15, through which test pressure enters the cavity of the controlled valve 13. In this case, an elastic rubber sleeve 12 is put on the rod 10. After the compressed air is supplied to the membrane chamber 15, the rod 10 moves downward. In this case, the rubber bushing 12 is compressed and, increasing in diameter, fits snugly against the inner surface of the controlled valve 13, providing a reliable seal of the joint during the test. The unlocking of the controlled valve 13 and the preparation of a clamping device 11 for the installation of the next valve for the installation of the next valve is carried out by switching the pneumatic switch 4. The operation of the circuit of this device can be described by the following equations: for objects of control with an allowable leakage of the test gas, that is, which are considered hermetic t⋅ At pi - ≥ pe V for test objects with test gas leakage exceeding the permissible one, i.e., which are considered to be leaky t⋅U pi -< pэ, V где У – суммарная утечка индикаторного газа; t – время контроля; V – контролируемый на герметичность объем в объекте; pи – давление в измерительной цепи; pэ – величина давления в эталонной цепи. 67 На рис. 2 приведена принципиальная схема устройства контроля герметичности изделий, имеющих две смежные полости, между которыми возможна утечка газа. Устройство состоит из системы управления, которая содержит реле времени 1, триггер со счетным входом 2 и коммутирующую кнопку 3. При этом реле времени 1 подключено к электромагнитным приводам вентилей. 4 и 5, инверсный выход триггера 2 – к приводам клапанов 6 и 7, каналы которых соединены с датчиками давления 8 и 9, а также с полостями П1 и П2 контролируемого изделия 11. Выходы датчиков 8 и 9 подключены к отсчетному блоку 10. Устройство работает следующим образом. После выдачи входного сигнала кнопкой 3 на реле времени 1 открываются вентили 4 и 5. Этим обеспечивается подключение полости контролируемого изделия 11 через нормально открытый канал клапана 6 к источнику вакуума и полости П2 через нормально открытый канал клапана 7 – к источнику избыточного давления газа. Рис. 2. Схема с изменением направления перепада давления в контролируемом изделии После того, как в полости П1 создастся заданный требованиями контроля уровень вакуума (0,015 МПа), а в полости П2 – заданный уровень избыточного давления (0,015 МПа), происходит срабатывание реле времени 1 и отключаются вентили 4 и 5. С этого момента начинается процесс контроля герметичности изделия 11. Результат контроля определяется по показаниям отсчетного блока 10, сравнивающего сигналы от датчика 8, контролирующего повышение давления в полости П1, и датчика 9, контролирующего понижение давления в полости П2. В случае обнаружения негерметичности испытание прекращается и изделие бракуется. Если датчики 8 и 9 не регистрируют на- рушение герметичности изделия 11, то осуществляется второй этап испытания. Выдается повторный входной сигнал на реле времени 1 и триггер 2. При этом сигнал управления появится на инверсном выходе триггера 2 и переключит клапаны 6 и 7, а реле времени 1 повторно включит вентили 4 и 5. Полость П1 контролируемого изделия 11 окажется подсоединенной к источнику избыточного давления газа, а полость П2 – к источнику вакуума. На этом этапе испытаний в полости П1 контролируется понижение давления, а в полости П2 – повышение давления газа. Если датчики 8 и 9 не зарегистрируют негерметичность изделия 11 и на втором этапе испытаний, то оно считается годным. 68 ИЗВЕСТИЯ ВолгГТУ Особенностью реализуемого в устройстве (рис. 2) способа контроля герметичности является создание двукратного изменения направления перепада давления в контролируемом изделии, т. е. проведение испытаний в два этапа для учета different conditions gas outflow in different directions through microdefects in the sealing element of the controlled item, if any. In addition, the creation of a vacuum in one cavity and an overpressure in an adjacent cavity does not exceed the absolute value of the allowable pressure on the sealing element, but at the same time creates twice the pressure drop in the places of possible gas leakage. This makes it possible to increase the reliability and accuracy of the gas valve tightness control, and to reduce its duration. The schemes and principle of operation of the considered devices allow the automation of the process of monitoring the tightness of gas fittings, which will significantly increase the test productivity and practically eliminate the release of leaky products. REFERENCES 1. GOST 18460–91. Household gas stoves. General technical conditions. - M., 1991 .-- 29 p. 2. Barabanov, VG On the study of the manometric method of testing for tightness / VG Barabanov // Automation of technological production in mechanical engineering: interuniversity. Sat. scientific. tr. / VolgSTU. - Volgograd, 1999. - P. 67–73. 3.A.S. No. 1567899 USSR, MKI G01M3 / 26. Method of testing a two-cavity product for tightness / G. P. Barabanov, L. A. Rabinovich, A. G. Suvorov [and others]. - 1990, Bul. No. 20. UDC 62–503.55 NI Gdanskiy, AV Karpov, Ya. A. Saitova INTERPOLATION OF THE TRAJECTORY IN THE CONTROL OF THE SYSTEM WITH ONE DEGREE OF FREEDOM GOUVPO Moskovskiy State University Environmental Engineering E-mail: [email protected] When using forecasting in the control of one-degree systems, it becomes necessary to construct a trajectory passing through previously measured nodal points. A piecewise polynomial curve consisting of Fergusson splines is considered. The article provides a method for partial calculation of spline coefficients, which requires significantly fewer computational operations compared to the traditional method. Key words: load models, forecasting, splines. It is necessary to construct the trajectory, which passing through the previously measured nodal points, when using the prediction in control systems. For this purpose, polynomial piecewise curve consisting of Ferguson spline is used. This paper presents a method for calculating the coefficients of these splines, which require significantly fewer computational operations than the traditional method. Keywords: model the external load acting, prediction, splines. In digital motion control systems in one-stage systems, it is proposed to model the external load M (t, φ (t)) along the φ coordinate in the form of a set of constant coefficients M k. In this case, the instantaneous value M (t, φ (t)) is the scalar product M (t, ϕ (t)) = M k, ϕk (t), in which the vector () torus ϕk (t) depends only on t and derivatives of ϕ with respect to t. With this method of representing the external load, to calculate the control action in this system, work A is used, which must be performed by the drive for a given control period: Ai = ti +1 ∫ (М k, ϕk (t)) ϕ ′ (t) dt. ti As follows from general view formulas for М and Аi, they do not explicitly contain the function ϕ (t), but only its derivatives. This general property of the solution method can be used to simplify the auxiliary task of interpolating the trajectory of the shaft movement along its nodal points. Suppose we are given an ordered array of trajectory nodes Рi = (ti, ϕi) (i = 0, ..., n). To construct a piecewise polynomial curve ϕ (t) of the second degree of smoothness passing through
One of the ways to solve the problem of automation of control of the tightness of hollow products, for example, shut-off valves, is the development of a multi-position readjustable stand for automatic control of the tightness of products with compressed air, according to the manometric method. There are many designs of such devices. Known automatic control of the tightness of products, containing a table with a drive, an elastic sealing element, a scraping device, a source of compressed gas, a copier and a device for clamping the product.
However, the automation of the process is achieved due to the significant complexity of the machine design, which reduces the reliability of its operation.
Known machine for monitoring the tightness of hollow products, containing sealing units with leak sensors, a test gas supply system, mechanisms for moving products and a rejection mechanism.
The disadvantage of this machine is the complexity technological process control of the tightness of products and low productivity.
Closest to the invention is a stand for testing products for tightness, containing a rotor, a drive for its step movements, control blocks placed on the rotor, each of which contains a comparison element connected to a rejecting element, a product sealing element containing an outlet tube and a drive for its movement, which is made in the form of a copier with the ability to interact with the output tube.
However, this device does not allow increasing productivity, since it reduces the reliability of product testing.
Figure 1.6 shows an automated chamber-based leak test device. It consists of chamber 1, in the cavity of which the controlled item 2 is located, connected to the air preparation unit 3 through a shut-off valve 4, a diaphragm seal 5 with a membrane 6 and cavities A and B, a jet element OR NOT OR 7. Cavity A of a diaphragm seal 5 connected to the chamber cavity 1, and cavity B through nozzle 8 - to outlet 9 OR of the jet element 7. To its other output 10, NOT OR connected to the pneumatic amplifier 11 with a pneumatic lamp 12. Cavity B is additionally connected by channel 13 to the control input 14 of the jet element 7, atmospheric channels 15 of which are equipped with plugs 16.
The device works as follows. The controlled item 2 is supplied with pressure from the air preparation unit 3, which, when the test level is reached, is cut off by valve 4. At the same time, when power is supplied to the jet element 7, the air stream through outlet 9 OR and nozzle 8 passes into cavity B of the diaphragm separator 5 and through channel 13 - to the control input 14 of the jet element 7. Thus, in the absence of leakage from the controlled item 2, the jet element 7 is in a stable state under the action of its own output jet. In the presence of a leak from the product 2, an increase in pressure occurs in the inner cavity of the chamber 1. Under the action of this pressure, the membrane 6 bends and overlaps the nozzle 8. The pressure of the air stream at the outlet 9 of the jet element 7 increases. At the same time, the jet disappears at the control input 14, and since the jet element OR - NOT OR is a monostable element, it switches to its stable state when the jet exits through the output 10 NOT OR. In this case, the amplifier 11 is triggered and the pneumatic lamp 12 signals the leakage of the product 2. The same signal can be fed to the jet control system of the grading.
This device is built on the elements of jet pneumatic automation, which increases its sensitivity. Another advantage of the device is its simplicity of design and ease of adjustment. The device can be used to control the tightness of gas fittings by compression methods at low test pressure, if the diaphragm seal is used as a sensor connected directly to the controlled item. In this case, the presence of abnormal leakage can be monitored by opening the diaphragm and nozzle.
Figure 1.6? Leak test device
Figure 1.8 shows a device that automates the control of the tightness of pneumatic equipment, for example, electropneumatic valves, that is, products similar to the gas fittings considered in the thesis.
The test item 1 is connected to the pressure source 2, the electromagnetic bypass valve 3 is installed between the outlet 4 of the item 1 and the exhaust line 5. The electromagnetic shut-off valve 6 is connected with its input 7 during the test process to the output 4 of the item 1, and the output 8 is connected to the pneumatic input 9 of the converter 10 of the system 11 for measuring leakage, which is made in the form of a thermal flow meter. The system 11 also contains a secondary unit 12 connected to the control input 13 of the converter 10, the pneumatic output 14 of which is connected to the exhaust line 5. The valve control unit 15 contains a multivibrator 16 and a block 17 for delaying and generating pulses. One output of the multivibrator 16 is connected to the control input 18 of the shut-off valve 6, the other to the control input 19 of the valve 3 and block 17. connected during the control process to the actuator 20 of the item under test 1. The calibration line 21 consists of an adjustable throttle 22 and a shut-off valve 23. It connected in parallel with product 1 and is used to configure the device.
Leakage control is carried out as follows. When the valve control unit 15 is turned on, a pulse appears at the output of the multivibrator 16, which opens the valve 3 and the unit 17 for delaying and shaping pulses. The same pulse opens, after a set delay time, the test item 1 by supplying an electrical signal from unit 17 to actuator 20. In this case, the test gas is vented through valve 3 into the exhaust line 5. After a time set by the multivibrator 16, the pulse is removed from valve 3, closing it, and is fed to the inlet 18 of the shut-off valve 6, opening it. In this case, the gas, the presence of which is due to the leakage from the product 1, enters the leakage measurement system 11 and, passing through it, generates in the converter 10 an electrical signal proportional to the gas flow rate. This signal is sent to the secondary unit 12 of the leakage measuring system, in which it is corrected, and the amount of gas flow through the closed test item 1 is recorded.
The disadvantages of this device include the following. The device is designed to control the tightness of gas fittings of only one type, equipped with an electromagnetic drive. Only one product is monitored at a time, that is, the process is ineffective.
Figure 1.8 shows a diagram of an automated device for monitoring gas leaks by the compression method with a pneumo-acoustic measuring transducer. The device consists of intermediate blocks and, providing control of large leaks (more than 1 / min) and a pneumo-acoustic unit for monitoring small values of leaks (0.005 ... 1) / min. The pneumo-acoustic unit of the transducer has two amplifying gauge stages, consisting of micromanometers 1, 2 and acoustic-pneumatic elements 3, 4, connected to each other through a distribution element 5. The measurement results are recorded by a secondary device 6 of the EPP-09 type, connected to the unit through distributor 7. The controlled item 8 is connected to the test pressure source through the K4 shut-off valve. The operation of the device is carried out in a continuous-discrete automatic mode, which is provided by the logical control unit 9 and valves -. The monitored product 8 with the help of block 9 is sequentially connected to the blocks and, by correspondingly switching on the valves and, where the preliminary value of the test gas leakage is determined. In the case of a small leakage value (less than 1 / min), the product is connected via a valve to the pneumo-acoustic unit, where the leakage value is finally determined, which is recorded by a secondary device 6. The device provides gas leaks control with an error of no more than ± 1.5%. The pressure of the supply and the tube-tube element in the block is 1800 Pa.
This device can be used for automatic control of gas fittings with a wide range of permissible gas leaks. The disadvantages of the device are the complexity of the design due to the large number of measuring units, as well as the simultaneous control of only one product, which significantly reduces the productivity of the process.
Figure 1.8 Automated device for gas leakage control by compression method.
Devices that provide simultaneous testing of several products are promising for monitoring the tightness of gas fittings. An example of such devices is an automatic device for monitoring the tightness of hollow products, shown in Figure 1.14. It contains a frame 1, fixed on racks 2 and closed with a casing 3, as well as a rotary table 4 with a drive 5. The rotary table is equipped with a faceplate 6, on which eight slots 7 are evenly located for products 8. The slots 7 are removable and have cutouts 9. Sealing nodes 10 are fixed on the frame 1 with a step twice the pitch of the sockets 7 on the faceplate 6. Each sealing unit 10 contains a pneumatic cylinder 11 for moving the product 8 from the seat 7 to the sealing unit and vice versa, on the rod 12 of which there is a bracket 13 with a sealing gasket 14 In addition, the sealing unit 10 comprises a head 15 with a sealing element 16, which is communicated by means of pneumatic channels with the air preparation unit 17 and with a leakage sensor 18, which is a membrane pressure sensor with electrical contacts. The rejection mechanism 19 is installed on the frame 1 and consists of a pivot arm 20 and a pneumatic cylinder 21, the rod of which is pivotally connected to the lever 20. Good and rejected products are collected in appropriate bins. The machine has a control system, the current information about its operation is displayed on the display 22.
The machine works as follows. The controlled item 8 is installed at the loading position into the slot 7 on the faceplate 6 of the turntable 4. The drive 5 performs stepwise rotation of the table by 1/8 of a full turn at certain time intervals. To control the tightness by actuating the pneumatic cylinder 11 of one of the sealing units 10, the product 8 rises in the bracket 13 and is pressed against the sealing element 16 of the head 15. After that, a test pressure is supplied from the pneumatic system, which is then cut off. The pressure drop in the product 8 is registered by the leakage sensor 18 after a certain monitoring time, which is set by the step of the table 4. The stop of the table 4 serves as a signal allowing the implementation of the corresponding operation in positions I - VIII during the standing of the table. Thus, when the table is rotated one step at each of its positions, one of the following operations is performed: loading the product; lifting the product to the sealing unit; tightness control; lowering the product into the socket on the faceplate; unloading suitable products; removal of defective products. The latter arrive at position VIII, while the lever 20 under the action of the pneumatic cylinder rod 21 rotates in the hinge, and with its lower end passes through the cutout 9 of the socket 7, removing the product 8, which falls under its own weight into the hopper. Suitable products are unloaded in the same way at position VII (unloading device is not shown).
The disadvantages of the device are: the need to lift the product from the faceplate into the sealing unit to control the tightness; use of a diaphragm pressure transducer with electrical contacts as a leakage sensor, which has low accuracy characteristics in comparison with other types of pressure sensors.
The studies have shown that one of the promising ways to improve the manometric method of tightness control is the combined use of bridge measuring circuits and various differential-type transducers.
The pneumatic bridge measuring circuit for leakage control devices is based on two pressure dividers (Fig. 1.9).
Figure 1.9
The first pressure divider consists of a constant fli throttle and an adjustable throttle D2. The second one consists of a constant choke Dz and a control object, which conditionally can also be considered a choke D4. One diagonal of the bridge is connected to the source of test pressure pk and the atmosphere, the second diagonal is measuring, a PD converter is connected to it. To select the parameters of the elements and adjust the bridge circuit, consisting of laminar, turbulent and mixed chokes, the following dependence is used:
where R1 R2, R3, R4 - hydraulic resistances of elements D1, D2, D3, D4, respectively.
Given this dependence, the possibility of using both a balanced and unbalanced bridge circuit, as well as the fact that the hydraulic resistance of the supply channels is small in comparison with the resistance of the chokes and therefore it can be neglected, then on the basis of the given pneumatic bridge circuit it is possible to build devices for monitoring the tightness of various objects. At the same time, the control process is easily automated. The sensitivity of the device can be increased by using unloaded bridge circuits, i.e. install transducers with R = in the measuring diagonal. Using the formulas for the gas consumption in the subcritical mode, we obtain the dependences for determining the pressure in the inter-throttling chambers of the unloaded bridge.
For the first (upper) branch of the bridge:
for the second (lower) branch of the bridge:
where S1, S2, S3, S4 are the flow area of the channel of the corresponding choke; Рв, Рн - pressure in the inter-throttle chamber of the upper and lower branches of the bridge, рк - test pressure.
Dividing (2) by (3) we get
From dependence (4) follows a number of advantages of using the bridge circuit in devices for monitoring the tightness by the manometric method: the pressure ratio in the throttle chambers does not depend on the tester ...
Consider schematic diagrams devices that provide leakage control by the manometric method, which can be built on the basis of pneumatic bridges and various types of differential pressure-to-electrical converters and other types of output signals.
In fig. 1.10 shows a diagram of a control device in which a water differential pressure gauge is used in the measuring diagonal of the bridge.
Figure 1.10 Diagram of a control device with a measuring diagonal of the bridge - water differential pressure gauge
Test pressure pk is supplied to two lines through constant throttles. One line - the right one is measuring, the pressure in it changes depending on the leakage value in the controlled object 4. The second line - the left one provides a reference back pressure, the value of which is set by an adjustable throttle 2. This element can be used typical devices: cone - cone, cone - cylinder, etc. Both lines are connected to differential pressure gauge 5, in which the difference in the heights of the liquid columns h is a measure of the pressure drop in the lines and at the same time makes it possible to judge the amount of leakage, since proportional to it:
It is possible to automate the process of reading the readings of a water differential pressure gauge through the use of photoelectric sensors, fiber-optic converters, optoelectronic sensors. In this case, the water column can be used as a cylindrical lens that focuses the light flux, and in the absence of water, scatter it. In addition, to make it easier to read the readings, the water can be tinted and serve as an obstacle to the luminous flux.
This device provides high precision leak rate measurement and can therefore be used to calibrate other instrumentation and qualify test leaks.
In fig. 1.11 shows a device for measuring leakage in object 4, in which a jet proportional amplifier 5 is used in the measuring diagonal of the bridge. Test pressure pk is fed through constant throttles 1 and 3 to the backpressure line and the measuring line connected to the corresponding control inputs of the amplifier. Under the action of the pressure of the jet leaving the amplifier, the arrow 6 is deflected, loaded with the spring 7. The deflection of the arrow corresponds to the amount of leakage. The counting is carried out on a graduated scale 8. The device can be provided with a pair of closing electrical contacts, which are triggered when the leakage exceeds the permissible value. The use of a jet proportional amplifier facilitates the adjustment of the device to a given leakage level and improves the control accuracy.
Figure 1.11 Schematic of a control device with a jet proportional amplifier
However, given that the amplifier has a hydraulic resistance Ry0, the bridge circuit is loaded, which lowers its sensitivity. In this case, as an adjustable tuning throttle 2, it is advisable to use a bubbling tank 9 filled with water and a tube 10, one end of which is connected to the throttle 1, forming a backpressure line with it, and the other end has an outlet to the atmosphere and is immersed in the tank. Regardless of the value of the test pressure pk in the tube 10, the pressure pp will be established, which is determined by the dependence:
where h is the height of the water column displaced from the tube.
Thus, the back pressure in the bridge circuit is regulated by setting the appropriate h and immersion depth of the tube. Such an adjustable throttle device ensures high accuracy of setting and maintaining back pressure. In addition, it is practically waste-free. However, regulating throttles of this type can be used in circuits operating at low pressure (up to 5-10 kPa) and mainly in laboratory conditions.
The use of bridge circuits with pneumo-electric membrane transducers in tightness control devices ensures their operation in a wide range of pressures pk with sufficient accuracy. A diagram of such a control device is shown in Fig. 1.12.
It consists of constant chokes 1 and 3, as well as an adjustable choke 2. A diaphragm transducer 5 is connected to the measuring diagonal of the bridge, while one of its chambers is connected to the measuring line of the bridge, and the other to the back pressure line. At the beginning of the process of monitoring the tightness of object 4, the membrane b is in the resting position, balanced by the pressures in the inter-throttle chambers of the bridge, which is fixed by closing the right pair of electrical contacts 7. When the object is leaking, i.e. when a leak occurs, there will be a pressure difference in the transducer chambers, the membrane will bend and contacts 7 will open. If a leak appears greater than the allowable value, the amount of deflection of the membrane will ensure the closure of the left pair of electrical contacts 8, which will correspond to the defective product.
Figure 1.12 Schematic diagram of a control device with a pneumatic diaphragm transducer
The relationship between the membrane travel and the pressure difference in the chambers in the absence of a rigid center and a small deflection is established by the dependence:
where r is the radius of the membrane, E is the modulus of elasticity of the membrane material,
Membrane thickness
Taking into account the dependence and leakage Y according to the formula, the dependence can be chosen structural elements and the operating parameters of this converter.
Transducers with flat membranes, in addition to electrical contacts, can be used in conjunction with inductive, capacitive, piezoelectric, magnetoelastic, pneumatic, strain-gauge and other output transducers of small displacements, which is their great advantage. In addition, the advantages of flat-diaphragm pressure transmitters are their design simplicity and high dynamic properties.
In fig. 1.13 shows a diagram of a device designed to control tightness at low and medium test pressures.
Figure 1.13 Schematic of a control device with a two-input three-membrane amplifier
Here, in a pneumatic bridge consisting of constant chokes 1 and 3, an adjustable choke 2 in the measuring diagonal, a comparison element 5 is used, made on a two-input three-membrane USEPPA amplifier of P2ES.1 type, the blind chamber A of which is connected to the back pressure line, and the blind chamber B is connected with measuring line. The output of the comparison element is connected to the indicator or pneumo-electric converter 6. The power supply of the comparison element is carried out separately from the bridge and at a higher pressure. With the help of an adjustable throttle 2, the differential pressure between the measuring line and the back pressure line is set, proportional to the maximum permissible leakage. If, during the control, the amount of leakage through object 4 is less than the permissible value, then the pressure p in the measuring line will be higher than the back pressure pp, and the signal at the output of the comparison element will be absent. If the leakage value exceeds the allowable value, the pressure in the measuring line will become less than the back pressure, which will lead to the switching of the comparison element and a high pressure will appear at its outlet, this will cause the indicator or pneumatic electric transducer to work. The operation of this scheme can be described by the following inequalities. For objects of control with an admissible leakage value:
For objects of control with a leakage exceeding the permissible:
This device can be used in automated stands to control the tightness of shut-off valves. An additional advantage is the simplicity of the design implementation on standard elements of pneumatic automation.
In fig. 1.14 shows a device for measuring and monitoring leakage in object 4, in which a differential bellows transducer is connected to the measuring diagonal of the bridge 5. Test pressure pk is fed through a constant choke 1 to the bellows b of the backpressure line, and through a constant choke 3rd bellows 7 of the measuring line. The pressure value corresponding to the permissible leakage is set by the adjustable throttle 2.
Bellows 6 and 7 are interconnected by a frame on which an indication system is fixed, consisting of an arrow 8 with a scale 9 and a pair of adjustable closing electrical contacts 10. The device is configured in accordance with the relationship:
Figure 1.14 Diagram of a control device with a differential diaphragm transducer
In the event of a leak, the pressure ri in the bellows 7 begins to decrease, and it is compressed, and the bellows 6 will stretch, because pp remains constant, while the frame will begin to move and the arrow will show the amount of leakage. If the leakage exceeds the allowable value, then the corresponding movement of the bellows will close the electrical contacts 10, which will give a signal about the rejection of the controlled object.
This device can operate at medium and high test pressure. It can be used in automated stands for testing the tightness of high-pressure shut-off valves, where relatively high leakage values are allowed and their absolute values must be measured.
