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10PRECISION ADJUSTABLE VACUUM LEAKS OF GAS AND VAPOUR MICROFLOWS FOR CONTROL OVER LEAKAGE OF ENERGY OBJECTS
(BRIEF REVIEW)
A.L. Gusev
OOO STC TATA, 29 Moskovskaya Street, PO Box 687, Sarov, Nizhniy Novgorod Region 607183 Russia Phone/Fax: +7-(83130) 90708; 63107, E-mail: gusev@hydrogen.ru,
Russian Federal Nuclear Centre - All - Russian Scientific and Research Institute of Experimental Physics (RFNC-VNIIEF),
607190, Russia, Sarov, Nizhni Novgorod region, Pr. Mira, 37
Education: A.F. Mozhaisky Military Engineering Institute (A.F. Mozhaisky Military Space Academy) in Saint-Petersburg in 1983; post-graduate studentship at VNII of "Cryogenic techniques" of KHIMNEFTEMASH, USSR on vacuum-cryogenic theme in 1995. (Scienti fic adviser: Professor V.I. Kupriyanov, USSR prize laureate).
Participation in scientific conferences and workshops more than 50. Participation in Editorial boards of international Electronic journals and Information systems Editor-inChief of the Electronic International Scientific Journal for Alternative energy and ecology since 2000 (http://isjaee.hydrogen.ru/). Editor-in-Chief of the International Information Portal "Vodorod" («Hydrogen») (http://www.hydrogen.ru).
Expert for Federal Scientific Program (FCNTP) - in the Field of Hydrogen Technology: http://www.fcpir.ru/; Number of papers in referred journals: 220; Number of reports and Abstracts on scientific meetings: 65; Number of patents: 35
Alexander Leonidovich Gusev
Introduction
The present state of the measuring technique, particular gasometric one, is characterized by a signi ficant hardening the operation conditions defined by the effect of temperature, pressure, humidity, supply voltage, soiled and tough probes being analyzed, a higher level of radiation, that vary within wide limits. Requirements to the accuracy, reliability, sensitivity and fast-action of the gas analyzing apparatus are increased with it. In this connection, additional requirements are imposed to these gas analyzing and gas analytical systems (R.T. Franko, 1983):
- maintaining the serviceability in severe conditions at limited operation opportunities that can be achieved by improving the measurement process automation, steep improvement of repairability, and reserving of less reliable units and components;
- offering the prospect of automatic test of basic met-rological parameters and serviceability;
- affording the self-control and forecasting the service-
able condition of the measuring guades and instruments as a whole;
- a higher stability of the measuring guade conversion coefficients (metrological reliability);
- improvement of the gas analytical apparatus fast-action.
The sampling device, a vacuum leaks in the case in question, is considered to be the most responsible and vulnerable unit of any gas analytical system.
The vacuum leaks, according to the cost, can be taken equal to few in number products, falling into high-tech category. If by convention, the products are estimated by a specific parameter cost/mass kilogramme, then for the high-tech products this parameter will be 1000 dollars per kilogramme.
Research on vacuum leaks to create precision adjustable products was started by the author of the review and his collegues when performing programmes on space exploration. Among those were leak tests in a vacuum chamber,
— - Международный научный журнал «Альтернативная энергетика и
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definition of total leakage of spacecrafts and sealed suits, and also search for local leaks. The author of this review was one of the participants of the "Mir"space station leak testing, and the leader of the vacuum loading system test series for the nonconsumable transport space system (NTSS) "Energiya-Buran"
A high quality of the vacuum process of heat insulating cavities of the cryogenic equipment made it possible to pioneer in the world practice the loading the "Buran" system with greatly overcooled fuel additives. However, the vacuum test method is considered to be one of the most labour consuming and expensive processes, difficult-to-automation and often restricting the period of the cryogenic systems to get started. This process is greatly complicated by exposure the external surfaces of the heat insulated cavities to rainfalls, vibrations, a wind load, radiation, insolation, dust, variations in temperature and humidity. Imperfectly delicate adjustment of the mass spectrometer leaks results in making the cathodes inoperative, and significantly increases maintenance costs, associated with the cathode replacement, and recalibration of mass spectrometers.
Nowadays,a highly effective precision leak can not be found. This situation is due not only to the national industry "relaxation", but also a technology niche in the worldwide production of universal precision adjustable leaks. "Edwards" and "Balzers"(Liechtenstein) companies are the major manufacturers of precision leaks in the world. Against the apparent simplicity, the precision adjustable leaks require call for a detailed look and improvement.
Precision adjustable vacuum leaks are devices designed for the adjustment of gas microflows. These devices shall operate properly under unfavourable conditions, providing precision adjustment, an increased capacity at a higher gas flow stability, and conform to the requirements for control process automation, and keeping the capacity at a high level.
Vacuum leaks have gained acceptance in cryogenic techniques used for testing evacuating systems, calibrating metering devices equipped with mass-spectrometric leak detectors, adjusting the sense of mass-spectrometric leak detectors, and also for checking local leakage, and for gas sampling in chromatography.
The adjustable gas leaks such as these, are considered to be indispensable in automobile industry, both in the vehicle design itself and in programme controlled units used to paint vehicles, and in gas welding equipment as well. A number of leading automobile manufacturers have aleady been equipping their motor cars with a fuel and oxidizer injection control system, that enables stoichiometric combustion [1]. Precision fuel injection systems such as these, are equipped, as a rule, with a feedback system, the key components of which are two devices: a gas meter installed at the outlet pipe, and an adjustable leak (either of gas, or liquid) an injector. The injector acts as a combination of the adjustable precision leak and a nozzle. At the present time, a broad spectrum of injectors: with a mechanical
injection K(KE)-Jetronic (Mercedes-123); electromechanical L-Jetronic (Mercedes-124), and electronic Motronic (Audi, VW, Opel, Renault, Lucas(Jaguar), Marelli (Fiat, Alfa Romeo) having been manufactured.
The precision adjustable gas leak is one of the key components in the space sealed dresses [2], aqualungs, life-support systems in spacecrafts and stations.
The precision adjustable leaks are required to build up automatically a higher gas pressure in cables with a protective gas medium, produce breathing mixtures for medical purposes, in energy emission gas control systems of the nuclear reactor fuel channel, and in other important science and technology fields.
Leak types, main requirements and operation conditions
Leak types and gas flow control systems
The majority of state -of-the-art gas-analytical devices are subjected to the influence of pressure instability, or the gas probe flow used for analysis. The stabilization of pressure or flow of the mixture being analyzed in the mass spectrometric chamber, is one of the ways of reducing the influence of pressure drop on the gas-analytical device indications. Thus, the most important stage of the sample preparation is to maintain the required pressure or gas sample flow. The vacuum leak has been designed for this purpose.
The most widespread leaks are devices with an adjustable conical needle and a respective hole, which are known as needle seals. Mechanisms for the precision adjustment of movable parts feed are necessary, but hard-to-make components [3-5]. In the development of the devices as these, it is necessary to provide a delicate leak adjustment, high accuracy, leak stability, reliability, fabrication
Fig. 1. Needle leak [8]: 1- thin steel needle with an angle of 2-6o; 2- seat; 3 - differential feed screw mechanism.
simplicity, repairability, high reproducibility of results, and the required throughput control. The movable system of the needle seal is driven usually by threaded mechanisms [4-7] (Fig.1).
Experimental studies of the needle seal with a movable threaded system, have shown that it, as a rule, has a number
36
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02SDHI
of disadvantages. Among them are:
a) the inability to maintain the required small pitch of the gas flow shift;
b) deformation of the threaded joint and microgap as a result of the exposure of the movable system to dynamic effects (shocks, vibrations), with a consequent uneven resetting-up of the adjustable throttle;
c) spontaneous uneven resetting-up of the adjustable throttle due to temperature fluctuations;
d) the typical thread adjustment mechanism does not enable the needle to delicately approach and touch the seat surface. This leads to the generation of a giant pressure, ranging up to about millions of atmospheres, between the contact surfaces, causing metal particles to be removed from the seat surface when the needle backs it off. Hence, during the next adjustment cycle, the reproducibility of the flow is lost. Moreover, in the vacuum leak design with a thread adjustment there is no opportunity for automated
Q
380
4—I
14
T
I
I I
17
T
3Ü--
25 -■ -
15
I
10---
01
12 13 1011 ■ ■
16 I
Fig. 2. Throughput ranges for leaks of various types [3], ^m Hg/sec.: 1 - hole in a platinum disk (0.6-30); 2 - conic capillary (1-0.0009); 3 - crack in glass capillary tube (0.0021.8); 4 - crack in glass tube (0-380); 5 - capillary of circular section (7 10-7- 0.02); 6 - collapsed copper tube ( 0.003-3);
7 - capillary tube in combination with a glass needle (0.02-5);
8 - insert (plug) of a porous metal (0.00005-0.01); 9 - porous ceramic rod (0.001-10); 10 - device with ring-shape inserts,
operated at load control ( 0.002-0.00005); 11 - device with a springing washer (0.0005-0.07); 12 - device with a steel ball on a spheric seat (0.7-0.007); 13 - needle leak (0.02-1);
14 - needle leak with a sylphon seal (0.00006-40); 15 -platinum wire expanded in the glass capillary (0.03-0.25); 16 - a tangsten rod expanded in the steel clad (0.1-0.9); 17 - a heated capillary (4-40).
precision adjustment of the gas flow. At the same time, the conic needle leaks are easy in operation: they have the widest range of adjustment (Fig. 2), and means for delicate adjustment with an acceptable pitch control.
The capillary leak designs [6], especially when used in a set, enable to increase the throughput somehow. However, designs as such are intended chiefly to generate discrete leaks to vacuum volume.
The slit throttles [5] (Fig. 3) being adjustable by changing the tube deformation degree, have disadvantages that are typical to leaks with the threaded movable system.
The throttles with porous components and a mercury seal [5], being adjusted mechanically, are also flow unstable under disturbing effects (Fig. 4).
Fig. 3. Slit leak [7].
Fig. 4. Mercury leak with porous partitions [8]: 1 - porous ceramic plates; 2 - mercury
A threadlike leak [8,9] (fig.5) can be used for a precision dozing in a relatively narrow range.
1
€
1
2
/
L
r
t
Fig. 5. Threadlike leak: 1 - glazed iron weight; 2 - thin rod; 3 - capillary tube
Throttles operated due to temperature drop [5,10], enable extremely delicate adjustment through the temperature drop of the product in which the external and internal tubes are made of materials with different coefficients of thermal expansion. However, the results of experimental studies testify a narrow adjustment range of the flow, as well as instabilities governed by the leak temperature hysteresis
[3,11].
In the diffusion throttle designs [3,5,12,13], a gas permeability of solid materials is used (fig.6). Their diffusion through some materials can be used to supply vacuum system with pure gases.
For example, the hydrogen diffuses freely through palladium and its alloy with argentum, the oxygen spreads through argentum, helium diffuses through a thin quartz glass, nitrogen and carbon dioxide spread through iron, etc. [8].
Fig. 7. Thermoffusional leak [15]: 1 - palladium tube; 2 -glass cylinder; 3 - heater
It is well known, that thin-walled quartz vessels are permeable to helium, hydrogen, and some other gases, particularly at high temperatures (Dashman, 1964).
Fig. 8 illustrates a laboratory combined diffusional and thermodiffusional leak equipped with a working part of the thinwalled quartz spheres placed in series with a quartz capillary. The illustration at the left, it is presented in assembled state, and at the right it is shown as dismantled. The design as such, can withstand temperatures up to 7000C. The flow of gas can be governed by both pressure, and temperature, through the thin-walled quartz envelope. The leak can operate with and without being heated.
Fig. 6. Diffusion leak: 1 - gas cylinder; 2 - diffusion tube [15].
The drawbacks to these leaks are a small gas flow being generated, and a gas flow narrow measurement range.
The basis for operation of thermodiffusional leaks (fig.7) is the phenomenon of gas selective diffusion through some materials when heated. The drawbacks to these leaks are a small gas flow being generated, and a gas flow narrow measurement range.
b)
Fig. 8. Combined diffusional and thermodiffusional quartz leak: a - in assembled state; b - dismantled
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Let us denote the volume in cubic centimeters (at 00 and 760 torrs) by q , diffusing through the wall of 1 cm2 per 1 sec. As q is proportional to gas pressure (Dashman, 1964), and in reverse proportion to the wall thickness d , one can write:
q =
KP_ d
(1)
where K is commonly termed as permeability. In the literature on diffusion, the pressure P is generally expressed in centimeters of Hg (Pcm), and wall thickness d - in millimeters.
The coefficient K increases as the temperature rises:
K = K 0e
- E / RT
(2)
where E is the activation energy, cal./mol.
The devices having been used to inject protium, deuterium and inert gases into the discharge chamber, and based on a piezoelectric drive with the time operation of about 5 ^sec., are widely known. The injection is performed through membrane filters made of palladium [16, 16a]. Electric-driven impulse valves are employed to feed gas to injector ion sources. Over 4 m3-Pa of protium or deuterium are injected into each source in a time of the pulse duration of 2 sec.[17].
In the course of the investigations, the author has developed a device [18] based on the invention features [19], having a body with a conic hole and conic needle, rigidly connected to a cylinder filled with a liquid nitrogen. The device has been made by the author of this review, based on a leak-detector employed in PTI-10 leakage detector. Fig. 9, as well as works [20,21], present the external view and desription of this device.
In this device, the mass flow is governed by a small thread in the cylinder tail part, and an adjustable nut. The fixed clearance is maintained by a stabilizing action of the cylinder with a liquid nitrogen to the temperature of the needle and conic hole. To protect the intake hole of the leak feeler against the migration of a cloud formed by adjacent leaks during searching for leakage, the device is equipped with a gas screen. This screen is produced by feeding a vapourised nitrogen through peripheral nozzles located close to the microhole.
Statement of the problem. Main requirements and operation of precision vacuum leaks when used as sampling devices in gas analyzing systems
The operation process of an ideal adjusted leak can be written as follows:
QH„=Qo ± n -hQv
(3)
where Qh „ - is the theoretical value of the gas flow through the leak when an adjustment element travels for a distance of n pitch, m3-Pa/sec. (p. Hg/sec.); Q0 - is the optimal (medium flow) gas flow through the leak, m3Pa/sec. (p. Hg/sec.);
n - is the pitch number of the adjusting needle travel mechanism;
AQp - is the programmable pitch increment of the leak flow, m3Pa/sec. ( p. Hg/sec.).
Two factors, however, affect the leak operation accuracy: disturbances caused by the adjusting needle travel system, and the needle itself when moved (physical fields defined by the type of power drive - thermal, electrical, vibrational, electromagnetic, etc.), seat material damage due to the contact with the needle scorings, working adjus-ment feeding feeding mechanism wear,etc.; environment
Fig. 9. Leakage detector feeler with a cryogenic cylinder and gas screen.
disturbances both natural origin daily variations of temperature, pressure, insolation, humidity,etc., and artificial (dynamic and static) one: optical, vibration, thermal electrical, magnetic effects, electromagnetic fields, and artificial concentration variations of gas atmosphere.
Thus, the process of the leak operation can be described by a more composite formula:
e* = qh„ ±AQ„ + aq
■'ph
(4)
where Qh „ - is the total gas flow through the leak, m3-Pa/ sec ( l-^m Hg/sec);
Here c1 and c2 are scalar weight multipliers for M and J; Y° and YL are upper and lower limits of the vector component Y, and G is the vector of functional limitations (static and dynamic displacements, forces increased by the drive, field frequency,etc.).
In this work, the total sum of the amplitudes of steady-state fluctuations of the adjusting element displacement velocity, various disturbing factors and power force errors increased by the drivers Fdr. and disturbing factors due to the drivers' operation, has been taken as a vacuum leak quality parameter. This factor is presented by the expression:
J (Y ) = Ju (Y ) + J F (Y ) = £ £ Qjk\ujk (Y )| + Z Z Rk\FnPlk
(8)
k=1 je Nu
AQH - is the change of the gas flow through the leak due to the environmental impacts, m3-Pa/sec( l-^m Hg/sec);
AQph - pitch leak flow change caused by hysteresis phenomena, m3-Pa/sec (l-^m Hg/sec),
AQph = fi( J (Y)) (5)
where J- is the quality control factor; Y - is the vector of project parameters (includes both parameters specifying the structure elements sizes, quality of these components, and also parameters prescribing gain coefficients to velocity and displacement in the feedback circuits).
The influence of environmental effects both natural, and artificial origin, is defined by the relationship
AQHe = f2(ATe, A^, APe, A^, ACPGe)
where A Te are environmental temperature variations, 0C;
A$e - environmental relative humidity changes;
AP - are environmental pressure variations, Pa;
A®e - is the environmental vibration pattern change, Hz;
and A CPG - is the probe gas concentration change in the ambient atmosphere (probe gas cloud evolution).
Hence, the aim of the investigation into the precision adjustment of gas microflows is the minimization of A QH , AQ ph , structure mass , and time to compensate negative disturbance effects. In addition, it is important to tackle such trivial problems as the reduction the leak overall dimensions, energy consumption, cost, to a minimum; the improvement of reability, serviceability coefficient, ergonomiciy and safety.
As the majority of these parameters can be summarized by key optimization factors, the combination of the best leak design and at the same time the choice of the management law can be presented as follows:
k=1 ieNF
min c1M (Y ) + c2 J (Y ), G(Y) < 0, Yl < Y < YU
(7)
where JF and J° are items of the quality parameter based on power load errors, encreased by the drivers Fnp, and disturbing factors based on the drivers' operation, respectively; kd is the number of disturbing factor conditions; NF is the set of disturbing factors which are to be taken into account in J calculation; N is the set of
F ' u
control forces which are necessary to be taken into account in Ju calculation; u.k is the driver displacement velocity; Qjk, Rjk are weight coefficients for amplitudes of displacement and forces increased by the drivers which are considered to be equal to the reverse values of the maximum allowed ones of these amplitudes.
The substitution of the expression (8) into (3) presents a complete statement of the problem to combine the device design and system of adjustment. Although the solution of this problem at the given values of c and c2 does not present any fundamentally difficulties, the choice of respective coefficients 'a pr'ior'y will be a problem. This problem is often solved with the use of various values for c1 and c2 until reasonable values for M and J will not be obtained.
Another approach is to consider two specific cases: 1) c1=1, c2=0, and 2) cx=0, c2=1. In the first case, the aim function is the design mass. The statement of the problem is supplemented with two limitations setting up maximum allowable values J„ and Ju, as these factors have not been already included in the aim function. As a result, the following statement of the problem is obtained:
min M (Y)
Y
G(Y) < 0, J u (Y) < JUU ,
Jf (Y) < JU Yl < Y < YU (9)
where J° and J° are the upper limits for Ju and JF, respectively.
The second case has two statements of the problem:
myin Ju (Y X
G (Y ) < 0, M (Y ) < MU, JF (Y) < JU, Yl < Y < YU
(10)
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where the limitations for M and Ju above are included to counterbalance the omition of the design mass, and control action measures from the aim function expression, and
min J F (Y),
G(Y) < 0, M(Y) < MU, (11)
Ju (Y) < JU, YL < Y < YU
where the limitations for M and Ju above are included to counterbalance the elimination of the design mass, and reduce the disturbance factors from the aim function to a minimum.
The problems of combinating the vacuum leak design and at the same time the choice of a control law, having been written in the form of (7) - (9), present a set of problem statements applicable to design a wide range of devices with respective operative control systems. Each of the problem statements is most suitable for obtaining one of the three engineering objectives: (7) - reducing the design mass to a minimum, (8) keeping the action of disturbing factors, based on the selected degrees of freedom,to a minimum, and (9) minimizing the measure of control action.
Operating conditions and requirements for leaks
reliable treatment for removing mechanical impurities and aerosols; reduction the moisture content (by drying) to a required level; stabilization of pressure, temperature, flow rate and other parameters; the absence or keeping to a minimum the sorption or desorption character-ristics, minimum transport delay, and a higher parametric reliability [23].
One of the key and difficult-to-meet requirements placed upon the probe preparation system, is to provide the representativity of a probe being pumped to the measuring transducer, that is the garanted safety of its composition, primarily regarding to a component (or components) being defined.
Precision adjustable gas leak is one of the main parts of the probe preparation system. The analysis of the measuring transducer and gas analyzer operation, performed in [23], shows that the majority of state-of-the-art gas analytical apparatuses are exposed to the analyzed mixture pressure and flow rate instability. One of the ways to reduce the influence of the sample pressure variation on the gas analyzer's indications is the stabilization of pressure or flow rate of the mixture being transferred to the measuring transducer for analysis. Thus, the most important stage of the probe preparation process is to maintain desired pressure or gas probe flow rate [23].
For the majority of gas analizing systems, the operating conditions significantly differ from normal ones (ambient temperature 20± 50C, pressure 101±3 kPa) [22].
As sampling devices in the gas analyzing systems are often located outside the premises, the ambient temperature may vary in the range of -50 to +500C. The location of the gas analyzing equipment underground, on board the air-crafts, also significantly increases the range of environmental pressure (of hundredth parts to 130 kPa and over) [23].
Clearly, when developing sampling apparatuses, it is necessary to include devices of precision thermostabiliza-tion with accuracy of 0,01 - 0,0010C. The precision of the flow rate regulators must be about 0,1 - 1%.
The probe being sampled for the analysis, has a wide range of parameters, on a dust content, pressure, temperature, humidity, nonmeasured component content, the presence of harmful and chemically active materials. Measuring transducers having been used in gas analytical apparatuses, require, as a rule, to be supplied with a purified and desiccated analyzed probe, having stable physical parameters (temperature, pressure, flow rate, etc.). That is the reason, that the sample preparation system must be installed between the body under measurement or control, and the measuring transducer. These facilities are designed for intaking, handling, purification, desiccating, and pumping the analyzed mixture through the transducer measuring chamber, and also for stabilizing (if necessary) its temperature, pressure, flow rate, and other parameters [23].
A number of requirements are set forth to the probe preparation systems. Among them are : effective and
The influence of disturbing factors on the stability of a flow through a microhole
Types of disturbing factors
All the disturbing factors affecting the throttle hole adjusting element due to the intensity of the action on the adjustable flow, are subdivided into intensive and relatively stable factors. The basic contribution to intensive attacks showed by various mechanical and dynamic disturbances, and also variable thermal fields changing with a higher frequency.
Among relatively stable factors are: daily ambient temperature and relative humidity variations. During short-run operations, relatively stable factors do not in effect act upon the stability of the adjustable microflow, so they can not be taken into account. On the contrary, intensive factors affect vigorously the microflow stability. Dynamic disturbing factors can act upon both the adjusting needle, and the seat. The intensity of thermal factors is, as a rule, less in comparison with the dynamic ones. Nevertheless, they can have an active disturbing effect primarily on the seat.It should be noted that thermal factors can just effect on the gas microflow, changing its current behaviour through the adjustable slot. It was enough in winter time to take a PTI-10 (Tx=303 K) leak detector probe body near the seat with hands, as the gas microflow immediately changed significanly (up to 50%). This fact can be proved by some simple manipulations. If the portion of the probe with the initial temperature (Tx=278 K) is warmed up with heat via
the contact with an operator to temperature (T2=303 K), then according to the formula for the viscous flow from [24] we shall have
Qtl n x TL
QT2 n Ti
(12)
where QTi and QTi are the leak flows (leakage) of a probe gas at temperatures T, and T2, respectively, m3-Pa/sec. (l-^m Hg/sec.).
For example, at a relative humidity ^=70% and atmospheric pressure for the moist air temperature T1=278 K and T2=303 K, having defined from the tables in [25] the dynamic viscosity coefficient nT=17.48 ^Pa-sec. and nT=26.44 ^Pa-sec., we have:
Qt, 26,44 303 1 ^
—- =-x-= 1,65
QTi 17,48 278
Qtj = 1,65 x Qt2
Thus, it has been emphatically shown that if the ther-mostabilization of the adjustable throttle seat is not effected, there will be no possibility to obtain a higher accuracy at setting up and maintaining a desired microflow.
When long-duration works were performed,for instance at the time of searching for leakages in a large-sized hermetically nonsealed spacecraft, the role of relatively intensive factors was also noted: daily ambient relative humidity and temperature variations. These variationns, especially in circumstances when the test room is conditioned for the sake of reducing the control gas background, can run to higher values. For example, in "Baikonur" space port case, the daily temperature variations late in
a)
b)
Fig. 10. (a) Experimental arrangement for total product non tightness test in a low-pressure chamber; (b) a large-sized
product for local leakage test via a probe method: G,G1,G2 are leakage indicators; PA - ionizational thermocouple; V1 - V4,V12 - gas valves; V5 - V10 - vacuum locks; V11 - vacuum valve; PD, PD1 - manovacuum meters; PD2 - PD6 -manometers; ND1- ND3 - streamjet pumps; NI 1, NI 2 - mechanical pumps; VF - leak
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May attain AT =30 - 400C, and relative humidity ones are A^=70 - 75%.
Fig. 10 a, b presents configurations to control leak-proofness.
When implementing these control systems, the flow instability is noted. The principle of the tightness test using a mass-spectroscopic method is as follows: A pressure drop is produced at the opposite sides of the tested body. A probe gas is pumped to the high pressure zone. This gas on exposure to pressure difference finds its way through the product leaks to a low pressure zone which is connected with a mass-spectroscopic leakage detector. The product tightness is estimated by the remote leakage detector indications [24].
The state-of-the-art test systems have a large length. To provide a desired operational efficiency, a leakage detector shall be used in each field of control. That is why some mass-spectroscopic leakage detectors shown in the figure above are connected to the low pressure chamber. The solution of a problem as this, enables one to reduce the tested system response time.
The low pressure chamber containing a body under control is evacuated by a pumping system. The process of the evacuation for a working pressure in the spacecrafts (SC) to be achieved, is rather long, and takes it almost the same time to evacuate vehicles of the same type. For example, the spacecraft "Souz" was pumped out in the SM-702 low pressure chamber for 24 hours, and "Mir" station - for 48 hours. It should be taken into account that depending on the vehicle storage conditions (humidity) the pumping out time for SC of the same kind may be different. When the required working pressure was achieved, a zero background of helium or another control gas - determination of "zero characterisics", in the low pressure chamber is estimated. Having found a stable background in the low pressure chamber where a SC is installed, the measuring system is completely calibrated. The precision leaks being a part of the leak control system are used for this purpose. Following the measuring the microflow parameters by means of a special microrotameter, a desired control flow corresponding to a minimum flow, being critical for the given SC section, is specified by the precision adjustable leak. The operation as this is performed for each SC controlled section.
Then these sections which have their specific features are filled with a helium-air and/or helium-nitrogen mixture (a pure gas or mixture of any controlled gas and inert gas). Once the controlled mixture is fed under pressure to the test section, the total tightness of the section is tested. If the control gas background in the low pressure chamber exceeds the allowable level, the procedure of the search for total nontightness in each SC section stops, and one starts searching for a local leakage in the vehicle. With this aim in view, a portion of actual air or another gas (as a rule desiccated) is injected into the low pressure chamber, and the search process for local nontightness begins (Figure
10 b). The high-sensivity magnetic mass-spectrometric leakage detector, specially calibrated to search for local leakage via "probe" method, is used as a rule to determine a local nontightness.
The leakage detector designed to be operated on the "probe" principle is adjusted (calibrated) to a microleak - a special standardized (GOST) "Gellit" leak. The "Gel-lit-1" leak flow is qual as a rule to 5-10-5 - 5-10-4 l-^m Hg/sec, but for different problems there may be produced control leaks with various flows. The operator familiar with this flow and leakage detector signal, determines his own leakage detector sensitivity (each instrument has its own sensitivity: the higher this parameter, the more stable operation of the probe-leak, and the less adjustment pitch of the throttle hole and flow, respectively.
The less the throttle hole, the higher influence of quantum effects on the flow stability. The flow significantly depends on the throttle gap stability. At these so small flows, the gap stability depends on the fluctuations of atmospheric relative humidity, ambient temperature, atmospheric pressure, dust content, and vibrations being externally applied to the leak body.
From what has been said, it is seen that the calibration of a leakage detector via "probe" method is very sophisticated, and it needs to be steadily controlled, as up to now there has not been devised a perfect intellectual system which could track the leakage detector sensitivity. The leakage detector sensitivity is the relashionship of the variation of leak detector signal to the measured value that causes its change. The leak detector sensitivity is greatly dictated by the fluctuations of the evacuating system capacity which in turn is specified by the variation of electric circuit characteristics, ambient temperature, vibrations, and serviceable condition of the electronic system of analogue control signal processing, etc.
The control gas concentration leakages, moving in the space in the form of "control gas clouds", and fluctuating in the vicinity of the leak search, in the test room may greatly affect the tightness control accuracy. All these factors significantly influence on the duration and cost of the SC tests. It is common knowledge that all the pre-flight SC test expenditures accounted for 30% of the tightness tests. It is generally recognized that these works are the most cumbersome of all the pre-flight procedures on the SC.
A key part of the equipment designed for tests as these, is a precision adjustable vacuum leak. This work is devoted to the improvenent of this device.
Dynamic disurbing factors. A model of dynamic vibrations in the regulator needle
To estimate the effect of dynamic disturbing factors on the device, a scheme of virtual vibrations of the regulator needle is presented (Fig. 11). For this arrangement (Fig. 11a) the gap area is determined by the gap sizes f and f2.
For the arrangement (Figure 11b), f1<f2. The regulator
needle vibrations reduce the area for the gas microflow to pass freely through the vehicle, upset the normal stream mode, and cause pulsations.
Fig. 11c shows a vibration -collision pattern. At some vibration frequency, the needle speed may exceed the sound speed in the air. It is known that the average molecule speed is commensurable with these speeds, notably at lower temperatures:
where var is an average arithmetic speed of a molecule
(13)
1
8 x k x T
s s
nx m„
g
for the analyzed gas, m/sec; kg is Boltzmann constant (k=1.38-10-23 J/K); Tg is the gas temperature, K; mg is a gas molecule mass, kg.
Tightening effect. Thus the tightening effect may well influence the flow variation even at room temperatures. The heavy gases - CO2, Kr, and SF6 exhibit this effect. The area of the free passage for the gas microflow significantly reduced. Under these conditions, the seat vibrations which disturb the stable operation are observed. The vibration model of the device under consideration is a flexible system installed between the retainers (Fig. 12).
The equation for the linear flexible system with a
friction on the interval between the collisions at the retainers in a dimensionless form is [26]:
x + 2 • C 4 • x + 4 • x = 4 • sin(t + p)
(14)
k * x = — x
P
t = m • t, C = ■
C
2 • m •m
4 =
COq_
m =
Here is the static displacement of the flexible system. General equation (14) solution for C <1 takes the form:
x = Q • sin(r + p + d)+ C • e C^'T • sin(^r1 T + p(
Q = ~-~2-=£• Vi-C2;
i) + 44
J 42
2 • c 4
W2"
(15)
a b c
Fig. 11. A view of the regulator needle virtual vibrations: a - without vibrations; b - vibrations without collisions; c - vibrations
with collisions at the lock valve seat.
Fig. 12. Vibration model of the flexible system between the retainers
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The boundary conditions of the system movement on the interval from the shock at the left retainer to the same of the right one are:
x = —a; x = v при т = 0 x = a; x = U при т = п -e
(16)
From the plot [27] ax - £ we see that on the Bfl interval the vibroshock mode amplitude appears to be wider than that of the linear ones at the constant excitation amplitude. The tightening effect will be obtained due to shock interactions between the flexible system and retainers. Thus, for the leak stable operation it is necessary to avoid vibration absence in the frequency range of (£, £), and also to use a material with a high friction coefficient to make a seat. In addition, the structure vibroprotection, notably needles and bodies against dynamic loads normally directed to the leak axis are worth to be foreseen.
On designing a definite device, it is worthwile to perform a vibrodynamic test for frequency ranges with a minimum effect on the leak stability to be defined. It is desirable to carry out climatic chamber tests, and also field tests in cryostats and hydrostats to estimate the structure stability and operating characteristics at temperature and humidity variations. It should be noted that the body and port of the device may be exposed to vibrations.
Thus the structure with a thermal controller will obviously aid in reducing the tightening effect on the adjusted gas flow. This is especially important for the process control at low temperatures.
Thermal disturbing factors. Thermal fluctuations of the seat and needle cone result in the microhole area variations. As the working width of the ring-shaped microhole is commensurable with the molecule sizes, its variations greatly affect the gas microflow. The thermostabilization of the adjusted microhole is therefore is required. In addition, temperature variations at the microhole surface are transmitted by the flowing gas molecules, causing the pulsations of molecules' speed. Owing to different coefficients of gas conductivity, the adjustable throttle selectivity effect to specific gases can be observed. Here, temperature variations may cause sudden change in the gas flows having a higher coefficient of conductivity, and insignificant one for gases of a lower conductivity.
Throttle effect and adjustable gas medium variations. When reducing the leaks' overall dimensions, and choosing materials used in manufacturing of a seat and needle, it should be taken into account the possible throttle effect on the flow rate accuracy. The throttle effect error can be multiplied by a drastic change in composition of the gas medium under control. For example, a higher error may be observed when the leak is used to search for a local leakage in vehicles.
Let us consider a case when a probe consisting of a helium trapped from the migrating cloud, passes through the probe-leak. It is well known that when helium flows
through the throttle, a reverse throttle effect is observed. It resulted in microheating of the adjustable throttle and the gas sample being transported via it.
In some designs, temperature variations as these may be sufficient for a sudden microheating process to start just after a microcool-ing have been performed. But this may reduce the flow rate accuracy.
Groundings in theory of gas and vapour mixtures when moving through conic throttle
Mass transfer physical and chemical processes in capillary structures
Influence of relative humidity variations and electrical fields
The behaviour of an ideal gas when moving through conic gaps and microholes has been well described in literature (Heinze, Rosanov, Dashmann, Kucherenko). In this section we dwell on the gas -vapour and liquid phases moving through microholes in detail. It is well known that on the seat surface at a relative humidity exceeding 30% , the fluid film thickness will practically always undergo a change, responding immediately to the flow relative humidity variations [28].
Possible leaks through microholes are calculated according to Poiseuille formula for volum rate of the incompressible fluid of viscosity n0, flowing through a round channel of radius R, and length L under pressure drop AP:
Q, = nxAPxR4 /(8хц0 XL)
(17)
The process of the sample fluid flow through real microholes having a complex unknown geometry, can be described by filtration theory methods [29]. In particular, the flow rate at the stationary flow through a porous medium ( in our case a microhole) is expressed by Darci law [29]:
Qi = CD xAP/По
(18)
where is a constant defined by the flaw structure and does not depend on the flowing fluid characteristics. For the round channel this coefficient equals to and from (12) we have (11).
When analyzing the fluid flow in the round channel of radius R in a narrow space of the size h at the channel walls, the fluid characeristics significantly differ from volume ones [30], [31]. The thickness of this near-surface layer with transformed characteristics undego changes, according to various authors' estimations, from 1nm for simple liquids to 103 - 104 nm for liquid crystals, and depends both on the character of intermolecular interaction in a liquid, and interaction of liquid molecules with the same of the wall [32].
Microstructure of the liquid thin layers. The application of state-of-the-art computers has enabled to obtain new information on the liquid molecular structure using numerical experiment methods molecular dynamics and Monte-Carlo. These methods appeared to be especially convenient to investigate structure features of thin liquid interlayers, as the limitation of the computer memory and fast operation do not allow to operate a very large amount of molecules [33]. The main point of the application of numerical methods is as follows. Definite potentials of the intermolecular interaction and interaction of the liquid molecules with surfaces bounding the interlayer are given. Then one finds a time average distribution of molecules in the interlayer which corresponds to a minimum free energy of the system, and thus its equilibrium state at the given conditions. The molecular forces potentials are preliminary tested on the volume liquid models in the absence of the surface force field, and are selected such as to transmit closer their physical characteristics. For simple liquids, the molecules of which can be approximated by spheres of radius 8, the potential u(r) of Leonard-Johnson molecular interaction is considered to be a good approximation. Numerical experiment results [34-39] show that the microstructure of the water boundary layers near hydrophilic and hydrophobic surfaces is different. In particular, near hydrophilic surfaces the water density is higher, and the molecules' tangenial flow is low. Near the hudrophobic surfaces, on the contrary, the density is lower than that of the water in volume, and the molecules'tangential flow is higher.
Influence of the microhole sizes. M.I. Kalinin in his work [32] suggest to divide all the microhole range into four domains. The key criteria are R, h, X (characteristic molecule size).
Domain I. R >>h; for polar liquids R> 0.1^1 |j.m. Here one can ignore the influence of the near-surface layer with a changed character on its flow. For this domain the hydrodynamic approach is an approximate one. Formulas (17) and (18) allow precise calculations.
Domain II. h < R; for polar liquids (10-20 nm) < R< (102-103 nm). In channels as these the proportion of the boundary fluid phase is comparable with the same of the volume one, and the near -surface layer influence significantly on the capillary flow behavior. The hydrodynamic approach is becoming less reliable and precise. The use of volume values for viscosity n0 is not quite justied.
Domain III. X << R << h; for polar liquids (0.3-0.5 nm) <<R<<(10-20 nm). In simple liquids this domain is not observed. In channels with sizes as such, the liquid volume phase disappears.
All the fluid is present in a particular surface layer. The overlapping of the near-surface layers with the structure being changed occurs, and this results in surface phenomena of the second kind, the most important feature of which is the fact of a wedging pressure [32]. The influence of these effects on the flow in capillaries has not deeply studied yet. The existing methods do not enable to obtain reliability and
precision adjustment of such microhole sizes [35].
Domain IV. X<R. It is the domain where the continual approach is unsuitable. The fluid flow can be described based on statistic mechanics methods, which have not been presently developed for the domain of the given sizes.
M.I. Kalinin, analyzing the influence of specific behaviour of the liquid near-surface layers on the capillary flow for a liophilic solid wall, calculated the rate per formula [32,34]:
^ TTxAPRrx(R2 -r2 ,
Qi =-1-dr
2 x L
n(r)
(19)
in connection with the fact that near the liophilic solid wall the liquid viscosiy n is higher than its volume value n0 several times, and acts as a function of the distance to the interphase surface [32, 34].
In channels with liophobic walls, a liquid slip phenomenon [30,32] on the boundary with the solid wall is observed. The liquid rate is calculated per formula:
Qi =
nxAP 8 x n0 x L
x R4 x (1 +
4xn xY R
(20)
where y is the slip coefficient. Based on the estimation [29], for the water in the capillary wih hydrophobized walls, y ~ 10-5m3/(H-c) (h = 4n0Y ~ 10 nm).
The slip resuls in flow growth. In thin boundary layers of polar liquids, a phenomenon of liquid-crystalline orien-tational orderliness is detected [31, 40, 41]. The molecules in this layer near the liophilic surface are oriented with their long axis normally to he interface. The induced orientation as this will cause the viscosity of the liquid flowing along the surface to increase. M.I. Kalinin assumes that in cases, when near liophobic wall there occurs the slip phenomenon, the polar liquid molecules are oriented parallel to the surface, that causes the viscosity as compared to volume values to decrease [31].
The experiments show [32] that as the temperature increases to 70-750C, the change of the boundary layers' behaviour in comparison with the volume ones is not observed. The reason is that the intensive thermal flow attacks the specific liquid structure, being formed by the surface forces. The temperature of the transition to the state of an unordered thermal flow depends on the energy of interaction between the liquid molecules and surface.
In parallel with the change in liquid behaviour and conditions at the boundary of a solid body, various physical and chemical processes, affecting the capillary flow in narrow nearboundary layers take place. When salts are present in the water, an adsorption phenomenon occurs. At negative adsorption , the concentration of a dissolved material at the microhole outlet will be significantly decreased. Filtration as this may cause an "apparent" decrease of the rate, and
46
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in general an "apparent" clog [32]. At positive adsorption, the increase in concentration results in the boundary layer viscosity decrease to a volume value [42,43], and thus to increase. Alternatively, in capillaries and especially at the outlet, crystallization processes may occur [44].
Effect of electro-kinetic phenomena. The fluid flow behaviour is significantly affected by electro-kinetic phenomena [45]. The electric double layer causes an electro-viscosity effect,decreasing the fluid. The theory of electro-kinetic phenomena has long and successfully been developed.Its application for the leaks used to search for local tightness of the equipment is necessary and possible in regions I, and partially in II ones. However, in regions III and partially in II, the theory status is far from the completion and practical implementation.
It is M.I.Kalinin's opinion [32] that the capillary adsorption process is affected by various factors: anomal behaviour of surface layers of liquids, dependance of the surface tension coefficient on external fields, relationship between the edge angle, flow velocity, and solution composition, existance of an adsorption film on the walls, possible nonuniformities of the walls, microdefects, and capillary end effects [46-48].
The prospects for development of the theoretical basis. M.I. Kalinin in the cited work [32] has shown that to improve methods and equipment for leak control, and metrological ware there is a need to provide the grounding in theory as follows:
1. In regions II, and in part of I one, it is necessary to investigate the mechanism of capillary flow taking into consideration the abnormality of the fluid boundary layer haracteristics viscosity, diffusion coefficients, solubility, and dielectric permeability), physical and chemical proc-
esses occuring in thin channels (adsor- tion, electrokinetics). Today the tackling of these problems is pos- sible on the phenomenological level via a computer simulation, as there are no reliable and complete data on the boundary layer be- haviour at hand.
2.In regions II and III it is advisable to elaborate statistical investigation techniques for studing liquids and gases (pure and mixtures) at the boundary with a solid body, to assess their behaviour which is of great importance at leak tests.
3.In regions III and IV it needs to research in fluids' behaviour and mechanism of their flow by means of computer experiments.
The data available in literature show that for the majority of polar liquids the variations in liquid behaviour observed at the near-surface layers regarding their volume values disappear with
increase in temperature to 70-750C. In this case, the liquid flow may be described by a hydrodynamic equation , and in particular the rate will be defined by formula (17) or (18). The increase of the flow through the microholes with the size of 20-30 nm due to viscosity decrease to a volume one is assumed to be 30-40% [49] and owing to an electroviscosity effect decrease it may be 10-20% [44]. For microholes with sizes of ~ 10nm, according to [45,49], the increase of the flow with the temperature growth may be 150-200%.
Experimental arrangements, designs and operation of tested leaks
Description of measuring arrangement
Fig. 13 shows the measuring arrangement used to test
Fig. 13. Measuring arrangement: NL1 - forvacuum pump; ND1 - diffusion pump; B1 - B6 - valves; PT1, PT2 - thermal vacuummeters; PA1,PA2 - ionization vacuummeters; VF1 - leak; PD1 - deformation vacuummeter; R1 - low pressure reducer; M1, M2 - manometers; V1 - V3 - vacuum chambers; V4 - chamber containing the gas under control.
and calibrate the leak presented in Fig. 9 [21]. The pressure differential at the leak was built up between a vacuum volume V2 and the same of the gas under control V4. The gas flow through the leak was measured by the pressure increasein the vacuum volume V3 with the valve B2 close. The leak tightness with allowance made for all errors was determined within the accuracy of 5 %.
Experimental results show that the tested prototype fails to afford continuous adjustment. The volatizing cryogenic component can not thermostabilize the adjusting needle for long. The design of the arrangement becomes more complicated when a cryogenic product is fed via a pipeline. Besides, ice crystalls having formed in the throttle in the course of the experiments cause the upset of the stabilized microflow.
The moisture content in the leaking air which is commensurable to the relative air humidity acts as a factor responsible for significant errors in the given leak maintenance. During the course of the experiments there was observed the influence of low vibrations acting both along the leak axis and perpendicular thereof.
Description of the apparatus for the adjustment of the gas flow under control based on thermal drive
In the course of the investigations in terms of the experimental results, the apparatus for the adjustment of a gas flow under control [50,51] (Fig.14).
Version I. The needle guide 3 consists of a container with a sylphon 4 being rigidly fixed to the body 2 by means of a heat insulating plug 9, and filled with liquid 5. Liquid 5 acts as a working body for the guiding the needle. The conical needle3 is rigidly fix- fixed to a movable wall of the sylphon 4. The container with the sylphon is equipped with a heating unit, comprising a heat tube in the form of a finned sleeve 6 with double walls. The space between the sleeve walls 6 is pressurized and filled with a capillary
structure impregnated by a liquid. Inside the sleeve 6 there is installed an electrical heater 7 connected to the temperature regulator being a bimetallic spiral 8 with a contact system installed on the turning limb 12. The spiral 8 and contact system 10 are connected by means of disconnector 13 fixed on the movable spiral 8 end. The bimetallic spiral 8 is located in the cylinder 14 with double walls which is installed in the container with the sylphon 4 axially to the heat tube. To protect the apparatus against external temperature fluctuations, the body 2 and conical needle 3 are made of a material with a linear coefficient of thermal expansion not more than 1.0-10-6 K-1, for example of 32 NKD alloy; and on the body there is installed a heat insulating housing consisting of two parts 11 and 16. The front part 16 has a cylindrical socket 17 located at the conical inlet. The rear part of the housing 11 is made removable , and installed on the body 2. The heat tube designed to remove the working fluid heat to the environment, is equipped with perforated ribs 19. The working fluid 5 filling the container with the sylphon 4, changes its volume when the temperature varies. Here the conic needle 3 movement along the axis of the hole 1 (at temperature variation by 1K) is determined by parameters, as follows: volume coefficient of thermal expansion of the liquid P, container volume 4, medium cross-section area of the container 4 sylphone. The elecri-cal circuit is connected to the mains such as 12 B. Based on the required control gas supply conditions, one selects the working fluid 5 temperature which is then specified by the turning the limb 12. A thermal flow from the electrical heater is quickly and evenly distributed over the sleeve 6 surface, and due to the perforated edge transmitted to the liquid 5. This liquid changes its volume according to the temprature variation ,causing the deformation of the container 4 sylphone,and the conical needle 3 travel along the hole 1 axis, and consequently, the variation in the gap
16 15 la 14 13 12 11 Version 1
Version 2
Fig.14. Apparatus for the adjustment of a gas flow under control: 1 - adjustable throttle in the form of a conic hole; 2 - heat insulated body; 3 - adjustable conic needle; 4 - container with a sylphon; 5 - working fluid; 6 - finned sleeve with double walls; 8 - temperature regulator in the form of a bimetallic spiral; 9 - heat insulating plug; 10 - disconnector; 11 - rear part of the heat insulating housing; 12 - turning limb; 13 - contact system; 14 - double wall cylinder; 15 - control gas discharge pipe; 16 - front part of the heat insulating housing; 17 - cylindrical socket; 18 - pressure sleeve; 19 - adjustable heat perforated tube; 20 -
sylphon filled with an inert gas
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between them resulting in the control gas flow. When the given temperature is reached, the heat sensitive bimetallic spiral 8 via disconnector 13 de-energize the terminals 10 of the electrical heater 7 power supply.
So, during the apparatus operation the temperature regulator and electrical heater 7 maintain the preset temperature of the working fluid 5, the surface of the container 4 with sylphone, and the conical needle 3, that must ensure the stability of the ad- adjustable throttle hole geometry. For this purpose the conical needle 3 and body 2 are made of a material with a low linear co- efficient of thermal expansion.
Further, to afford automatically controlled process and higher capacity of the apparatus, a construction of the device to regulate a gas flow (Fig.14, version II) [52] has been developed.
Version II. The device designed to regulate a controlled gas flow consists of a container with a sylphon 4, filled with a working fluid. A working body in the form of a conical needle 3 is fixed on a movable edge of the container sylphon 4 and can travel due to the working fluid thermal expansion (compression). At the end of the advancing end of the heat tube 6 coaxially to the latter there is installed a pressure sleeve 18 and sylphon 20 charged with inert gas. The external movable wall of the sylphon 20 is in contact with the inner surface of the pressure sleeve 18 edge, incorporating a locking device. The sylphon 20 and sleeve 18 are used to adjust the liquid-vapour boundary in the tube 6, that is, to control the emission of heat to the environment, and as a consequence, the needle3 travel velocity.
The apparatus [52] as opposed to the same of [51] affords better heat removal and, as a result, cutting short the time of the locking body move when the throttle opens, higher accuracy of the preset heat removed to the environment and a desired gas flow. This enables to increase the apparatus efficiency in adjusting the gas flow under control.
Thermal adjustment of a gas microflow
Fig. 15 shows a kinematic scheme of the developed apparatus. The adjustment process is characterised by two ranges: 1 - high and 2 - low leaks.
Assume, that in the range of high leaks the adjusting needle does not come in contact with the locking body seat, and in the range of low leaks a conical part of the adjusting needle interacts with the conical hole surface.
For the first range
У = Bc xaL x TI,
x = Bc xaL xT7 xRIл!H2 + R2
(21)
where y is the axial displacement of the adjustinh needle; x is a lateral displacement of the adjusting needle; Bc is the lengthe of the sylphone; aL is the coefficient of volume thermal expansion; T1 is the temperature of fluid in the first range; H and R are the length and radius of the conical end of the needle cylindrical part, respectively.
Thus,
where f0 is the setting clearance at normal conditions; f
f = fo - x = fo - Be xaL x T x R lV H2 + R7
(22)
is the clearance between the inner surface of the conical hole and the same of the needle conical part.
For the second range, the displacement of the adjusting Fig. 15. Kinematic diagramme of the shutting-off device
needle will be defined by the fluid liquid thermal expansion, and the joint thermal deformations of the adjusting needle and body on heating.
All these processes considered, one can determine the working temperature the working fluid is to be heated to reduce the leak to the given value. The force acting on the sylphon movable end is found from
Fi = zL x PL x TI
(2З)
where zL ,pL, RL is the compressibility, density and gas constant of the working fluid. The projection of the area of the adjusting needle conical end with the leak seat to the plane perpendicular to the acting force Fl is defined as:
„ ru R (x - fo )xV H2 + R2 .2
Fcon =ПХ ( Hs x H + ---+ r)2 -
■nx
r +
(x - fo)xVH2 + R H
2
(24)
where HS is the seat hight, r is the conical hole least section radius.
Then the force occuring as a result of the needle injection into the body will be equal to
F2 = Fcon x(<T)K =nx(<T)K x
(x - fo)xVH2 + R
HS x — + S H
R + (x - fo)x Vh2 + R2
H
-nx (<T)K x
H
(25)
lr
where (aT)K is the flow limit of the body material. Hence we have the relationship between the clearance in the second region and the fluid temperature TII:
fi =
TII x Fc x zL xpL x Rl
„ HS x R Vh2 + R2 2 x —S-x-xnxa7
(26)
H HS x R
H r x H
2 xV H2 + R2 -H+R
Once the thermal flow was transmitted to the working surfaces of the shutting-off device, the deformation of the adjusting needle and leak seat occurs, and as a result the clearance follows the magnitude of the additional displacement through thermal conductivity Af The relative deformation of the adjusting needle and seat is found from:
HS x — + S H
R fn xV H2 + R2
H
+ r
- (r +
fi xV H2 + R2 H
Fk x EK + Eu =
, „ „ „ =aK x TK aU x TU J
)2 ]x
(27)
where FK , Fv.
E , and ETT are the sectional area and elastic
modula of the body and needle, respectively; are
the coefficients of the thermal expansion of the body and needle, respectively; TK and Tv are the temperature of the body and needle, respectively. So we have:
Thus, the total clearance in the second region of the adjustment will be
Device for adjusting a gas flow under control based on a threaded hydraulic reducer
Fig. 16 shows the vacuum leak fitted with a special precision drive [53].
The main idea of the apparatus in question lies in the fact that to reduce the influence of the instrument error on the precise movement of the needle, and also to improve the accuracy at setting the required gap through reduction the driving torque co- efficient of the nut to the needle translational motion, the conical needle guide threaded mechanism is connected indirectly to the needle via the second adjuster (a hydraulic reducer) with the given transmission coefficient.
Figure 16. Vacuum reducer with a hydraulic reducer: 1 - heat insulated body; 2 - adjustable throttle; 3 - shutting-off device in a conical needle form; 4 - large sylphon of the hydraulic reducer; 5 - conical needle threaded adjuster; 6 - adjusting nut; 7 - adjusting nut chamber; 8 - lock-nut; 9 - washer; 10 -hydraulic reducer small sylphon; 11 - small sylphon movable end; 12 - working fluid; 13 - large sylphon movable end;
14 - seat
Af =
aK x TK
a T
w-U1 U
2xnx (aT)K x HS x R x Vh 2 + R
2 x* K
F x E^ + FTI x ET
FK x EK x FU x EU
HS x R
2xVH2 +R2
r x H
Vh2 + r2
(28)
s = nx
X X
T
fn = fn +¥ (29)
In such a manner, the proposed apparatus is characterized by the throughput thermal control, needle heat stability, and a dynamic stabilization of the needle in axial direction. The apparatus is suitable to be equipped with an automation system of the process control and maintenance of throughput. A system for protection against large forces applied to the needle and seat when the valve is locked, may be easily realized in this apparatus.
As compaired to the well-known leaks with the thermal control, the effects of thermal and force hysteresis are significantly low in this device. Moreover, the apparatus construction has been protected against dynamic disturbances occured in the plane perpendicullar to the needle axis.
Let us consider the apparatus presented in Fig.16 in detail. The smaller diameter sylphon is installed in the sylphon of a larger diameter, and because of this an air-tight cavity of variable volume is formed, which is filled with a working fluid having a small thermal expansion coefficient and high heat capacity. The combined end of the sylphons is fixed to body. The large diameter sylphon movable end is rigidly coupled with the conical needle, and the small sylphon movable one is connected to the conical needle guide threaded mechanism. The seat is made of elastomer, such as polyetrafluoroethylene.
Liquid with a small thermal expansion coefficient aT and high heat capacity to reduce the influence of heating process regime variation of the apparatus can be used as a working fluid:
Al = laT •((-10) (30)
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02SDHi
where Al is the overall dimension increment due to thermal expansion; l is the initial overall dimension of the device; t-t0 is the ambient temperature variation.
The threaded adjuster is made of steel U8A or U10A being chilled to a hardness of NRS 50-55 followed by grounding. The finished surface hardness of the chilled screw is of P1.6 to P 0.8.
In order to avoid the influence of the apparatus's operation heating schedule due to the environmental temperature, the body 1 is coated with an insulating material. Moreover, the body 1 and conical needle 3 are made of the material with a linear thermal expansion coefficient of no more than 1.0-10-6K-1, such as 23NKD alloy.
In order to avoid the adjusting nut gap, washers on the both sides of the nut along the screw axis in the nut slide stop chamber are installed. To fix the threaded adjuster after the required gap between the needle and seat has been set, a locking nut 8 is mounted on a free end of the threaded adjuster outside the chamber 7.
The principle of operation of vacuum leak with a reducer.
A required rate of the gas flow through the apparatus is specified as follows. Having released the lock-nut 8, the adjusting nut 6 is turned from side to side to set the wanted gap between the seat 14 and needle 2. While turning the adjusting nut 6, the threaded adjuster 5 transfers to the distance l1 the movable end11of the small diameter sylphon with an area of fc . The volume of sylphon 10 contained in the working fluid 12 is changed by the magnitude AV1. In response to this, the incompressible working fluid 12 trying to retain its volume exerts some hydraulic action on the inner surface of the large sylphon4. The movable end 13 with an area of Fc tavels the distance l1 changing the volume having been formed by the large sylphon by A V2. Here, owing to the travel of the conical needle 3 which is rigidlly connected to the movable end 13, the working gap of the adjusting throttle 2 varies. As the fluid is considered to be incompressible, then AV1 = AV2. But
AV = fc lp and AV2 = Fcl2. ^ fc l! = Fcl2 ; l1 = fJJ
Fc or ll = Kl2,
where K = f/ Fc is the geometrical parameter specifying the sensitivity of the needle second adjuster.
To improve the acuracy the reducer sensitivity must be reduced. When designing the apparatus construction, the ends' area ratio is specified based on the device sensitivity. The design of the adjusting needle rigid fastener practically eliminates the needle vibration in the perpendicullar direction to its axis altogether.
Investigation into processes occuring in microholes under changes of the environment physical parameters and effects of external man-made noises, is of great importance now.
Nowadays in Russia there have been manufactured
several types of leaks: NDZ; NET; NMB-1; NK-2P; NRP-1,6 and NK-2,5 and KN-2m. One way or the other, the disadvantages having been considered in this work are intrinsic in all these devices.
Precision adjustable leaks are called for modelling space environment systems [54,55], adjusting the gas mixture sampling in leak detectors [56, 57], certificating and calibrating the gas sensors [58,59], implementing a number of chemical processes [60], employing in rapidly growing nanotechnologies [61] , accelerator vacuum systems [62] and vacuum-tritium systems of the thermo- nuclear reactors [17], vacuum-cryogenic techniques [63,64], developing adsorbents and heters [65,66], and also to formate gas-alloyed metallic film structures [67].
Improvement of precision leak ergonomics to search for local leakage in environments not easily accessible
Minimization of the leak dimensions
Among the above mentioned characteristics, the leak detectors must have small dimensions. The leak detector manufactured at NPO "Energiya", equipped with a conical needle having a thread and wedge-type groove is shown in Fig. 17.
Fig. 17. Leak detector with a wedge-type groove
In this design the needle comes fully into the conical body, and the wedge-type groove runs the entire length of the needle.
The thread significantly reduces the vibration effects on the leak performance stability over the needle length.
An unconventional design of the wedge-type leak is presented in [68]. This design enables to measure gas or liquid flows over wide limits with given leak parameters according to the micrometrical head specifying the displacement of the changeable driver.
The widening of the adjusting range is provided just by replacing the leak driver without disassembling of the design, and specifying the law of flow change as a function of the drive displacement.
On operaing as a part of the vacuum system with the chamber of 100 l and basic pressure of 5 - 10-7 torr, the apparatus ensures a time stable pressure in the range of
5 - 10-7 to 1 torr. The leak
driver stroke is of 0 - 20 mm to the nearest 0.01mm. When the driver strokes over 0.1 mm, the pressure changes less than 10% of the vacuummeter scale [68].
The tendency to minimize mass-spectrometers has been noted worldwide. To do this would require to minimize not only the measuring transducer, pumping-off system, but also the sampling device. Scientists from Toronto University (Canada) and Michigan University (USA) propose to develop a pump based on a hollow carbon nanotube. Tube as this may be filled with atoms, such as helium. This proposal has been already demonstrated in the experiment [69]. The problem of atom transport may be solved through an external electric alternative field enabling the electromigration of atoms.
The seat of the precision vacuum leaks may be severely damaged, and as a result, the leak reproducibility is impaired. If the seat and needle is to be made of high-strength materials , the reproducibility will be increased. The seat must be made of a mateal of higher elasticity compared to the same of the needle. The opportunity to employ carbon nanotubes for the needles and their surfaces to be produced is of interest. The merits of carbon nano- tubes for this purpose are their extremely high mechanical strength which is specifically proved by direct measuring results [70]. The nanotube Young's module in the axial direction, for instance, is about 7000 Gpa, while for steel and iridium this parameter has the values of200 and 520 Gpa, respectively. It must be also noted the higher thermodynamic stability of the nanotubes and fullerenes that, as it was mentioned earlier, is of great importance to provide the stable flow through the throttle.
Conclusions
The procedure of synthesizing leak design and an active control system according to which there are considered independent multitudes of design parameters related to the construction and control.
A special feature of this work is the use of a great quantity of various limitations in the definition of the problem. The design improvement enables to significantly decrease control operations. Integrating the special designed control system may lead to drastically reduction in the design weight, also the influence of disturbances on the constant value of the adjustable flow both in static and dynamic modes.
The importance of further development of a new generation of the precision adjustable leaks is undeniable. The staff of the "New hydrogen and vacuum techniques" group of All-Russian Scientific and Research Institute of Experimental Physics (Sarov) has been undertaking the investigation on the development of high-precision leaks of the new generation, haracterized by their stability environmental effects and higher degree of the gas probe representation.
Investigations into the development of a multichan-
nel delicate leak detector of comparator type along with the mathematical processing of the signals based on the wavelet-analysis [71-73], and talking indication of the leakage direction for the operator have been planned. In addition, scientific and research activities in developing a leak detector equipped with a spacing magnetic-friction leak with the capacity of self-adjustment to any specified programme have been under way. The magnetic-friction leak driver is operated based on input signals from remote-acting throttles.
Bearing in mind the fact that in devices designed for remote micropositioning of small-scale facilities, piezo-ceramic or electrostriction elements [74-77] are used as an operating one, the opportunity to develop a leak based on a force element similar to these drivers has been planned.
It goes without saying that the precision adjustable leaks for gases (hydrogen in particular) will find commercial application in the development of a hydrogen vehicle and in the hydrogen filling system infrastructure as well [78-80].
The author is grateful to Shumilin Alexei Alexandrovich, the former Head of "Baikonur"spacedrome, and now the leader of the Russian programme on equatorial launches from the buoant start "Sea Launch"- "Sea start", who has provided a useful guide and paid a great attention to not easy work of the people testing space vehicles and suits in pressure chambers SM-702 and SM-357.The author is especially indebted to his collegue Kudryavtzev Ivan Ivanovich who displayed his talent in the joint development of new types of leaks.
The author is especially grateful to Samostrelova Svetlana Petrovna, Dyadyuchenko Yuri Pavlovich and Nemyshev Victor Ivanovich for their generous effort in putting this work into shape.
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