PRECISION 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 significant 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,
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экология» № 4 (82) 2008
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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
International Scientific Journal for Alternative Energy and Ecology № 4 (82) 2008
© Scientific Technical Centre «TATA», 2008
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
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Fig. 2. Throughput ranges for leaks of various types [3], ^m Hg/sec.: 1 - hole in a platinum di
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