School Work

Asignment water

of 22
All materials on our website are shared by users. If you have any questions about copyright issues, please report us to resolve them. We are always happy to assist you.
Related Documents
  There is a constantly growing need for products and technologies that for their realisationrequire hermetically closed elements, vessels and tubes. Envelopes with greater or smaller vacuum tightness had to assure a satisfactory isolation between external atmosphere and inside over- or underpressure. Crytical leak spots in closed systems are usually connections, gaskets, welded and bra!ed oints, defects in material etc. #egarding to quality demands in different production processes, technical people very often encounter with standards for tightness testing. $ishing to state if an element or system meets with corresponding standards we have to be familiar with the procedures enabling the following appointments - question if there is leakage or not- settlement of the leak si!e- detection of leak locality%or solving the mentioned problems different treatments and techniques are known but among them there is no an universal method, Each testing is suitable only for a selected leak rate or for fixed forms and technologies. &ur article presents the leak types, their si!es and various leak detection techniques. 'eywords tightness, leakage, trace gas, helium mass spectrometer leak detector, hermetic sealing 1. INTRODUCTION The words (leak( and (leakage( appear in the field of vessels) hermetical closing and do not confront only with vacuum technologists but also engineers working with high  pressures. * leak means an unintended crack, hole or porosity in an enveloping wall or   oint which must contain or exclude different fluids and gases allowing the escape of closed medium. The basic functions of leak detection are the locali!ation and si!e measurement of leaks in sealed products and systems %or ma ority of examples, a leak  test procedure is a quality control step to assure a device integrity, and is one-time nondestructive test.Tipical products in which the leak detection has to be used are vacuum chambers, T+- , and other cathode tubes, hermetically sealed electronic components, pressure vessels, aerosol containers, vacuum thermal isolation e.g. dewars, pumps, refrigerating systems, chemical and nuclear plants, beverage cans, products containing metal bellows, electron microscopes, peace makers, etc.n spte of modem technologies it is practically impossible to manufacture a sealed enclosure or system that can be guaranteed to be leakproof without first being tested. The main question is what is the maximum acceptable leak rate consistent with resonable  performance life of the product.$e can distinguish different tipes of leak sources.  Leaks caused by defects  in the containing envelope. %or example a too thin wall of a  plastic bottle becomes microscopic cracked at enough high pressure difference, or in canning industry if the score mark is too deep in ring pull-tab can top, or porous cast in machine housing metallurgy, etc.  Leaks in newly manufactured products are most commonly imperfect oints or seals by which various parts are assembled to form the final article. There are known demountableand fixed oints. /etween them the most often used are welds, bra!ed and soldered oints glass-to-metal and ceramic-to-metal seals, &-rings and other gaskets, etc.  Materials permitting gas diffusion and permeation  through the wall. Virtual leak, a special type of leak in vacuum technique, which is not really a leak but is the internal source of gas or vapor. These are cavities in a chamber wall with thin connections to the inner vessel space such as improper welds, closed threads and holes etc. 2. LEAK SIZES AND MEASUREMENTS OF LEAK RATES The shapes of leaks cracks, fissures, porosity, damages, etc ...  are very different, unknown and non-uniform. Therefore it is impossible to measure their si!es with any  geometrical dimension except of course in the case of an ideal or artificial leak as used for calibration. 0ow then to define the leak si!e1 * generally accepted method became the observation of gas or fluid flow through it in certain conditions of temperature and  pressure difference. Consequently leak rates can be defined in two ways o in terms of the application, e.g. 2 g of refrigerant #-34 in 4 years at 5 bar, or 65 mof oil per year at 67 8C at atmosferic pressure, etc... o in terms of vacuum leak detection method, e.g. 4 mbar pressure increase in  hour or 2x 37-9 mbarl:s helium using 0e-detection methodEach of the mentioned examples gives a legitimate description of leak rate but the generally accepted unit is the last one because of very simple and understandable helium leak detection. ts efficiency is the result of a fierce and highly competitive developments within the last 35-47 years.The basic experiment explaining the leakage of hermetic system increasing pressure method is presented in %igure 3. The tested vessel or system with known volume + o  is  by a valve connected to pump. *fter evacuation the connecting valve should be closed and then the pressure in volume registered for a suitable long time periode. There are more possibilities system is tight and clean a, fight and not clean b, not tight and clean, i.e. ideal leak c, not tight and not clean, i.e. combination of leak and degassing.*s we can see in each case except for a tight vessel we have pressure increase and regarding to the shape of diagram we can conclude the type of leak. ;sing the curve inclination data the leak si!e can be determined by the following formula Q = ^p * V o /^ t (m! #/$% This simple test helps us to describe the situation at the begining of leak tracing  procedure. +ery similar test can be reali!ed by pressuri!ing the the system and detecting the pressure decay but gives only data on leak si!e and is not used so often.The leak rate < does not only depend on the geometric dimensions diameter, lenght of the leak but also on the physical properties of the gas or the liquid, such as viscosity,  relative molecular mass and on the pressure difference. %or example in the same environment conditions helium flows through orifices 4.9 times faster than air. /ecause of different results if the same leak is measured by various mediums it must be always noted with which gas a testing was performed.The maximum acceptable leak rate for a given product depends on the nature of product. =ince the cost of leak detection and manufacturing too hermetic envelopes increases in inverse proportion to a leak rate, it follows that testing for unnecessary small leaks causesunnecessary rise of production costs. =ome examples of tolerated leaks in different elements and systems are shown in Table 3. $e can see very wide range of si!es from great with some tenths mbarl:s in rough vacuum devices, to milion- and more- times smaller in hermetically closed electronic elements. t is possible to state there are no ideal products without leakage. $e only can demand they have leaks smaller than the specifiedleak.Table 3. >eak rate specification for various elements and systems E#&m&'t o S$t&mM!). p& m$$#& L&!+!,&R&m! +  chemical process equipment37 -3  to 3 mbar l:sgreat process flows beverage can bottom37 -5  to 37 -6  mbar l:sretention of C& 4 dynamic pumped vacuum s.37 -5  to 37 -9  mbar l:spermanent pumpingC-package37 -9  to 37 -?  mbar l:s pacemakercca 37 -@  mbar l:slong time implanted in bodyclosed vacuum elements37 -?  to 37 -37  mbar l:se.g. T+- and Aray- tubes -. LEAK DETECTION METODES * few leak detection techniques are known. Their performances regarding to detecting sesitivity are presented in %ig 4. /ecause of their advantages we shall concentrate
We Need Your Support
Thank you for visiting our website and your interest in our free products and services. We are nonprofit website to share and download documents. To the running of this website, we need your help to support us.

Thanks to everyone for your continued support.

No, Thanks