Fundamentals of Vacuum Technology (1248463), страница 47
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individual leaks (local measurement) Ð sketches b and d in Figure 5.4,and registeringb. the total of all leaks in the test specimen (integral measurement) Ðsketches a and c in Figure 5.4.112HomeLeak detectionissuing from leaks is collected (extracted) by a special apparatus and fed tothe leak detector. This procedure can be carried out using either helium orrefrigerants or SF6 as the test gas.Helium5.4a: Integral leak detection; vacuum insidespecimenc: Integral leak detection (test gasenrichment inside the enclosure); pressurizedtest gas inside specimenHeliumHeliumb: Local leak detection; vacuum insidespecimenFig. 5.4d: Local leak detection; pressurized test gasinside the specimenLeak test techniques and terminologyThe leak rate which is no longer tolerable in accordance with theacceptance specifications is known as the rejection rate.
Its calculation isbased on the condition that the test specimen may not fail during itsplanned utilization period due to faults caused by leaks, and this to acertain degree of certainty. Often it is not the leak rate for the test specimenunder normal operating conditions which is determined, but rather thethroughput rate of a test gas Ð primarily helium Ð under test conditions. Thevalues thus found will have to be converted to correspond to the actualapplication situation in regard to the pressures inside and outside the testspecimen and the type of gas (or liquid) being handled.Where a vacuum is present inside the test specimen (p < 1 mbar),atmospheric pressure outside, and helium is used at the test gas, onerefers to standard helium conditions.
Standard helium conditions arealways present during helium leak detection for a high vacuum systemwhen the system is connected to a leak detector and is sprayed with helium(spray technique). If the specimen is evacuated solely by the leakdetector, then one would say that the leak detector is operating in thedirect-flow mode.
If the specimen is itself a complete vacuum system withits own vacuum pump and if the leak detector is operated in parallel to thesystemÕs pumps, then one refers to partial-flow mode. One also refers topartial stream mode when a separate auxiliary pump is used parallel to theleak detector.When using the positive pressure method it is sometimes either impracticalor in fact impossible to measure the leakage rate directly while it couldcertainly be sensed in an envelope which encloses the test specimen. Themeasurement can be made by connecting that envelope to the leakdetector or by accumulation (increasing the concentration) of the test gasinside the envelope. The Òbombing testÓ is a special version of theaccumulation test (see Section 5.7.4).
In the so-called sniffer technique,another variation of the of the positive pressure technique, the (test) gasLeak detection methodswithout a leak detector unitThe most sensible differentiation between the test methods used isdifferentiation as to whether or not special leak detection equipment isused.In the simplest case a leak can be determined qualitatively and, when usingcertain test techniques, quantitatively as well (this being the leak rate)without the assistance of a special leak detector. Thus the quantity of waterdripping from a leaking water faucet can be determined, through a certainperiod of time, using a graduated cylinder but one would hardly refer to thisas a leak detector unit. In those cases where the leak rate can be determined during the course of the search for the leak without using a leakdetector (see, for example, Sect. 5.4.1), this will often be converted to thehelium standard leak rate (Sect.
5.2.1). This standard leak rate value isfrequently needed when issuing acceptance certificates but can also be ofservice when comparing leak rate values determined by helium leakdetector devices.In spite of careful inspection of the individual engineering components,leaks may also be present in an apparatus following its assembly Ð be itdue to poorly seated seals or damaged sealing surfaces. The processesused to examine an apparatus will depend on the size of the leaks and onthe degree of tightness being targeted and also on whether the apparatus ismade of metal or glass or other materials. Some leak detection techniquesare sketched out below. They will be selected for use in accordance withthe particular application situations; economic factors may play an importantpart here.5.4.1 Pressure rise testThis leak testing method capitalizes on the fact that a leak will allow aquantity of gas Ð remaining uniform through a period of time Ð to enter asufficiently evacuated device (impeded gas flow, see Fig.
1.1). In contrast,the quantity of gas liberated from container walls and from the materialsused for sealing (if these are not sufficiently free of outgassing) will declinethrough time since these will practically always be condensable vapors forwhich an equilibrium pressure is reached at some time (see Fig. 5.5).
Thevalve at the pump end of the evacuated vacuum vessel will be closed inpreparation for pressure rise measurements. Then the time is measuredduring which the pressure rises by a certain amount (by one power of ten,for example). The valve is opened again and the pump is run again forsome time, following which the process will be repeated. If the time notedfor this same amount of pressure rise remains constant, then a leak ispresent, assuming that the waiting period between the two pressure risetrials was long enough.
The length of an appropriate waiting period willdepend on the nature and size of the device. If the pressure rise is moremoderate during the second phase, then the rise may be assumed to resultfrom gases liberated from the inner surfaces of the vessel. One may also113HomeLeak detectiona shut-off valve, then one may expect an effective pumping speed of aboutSeff = 30 l/s. Thus the ultimate pressure will bepend =QLSeff=6 ⋅ 10 – 5 mbar ⋅ ` ⋅ s – 130 ` ⋅ s – 1Pressure= 2 ⋅10 – 6 mbarNaturally it is possible to improve this ultimate pressure, should it beinsufficient, by using a larger-capacity pump (e.g. the TURBOVAC 151) andat the same time to reduce the pump-down time required to reach ultimatepressure.Time1 Leak2 Gas evolved from the container walls3 Leak + gas evolutionFig.
5.5Pressure rise within a vessel after the pump is switched offattempt to differentiate between leaks and contamination by interpreting thecurve depicting the rise in pressure. Plotted on a graph with linear scales,the curve for the rise in pressure must be a straight line where a leak ispresent, even at higher pressures. If the pressure rise is due to gas beingliberated from the walls (owing ultimately to contamination), then thepressure rise will gradually taper off and will approach a final and stablevalue. In most cases both phenomena will occur simultaneously so thatseparating the two causes is often difficult if not impossible. Theserelationships are shown schematically in Figure 5.5.
Once it has becomeclear that the rise in pressure is due solely to a real leak, then the leak ratecan be determined quantitatively from the pressure rise, plotted againsttime, in accordance with the following equation:∆p ⋅ V∆tQL =(5.3)Example: Once the vacuum vessel with a volume of 20 l has been isolatedfrom the pump, the pressure in the apparatus rises from 1 á 10-4 mbar to1 á 10-3 mbar in 300 s. Thus, in accordance with equation 5.2, the leak ratewill be1 ⋅10 – 3 − 1 ⋅ 10 – 4 ⋅ 20QL =5.4.2 Pressure drop testThe thinking here is analogous to that for the pressure rise method (Section5.4.1).
The method is, however, used only rarely to check for leaks invacuum systems. If this is nonetheless done, then gauge pressure shouldnot exceed 1 bar since the flange connectors used in vacuum technologywill as a rule not tolerate higher pressures. Positive pressure testing is, onthe other hand, a technique commonly employed in tank engineering. Whendealing with large containers and the long test periods they require for thepressure drop there it may under certain circumstances be necessary toconsider the effects of temperature changes. As a consequence it mayhappen, for example, that the system cools to below the saturationpressure for water vapor, causing water to condense; this will have to betaken into account when assessing the pressure decline.300mbar ⋅ `9 ⋅10 – 4 ⋅ 20= 6 ⋅ 10 – 5300s=The leak rate, expressed as mass flow ∆m / ∆t, is derived from equation5.1 at QL = 6 á 10-5 mbar á l/s, T = 20 ¡C and the molar mass for air(M = 29 g/mole) atQL =Today leak tests for vacuum systems are usually carried out with heliumleak detectors and the vacuum method (see Section 5.7.1).
The apparatusis evacuated and a test gas is sprayed around the outside. In this case itmust be possible to detect (on the basis of samplings inside the apparatus)the test gas which has passed through leaks and into the apparatus.Another option is to use the positive-pressure leak test. A test gas (helium)is used to fill the apparatus being inspected and to build up a slight positivepressure; the test gas will pass to the outside through the leaks and will bedetected outside the device. The leaks are located with leak sprays (orsoap suds, 5.4.5) or Ð when using He or H2 as the test gas Ð with a leakdetector and sniffer unit (5.7.2).mbar ⋅ `g∆m= 6 ⋅10–5⋅⋅ 29⋅∆tsmol⋅mol ⋅ Kg= 7 ⋅10 – 82s. mbar ⋅ `⋅ 293 ⋅10 K83145.4.3 Leak test using vacuum gaugeswhich are sensitive to the type ofgasThe fact that the pressure reading at vacuum gauges (see Section 3.3) issensitive to the type of gas involved can, to a certain extent, be utilized forleak detection purposes.
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