Leakage Detection and Prevention in Vacuum Systems

Leaks pose biggest challenge to engineers handling vacuum systems. Leakages cripple the workand are invariably time consuming to detect.

This blog post is about two major issues related to leakage - Detection and Prevention.

Leakage Detection

Following are three methods of leakage detection-

1. Physical Verification: - This should be used if even roughing vacuum (1 x 10-1 Torr)cannot be reached in the vacuum system. In this method the person has to movearound the vacuum system slowly, paying attention to any hissing or whistling sound.This sound indicates location of leak.

Though method is very simple, in this method experience of the person is veryimportant and this way of leakage detection is suitable only for detecting large leaks.

2. Pirani Gauge and Solvent Method: -

If vacuum climbs above roughing stage then obviously the leakage is small. For suchleaks this method is used. In this method vacuum pumps are allowed to operate toreach a stable vacuum level. At this point a solvent, normally acetone, is sprayed(generally with a syringe) at the suspect joint. Since system is under vacuum if a leakexists at that joint then solvent is quickly drawn in. As we understand pirani gauge isvery sensitive to change of gas compositions. Pirani gauge attached with the systemtherefore quickly picks up presence of solvent and its reading changes (vacuum drop).After a while the reading returns to the previous reading as solvent has been suckedaway by vacuum pumps.

If there is no change in reading of Pirani gauge then next suspect joint is tried and so ontill leakage is detected.

This is a very inexpensive method as it requires no special equipment. Expertise ishowever required for proper results.

3. Helium Leak Detector: - Though expensive this is by far the most accurate of leakagedetection methods. In this method a helium leak detector is employed. Using a bottle of

Helium gas, Helium is squirted at the suspect joint by a nozzle. Since system is undervacuum, if a leak exists then helium is quickly drawn into vacuum system. After this aspectrometer is used to detect helium.

Vacuum Leak Detection With Helium

Spectrometer consists of the following:-

(a) An ionizer to ionize gas

(b) Analyzer to separate the gas and

(c) A detector to detect the composition of gas.

The spectrometer is connected to vacuum line from which vacuum pumps are drawingaway the air. Since spectrometer has an excellent sensitivity, by using the spectrometer,even minutest of leakages can be detected.

If spectrometer does not detect any helium, then next suspect joint is tried, and by hit and trial, the leakage is identified.

Precaution-

Since Helium is a light gas care needs to be exercised during leak detection to move from topto bottom of vacuum system and not the other way round. This is because Helium squirted at alower joint can always quickly travel up and enter system from an upper leaking joint. This canabsolutely sabotage the procedure.

Leakage Prevention

As the saying goes “prevention is better than the cure”. It is better if we are able to preventleakage in the first place.

Based on my engineering background and experience of about 20 years in vacuum line a fewpractical tips for leakage prevention are given below-

1) Proper Maintenance of Gauges: -

Vacuum Gauge

Many times faulty gauges result in assumption of a leak. Since gauges are highlysensitive equipment hence maintaining these properly is very important. This willprevent any false alarms and wastage of precious production time in hunting a leak.

2) Virtual Leak: - Many times the design of system is such that from some part air is notable to escape freely (As in diagram below). This results in pressure differential withinvacuum chamber. For example, vacuum chamber may have been pumped down to 1 x10-5 Torr but a part of chamber from which free escape of air is not possible may havevacuum of only 1 x 10-2 Torr. Slowly when the vacuum will be equalized in thechamber, then this will show as a vacuum drop, resulting in time wastage on leakhunting. To prevent this, proper system design is important and if such a pocket existsin chamber from which free flow of air is not possible then that part must be givenmore openings for faster evacuation.

3) Water Vapour: - As vacuum engineers know, water vapor is most difficult to be pumpedout. Because of the polar nature of water molecules, they stick to the walls of systemforming strongly attached multiple layers. With time water vapor will be released fromwalls and cause vacuum drop. To take care of this, cryo panels are used in vacuumsystems to freeze water vapor. Another way is to push inert gas in vacuum system athigh speed. This dislodges water molecules and with inert gas these water moleculesare also drawn away by vacuum pumps. Argon and helium are cheaper in the list ofinert gasses and are therefore commonly used for this purpose. One could however useany other inert gas also like Neon, Xenon, Krypton or Radon.

4) Proper Maintenance of ‘O’ Rings: -

O-Rings

Since metal to metal surface contact is unable to hold vacuum so O rings are employedto seal vacuum. Using good quality Viton O rings with good compressibility solvesproblems of leakage to a large extent. Another precaution is to discard O rings if theyharden, soften too much or swell up.

5) Use of high vacuum silicone grease: -

Supervac High Vacuum Silicone Grease SV-G9

Good quality Vacuum grease applied to O rings goes a long way in preventing leakages.This is because not only does it provide additional vacuum sealing but also protects theO rings from hardening and damage. This grease should have a very low vapourpressure and excellent tackiness. Supervac manufactures High Vacuum Silicone GreaseSV-G9 which is widely used by vacuum industry all over the world.

6) Baking:-If Possible the vacuum system should be baked at a temperature of 250o C forminimum 12 hours under vacuum from time to time. This expels the volatile gassesattached to vacuum system walls.

Conclusion

Leakage in a vacuum system is a nightmare, but observing a few basic precautions like propercare of pumps, replacement of O rings as they age, application of Vacuum grease and cleaningof flange surfaces etc. go a long way in containing the problem.If a leakage occurs then immediate action to detect it before it snowballs into a major problemis important. Also people performing leakage detection should be adequately trained and givenrequired tools to be able to solve the problem quickly and save precious production time.