Abstract: Leak testing large vacuum systems
Authors: Wilson Norman G. 
Published: July 2002

Testing of large vacuum systems for leaks may best be described as a problem in high vacuum engineering with leak detection methods as a tool for its accomplishment. Before any leak testing is performed, it is necessary to determine the operating limits for the vacuum system which in turn are used as the boundary conditions for specification of acceptable leak rates. After determination of the leak rates which are acceptable to the overall vacuum system operation, a leak testing system design and procedure is developed to satisfy the leakage criteria. The leak testing philosphy to be followed is little different than any other leak testing problem.

Differences found in leak testing of large vacuum systems as compared to laboratory systems are primarily ones of physical magnitude and emphasis. For example, the roughing cycle commonly used on small components, i.e., laboratory vacuum systems and components of less than 50 ft 3 volume, is of little concern when making use of a mass spectrometer leak detector with an internal roughing pump of 10−cfm pumping speed. At the worst, a few minutes are required to ’’rough’’ the components to leak testing pressure. In the example, approximately one hour would be required to achieve Tracer Probe Mode, 1 mass spectrometer leak testing pressure of 1×10−2 Torr in the 50 ft3 component, using the 10 cfm roughing pump. All the common leak testing methods, in addition to mass spectrometer leak testing methods, can be applied; however, some become impractical as the size of the components increases. For example, if the vacuum system or component size is several thousand cubic feet in volume (which represents several hundred feet of pipe several feet in diameter), it is not practical to use pressurization of the component with a tracer gas and the Sample Probe Mode2 of mass spectrometer leak detection.

Experience has shown the following leak testing methods to be useful in testing of large vacuum systems and components: (a) Rigorous comparison of operating vacuum system history with engineering prediction. (b) Use of rate−of−rise pressure changes for comparison with engineering prediction. (c) Installation of residual gas analyzer instrumentation and subsequent monitoring of possible leakage components. (d) Specific design of components in large system to permit testing of as small a component as possible. (e) Use of a mass spectrometer leak detector with high capacity auxiliary pumping system and the Tracer Probe Leak Testing Mode. Rigorous comparison of vacuum system operating conditions and rate of pressure increase after pump isolation have been found by Schalla 3 to give a useful determination of in−leakage permissible for vacuum system operating pressure in the millitorr range. In systems of this type, engineering judgement is necessary in the design of the vacuum system which considers the cost of providing large pumping systems and high leakage rates vs the cost of precision fabrication and low leakage rate, and component leak testing at high sensitivity. Schalla describes systems of up to 4.5×104 ft3 with total leak rates of 10 atm cc/sec or more, and several 18000 liter/sec diffusion pumps to produce 6×10−4 Torr operating pressure.

In applications where operating pressures are much lower, i.e., less then 1×10−5 Torr to 1×10−8 Torr, the use of rigorous classical vacuum engineering design and adaption of mass spectrometer leak testing and residual gas analysis is necessary. Similar procedures are followed when there are limitations on physical size of sustaining vacuum systems and overall installed vacuum system costs. In these cases, engineering design criteria include the necessity of careful seal design, fabrication, and assembly, rigorous inspection of weld quality, and limitation on individual component size so as to permit their testing for leaks with classical Tracer Probe Mode vacuum leak detection methods. In such systems, leak testing is accomplished with maximum acceptable leak rates of 5×10−9 atm cc/sec for individual components. A typical system, 4 tested to this level, composed of many individual components of 5300 ft3 total volume, was equipped with one 7 liter/sec two stage rotary piston mechanical pump; a pressure of less than 5×10−2 Torr was sustained. In this example a high capacity auxiliary pumping system of 3000 cfm Roots blower capacity and a 300 liter/sec high speed diffusion pump leak detection station were used to pump down and leak test the final vacuum system installation to the allowable maximum leakage of 5×10−9 atm cc/sec. Similar procedures have been used on systems of greater than 2000 ft3 where pressures of less than 1×10−3 Torr have been maintained by a 150 liter/sec Roots blower mechanical pump group.

Where large systems are required to operate at design pressures of less than 1×10−5 Torr, an installed mass spectrometer residual gas analyzer is used in concert with engineering design procedures and classical high sensitivity mass spectrometer leak testing methods for system leak detection. A typical space environment simulation system may contain 4×105 ft3, yet be required to operate at ’’non−cryogenic’’ pressures of less than 1×10−6 Torr and ’’cryogenic’’ pressures of less than 1×10−8 Torr.

Testing for large vacuum system leaks is treated with the same engineering analysis and judgement as the engineering design approach for the vacuum system itself. Where design operating pressures are modest, it is possible to make use of operating condition comparison with engineering prediction to evaluate the in−leakage to the vacuum system. Where classical vacuum engineering design and leak detection methods are desirable and practical, they can be applied with full confidence and facility with the use of proper auxiliary pumping systems.