- Cleanliness. This is essential to reach and maintain a high vacuum. Any dirt, liquid or oil will slowly offgas unless it has a very low vapor pressure, degrading the vacuum. So, ordinary oils and lubricants won’t do. Before use in the high vacuum chamber they all must be “pre-outgassed” or held in a deep vacuum until all volatilization has ceased. Even small smudges or fingerprints on the vacuum side of a vessel will outgas.
- Metal surface smoothness. Small nooks and crannies on the metal surface can adsorb air and other gases, which then offgas slowly, degrading vacuum level. To prevent adsorption, deep vacuum vessels usually must have the surfaces facing vacuum polished.
- Elastomers. Flexible materials used in gaskets, o-rings and seals can outgas at deep vacuum. Elastomers also can lose their dimensional memory as they expand and contract with the cycling of the vacuum system, and thus not completely seal all surfaces. In fact, metal gaskets or special o-rings called sealing discs (o-rings with metal “winglets”) often are needed to prevent leaks in joints, flanges, etc.
The techniques used in so-called high vacuum design become necessary when the operational pressure level falls below 1 Torr. They really constitute the best available technology and should be considered to improve the quality of any vacuum system.
Unfortunately, they all too often are dismissed as too expensive. The difference in cost between a flanged connection and a threaded connection may be the most extreme example. However, low-pressure-drop layout of the piping, inside wall smoothness, hard elastomers, etc. can make any system, regardless of its operational vacuum level, operate more efficiently. That is, they help deliver more of the vacuum source’s capacity to the user equipment and thus contribute to the overall economics of the vacuum system.
It’s essential to periodically check your vacuum system for the development of in-leak.
The most common method is the “rate-of-rise” test. In it, the system is pumped down below its usual operating pressure and valved off (isolated from the vacuum source).
So, the only way the pressure can rise is due to the in-leak of air. The deeper the operational vacuum used the less air in-leak that can be tolerated. Industrial systems shouldn’t increase more than 10% from the base pressure in 24 hours. High vacuum systems need a more strenuous standard, especially if the processes can’t tolerate the presence of oxygen (as in most semiconductor systems). Here, the generally accepted standard as a 1% rise from the base pressure in 24 hours.
There’s a temptation in industrial or “rough” vacuum operations to use pressurization methods to test the system (either with water, as in a pressure or hydrostatic test, or with compressed air). However, they only will give a very rough indication of the tightness of a system. Water is a liquid and its surface tension reduces in-leak. In addition, many components such as seals in a vacuum system are directional in nature and pressure testing is looking for flow in the wrong direction (vacuum leaks are in-leaks).
Henry H. Hesser is a staff technical specialist for Busch, LLC, Virginia Beach, Va. Reach him via e-mail at firstname.lastname@example.org.
Vacuum systems will provide better performance if you follow these eight tips:
- Minimize bends in the piping layout. Every bend, every change in direction, etc., adds to pressure drop.
- Wherever elbows are required, use long-radius instead of short-radius ones to reduce pressure drop.
- Wherever two lines come together, use a wye instead of a tee to lower pressure drop.
- Make sure that the overall pressure drop of the piping (farthest use point to the source of vacuum) doesn’t exceed 10% of the operating pressure.
- Eliminate unsealed threads wherever they are used. They are a large source of leaks. Always remember that whatever leaks into the vacuum system expands greatly. Two ways to lessen the impact of threads are to use a sealer in the gaps of normal NPR threads or use NPT “fine cut” threads.
- Tighten flanges, regardless of type, by working in turn on “opposites” or “facing” bolts, not clockwise or counterclockwise in succession.
- Check every elastomer (gasket, o-ring, etc.). Each should be “hard” (greater than 90 durameter) to eliminate leaks arising from pressure cycling (up, down, up, etc.).
- Keep the rate-of-rise over 24 hours of the piping system to 10% or less of the operating pressure and test regularly.