How to Choose a Valve.
For decades, high-vacuum valve selection has had limited options. Applications have been limited to two basic types of valves with a variety of options--the high-vacuum gate valve and the high-vacuum poppet valve. The ball valve has never been considered a good choice because of inherent virtual leak possibilities, stem leaks, and gas entrapment. Now, the advent of high-tech, high-vacuum ball valves is a reality.
New valve designs have provided ball valves that work well from 10-7 torr to 6.8 positive atmospheres, about 100 psig, while maintaining a leak rate of less than 1 x [10.sup.-9] Std/cc He sec. However, as with other valves, read manufacturers' specifications carefully, as not all high-vacuum ball valves are created equally, With the elimination of gas entrapment, stem seal improvements, and no possibility of virtual leak, the high-vacuum ball valve has proven itself to be a component worthy of consideration for high-vacuum valve selection.
If you need proof and you're in the semiconductor industry, take a close look the next time a process tool is wheeled out onto the fab floor. Chances are it will have ball valves where previous models had gate, butterfly, or poppet valves. The super-selective OEM tool/reactor manufacturers have begun turning to the high-vacuum ball valve for roughing and high-vacuum foreline applications because the valves work. They can be heated with the foreline to keep gases from condensing until they are trapped in a desired location. They are cost effective, offer longer cycle life, are easy to maintain, and can be replaced with a back-up valve in seconds with quick connect flanges to reduce downtime and increase production and yields.
With only two moving components in the manual version and separate, independent, removable/ replaceable actuators (pneumatic, electro-pneumatic, and electronic, with or without speed controls or talk-back for computer drive/confirmation and a bevy of other options), the high-vacuum ball valve has far fewer "rules" to obey than other high-vacuum valves.
Material construction is also a consideration. Most ball valve manufacturers offer 316-L corrosive-resistant stainless steel as standard (most poppet and gate valves are constructed from 304 SS), some offer brass, and one will offer aluminum construction in the near future.
While the high-vacuum ball valve doesn't fit every application, it does fit more vacuum and fit-up requirements than most other types of valves. The next time a valve is causing problems, consider replacing it with a high-vacuum ball valve. If a supplier tells you that other valves are better or the ball valve is not an option, confirm that the company sells newer types of high-tech vacuum ball valves.
Comparing Valve Types High-vacuum High-vacuum High-vacuum ball valve poppet valve gate valve Maximum conductor Yes No Yes Minimum line length No No Yes Low to moderate particular Yes Yes No Moderate to high particular Yes No No Bakeable to 100 [degrees] C Yes Yes Yes Bakeable over 100 Yes 1 Yes 1 No [degrees] C Large, full bore Yes 2 No Yes pass-through capability Positive pressure Yes 3 No No holding capability Pressure range ATM to Yes Yes 4 Yes [10.sup.-7] torr Corrosion resistance Yes 5 No No Pneumatic actuation Yes Yes 6 Yes Electric actuation Yes Yes No Seats wiped clean Yes No No during cycling In-line re-billability Yes No No Ease of re-build Yes Yes No Catastrophic fail safe Yes 7 No No closed or open Adjustable cycle time Yes No No with actuation Yes = recommended No = not recommended 1 = Viton begins to outgas and set at 150 [degrees] C. Other seals can be substituted for higher temperature (200 [degrees] C). Metal seal poppet valves go up to 400 [degrees] C 2 = Large bore pass-through capability of up to 5.8 available with tube bore high-vacuum ball valves. 3 = Up to 6 ATM (100 psi) 4 = Metal seal poppet valves up to [10.sup.-10] torr 5 = High-vacuum ball valves manufactured from 316 L. Poppet and gate 304 6 = Exception-metal-sealed valves 7 = With pneumatic fail safe actuator WEB RESOURCES FOR VALVE TECHNOLOGY: www.ancorp.com www.avs.org www.avem.org
Tourigny is national sales manager and Burke is international sales manager at A&N Corp.
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|Title Annotation:||high-vacuum valve selection|
|Comment:||How to Choose a Valve.(high-vacuum valve selection)|
|Author:||Tourigny, Bob; Burke, Paul R.|
|Publication:||R & D|
|Article Type:||Brief Article|
|Date:||Jun 1, 2001|
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