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How to Choose a Valve.

Vacuum: A space in which the pressure is far below normal atmospheric pressure so that the remaining gases do not affect processes being carried on in the space.

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
Geographic Code:1USA
Date:Jun 1, 2001
Words:721
Previous Article:How to Scan at [is greater than] 250 [degrees] C.
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