Manual turbo purge valve on RIE and why

2004-05-06, A. Liljeborg, A. Holmberg

There has been a long history of problems with the turbo not staying at speed during venting of the system. During the venting the turbo is "closed in" at some steps, thus having no backing and not pumping the chamber. The state is: During this time the turbo experience a large load, it draws current close to its maximum, usually 3.10 - 3.20 A. It quite often exceeds its maximum current and stops. Then the user has to go into service mode, which is always risky, and do a "manual" venting.

The standard purge valve delivered by Oxford has been replaced three times without curing this problem. This valve is controlled by compressed air in sync with the backing valve.

What we now think we have proved is that the nitrogen purge gas is still present when the turbo is "closed in". It is not a leak, the leak-tester has been used several times without finding any leak.

The manual valve added before the standard purge valve gives the posibility to shut off the purge gas flow before closing the backing valve, "closing in" the turbo. It was shown that more than 30 seconds was needed between the closure and the closure of the backing valve, before the turbo showed normal load (1.0 - 1.5 A) in the "closed in" state.

We think this could be caused by a partially blocked entry for the purge gas into the turbo, thus taking a very long time to pump away the rest of the purge gas after the standard purge valve has been closed. The little standard valve closes faster than the backing valve, that can be seen from the outside of the backing valve. The red pin indicator is taking about a half a second to open or close.
The purge entry could be checked at the next service of bearings in the turbo.

Anders Liljeborg Nanostructure Physics, KTH.


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