Problem tylannitride 2001-01-25 18:18:14: DCS Excess Flow Valva

shott at shott at
Thu Jan 25 06:18:17 PST 2001

Ray, Dick, Mike, et al:
This evening Helen Kung was running a low-stress nitride process and began to "run out of DCS".  She contacted Cesar who contacted me.  I remembered that earlier this week we had seen an excess flow valve condition ... so that was my firrst place to lool.  Sure enough, we had another excess flow condition.
I basically reset things in the DCS cabinet ... and then left the valve in "Purge Mode" so that another excess flow condition won't shut things down this evening.  While I don't think we should do that as a matter of habit, I concluded that in this case it was probably OK becuase if we got a complete line rupture, the comparatively low vapor pressure would keep the flow from getting too hhigh and if there was a leak to the outside world, the detector would shut down the cylinder.
Helen is running a low stress process that, I think, uses a higher flow that most "normal" nitride processes.  For starters, it would be interesting to know what the excess flow valve for that tube was originally supposed to be ... I've got a suspicion that it was selected and installed during the days when "normal" (high stress) nitride was run ... and that this higher flow is right at the threshold of its shutoff point.  But that is only a guess ...
Certainly we need to understand why we appear to be getting excess flow alarm conditions under apparently normal (but high flow) situations.
Helen is going to provide a list of all of the DCS and ammonia flows in our existing library of recipes to see if this is indeed close to the maximum flow of all recipes.  Trying to find out what that valve was set at would be a good idea ... and then maybe we can determine whether it has drifted from its original setting.
That's all for now ... but that is why the tylan nitride DCS gas cabinet was left in purge mode.

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