Yesterday's toxic gas alarm ....

John Shott shott at stanford.edu
Fri Aug 6 08:02:40 PDT 2010


SNF Lab Members:

As many of you are aware, yesterday morning we had a toxic gas alarm 
that evacuated the lab and building.  We believe that this was triggered 
by phosphine gas from the Tylan LTO tube running the PSG400 recipe.  As 
only some of you know, this past Sunday we also had detectable amounts 
of phosphine when this same recipe was running that did not reach the 
alarm trigger point.  After that first incident, we disassembled and 
tested the components that we felt were at fault and believed that we 
had resolved the issue.  We clearly did not and, as a result, are taking 
the system down for a more extensive set of checks and component 
replacements.

We have an extensive toxic gas monitoring system that has been carefully 
designed to alert us in the event of the release of potentially 
corrosive, flammable, or toxic gases.  While it is certainly true that 
the process gas SHOULD BE confined to the process tube and vacuum 
system, this experience clearly shows that components can fail in ways 
that allow gas to be in places where it should not be.  We believe that 
the toxic gas monitoring system has functioned as it should.

Our toxic gas system is monitoring close to 100 points in the lab and 
gas bunkers at all times.  How are the alarm levels set and how is that 
checked?  This system is permitted by Santa Clara county and there are 
regulations that control when we need gas detection, the levels that 
should trigger and alarm, etc.  During the annual holiday shutdown, we 
hire an independent third-party (Industrial Hygiene Services) to come in 
and test each sensor with a calibrated concentration of an appropriate 
live gas.  During that test, checks are made to insure that each sensor 
responds as it should, that alarm set points are set to appropriate 
levels and that each alarm triggers the appropriate response (calls the 
Fire Dept, sounds a local alarm, etc).  All of the hydride sensors were 
checked with a calibrated standard containing 1000 ppb (parts per 
billion) of phosphine and responded as they should.  Note: this test 
typically takes three days to conduct.  The entire test is witnessed by 
a Hazardous Materials Specialist from the Santa Clara County Hazardous 
Materials Compliance Division.

We are working hard to identify and replace the failed component and 
hope to return tylanbpsg to service in the near future.  While we are 
sorry that we had any form of leak, we are certainly glad that we have 
appropriate monitors in place to alert us to this serious problem.

Please let me know if you have any additional questions,

John







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