CCD-world: pump oil backstream protection

[Tom Droege]

The following was posted to CCD-world:

This is a standard controls problem that can even be solved with relays.
OK, I do read this as a warning rather than a call for help.  The idea is
that you really have to think through any control system.  The standard
"fix" here is to always cause the valve to close on loss of power, but
require a push button - AND time delay - AND fore pump vacuum - before you
allow the valve to open again.  

The big thing is to think everything through.  As with everything there are
computer design tools that help with this task.  Sorry that I cannot
recommend any.  You need to talk to a "controls" person.  

Tom Droege

At 04:17 PM 8/16/99 -1000, you wrote:
>The following was posted to CCD-world:
>
>Aloha Everyone,
>
>I was investigating the CFHT turbopump protection circuitry for our
>Varian Turbopumps.   We're using an oil filled roughing pump in front of
>the turbo and the system has the Varian automatic controls, it's a nice
>convenient setup.  One of our staff had rigged up a pneumatic gate valve
>which opens or closes when a loss in AC power is detected.  This is
>great for a loss in AC power, the valve shuts quickly and it would
>appear to prevent oil backstreaming.  The problem occurs when the AC
>power comes back on, the gate valve immediately opens, not so good. 
>Since the dewar and associated pump segments down stream of the gate
>valve remain under vacuum, and the turbo (ours autostarts even) takes a
>few minutes to come up to speed we have a situation where the dewar
>maybe pumping the turbo instead of vice versa. Definitely not good. Now
>the dewar I needed to pump down was the CFH12k with 12 thinned CCDs
>installed, needless to say I wasn't too happy to see my defense system
>had a big hole in it.   I've seen other arrangements where people were
>using solenoid actuated valves instead of pneumatic ones, but the
>situation is the same, the valve opens and closes on AC power, not on
>pressure differentials.  Also these AC power detection circuits offer no
>protection if you have a pump failure, which we have experienced  in the
>past. So, take a look at your systems and see if you are as protected as
>you think you are, I was operating under a false sense of security. 
>
>Barry
>-- 
>_____________________________________________________________________________
>
>Barry Michael Starr                                    
>starr at cfht.hawaii.edu
>Canada France Hawaii Telescope                          Ph: 808-885-3139
>PO BOX 1793  Kamuela, HI 96743                          Fax:
>808-885-7288
>_____________________________________________________________________________
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