CCD-world: pump oil backstreaming cont.

[Tom Droege]

The following was posted to CCD-world:

Why buy a new pump when you already have all the pieces needed to make a
good control system?  As Willie Koorts points out, the way to control this
is pretty standard.

You just need a few dollars worth of relays.  (OK, I hate relays, but there
are some applications where they are the right thing to use.)

a) Wire one relay so its contacts close when the vacuum is high enough.
Hopefully the vacuum gage you already have has such contacts available.
Possibly you want two vacuum relays, one on the rotary pump and one on the
turbine pump.  
b) Wire one relay so its contacts close a suitable time delay after the
pumps are turned on.
c) Wire one relay so its contacts close when a push button is pushed.  This
relay is wired to hold on (as long as the power is on) using a second set
of contacts.

Wire these three sets of contacts in series to open the valve when they are
all closed.  

This will:

1) Close the valve whenever the power fails due to c) above.
2) Prevent opening the valve until you command opening due to c) above.
3) Prevent opening the valve until the pumps have had time to settle into
normal operation due to b) above.
4) Prevent opening the valve until the pumps have produced sufficient
vacuum due to a) above.  

Hopefully the valve is designed so that it requires power to keep it open.
This is just one more safety level.  Always think about the most common
event.  A brief power dip.  You can always get a dip that drops out the
rotary pump but does not drop out c) above.  You have to think about such
things, and design the rotary pump lock on relay to drop out slower than
the c) relay above.  In this case one still has the vacuum relay for
protection but this reduces the redundancy of the system.  

As I say, this is very standard controls stuff.  If I know about it, it is
pretty standard.  You also need to think about how things fail.  Relays
fail.  They like to weld their contacts shut so they do not open when they
should.  To be more conservative you can wire several sets of contacts on
one relay in series.  

Remember your new pump will come with a price tag AND a new set of problems
that you don't want to learn about.  Better the devil you know and
understand than a new one.  

OK, controls people make diagrams with a bunch of things that look like
capacitor symbols.  This is a normally open relay contact.  A slash through
it signifies a normally closed contact.  To design such stuff, you need to
find someone familiar with controls.  You will probably find such a person
in the machine shop where they work on the control systems for machine
tools.  This is just one more art with it's own set of problems.  Better to
find someone with some experience if you don't want to make the normal dumb
mistakes as was apparently already made when the valve was put in.  Not to
knock the person that did it.  It is just one of the standard mistakes that
anyone with just a little experience would not have made.  

If this mistake was made, then there may be other problems lurking.
Possibly it will be worth the effort required to review how you do the
controls and interlocks on your systems.  You don't want to lose that dozen
thinned CCDs.  

Tom Droege

At 11:40 AM 8/17/99 -1000, you wrote:
>The following was posted to CCD-world:
>
>Aloha Again,
>
>Thanks for the quick responses gentlemen.  And thanks to Tim Abbott for
>maintaining this service, it's a great forum, I haven't always commented
>but I've been a faithful reader.
>
>Our current plan of action is to replace the oil-filled rotary pump with
>a dry pump, the problem is they're a bit expensive, and squeezing the
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