Re: [foxboro] GDEV functionality

Tom,

We're talking about a full-port ball valve with a fail-close quarter-turn
actuator. The single air supply that would open the valve actually has a
normal high-pressure supply for 100% open AND a regulated supply for
partial open (let's say 50%). If you wanted to go from 0% to 50%, the lower
air pressure often has trouble overcoming the initial actuator spring
tension, but we operate 0% > 100% > 50% > 0% (to prevent hammer and meter
overshoot) which works nicely. As for limits, a no-limit feedback condition
is equivalent to a 50% position. I know this is not completely accurate,
but we're not overly interested in the actual position during partial open,
just that it's not fully open. The valve operation is controlled by a flow
AIN and ACCUM with a preset of, say, 95%, so the last 5% goes at a reduced
rate. This prevents hammer in the pipe, and also overshooting the ACCUM. We
diagnose the regulator performance by tracking the AIN value.

We do have 1 ball valve that we needed to have fast-opening and
slow-closing ... for that we used an AOUT to ramp down the air pressure
before closing the GDEV. Obviously, that required a long TOC (open-close
time) to avoid tripping the mismatch alarm.

We could have used control valves and continuous or discrete position
indicators, but the extra cost couldn't be justified.

This reminds me of a riddle ... Which takes longer - raising a flag all the
way up a flagpole, or to half-mast? Proper flag etiquette says you should
go to half-mast by going all the way up and then lowering it to half-mast.

Brad Wilson
Process Control Engineer
ExxonMobil Chemical Co
Edison Synthetics Plant
732-321-6115
732-321-6177 fax
Brad.S.Wilson@xxxxxxxxxxxxxx

>>>
when you are talking about a valve with two solenoids and only two limit
switches how do you tell if the valve is half open and how do you hold it
in that position?  Do you wait until neither limit is made and then turn
both the COUT's off?  We have a call for "Fail Last" valves that utilize
two solenoids and two COUT's but have never used a block valve in a three
position mode
<<<

 
 
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