I am doing an automation upgrade to a steam superheater at our gas plant. Upgrading from pneumatic control and hard wire logic to DCS and PLC. I have a question about opinions on where to put valve limit switches. I have a Fisher E body valve and plan to use Topworx Go Switches (magnetic).
The steam vent valve in on the outlet piping of the heater, and is programmed to open when the only steam user, a large turbine shuts down. This protects the superheater from overheating. I want to put a limit switch on this valve to bring the position of it into the contol room (It is solenoid operated, not positioner). I can put it on the open position, closed position, or both.
My thoughts:
99.9% of the time the valve is closed, and the valve is fail open. I would put the switch on the closed position as if the valve is to open it should go all the way open, unless stuck for some reason. Very clean process, and steam tracing and insulation on the valve makes this unlikely. Also, if the switch fails, I would know right away because 99.9% of the time the switch will be used.A Co-worker's thoughts: It is more important to know that the valve is wide open rather than closed, due to the catastrophic failure that can occur if the vent valve is not open when it should be. The valve does have the small potential to come off the closed switch without travelling fully open.
Option 3: Put a switch on both open and closed limits. I could do this. The cost would not be huge, two pair teck cable instead of a single pair and another switch. But am I "over-instrumentating" this valve? Something likely done in our industry too often. The valve is outside, and Northern Alberta winters are harsh. I don't want to hang to much stuff off of it that could break.
Anyway, any thoughts would be great.
Thanks.
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