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
These switches will work well for us, and are pretty much a standard
at our plant. I don't really want to use anything else, as we already
stock these in our warehouse.
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.
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.
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.