Where to put valve limit switches

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).
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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.
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.
Curtis Keyes
Reply to
Curtis
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When I interview programmers, I ask them a question similar to that. I don't hire the ones who think one switch is enough.
I.e., in real systems, with mechanical parts, "NOT closed" is NOT the same as "OPEN".
-Mike-
Reply to
Mike Halloran
Curtis,
I prefer two switches. If a switch sticks or the wire breaks, it is more likely that you will detect the failure. I am not familiar with your particular valve to know the chance that the valve will be stuck just slightly open, but I assume that it is possible.
I usually prefer to wire the switches so that the open switch (ZSO-xxx) opens (off) when the valve is fully open and the closed switch (ZSC-xxx) opens when the valve is fully closed. The logic would then resolve the two digital inputs as: OFF, ON to be Valve Closed, ON, OFF to be Valve fully open, ON, ON to be transition or midway, and OFF, OFF to be valve fully open and fully closed, or clearly a failure of a switch. In my experience the most common failure is an open circuit due to fuse, broken wire, or bad switch. It is possible, but less common, for a switch to be stuck on when it should be closed.
John Shaw Process Control Solutions
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Reply to
John Shaw
I second this opinion. If the two sensors do not agree then you can diagnose a system problem. If you only have one sensor, you have the possibility of not knowing that you don't know what a situation is. Second order ignorance is to be avoided.
Just for giggles, backing the sensor up with some temperature and pressure sensors which the operator or control system can follow to make sure that the valve has done its job is a very good idea. Again, this lets you know what is happening better.
Michael
Reply to
Herman Family
Regardless of you choice of a protection scheme, I'd be cautious about using magnetic limit switches if the area they'd be used in experiences vibrations a lot, or would experience vibrations in event of the valve's usage. I used to work with a utility co., and we used mechanically-operated limit switches from Namco (don't know if they're still around; maybe they were bought out), which worked well in the tough application of monitoring the open and closed positions of turbine valves (where heat, vibration and dust made use of other devices untenable or questionable.)
Mike
Reply to
M. Hamill
I've always been partial to 2 switches, because I may also want to know that it is in between states. For example, solenoid operated knife gate vales.
What if your valve is 90% open and just misses the 100% open limit switch?
If only one switch, definately the closed limit. The one time a year it is needed will the switch function?
Reply to
dummy

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