Pressure Switch Problems

Hmm, I assume you've been working on this issue for a while, analyzing the system, checking out your instrumentation, looking for leaks and pressure failure points, checking connections, etc., all to no avail. So why would you think we who haven't engaged in any of those processes could solve your problem based on a 1 sentence question.

Tell us a little more, man - what are you working with, what ahve you looked at, what have you tried, what didn't work, did you just install it or has it worked before and is now not working?

Help us to help you!

So, isolate the pressure switch onto a test rig at an instrumentation workshop. Give it a squeeze. Above the set pressure, it opens, and below it closes. Is this correct? No? Then it's knackered and needs changing.

However, if it does work correctly then the fault is elsewhere. Look for errors in and additions to the system wiring. Sometimes the most dangerous thing on a plant is a piece of wire about 50mm long with two stripped ends. These are sometimes known as "frigs", and they are a frustrated operator's way of making sure that the plant continues to run in the absence of any technical support from the Instrumentation Department in the dead of night.

Uhm, because it's broken?

There could be several reasons why. I'll give you one example.

Someone, way before my time, installed and connected a pressure switch during the shutdown. During the startup, the pressure switch kept alarming, though the pressure was fine (below the setpoint). Several technicians replaced it and wired it exactly the same way as before, but it still kept alarming. They had the switch bench-tested and it checks fine.

Another guy comes along and thinks, "hey, they've replaced the pressure switch a couple of times already, so it might not be the switch after all". He replaces the cable, but still the alarm kept coming. The situation lasted for a week.

Now, if the switch and cable are new, what's the cause of the problem?

The switch is supposed to be wired as normally closed. In this case, it was wired as normally open. Succeeding technicians replaced it but wired it the same way as before...still as normally open, probably because they think "I found it that way so that's how it probably should be wired". Changing the cable still didn't work because the switch was still wired erroneously.

Sometimes, I think people are just too lazy to think things through.

I'm not saying that you may have the same problem. There could be other reasons. Endure a little sweat from time to time.