It was recently recommended in our PHA revalidation that we install NH3 detectors in our relief headers. I've done some research and talked to some technicians who say that these 1-2% relief line sensors are notoriously unreliable.

A consultant heard that some people are placing a small stub of pipe on the relief header. At the far end of the pipe is a pressure sensor with a 10# trip. At the end of the pipe that is welded to the relief header they are placing a 10# burst disc. Has anyone heard of such an application? I can't see a reason that this wouldn't work - of course it would not detect a weeping pressure relief valve, but we are primarily concerned with larger releases.

It would certainly be less $$$ than a $2000 manning sensor and likely would not need to be frequenly excercised and calibrated as NH3 sensors must.

Then again, since we have pressure burst discs on all the relief valves going into those headers, do we really need a relief NH3 sensor at all?