PSHL

No.

Generally, pressure switches should be selected to operate well before the maximum allowable piping pressure is approached. For example, on my refrigeration compressors this sequence occurs: pre-alarm computer screen warning, compressor unloads on increasing pressure, compressor unloads on high motor amps, compressor shuts down, relief valve lifts; and only after all that occurs does discharge pipe leak or break.

"Generally, pressure switches should be selected to operate well before the maximum allowable pressure is approached"

That is mean PSHL pressure is lower than Maximum allowable pressure (MAWP). Then, the pressures (PSHL and MAWP) should be higher than maximum operating pressure (MOP). Generally in my case PSHL pressure is lower than design pressure.

In addition to what mentioned, the piping system shall be furnished with a safety relieving device to be designed to protect the piping system against any pressure attained equal to or in excess of the MAWP (design pressure). And the pressure at which the safety device designed to open is called "Set Pressure". Hence, the PSHL must be selected to be lower than not only the MAWP, but also must be less than the Set Pressure of safety relieving device.

Thanks

then, is that wise if I assume that set pressure at PSHL equal to maximum operating pressure and set pressure at PSV equal to MAWP (derating Design Pressure)

No.

The PSV (pressure safety valve) should be set somewhat less than the MAWP of vessels or piping. (Maybe something like 10-15% less.)

In turn, any pressure switches (such as PSHH > PSHL) should be set somewhat less than the PSV setting. (Again something like 10-15% less.)

If this is done correctly, it results in a series of control actions, each of which anticipates the next control action. The whole scheme thus gives several levels of redundancy, as in the five or so steps I mentioned in the refrigeration compressor example.

If the high setting of the <...PSHL (Pressure switch high low)...> were set the same as <...same as maximum operating pressure in piping...> then there is a less than 100% chance of it operating before the pipe ruptures! That's insane.

Refer to the Process Description document, the Control Philosophy document or the HazOp Study document to determine the correct setting. If all else fails, ask the Process Engineer why the thing was installed and what the setting should be.