Is there a rule of thumb of determining the PSHH and PSLL when determining the setpoint of a pressure switch? I am installing a Pressure Switch on a line where the operating pressure is 950psi. My pressure switch is 0-1500. What should my PSHH and PSLL setpoint be?
What other major factors should I also be considering?
I believe 20% above and below your normal operating pressure is the standard. However, the HIHI should be set below the relief valve setting. The relief valve should be your last line of defense. Your PSHH needs to be set below the MAWP of the line or equipment it is protecting. So you need to find the weakest link in your line (e.g. flange ratings, gaskets, pipe thickness, valves, fittings, vessels) and set the switch accordingly. After you have determined the maximum pressure you can safely shutdown at, then you can consider the operational benefits of the switch. It may be beneficial to you to shutdown at 1000# even though your line is rated for 1440#. Especially if you are dealing with compressors (rod load, engine overloading, and non rod reversal are all problems that could be caused by high discharge pressure). This could give you an early indication of blockages or other unfortunate problems. As far as the low side, you are wanting to protect the upstream and downstream equipment from starvation. Find the minimum allowable operating pressure and then consider the operational benefits of the switch. A slightly lower pressure could indicate a rupture, a leak, or various other problems concerning your equipment. Protection of life first, equipment second, and the environment third.
I suggest that you follow the following:
1- Check your line rating and consequently you will come to know the line design pressure. Assume this pressure is X.
2- PSV set point would be the same = X
3- Maximum PSHH value would be 0.8 X
4- Normal Operating pressure would be 0.4 - 0.6 X
5- PSLL would be defined to avoid vacuum conditions or would be defined as per the driving machinery (pump / compressor) low suction pressure which is defined by the machinery manufacturer.
I hope that this would help.