PC Pumps (API676)

If the process discharge pressure is 2.4Barg and the pump maximum discharge pressure is 17Barg, a relief valve set to anywhere between 2.5 and 17Barg should be the business.

Clearly, if the fluid contains flammables, spraying them willy-nilly is absurd; returning the relieved fluids to the tank at the suction is appropriate. However, if the discharge piping has nothing in it that could cause excess pressure over and above 2.4Barg then a relief valve would be unnecessary.

Best thing - subject the installation to a formal HazOp Study, and see what happens, simply because the forum cannot see the current issue of the P&ID.

If the suction pressure is 13.6 barg, it would stand to reason that, even if there was no pump in the line, there would still be a pressure of 13.6 barg in the system. Therefore, the relief valve would have to be set at or above that pressure. Frankly, unless I am missing something, I don't know how the writer can have an inlet pressure of 13.6 barg and a discharge pressure of 2.4 barg unless the PC pump is acting as not only a delivery mechanism but a pressure reducing mechanism as well, however unlikely that may sound. A PC pump is, by definition, a semi-positive displacement pump. That being said, if the pump rotates, something has to move. And when the pump turns, the pressure will continue to rise until something "lets loose". That is why it is always advisable to have a relief valve in the line of a PC pump. If something closes the discharge pipe completely and the pump continues to run, the only protection left is electrical (overloads on the motor starter) or mechanical as in ruptured lines, motor burn up or something equally as catastrophic. You are correct in that whatever the setting of the relief valve, the location of the discharge of that valve must be contained some how.

You set your limit to slightly below the max pressure of the discharge tubing, do not use a PSV, use a shut down trip. STOP THE PUMP!

PCP's will continue to build pressure even if the discharge is closed, as I'm sure you know.. so stop the motor! No more pressure. I have seen a PCP split steel tubing in the oil fields...

I have no idea what you are pumping as a PCP will pump anything from live fish to peanut butter. Whatever you are pumping you don't want to have to clean it up!