Why have redundant pressure relief on piston/diaphragm pumps?

[hollerg]

I have simplex diaphragm pumps from Milton Roy that are equipped with internal relief valves. I see piping diagrams that specify external relief, from them and other vendors. In my case I would relieve back to the suction line with a pulsation dampener to absorb the minimal thermal expansion, at my 450 lb/hr pump rate.

Why the dual relief? Is there a code to comply with in chemical service?

Is this a case where the vendors do not want the internal relief device to be a safety device for the piping system, but rather only equipment protection?

 

[JJPellin]

Yes. The internal relief valve is relieving oil pressure not product pressure. Because of the design of the pump, it cannot produce product pressures any higher than the oil pressure. But the internal relief is there to protect the pump. It is not designed to the standards necessary to be considered adequate to protect downstream equipment. It cannot meat the testing requirements for protection of process equipment. So, when I have attempted to press this issue, I was forced to keep both. The internal relief protects the pump and the external protects the piping and downstream equipment.

Johnny Pellin