What is the function of a pump internal relief valve on positive displacement pumps?
Ask most pump people “what is the function of an internal relief valve?” and the most frequent answer is that “the valve is there to protect the pump should overpressure occur”. While not entirely incorrect, consider the consequences of an internal relief valve in full bypass. Against a closed valve or other downstream blockage the pressure will quickly spike to the full bypass pressure setting of the internal relief valve and the same volume of liquid is recirculated internally. As the pressurized liquid passes the poppet to the low pressure side of the valve, its energy is converted into heat. The vast majority of input power supplied to the pump from the driver is also transformed into heat. This heat then raises the temperature of a relatively small volume of liquid circulating within the pump and the pump itself. For some applications this added heat may be inconsequential, but for others it may damage the pump or product inside. These include, but would not be limited to, mag drive pumps, shear sensitive liquids, liquefied gases or liquids with a tendency to thermoset, coke, or liquids that may be otherwise damaged by high temperature.
The purpose of the internal relief valve is to provide system protection from overpressure conditions resulting from blockage of the pump. A pump with an internal relief valve may still fail due to higher valve settings, long term valve bypass and the reasons and examples above.
When supplying a pump with an internal relief valve consider the following questions:
- Should other secondary means of overpressure protection be included to protect the pump as well as the system?
- Is an internal relief valve the best option or would a return-to-tank or inline relief valve be a better solution?