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Since positive displacement pumps are not self- limiting, i.e., they can produce increasing pressure if sufficient driver power is available, a device to limit pump pressure and horsepower is required. The function of a relief valve as a protection device is to limit pump discharge pressure and horsepower to a specified value without generating any valve instabilities and to positively reseat. While the function of a relief valve is simple enough, valve chatter (instability) and failure to positively reseat can cause the shutdown of the critical equipment. Relief valve chatter can cause high pressure pulses that will activate shutdown pressure switches and damage valve seats and plugs. The inability to reseat properly will introduce an ‘equivalent orifice’ into the system that will reduce or totally eliminate the system flow to the critical system components.
Experience has shown that a sliding piston type relief valve, which is a modulating device, as opposed to a spring loaded poppet valve, which is an on-off device, meets the requirements of stability and positive shutoff for liquid auxiliary system service.
Relief valve set pressure is usually set 10% above the pump maximum discharge pressure. However, the maximum pressure ratings of all system components must also be considered. Given the maximum pump flow and the relief valve set pressure, the maximum system pressure can be determined as follows: Maximum system pressure = relief valve set pressure + relief valve overpressure. Relief valve overpressure is the valve pressure drop necessary to pass full pump flow. For the present example using a 2” valve: 1. Maximum pump discharge pressure = 200 PSIG 2. Relief valve set pressure (cracking pressure) = 1.1 × 200 = 220 PSIG 3. Maximum system overpressure = 220 PSIG + 25 PSIG = 245 PSIG.
Note that the overpressure values are viscosity sensitive and can be used up to a viscosity of 500 SSU. Above this value, the overpressure can be estimated to vary by the relationship: (viscosity/500 SSU)1/4. Therefore, the maximum pressure at 1000 SSU will be 262 psi or 19%. Relief valve overpressure expressed as a percentage of relief valve set point is defined as accumulation. Typical values of accumulation vary between 10 and 20%.