Size is determined by the amount of flow required to lower the inlet pressure. This relief flow can be difficult to determine, so a general guideline is to use 60% of the rated pump flow. The 108 Series valve is capable of intermittent flows up to 45 ft. per second. Relief valve sizes are typically 50-60% of the mainline size.
BYPASS PRESSURE CONTROL
Bypass pressure control valves are sized based on maximum flow and pressure drop across the valve. The maximum flow through the valve is the pump flow at the desired set point (from the pump curve) minus the minimum system flow. The pressure drop across the valve is the set point minus the pressure at the valve discharge (typically pump suction or storage tank head). Determine the valve’s operating Cv using the maximum flow and pressure drop from the formula:
From the chart below, pick the smallest valve that has at least the Cv determined above, and where the velocity does not exceed 25 ft/sec.
- Qmax = maximum flow rate, gallons per minute
- DP = pressure drop, psi
- sg = liquid specific gravity (water = 1.00)