Water Relief Valve Sizing

Pressure relief valves are used to protect equipment from excessive overpressure.

Water relief valve sizing. Properly sized relief valves will provide the required protection while also avoiding issues with excessive flow rates including. The valve s relief of elevated liquid gas or steam pressures prevents damage to the system. Correct relief valve sizing is a complex process and there are many methods and tools available to size such devices. Correct and comprehensive pressure relief valve sizing is a complex multi step process that should follow the following stepwise approach.

We offer a wide selection of relief valves for any application. Valve sizing coefficient determined experimentally for each style and size of valve using water at standard conditions as the test fluid p pressure differential in psi g specific gravity of fluid water at 60 f 1 0000 thus c v is numerically equal to the number of u s. Similarly for a 4l6 valve suction nozzle is 4 and discharge is of 6 size. Auxiliary type relief valves which can be placed anywhere on the system and set approximately to 10psi below the water heater relief valve setting.

The following is a sizing method created back in 1996 for a newsletter i called the deppmann connection. Two common standards used for the sizing of pressure relief valves are iso 4126 american petroleum institute api rp520 part i. Therefore a water heater with a mawp of 160 psi still requires a t p relief valve set at 150 psi even though the vessel is designed and built to withstand a higher pressure. The relieving capacity of the valve must be equal to or greater than the btu hr.

Relief valve sizing methods for chilled water systems are not easy to find. Model codes require the t p relief valve be set no higher than 150 psi. Relief valves are designed to open at a preset pressure or temperature level and relieve the system when it has exceeded the desired level. For a 3l4 valve suction nozzle is of 3 size and discharge nozzle of 4 size.

Available valve sizes are 3l4 and 4l6 which correspond to different nozzle sizes for suction and discharge nozzles of the pressure relief valves. Every closed hydronic system must have a compression or expansion tank. The equation methodology on this web page mostly follows the american petroleum institute standard 520 for pressure relief valve sizing for liquids in order to protect equipment having a maximum allowable working pressure of 15 psig 103 kpa gage or greater api 2014 p. They allow the auxiliary relief valve to take the brunt of thermal expansion relieving and prevent unnecessary relieving cycles by the t p relief valve.

Possible valve damage impaired performance undersized discharge piping and effluent handling systems and higher costs. The latter standard is the most widely used for relief valve sizing. Pressure relief valve sizing craig spears november 10 2015. Each piece of equipment in a process should be evaluated for potential overpressure scenarios.