National Fire Fighter Corp. carries a full line of Pressure Relief Valves for relief of excessive pressure or line surges. When the nozzle is shut off or the hose kinked, or when other restrictions occur, a spring loaded valve opens and water is recirculated back to the pump. This is especially critical in positive displacement pumps where excessive pressure can cause serious damage. Fully adjustable, set at 150 degrees. 1" NPSH with 1" bypass. 300 PSI. Aluminum construction.