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Relief valves are used in most pumping systems to prevent excessive pressures caused by changes in the conditions under which the system is operating. There are many different kinds of relief valves and they function in different ways depending on the type of pump, the characteristics of the pump driver (engine or motor, etc.), and the operating conditions. This discussion is restricted to pilot–operated valves used with centrifugal pumps driven by internal combustion engines, feeding one or more discharge lines, any or all of which may be shut off without the prior knowledge of the pump operator –– in other words, to relief valves used on fire department pumpers.