Scenario: you have been assigned as one of the pumpers in a relay that is supposed to be pumping 1,500 gallons per minute (gpm). As you set up, you remember the lesson on relay pumping from the pump operator's class that you took—since you have a 1,500-gpm pump, you know that the design data name plate will give you the correct engine speed in revolutions per minute (rpm) with a' pump discharge pressure (PDP) of 150 pounds per square inch (psi) to achieve a flow of 1,500 gpm. When pumping at overload, you are able to increase the PDP to 165 psi. You also recall from the class that the friction loss in five-inch large diameter hose (LDH) is dependent on the volume of water being flowed and the distance of the stretch. A relay flow of 1,500 gpm will create friction loss of approximately 18 psi per 100 feet of hose. When relay pumping, you also remember that you want to deliver the water to the receiving pump with a 20-psi residual pressure to avoid cavitation. This equates to a maximum five-inch LDH hose stretch of 800 feet (eight 100-foot lengths at 18 psi = 144 psi, and a 20-psi residual requires 164-psi PDP).
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