State the general relationship among discharge pressure, friction losses, and nozzle pressure used in pump calculations.

State the general relationship among discharge pressure, friction losses, and nozzle pressure used in pump calculations.

• A. Discharge pressure equals nozzle pressure plus total friction losses in all attack and supply lines plus any elevation or other minor losses.
• B. Discharge pressure equals nozzle pressure minus total friction losses.
• C. Discharge pressure equals nozzle pressure multiplied by friction losses.
• D. Discharge pressure equals nozzle pressure divided by elevation losses.

Answer: (A)

Discharge pressure is the pressure the pump must supply at its outlet to deliver the desired pressure at the nozzle, after all losses along the hose and elevation are accounted for. Start with the nozzle pressure you need to achieve your flow, then add all the losses that occur as water moves from the pump to the nozzle. Those losses include friction losses in every attack and supply line, as well as any elevation losses if the nozzle is higher than the pump. Don’t forget other minor losses from fittings, valves, and adapters along the path. The sum of nozzle pressure, total friction losses, elevation losses, and other minor losses gives the discharge pressure to set on the pump. For example, if you want 50 psi at the nozzle, with 28 psi of friction losses in the hoses, 6 psi of elevation losses, and 4 psi of minor losses, you’d set the pump to 50 + 28 + 6 + 4 = 88 psi. Subtracting losses or multiplying/dividing by them would not reflect how pressure accumulates along the line, so those approaches don’t fit pump calculations.

Discharge pressure is the pressure the pump must supply at its outlet to deliver the desired pressure at the nozzle, after all losses along the hose and elevation are accounted for. Start with the nozzle pressure you need to achieve your flow, then add all the losses that occur as water moves from the pump to the nozzle. Those losses include friction losses in every attack and supply line, as well as any elevation losses if the nozzle is higher than the pump. Don’t forget other minor losses from fittings, valves, and adapters along the path. The sum of nozzle pressure, total friction losses, elevation losses, and other minor losses gives the discharge pressure to set on the pump.

For example, if you want 50 psi at the nozzle, with 28 psi of friction losses in the hoses, 6 psi of elevation losses, and 4 psi of minor losses, you’d set the pump to 50 + 28 + 6 + 4 = 88 psi. Subtracting losses or multiplying/dividing by them would not reflect how pressure accumulates along the line, so those approaches don’t fit pump calculations.