On horizontal pumps, the difference between suction and discharge pressures should match which pressure as shown on the fire pump nameplate?

• A. Shutoff pressure
• B. Discharge pressure
• C. Suction pressure
• D. Churn

Answer: (D)

The key idea is that the shutoff pressure on a fire pump nameplate is the head the pump develops when there is no flow. For a horizontal pump, the difference between suction pressure and discharge pressure at zero flow should match that shutoff pressure. When the discharge is effectively blocked, the pump works up to its maximum pressure, and the suction-to-discharge pressure difference reaches the nameplate’s shutoff value. This confirms the pump is delivering its rated no-flow head.

Discharge pressure alone varies with flow, so it isn’t the constant reference you compare to the nameplate. Suction pressure also changes with suction conditions and isn’t the fixed no-flow head. Churn isn’t a pressure measurement shown on the nameplate; it’s a term referring to a condition, not a spec to be matched.