Which type of fire pump must be prime using an external priming device?

• A. Centrifugal
• B. Rotary gear
• C. Piston
• D. Diaphragm

Answer: (D)

Diaphragm pumps are positive-displacement devices that move water by flexing a flexible membrane. When the suction line isn’t already filled with water, air in the line prevents the diaphragm from pulling water into the pump, so the pump cannot prime by itself. Because of that, an external priming device is required to create a prime—removing air and filling the suction path—before the diaphragm pump will deliver water reliably. Once primed, it can continue pumping effectively.

The other pump types rely more on filling the suction line or have different inherent priming behavior, so they’re not defined by this same requirement in the way diaphragm pumps are.