The service tank must be how full at all times?

• A. 10%
• B. 50%
• C. 75%
• D. 95%

Answer: (D)

The service tank must be kept almost full to ensure a reliable source of water for firefighting systems and to prevent loss of pump performance. When the tank is around 95 percent full, there’s enough water head to feed the fire pumps and maintain a flooded suction, which helps avoid air pockets and cavitation that can occur if the tank is too low. That near-full level also leaves a small margin for thermal expansion and sudden water withdrawals without dropping the supply. If it’s only a fraction full, the pumps can struggle to draw water, delays can occur, and air can enter the system. Levels around 75 percent or 50 percent don’t provide the same safety buffer for rapid demand or temperature changes, whereas 95 percent balances readiness with minimal wasted space.