What is residual pressure in a firefighting hydraulic system?

• A. The pressure present in the line while water is flowing, after the system has started delivering water
• B. The pressure when no water is moving
• C. The pressure at the pump intake
• D. The pressure lost to friction in the hose

Answer: (A)

Residual pressure is the pressure in the line while water is flowing and the system has started delivering water. As the water moves through the pump, hoses, valves, and nozzle, friction and elevation losses reduce the pressure from the initial discharge. The remaining pressure you measure on active flow is the residual pressure, and it reflects the actual working pressure at the nozzle to ensure adequate flow. It isn’t the static pressure when no water is moving, nor the pump intake pressure, nor simply equal to the hose friction losses—the residual is the pressure that remains during active flow after those losses.