What is the difference between static pressure and residual pressure in hydrant operations?

• A. Static pressure is the pressure when water is flowing; residual pressure is the pressure observed while water is flowing.
• B. Static pressure is the pressure when no water is flowing; residual pressure is the pressure observed while water is flowing.
• C. Static pressure and residual pressure are the same measured value.
• D. Static pressure is the pressure at the discharge and residual pressure is at the intake.

Answer: (B)

Static pressure is the pressure in the water system when no water is moving. It shows how much pressure is available from the source with taps closed, useful for assessing the supply before you start flowing water. Residual pressure is the pressure you observe while water is flowing; as water moves through pipes and fittings, friction and turbulence reduce the pressure, so the line pressur e drops to a lower, steady value. In firefighting, you use static pressure to estimate overall supply capacity and residual pressure to ensure there’s enough pressure at the nozzle during flow. They aren’t the same because starting and maintaining flow introduces losses that lower the pressure from the static value.