Residual pressure is defined as:

• A. Stored potential energy available.
• B. The pressure remaining in a water supply system while water is flowing.
• C. Forward velocity pressure at the point of discharge.
• D. The minimum pressure required in a residential area.

Answer: (B)

Residual pressure is the pressure in a water supply system while water is flowing. It shows the pressure that remains after losses from friction, elevation, and other factors as water is being delivered, which helps verify there’s enough pressure to fight a fire. This is different from static pressure, which is the pressure when no water is moving. The idea of stored potential energy isn’t what residual pressure measures—it’s about what’s left during active flow. Velocity pressure is the dynamic pressure from the water’s speed, a separate concept. And a minimum pressure for a residential area is a design target, not the pressure observed during flow. So the correct understanding is that residual pressure is the pressure remaining in the system while water is flowing.