Which statement correctly defines static pressure and residual pressure on a hydrant?

• A. Static pressure is the pressure when no water is flowing; residual pressure is the pressure under flow.
• B. Static pressure is the pressure under flow; residual pressure is the pressure when no water is flowing.
• C. Static pressure is the pressure when water is flowing; residual pressure is the pressure when no water is flowing.
• D. Static pressure equals residual pressure.

Answer: (A)

The main concept here is understanding the difference between static pressure and residual pressure in a hydrant system. Static pressure is the pressure in the water distribution system when no water is moving; it shows the available pressure at the hydrant with the valve shut. Residual pressure is the pressure observed while water is flowing through the system as water is being drawn from the hydrant; it accounts for friction losses in the pipes and elevation changes. The statement is correct because it states static pressure occurs when no water is flowing and residual pressure occurs during flow, which is exactly how these terms are defined. The other ideas mix the states incorrectly—static pressure is not measured during flow, and residual pressure is not simply the same as static pressure; in real systems friction reduces pressure during flow, so residual pressure is typically lower than static pressure. Context: knowing these helps determine if there is enough pressure to deliver a fire stream by comparing static and residual values and understanding how much pressure drops as water moves through the system.