Available flow from hydrants is estimated by determining the percent drop between the static pressure and which pressure?

• A. Residual pressure
• B. Dynamic pressure
• C. Atmospheric pressure
• D. Hydraulic pressure

Answer: (A)

The key idea is that hydrant flow potential is judged by how much pressure falls when water starts to flow. You first measure static pressure, which is the pressure in the system with no water moving. Then you open the hydrant and measure residual pressure, the pressure at the same point once full flow is established. The percent drop between static and residual pressures reflects how much head is lost to friction and flow in the system as water is drawn. That drop is then used with standard hydrant rating methods to estimate the available flow (GPM). The other pressures don’t indicate how much flow the hydrant can deliver: dynamic pressure is tied to velocity, atmospheric pressure is just ambient, and hydraulic pressure isn’t the metric used for this estimation.