Hydrant Flow Check
You are laying out a supply line from a hydrant to a pumper. The hydrant is a standard 2½-inch outlet. You static-test the hydrant and read 62 psi. With a single 2½-inch line flowing through a pitot gauge, you get a pitot reading of 18 psi. A 2½-inch smooth bore nozzle with a 1⅛-inch tip is on the line. You need to estimate available flow to decide whether to lay a second supply line. The pumper is 300 feet away.
1. Estimate the flow from the nozzle using the pitot reading and explain the steps.
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Use the flow formula for a smooth bore nozzle: GPM = 29.7 × d² × √P, where d is the tip diameter in inches and P is the pitot pressure in psi.
Step 1: d² = 1.125² = 1.2656. Step 2: √18 ≈ 4.243. Step 3: GPM = 29.7 × 1.2656 × 4.243 ≈ 29.7 × 5.37 ≈ 159.5, so about 160 gpm.
That is the flow through the nozzle, not the total hydrant capacity. It is the starting point for judging whether the supply is adequate.
2. The pumper is 300 feet away. How does that distance affect the decision to lay a second line, and what else should you check?
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Friction loss in 2½-inch hose is roughly 2 psi per 100 feet at low flows but rises steeply as flow increases. At about 160 gpm, a 300-foot lay of 2½-inch hose will lose a meaningful amount of pressure, and if more lines are charged from the same pumper, the loss grows. The static-to-flow comparison matters: a 62 psi static dropping to a pitot of 18 psi on a single line suggests the hydrant is being drawn down substantially, which is a sign of limited available flow.
Check the residual pressure at the pumper while flowing, not just the pitot at the nozzle. If residual pressure drops below about 10-15 psi, the hydrant is near its limit and a second supply line or a different hydrant is warranted. Also confirm the hydrant's main size and whether other companies are using it.
3. If you lay a second 2½-inch supply line from the same hydrant, what happens to friction loss and available flow, and is that always the right move?
Read the worked answer
A second parallel line from the same hydrant splits the flow, roughly halving the friction loss per line for the same total flow, so the pumper receives more pressure and can flow more water. It improves the supply only up to the hydrant's capacity. If the hydrant main is small or the static pressure is low, two lines will just draw the hydrant down faster without adding usable flow.
The right move depends on the residual pressure test: if residual stays healthy, a second line helps; if residual collapses, find a second hydrant or a water source instead of adding hose to a weak hydrant.