Pump Discharge Pressure for a Second-Floor Attack Line
At 02:10 you are the operator on Engine 3, a 1,500 gpm single-stage pump, at a 2-story strip mall with fire in a second-floor unit. One attack line is deployed: 150 ft of 1¾-inch hose with a 15/16-inch smooth-bore tip at 50 psi nozzle pressure, flowing about 185 gpm. The nozzle is 12 ft above the pump. Your intake is from a hydrant with 72 psi static and 58 psi residual while this line is flowing; tank-to-pump is open and the tank is full. The pump discharge gauge reads 118 psi and the nozzle reports the stream is soft. There are no appliances in the layout. Your department uses standard friction-loss coefficients and 0.434 psi per foot of elevation.
1. What pump discharge pressure should this layout require, and how does your current setting compare?
Read the worked answer
Required PDP = nozzle pressure + friction loss + elevation + appliance loss.
- Friction loss for 1¾-inch hose: FL = C × Q² × L, with C = 15.5, Q = 185/100 = 1.85, L = 1.5 (hundreds of feet).
- FL = 15.5 × (1.85)² × 1.5 ≈ 15.5 × 3.42 × 1.5 ≈ 80 psi.
- Elevation: 12 ft × 0.434 ≈ 5 psi.
- Nozzle: 50 psi.
- Required PDP ≈ 50 + 80 + 5 = 135 psi.
At 118 psi the nozzle is getting roughly 118 − 85 = 33 psi, which with a 15/16-inch tip flows only about 150 gpm instead of 185. That matches the reported soft stream. The fix is to bring the engine up to about 135 psi, then confirm with the nozzle and recheck the intake gauge for a stable residual.
Note the calculation is iterative: if the actual flow is lower, friction loss is slightly lower, so 135 psi is the design point, not a precision guarantee. The nozzle report is the final check.
2. Twenty minutes later a crew adds 100 ft of the same 1¾-inch hose to reach a rear unit, and the line now passes over a chain-link fence, creating a kink. The nozzle still needs 50 psi at 185 gpm. What is the new required PDP, and what are the risks and alternatives?
Read the worked answer
The friction loss per 100 ft stays at roughly 53 psi, so 250 ft gives about 53 × 2.5 = 133 psi of hose loss.
New PDP ≈ 50 (nozzle) + 133 (friction) + 5 (elevation) = 188 psi.
That figure is a minimum, because a kink adds loss that cannot be read from any chart. The operator would have to push even higher than 188 psi at the panel to satisfy the nozzle.
Risks:
- Older 1¾-inch hose and couplings are being pushed toward their working-pressure margin; a burst line injures crews.
- High panel pressure with modest flow tempts the operator to over-throttle, and nozzle reaction rises.
- A kink makes the gauge reading meaningless as an indicator of what the nozzle is receiving.
Better options, in order: remove the kink or re-route the line; replace the 100-ft extension with a 2½-inch or 3-inch feeder to a wye; move the engine closer; or stretch a second line. Raising the panel to 188 psi should be the last choice, not the first.
3. The fire is knocked down and all lines are shut off for five minutes while crews open up. What do you do at the pump, and why?
Read the worked answer
A pump with no discharge flow is deadheaded. The impeller is still churning water, and the energy goes into heating the water in the pump and the intake. The pump can overheat, steam, and damage seals or the packing within minutes.
Actions:
- Open the bypass or recirculation line (or crack the tank fill) so a small flow continues through the pump. A flow of a few tens of gpm is enough to carry heat away.
- Keep the intake open and watch the intake gauge for a stable reading.
- Keep an eye on pump temperature and listen for cavitation-like noise, which here would signal churn rather than a suction problem.
- Before lines are reopened, make sure nozzles are open so water hammer does not slam the piping and gauge.
Do not rely on a thermal relief device as the operating plan; it is a backup.