Setting pump discharge pressure for a first-floor attack line
You are the pump operator at a two-storey commercial building fire. The officer in charge has called for one attack line: 60 m of 45 mm hose with a combination nozzle flowing 230 L/min at 700 kPa nozzle pressure. Your brigade's pump card gives friction loss in 45 mm hose at 230 L/min as 3.5 kPa per metre of hose. The branch will be worked on the first floor, with the nozzle 6.0 m above the pump outlet. Allow 50 kPa for losses in the appliance piping and the gauge line. The pump is a single-stage unit rated at 2,000 L/min at 1,000 kPa, falling to about 1,100 L/min at 1,500 kPa. The previous driver left the relief valve set at 1,200 kPa. The officer in charge wants the correct pressure at the branch, not merely a number on the pump gauge.
1. Calculate the correct pump discharge pressure for the attack line as positioned.
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Build the answer from the four components, in the order they affect the branch:
- Nozzle pressure = 700 kPa.
- Friction loss = 3.5 kPa/m × 60 m = 210 kPa.
- Elevation = 10 kPa per metre of lift × 6.0 m = 60 kPa.
- Appliance and gauge-line allowance = 50 kPa.
Pump discharge pressure = 700 + 210 + 60 + 50 = 1,020 kPa.
Rounding to 1,020 kPa (about 10 bar) is correct; leaving it at 1,000 kPa would under-supply the branch by about 20 kPa, which shows up as a weak, short stream when the crew most needs reach. The relief valve at 1,200 kPa is set above the required pressure, which is right in principle, but 180 kPa of headroom is loose for a crew working internally - if the branch is shut suddenly, the pump will climb toward the relief setting before it dumps. Set the relief just above the calculated operating pressure and confirm it with the gauge as the line is charged.
2. The branch is moved to the second floor, 3.2 m higher, and a second identical line is put to work. What pressure and flow are now required, and can this pump deliver them?
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New elevation: 6.0 + 3.2 = 9.2 m, so 9.2 × 10 = 92 kPa.
Pump discharge pressure = 700 + 210 + 92 + 50 = 1,052 kPa per line. Both lines are identical and are fed from the same pump, so the pressure is the same and the flows add:
- Flow = 230 + 230 = 460 L/min.
- Required pressure = 1,052 kPa.
Check against the pump curve given: it will produce 2,000 L/min at 1,000 kPa and about 1,100 L/min at 1,500 kPa. At roughly 1,050 kPa the pump will comfortably supply 460 L/min, so capacity is not the constraint.
The real risks are elsewhere. Two charged lines plus the extra height mean the relief valve setting must be re-checked rather than left at a fixed 1,200 kPa, and the supply to the pump must be confirmed - if the pump is working from the tank, 460 L/min empties it quickly and the pump will cavitate, dropping pressure and flow regardless of throttle. Confirm the water source before adding the second line.
3. The branch is fully open but your gauge reads 780 kPa. What could explain it, and what do you do?
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The gauge is telling you the system is not delivering what was calculated. Do not simply open the throttle to force the needle up - that hides the fault and can injure the crew if the restriction clears suddenly.
Likely causes:
- A kink or a flattened section in the 60 m layout, which adds friction loss the pump card does not account for.
- A partly closed branch, hose cock or delivery valve, or a coupling not fully made.
- A leaking or burst section losing water before the branch.
- Restricted supply to the pump - a partly open tank valve, a blocked suction strainer, or a hydrant not fully turned on - causing cavitation and a falling gauge.
- A faulty gauge, which is why you cross-check with the compound gauge and the pump's behaviour rather than trusting one dial.
Actions: hold the current setting, tell the branch operator what you are seeing and ask whether the stream looks right, walk the line for kinks and leaks while a second person watches the gauges, check the supply, and only then adjust. Report the actual reading and the fault to the officer in charge - the crew's safety depends on knowing the true pressure at the branch.