Combined Domestic and Fire Water System in a 14-Storey New Cairo Tower
A 14-storey residential tower in New Cairo (45 m to the roof) has a combined domestic and fire water system. The basement houses a domestic booster set (2 duty + 1 standby) drawing from a 120 m³ underground tank, and a fire pump taking suction from the same tank through a common manifold. The fire system is a wet riser with landing valves on every floor, plus a 6 m³ rooftop tank feeding a down-feed riser for the top four floors. On commissioning, at 1,950 L/min the fire pump produced 5.8 bar; shut-off head was 9.9 bar against a rated duty of 1,300 L/min at 7.0 bar. The jockey pump starts at 7.6 bar and the main pump at 6.5 bar. The contractor's sizing note counts the full 120 m³ as fire storage and assumes refill at 3 L/s from the 50 mm municipal connection. Civil defence inspection is booked for next week and the client wants handover.
1. Assess the fire pump test against recognised acceptance criteria and state what you require before the civil defence inspection.
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Flow/head relationship. At 150% of rated flow (1,950 L/min) the pump must still produce at least 65% of rated head: 0.65 × 7.0 = 4.55 bar required, 5.8 bar measured - comfortable pass, so the curve is not collapsing at overload.
Shut-off. Shut-off head must not exceed 140% of rated head: 1.40 × 7.0 = 9.8 bar. Measured 9.9 bar is 141% - a marginal non-compliance, and one that also matters for the pipework and component pressure rating and for the churn temperature rise.
What is missing. The 100% rated point (1,300 L/min at 7.0 bar) was never demonstrated, so you cannot confirm the pump is trimmed for the actual duty, and no flow-measuring device or test header is mentioned. Require: a permanent test header and flow meter sized for 175% of rated flow; a witnessed test at 0, 100%, 150% and churn; verification that the non-return valve, relief and sensing lines are correctly installed; and a pump curve comparison to decide whether the impeller needs trimming.
Also verify before the inspector arrives: dedicated fire suction with a vortex plate, no domestic draw on the fire reserve, correct pressure switch calibration, weekly churn test procedure, and a signed pump room signage and operating instruction set. Do not sign the pump as accepted on this test alone.
2. Recalculate the fire storage and refill assumption and state what must change in the tank arrangements.
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Demand. Two landing valves operating simultaneously at about 250 L/min each gives 500 L/min; for a 60-minute duration that is a 30 m³ minimum fire volume for the wet riser, before any domestic allowance. Volume alone is therefore not the problem - 120 m³ is generous.
Usable versus gross is the problem. The suction must retain submergence above the vortex plate and low-level cut-out, so the declareable fire volume is the usable volume, typically 10–15% below gross. The contractor's note counting the full 120 m³ as fire storage must be rejected and replaced by a marked usable volume with a level diagram.
Non-compounding. Domestic and fire must not share a compound reserve. The domestic pumps drawing on the same manifold will erode the fire reserve: require a dedicated fire compartment with an overflow weir or a fire-only suction with a vortex plate, and an interlock that stops the domestic duty pumps on a fire-pump-run signal.
Refill. 3 L/s = 180 L/min is well below the 500 L/min fire demand, so the tank is a one-shot supply. That is acceptable only if the usable fire volume genuinely covers the full duration, which must be shown arithmetically; the refill rate restores the reserve afterwards, it does not sustain the fire. Record the municipal connection's measured pressure and flow, not the nominal pipe size.
3. The contractor proposes jockey start at 7.6 bar and main pump start at 6.5 bar, and asks you to accept the 6 m³ rooftop tank as the sole source for the top four floors' landing valves. Give your decision with reasoning.
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Reject both.
Pressure switches. Settings should be read off the actual pump curve, not picked to suit the existing pipework. With a 9.9 bar shut-off: jockey stop about 9.6–9.8 bar, jockey start about 9.2–9.4 bar, main fire pump start about 8.3–8.6 bar. Starting the main pump at 6.5 bar means the system pressure is allowed to fall around 3.3 bar below shut-off before the fire pump runs - the jockey churns, the topmost landing valve residual is compromised during the critical first minute, and the low settings mask leaks rather than controlling them. Require calibrated pressure switches set from the curve, with a witnessed start/stop sequence.
Rooftop tank. A tank sitting at roof level gives the top-floor landing valve only about 0.1 bar per metre of drop - roughly 1 bar for a 10 m fall, far below the residual pressure required at the most remote landing valve. The down-feed arrangement therefore fails on pressure before it is even checked on volume: 6 m³ at 500 L/min is 12 minutes, against a 60-minute duration. There is also no shown automatic refill, so it is a single-shot supply.
Decision: delete the down-feed branch and extend the wet riser to all floors fed by the main fire pump, or provide a dedicated top-zone pump set with its own stored volume; resubmit the pressure switch schedule for review.