Canteen pipe sizing in a refurbished office tower
A 14-storey office building in Kwun Tong is being refurbished. The new eighth-floor canteen will have six 32 kW wok ranges, two 22 kW congee stoves and one 45 kW hot-water heater, all town gas. The gas meter room is at ground level; the equivalent pipe length from the meter outlet to the canteen manifold is 95 m, including fittings and two fire-rated penetrations. At no flow the meter outlet reads 1.6 kPa; each appliance is rated at 1.0 kPa minimum inlet. The tenant's contractor proposes a 32 mm copper run because it fits the existing riser sleeve and has quoted a pressure loss of 0.55 kPa at a diversified load of 60%. The building owner is concerned about the existing 50 mm riser sleeve and whether the meter capacity is adequate. No appliance diversity data has been provided beyond the contractor's assumption.
1. Calculate the connected and diversified demand in kW and m3/h, and judge whether the contractor's 60% diversity is defensible.
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Connected load = 6×32 + 2×22 + 45 = 281 kW. Using town gas at about 17 MJ/m3, that is 281 kW × 3.6 MJ/kWh / 17 MJ/m3 ≈ 59.5 m3/h. The contractor's 60% diversity gives 169 kW and about 35.7 m3/h. Is 60% defensible? For a canteen with six woks, diversity among woks may be high at lunch, and the water heater can coincide with cooking. A more realistic coincidence might be 0.7–0.8, i.e. 197–225 kW or 42–48 m3/h, unless metered data from a similar canteen supports lower. The design should use the appliance manufacturer's gas rates and the operator's schedule, not just connected nameplate. Also check the meter's maximum flow and pressure loss at 42–48 m3/h; a meter sized only for the diversified figure must still pass the peak. If the existing meter or regulator cannot pass that flow at the available inlet pressure, the project needs a meter upgrade before pipe sizing is finalised.
2. Evaluate the proposed 32 mm copper run against the available pressure and velocity, and recommend a sizing solution.
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Available pressure is 1.6 - 1.0 = 0.6 kPa (600 Pa), but that must cover the meter, regulator, manifold, appliance connection and any future fouling; allocating 550 Pa to the 95 m pipe alone leaves almost nothing. At the contractor's 60% flow, 32 mm copper is already marginal; at a more realistic 42–48 m3/h the velocity in a 32 mm bore is roughly 18–21 m/s, and pressure loss rises with the square of flow. A 50 mm bore would bring velocity to roughly 9–11 m/s at 48 m3/h and greatly reduce friction, but the existing 50 mm sleeve may not accept an insulated 50 mm copper pipe. Options are: ream and use the sleeve with a suitable lining, form a new fire-rated penetration, or use a smaller-length 40 mm section only if the hydraulic calculation proves the total loss stays below about 300–350 Pa with margin. The 0.55 kPa quotation is not acceptable as a design basis because it uses an unverified 60% diversity and ignores meter/regulator losses. Require a stamped calculation using measured inlet pressure, actual equivalent length, fitting losses and manufacturer appliance rates.
3. List the design and commissioning checks that must accompany the final pipe size.
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Before installation, confirm: meter and regulator capacity at 1.6 kPa inlet; allowable pressure drop allocation; fire-rated penetration details and sleeve ventilation; isolation valve accessible at entry to canteen; emergency shut-off; gas detection and ventilation interlock for the canteen if required; flue and combustion air for the water heater; and appliance connectors. On commissioning: strength/pressure test the new run, purge, tightness test at operating pressure, check regulator lock-up, measure operating pressure at the furthest appliance with all diversified appliances running, and record gas rate, flame safeguard, CO/CO2 and flue draught for each appliance. Confirm the canteen ventilation is balanced so the gas appliances do not back-draught. Provide as-built drawings, test certificates and operating instructions. If the measured pressure at the manifold falls below 1.0 kPa with the design diversified load, upsize or re-route; do not compensate by raising the regulator set point beyond appliance rating.