Six-Storey Hotel Direct-Supply Pressure Shortfall
A six-storey hotel is supplied directly from a 300 kPa municipal main through a 50 mm service. Each floor is 3.0 m high, and the highest fixture is a flush-valve water closet on the sixth floor, 18 m above the main. The owner reports weak flushing on floors 5 and 6, while ground-floor hose bibs have adequate pressure. A contractor proposes replacing the 50 mm service with 75 mm pipe and adding a pressure-reducing valve at the basement to 'stabilise' pressure. Existing peak-demand friction loss from the main to the top fixture is estimated at 70 kPa. Flush valves require 140 kPa minimum flowing pressure. The local authority caps static pressure at fixtures at 550 kPa. There is no booster pump or roof tank, and the building operates at full occupancy each morning.
1. Calculate the approximate residual pressure at the top-floor flush valve and decide whether upsizing the service alone will solve the complaint.
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Use the static main pressure and subtract elevation loss and friction loss. Elevation loss is about 9.81 kPa per metre, so 18 m costs roughly 177 kPa. Residual pressure at the top fixture is approximately 300 − 177 − 70 = 53 kPa. That is well below the 140 kPa flowing minimum for a flush valve, which explains weak flushing on the upper floors. Upsizing the service from 50 mm to 75 mm can reduce friction, but the elevation loss remains 177 kPa. Even if friction dropped from 70 kPa to 30 kPa, the residual would be only about 93 kPa, still short of 140 kPa. Therefore the 75 mm service alone is inadequate. The proposed basement pressure-reducing valve would make the upper-floor problem worse, not better, because it would lower the supply pressure feeding the riser.
2. Recommend a code-compliant supply arrangement and explain how zoning, pressure control, and booster selection should be handled.
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The elevation loss dominates, so the upper floors need a pressure booster or a gravity tank high enough to provide the required pressure. A practical scheme is to split the building into zones. The lower zone can remain on direct municipal supply if static pressure stays below the 550 kPa fixture limit. The upper zone should be fed by a booster pump sized for the morning peak demand, with an expansion tank and pressure-relief valve on the discharge. The booster must maintain at least 140 kPa at the top flush valve after all friction and elevation losses. A pressure-reducing valve may be needed on the lower zone only if static pressure there exceeds the code cap. Each zone should have backflow protection for the booster connection and isolation valves for maintenance. The designer should verify the municipal main can supply the booster without causing unacceptable pressure drop to neighbouring properties.
3. The owner suggests putting a roof tank on the sixth floor instead of a booster. Evaluate that option.
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A roof tank only works if its water level gives enough head above the highest fixture. A tank sitting on the sixth-floor roof might provide only about 3 m of head to a sixth-floor fixture, roughly 30 kPa. That is far below the 140 kPa needed for the flush valve, even before friction losses. To work by gravity alone, the tank bottom would need to be at least about 14.3 m above the highest fixture, plus extra height for friction, which is impractical on this roof. A roof tank could be used with a small booster pump, but then it is not a gravity-only solution. It would also add structural loading and maintenance obligations. The better answer is a properly sized booster for the upper zone, with pressure control and backflow protection, rather than an undersized roof tank that cannot meet fixture minimums.