Riser pressure drop in a four-storey extension
A four-storey hotel extension is added to an existing building. The incoming main delivers 3.2 bar static at the ground-floor stop valve, and the design flow is 1.8 L/s through 28 mm copper tube. The extension has two bathrooms per floor, each with a 9 L/min shower and a 6 L/min basin tap. The engineer proposes to run a single 22 mm riser to the top floor and tee off each level, arguing that 3.2 bar is 'plenty'. The building control officer flags the proposal. The longest branch from riser to the top-floor shower is 14 m with six elbows and a full-bore gate valve. Static head per storey is 3.1 m.
1. Using the information given, explain why the single 22 mm riser is likely to fail at the top floor, and identify the two design changes that should be made.
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First, calculate the available pressure at the top floor. Static head loss = 4 floors x 3.1 m = 12.4 m, which is about 1.22 bar. Remaining static pressure at top floor = 3.2 - 1.22 = 1.98 bar, before any friction losses.
Second, estimate friction. At 1.8 L/s through 22 mm copper, the velocity is roughly 4.7 m/s and the friction gradient is in the order of 0.9-1.1 bar per 10 m. Even for a 14 m branch with six elbows and a gate valve, the equivalent length is well over 20 m, so friction alone would consume most of the remaining pressure. A shower needs about 0.5-1.0 bar at the outlet to work properly; there is no margin.
Third, the simultaneous demand is underestimated. Two bathrooms per floor with showers and basins means the diversified flow at peak is higher than 1.8 L/s unless fixture units are properly counted. A single 22 mm riser cannot carry that flow at acceptable velocity.
The two changes: (1) upsize the riser to 28 mm or 35 mm and check velocity is below about 1.5-2.0 m/s; (2) install a booster set or break tank with pump to guarantee pressure at the top floor, because static head alone leaves too little margin for friction and simultaneous use.
2. State the minimum test pressure and procedure for the new pipework before handover, and explain why this matters to the building control officer.
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For cold and hot water services in a building of this type, the accepted practice is to pressure-test at 1.5 times the maximum working pressure, with a minimum of 6 bar for a duration of at least one hour, or as specified by the local code. The test should be carried out in sections, with the pipework capped, air expelled, and a calibrated gauge at the lowest point. A drop of more than about 0.5 bar over the test period, or visible weeping at any joint, is a fail.
The building control officer needs this because the riser will be concealed in service shafts and ceilings. Once concealed, a leak or a joint that fails under thermal cycling cannot be found without destructive access. The pressure test proves joint integrity before concealment, protects the hotel from water damage, and provides the documented evidence the officer needs to sign off the installation.
3. If the client insists on keeping the 22 mm riser for cost reasons, what compensating measures could be proposed and what residual risk remains?
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Compensating measures could include: reducing simultaneous demand by fitting flow restrictors and pressure-compensating aerators to all outlets; installing a small local booster pump on the top-floor branch; and converting the top-floor showers to low-flow heads. Each of these reduces the peak flow through the riser.
However, the residual risks are significant. Flow restrictors reduce performance and may be removed by hotel maintenance. A local booster on the top floor does not fix the velocity and noise in the riser itself, which at 4.7 m/s will be audible in guest rooms and will accelerate erosion-corrosion at elbows. The 22 mm riser also gives no margin for future additional fixtures.
A professional recommendation should be to upsize the riser. The cost of a 28 mm riser over a 22 mm riser is small compared with the cost of re-opening shafts after handover. Document the advice in writing so the client's decision is recorded.