Pressure Zoning and Water Hammer in a 20-Storey Residential Tower
A 20-storey residential tower is fed by a roof tank and a duplex variable-speed booster set. Pressure-reducing valves are installed at the 6th and 12th floor riser branches. Residents on the 18th floor complain of weak flow, while residents on the 2nd floor report pressures high enough to cause water hammer at washing-machine solenoids and single-lever mixers. The booster set hunts at low demand and cycles frequently at night. There is no bladder tank on the discharge header. The contractor proposes removing the 6th-floor pressure-reducing valve to improve upper-floor pressure and increasing the pump pressure setpoint. The owner asks the supervisor to review the proposal before the next progress payment.
1. Analyse the pressure distribution and explain why removing the 6th-floor pressure-reducing valve would not solve the upper-floor problem.
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The building is effectively a static-pressure zoning problem. A roof tank plus a booster set can produce high pressure at lower floors and marginal pressure at the top. The pressure-reducing valves at the 6th and 12th floors are intended to create zones so that fixture pressures remain within a usable range, typically a few bar, not the full static head.
Removing the 6th-floor valve would raise pressure below that point, worsening water hammer and the risk of component failure on lower floors. It would not necessarily improve the 18th floor because the upper zone is downstream of the 12th-floor valve and the booster set. If the 18th floor is weak, the causes may include an undersized riser, a mis-set or partially closed 12th-floor valve, a booster setpoint that is too low for the top zone, or excessive losses in the roof-tank outlet.
Raising the pump setpoint alone would over-pressurise the lower zones unless the pressure-reducing valves are correctly sized and operating. The supervisor should require a hydraulic check of each zone: static and residual pressures at the highest and lowest fixtures, valve settings, strainer condition, and pump curve versus actual demand.
2. What measures would you require to control water hammer and pump hunting, and how would you verify them?
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Water hammer at quick-closing solenoids and single-lever mixers is commonly caused by high velocity, insufficient damping, or missing air chambers. Pump hunting at low flow is consistent with a variable-speed booster without adequate minimum-flow or pressure stabilization.
Required measures:
- Verify and reset the pressure-reducing valves zone by zone; do not remove them.
- Install a bladder-type diaphragm tank on the discharge header, sized for the pump and system, to absorb pressure surges and reduce short cycling.
- Check pump minimum-flow setting, non-return valve operation, and the pressure sensor location. A sensor too close to the pump can cause hunting.
- Provide proper pipe anchoring and eliminate loose or undersized sections contributing to hammer.
- Confirm fixture pressures are within acceptable limits at both top and bottom of each zone.
Verification should include a pressure-logging test at night and at peak demand, a pump start/stop count over a defined period, and a quick-closing valve test at the most remote and lowest fixtures. The supervisor should witness the tests and record pressures before releasing the payment.
3. The contractor says the pressure-reducing valves are unnecessary because the fixtures are rated for higher pressure. How should the supervisor decide?
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The decision should be based on the actual system pressures and the fixture and pipe ratings, not on a general statement that fixtures can take higher pressure. Even if a fixture is rated for a higher static pressure, repeated surge from water hammer can damage solenoids, flexible connectors, and seals. Lower-floor pressures reported by residents are a symptom that must be measured.
The supervisor should require a pressure survey at the lowest and highest points of each zone, including surge pressure during rapid valve closure. If measured pressures exceed the applicable limits or produce hammer, the pressure-reducing valves are necessary. If a valve is genuinely redundant, the contractor must demonstrate that by calculation and test, not by assertion.
Because the proposal also removes a safety and serviceability feature, the supervisor should not approve it informally. A written technical assessment, owner acceptance, and a revised hydraulic calculation should be required. Payment certification for the affected work should be held until the zoning is resolved.