Water Hammer Prevention in Hydraulic Systems
What You Need to Know
Water hammer is the loud bang you hear when a tap or valve shuts fast. It sends a pressure shock wave through the pipes that can burst joints, crack fittings, and void warranties. AS/NZS 3500.1 (the Australian Standard for water services) sets the rules to prevent it. This memo covers the pressure limits, velocity caps, and devices you need to keep your pipework safe.
The Rules
- Maximum static pressure at any outlet must not exceed 500 kPa (AS/NZS 3500.1 Cl 3.3.4)
- Maximum water velocity in piping must not exceed 3.0 m/s (AS/NZS 3500.1 Cl 3.4)
- Where water hammer occurs, install a surge protector or water hammer arrester (AS/NZS 3500.1 Section 4)
- All water services must comply with AS/NZS 3500.1 as referenced by the NCC 2025 Volume Three
- Pressure limiting valves (PLVs) are required where supply pressure exceeds 500 kPa (AS/NZS 3500.1 Cl 3.3.4)
- Pipework must be fixed with brackets and clips at maximum spacings set by pipe material and size (AS/NZS 3500.1 Section 6)
What This Means in Practice
Take a 10-storey residential building with mains pressure at 700 kPa at ground level. Without a PLV, the lower floors cop the full 700 kPa. Every time a washing machine solenoid snaps shut, it sends a pressure spike well above 500 kPa through the pipes. That spike loosens joints, fatigues copper, and creates the banging noise that residents complain about. A PLV at the meter or at each floor entry drops the pressure to around 350–400 kPa and cuts the severity of every hammer event.
Velocity matters just as much as pressure. AS/NZS 3500.1 caps velocity at 3.0 m/s, but best practice is 1.5 m/s. When a pipe is sized too small for its flow rate, the water moves faster, and faster water creates bigger hammer shocks. Watch the internal diameter when swapping materials. A DN20 copper pipe has approximately a 17 mm bore, but a DN20 PEX pipe has only about 14 mm. That smaller bore pushes velocity up by about 50% for the same flow. PEX fittings can be worse still, with bores as small as 11.2 mm.
Water hammer arresters absorb the shock by letting a piston or bladder compress against a sealed air charge. They go upstream of quick-closing valves like dishwasher solenoids, washing machine inlets, and lever-action taps. Without an arrester at these points, every cycle of the appliance sends a shock wave back through the building's pipework.
Key Design Decisions
Pressure Limiting Valve Location
Install a PLV at the water meter where supply pressure exceeds 500 kPa. For multistorey buildings, install PLVs at each pressure zone boundary to keep every floor below 500 kPa.
Pipe Sizing for Low Velocity
Size pipes so flow velocity stays at or below 1.5 m/s, not the code maximum of 3.0 m/s. This cuts hammer intensity and reduces pipe noise.
Water Hammer Arrester Placement
Install arresters as close as possible to every quick-closing valve. This includes dishwashers, washing machines, and any solenoid-operated valves. Size arresters using the fixture unit method per ASSE 1010.
Check Valve Selection
Use non-slam (spring-loaded or silent) check valves instead of swing check valves on pump discharge lines and booster sets. Swing check valves slam shut on flow reversal and create severe hammer.
Who Needs to Know What
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References
- AS/NZS 3500.1:2021, Plumbing and drainage — Part 1: Water services
- National Construction Code 2022, Volume Three (Plumbing Code of Australia), Section B — Water Services
- ASSE 1010, Performance Requirements for Water Hammer Arresters
- PDI-WH 201, Water Hammer Arresters Standard (sizing and certification)
- CIBSE Guide G, Public Health and Plumbing Engineering (international reference)