Blog · · 2 min read
Water hammer: understanding the phenomenon and protecting your installations
A valve closing too fast, a pump stopping abruptly, and a whole pipeline takes a pressure wave capable of bursting it. Water hammer is well known, yet still too often underestimated when designing or operating a network.
A transient phenomenon
In steady state, water flows through a pipe at constant velocity. As soon as a device changes that velocity abruptly — valve closure, pump stop or start, pipe rupture — the fluid's kinetic energy must be absorbed. Since water is almost incompressible and the pipe barely deformable, that energy turns into a pressure wave that travels through the network at the speed of sound in the pipe, then reflects at the ends. That is water hammer: a succession of overpressures and underpressures that can last several seconds.
Estimating it in a few lines
The Joukowski formula gives the order of magnitude of the overpressure for a sudden change of velocity: ΔP = ρ · c · Δv, where ρ is the fluid density, c the wave celerity (around 1,000 m/s in a steel pipe, less in plastic) and Δv the change in velocity. For water stopped from 2 m/s in a steel pipe, that is about 20 bar of overpressure — far more than the working pressure of many networks.
This maximum is only reached if the manoeuvre is faster than the wave's round-trip time, T = 2L/c, where L is the pipe length. For a 500 m pipe, T is about one second: a valve closed in under a second produces the full overpressure; closed in ten seconds, far less. Beyond these estimates, Bergeron diagrams and then numerical simulation track the wave through a real network.
The most frequent causes
- Fast closure of a valve, especially ball valves and solenoid valves.
- Pump trip on power failure: the water column keeps moving, creates a depression, then comes back.
- Pump start-up on an empty or partly drained pipe.
- Air pockets moving or collapsing at high points.
Consequences
Overpressures above the admissible pressure of pipes and equipment, underpressures causing cavitation and sucking in seals, vibration and displacement of pipework, fatigue of supports, noise. In drinking water, an underpressure can also draw in outside water and contaminate the network. Incidents range from recurring leaks to outright pipe bursts.
Protecting an installation
- Act on the cause: slow down manoeuvres (valve closure times, soft starters and variable-speed drives on pumps), flywheels.
- Absorb the wave: surge vessels and air chambers, surge shafts, relief valves, air valves at high points, check valves.
- Size it: the choice and setting of a protection depend on the pipe profile, the flow rate, the celerity and the worst-case scenario; a calculation is needed before buying a vessel.
How to study it
For a simple network, a manual calculation and a Bergeron diagram give a reliable answer. For a branched network or a process circuit, 1D simulation with software such as Simcenter FloMaster lets you test manoeuvre scenarios and compare protections. Numersalis carries out such studies within its fluid and process simulation services and offers a two-day water hammer course so that your teams can recognise, calculate and prevent the phenomenon.
