Water Hammer, Cavitation and Troubleshooting

Water hammer

Closing a valve quickly stops a flowing liquid, but the column of liquid upstream still has its momentum. It piles up against the valve, the pressure jumps, and a pressure wave runs back up the pipe at the speed of sound in the liquid, reflecting at the reservoir and coming back. The bang, the shaking of the pipes and, in the worst case, a burst pipe or a wrecked valve are called water hammer (or hydraulic shock, or surge).

If the valve closes in less than the time for the wave to travel to the reservoir and back, 2 L ÷ a, the pressure rise is the full Joukowsky value:

Δp = ρ a Δv

Closure time	Closure type	Pressure rise
0.5 s	rapid (surge is the full Joukowsky value)	22.0 bar
0.9 s	rapid (surge is the full Joukowsky value)	22.0 bar
2.0 s	slow	5.0 bar
5.0 s	slow	2.0 bar
10.0 s	slow	1.0 bar

The speed of the pressure wave

In a rigid pipe the wave speed would be the speed of sound in water, √(K ÷ ρ) = 1 485 m/s (K = 2.2 GPa). A flexible pipe stretches, which absorbs some of the pressure and slows the wave:

a = √(K ÷ ρ) ÷ √(1 + K D ÷ (E t))

Pipe	D	t	E	Wave speed a	Surge at 2 m/s
Steel DN80	77.9 mm	5.5 mm	200 GPa	1 381 m/s	27.6 bar
PVC DN80	80 mm	4 mm	3 GPa	375 m/s	7.5 bar
Copper 22 mm	22 mm	1 mm	117 GPa	1 249 m/s	24.9 bar

Plastic pipes have a much slower wave and so lower Joukowsky surges, but their pressure ratings are lower too, and a surge that is within the rating can still shorten the life of a plastic line. Air in the liquid slows the wave dramatically (and softens the shock), which is why some old systems “don’t hammer” until the air is purged.

Preventing and limiting surge

• Close valves slowly: longer than 2 L ÷ a, and ideally 10 times that. Use motorized or hydraulic actuators with a controlled closing time; the last 10 percent of the closure is where most of the flow stops, so slow that part.
• Keep velocity down (1 to 3 m/s): the surge is proportional to velocity.
• Surge vessels and accumulators near the valve or pump absorb the flow change: a gas-charged bladder vessel is the common fix for a pump station.
• Surge relief valves, pressure relief and air valves to vent the pressure or admit air at the vapor-pressure dip.
• Pump trips: a pump that stops suddenly starts a reverse wave; fit slow-closing non-slam check valves and consider a flywheel on large pumps.
• Water-hammer arrestors on quick-closing taps and washing-machine valves: small sealed gas or spring chambers, sized to the flow and pressure.
• Analyze long pipelines: over several kilometers or with big pumps, use a transient program (such as the free EPANET extensions, or commercial surge software) rather than the hand formula.

Cavitation

Cavitation is the formation and violent collapse of vapor bubbles where the pressure falls to the vapor pressure: at the inlet of a pump, behind a throttling valve or after a sharp restriction, or where a surge pushes the pressure down. It makes a rattling or crackling noise, erodes metal (pitting that looks like a sponge), vibrates the machine and lowers the output. The cure is to raise the local pressure or reduce the local speed: see NPSH in Pumps, Pump Curves and NPSH, and for valves the maker’s cavitation index and anti-cavitation trim.

Air in pipelines

Air collects at high points, reduces the flow area, raises the head loss and can create surge or a “slug” that blows out. Vent it with air-release valves at high points and after pumps, slope the pipe gently towards them (1:500 or steeper), and keep velocities high enough (above about 1 m/s) to sweep bubbles along. Air entering a pump suction through a leaking joint or a vortex in the tank gives the same symptoms as cavitation: check the fittings and the submergence of the inlet.

Forces on bends and supports

The momentum of the fluid and the pressure on a bend or a closed end give a force on the pipe that the supports must resist. For a 90° elbow with the same pipe size at both ends, each leg gives a force of p A + ρ Q v along the pipe, and the two combine to a resultant √2 times as big:

F_leg = p A + ρ Q v    F_resultant = √2 × F_leg

For the DN80 example at 4 bar gauge: p A = 1 907 N and ρ Q v = 21 N, so F_leg = 1 928 N and the resultant on the elbow is 2.73 kN, pointing away from the corner. Under a water-hammer peak of 26 bar the pressure term alone would be 12.4 kN: surge loads are what break unsupported bends and push pipes out of their joints. Restrain bends and tees on buried and plastic lines, and use thrust blocks or tie rods at changes of direction.

Troubleshooting piping and pump problems

Symptom	Likely cause	What to check or do
Bang or shudder when a valve closes	water hammer	closing time, velocity, check valves; fit slower actuators, arrestor or surge vessel
Pump rattling, crackling, falling output	cavitation (NPSH too low) or air in the suction	NPSH available, suction line size and fittings, tank level and temperature, leaks in the suction pipe
Pump flow lower than the datasheet	system resistance higher than expected, wrong rotation, wear, air, speed	pressure at both ends, strainer blocked, valve position, impeller diameter, rpm
Pump flow higher than the duty, motor overloaded	system resistance lower than expected	throttle the discharge, trim the impeller, or slow the pump; check the motor current
Flow falls over months	fouling, scale or corrosion raising the roughness	calculate friction with aged roughness; descale or replace; add a pressure gauge to trend the drop
High pressure drop in a short line	undersized pipe, closed valve, many fittings, a blocked filter	check velocity, valve positions, the differential across strainers
Noise or vibration in a pipe	velocity too high, cavitating valve, pulsation from a positive-displacement pump, loose supports	velocity below 3 m/s for liquids; damper; support spacing; pressure behind the valve
Gauge needles flutter	pulsation, air, or cavitation	fit gauge snubbers or glycerine-filled gauges; check for air
Leaks at flanges and threaded joints after a pump stops	surge or pressure cycling loosening joints	torque and gasket, thrust restraint, surge protection

Related

• Previous: Compressed Air and Gas Flow. Back to the start: Fluid Mechanics and Pipe Flow.
• Pipe Flow and Pressure Drop Calculator.
• Pump Power and Sizing Calculator (includes NPSH available).
• Workshop Safety: pressure and stored energy.

FAQ

Can a surge burst a pipe rated for the working pressure?

Yes. A 22 bar surge on a 4 bar line would exceed the rating of most plastic and thin-walled pipe. Pipe pressure ratings include the working pressure plus an allowance for surge, but sudden closure can exceed it. Design for the surge or prevent it.

Does a bigger pipe reduce water hammer?

It reduces the velocity for the same flow, so the surge falls with it, but the wave speed and the surge for a given velocity do not change. Slow valves and surge control are more effective.

Is a check valve a cause or a cure for hammer?

Either. A swing check that slams shut when the flow reverses is a classic cause; a nozzle or spring-assisted check valve closes before the flow reverses and prevents it.

Why does my pump hammer when it stops?

Because the flow slows quickly and the column of liquid decelerates or reverses and slams the check valve. Fit a non-slam check valve, a surge vessel, or ramp the pump down with a drive.

References

• Joukowsky, N., “Über den hydraulischen Stoss in Wasserleitungsröhren”, 1898.
• Wylie, E. B. and Streeter, V. L., Fluid Transients in Systems, Prentice Hall.
• Thorley, A. R. D., Fluid Transients in Pipeline Systems, Professional Engineering Publishing.
• AWWA Manual M51 (air valves); Hydraulic Institute standards on pump intake design and NPSH.

Last reviewed: October 2026. Wave speeds and surge pressures are calculated with typical constants: use a transient analysis for critical pipelines.