When force is applied to a liquid to increase its pressure and move it from one
point to another it creates dynamic energy that is transmitted instantly across
the system because liquid is incompressible. When liquid moves, stops, or
changes velocity abruptly, the dynamic energy amplifies the normal system
pressure creating a sudden pulsating pressure surge, or spikes.
When the pressure surge accelerates and its sonic velocity approaches the
speed of sound, the surge becomes in acoustic resonance with sound waves.
The resonant condition amplifies the pressure surge many times over, in some
extreme cases as high as ten times the normal system pressure. This pressure
surge, which is time-dependent, can be very destructive to system components,
and is known as water hammer. It is characterized by very high vibration level
and loud noise.
The term water hammer comes from the pressure spike's hammering effect on a
system. It is analogous to the pounding of a hammer on a nail where dynamic
energy from the hammer is converted into pressure energy on the nail, strong
enough to cause the nail to be embedded into a hard surface. The term water
hammer is somewhat of a misnomer because it implies that the phenomenon
applies only to water as the medium.
In fact, water hammer can occur with most liquids that CENTRIFUGALPUMP.
COM and The EMA Project prefer to call this phenomenon as liquid hammer.
For instance, an oil pump that suddenly reversed its rotation due to system
failure, resulted in a liquid hammer so severe that it damaged the impeller, the
impeller wear rings, and the bolted-on suction splitter on its last stage volute. In
another reported incident of water hammer the shaft of a single stage horizontal
pump was bent and damaged beyond repair.
Some preventive actions to prevent water hammer are:
- Protect the pump by installing non-return check valve on discharge piping
to isolate the pump in the event of its sudden stoppage or reversal of its
rotation in an upset condition.
- Eliminate, or minimize, sudden constrictions and abrupt changes in
direction in piping systems.
- Open or close valves gradually. Opening or closing a valve fully in [ * ]
seconds or less, depending on valve size and pressure rating, is usually
considered too quick and can produce water hammer.
- Install surge suppressor, or pulsation dampener, (either with or without a
bladder) to suppress surge transients
The phenomenon of water hammer can be better understood with detailed
surge analysis but the analysis can be both tedious and time-consuming. There
are commercially available softwares for calculating water hammer, some of
which are available free online. These calculators may yield varied results,
depending on the desired level of accuracy because each program is based on
varying complexities of the system, the assumptions made, and the required
This simple water hammer formula can be used for making rough
estimate: [ * ]
Ph - water hammer pressure, PSIG
Pi - initial or inlet pressure, PSIG
Q - flow rate, gallons per minute
L - pipe length, feet
D - inside diameter of pipe (or system component), inches
t - elapse time, seconds
Example: What is the estimated water hammer resulting from the closure of a
solenoid valve in a 20-foot long upstream pipe, 1" ID, delivering 40 gallons per
minute, at 60 PSIG, if the solenoid valve closes completely in 50 milliseconds?
Solution: [ * ]
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Photo shows the catastrophic damage caused by water hammer on a pump rotor.