Water hammer or liquid hammer


Liquid is incompressible. A force applied to move and add pressure to a liquid
creates dynamic energy that is transmitted instantly across the system. When
liquid moves, stops, or changes velocity abruptly, the dynamic energy amplifies
the normal system pressure creating a sudden pressure surge, or spike.

When the pressure surge accelerates and approaches the speed of sound, the
surge becomes in acoustic resonance with sound wave. The surge is amplified
many times over, in some cases as high as [ * ] times the normal system
pressure. This pressure surge can be very destructive to system components,
and is known as water hammer.

The term water hammer comes from the pressure surge'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 surface. The term water hammer
is a misnomer as it implies that the phenomenon applies only to water.

In fact, water hammer can occur with most liquids that others prefer to call this
phenomenon as
liquid hammer. For instance, an oil  pump which suddenly
reversed its rotation, due to system failure, resulted in a liquid hammer so severe
that it damaged the impeller and bolted-on suction splitter on its last stage volute.
In another incident of water hammer the shaft of a pump was bent 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.


There are softwares for calculating water hammer, some are available online.
These calculators may yield varied results because each program is based on
different assumptions and inputs depending on the desired level of accuracy.


This simple water hammer formula can be used for making rough estimate: [ * ]







where:
Ph  -  water hammer pressure, PSIG
P
i   -  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: [ * ]








[ * ] Some information are excluded in this article.
Read more.


R0509-WAHA
Category: design, operation
File: water hammer
This rotor had catastrophic
damage when a system
upset caused severe water
hammer that forced the rotor
to reverse its rotation
abruptly.
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