Hammer
Water
hammer is a phenomenon that occurs in many pumping systems, sometimes having
minimal effect, and other times catastrophic.
If
not carefully planned for, water hammer can cause thousands of dollars in
damage and lost productivity.
So
what is water hammer anyway?
According to a Pumps and Systems article, published in
August 2008, “water hammer is a pressure surge that can arise in any pumping system
that undergoes an abrupt change in its rate of flow and usually results from
pump starts and stops, the opening and closing of valves, or water column
separation and closure.”
When these abrupt changes are inflicted on a system, they
cause the fluid to undergo a change in momentum.
The
flow of the water has been interrupted and has nowhere else to go, and collides
against the closed valve.
Because
water is not very compressible, it creates a pressure, or shock wave, then
reverses flow, back upstream.
Should
the wave then run into a pump upstream, the fluid will continue to travel back
and forth until friction losses cause the wave to settle.
Throughout this process, the shockwaves have sent vibrations
through the system.
The
size of the shockwaves is dependent upon 3 factors: the velocity of the fluid
in ft/sec (V), the length of the pipe between barriers in feet (L), and the
amount of time it took to close the valve in seconds (t).
The
intensity of the shockwave can be calculated by this equation, (offered in article from Blacoh U): P=0.07(VL/t), where P is the resulting additional pressure
generated by the shock wave.
(Additional water hammer calculators can be found in a
previous post, “3 Calculators For Water Hammer And Pressure Drop.)
Let’s
take our example and plug it into the equation.
The
fluid is moving at 5 feet per second, and the length of pipe between our two
valves is 500 feet. Our actuator has quickly closed the valve at .5 seconds.
The
resulting pressure from the shock wave is as follows:
0.07(5 x 500)/.5 = 350 PSI
Most systems are not designed to handle such great
additional pressures as those generated by water hammer.
For
a more in-depth study of water hammer, head out to Blacoh U and create an account for access to a
30 minute video "The Phenomenon Called Water Hammer".
When you abruptly close valves, listen for loud noises or
vibration and pulsation of pipes.
If
you hear or see what looks like the effects of water hammer, be sure to consult
an experienced engineer.
They
can help you put some fixes in place to keep your system intact.
Got a problem with water hammer? Ask us about
it! We gladly provide technical
assistance to businesses in Wisconsin and Upper Michigan.
Sara Peters leads Crane Engineering's blogging team, coming up with
fresh stories and insights for our readers to apply to their every day work.
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