10/8/2023
Water
Hammer
Fluid Mechanics
Definition
• Water hammer is a surge or pressure
wave that is created due to a sudden
change in flow velocity in a confined
system
• Water hammer is a pressure surge or
wave resulting when a fluid in motion is
forced to stop or change the velocity
• A water hammer commonly occurs when
a valve closes suddenly at an end of a
pipeline system, and a pressure wave
propagates in the pipe. It is also called
hydraulic shock.
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Reason
• abrupt opening or closing of valves
• starting or stopping of pumps
• failure of mechanical devices in a flow line
• Power outage
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A conceptual
schematic of
water hammer in
a simple system
The system consists
of a frictionless
horizontal pipe of
constant diameter,
which is fed by a
reservoir at constant
pressure, and is
connected to a
downstream valve
that is suddenly
closed.
Fluid Mechanics with Engineering
applications by Joseph B Franzini & 20XX 4
E.John Finnemore
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At 𝒕 = 𝟎, the pressure head is steady down the length of
the pipe, as shown by the constant hydraulic grade line
(shown in red), because friction was neglected, and the
flow velocity is 𝑣0.
As soon as the valve is shut-in, the fluid element closest to
the valve comes to rest, and this rate of change of
momentum causes a rise in the pressure head by +∆𝐻 . As
subsequent fluid elements come to rest, the high pressure
propagates upstream from the valve towards the reservoir
like a pressure wave.
𝐿
At 𝑡 = , where L is the pipe length and a is the wave speed,
𝑎
the high-pressure wave reaches the reservoir as all the fluid
in the pipe comes to rest. However, this causes a pressure
discontinuity at the boundary with the constant pressure
reservoir.
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To achieve pressure equilibrium at the reservoir, a pressure
wave of magnitude −∆𝐻 is reflected towards the valve and the
direction of the flow velocity reverses towards the reservoir. This
2𝐿
reflected wave reaches the downstream valve at 𝑡 = . This
𝑎
time is called the reflection time (𝑇𝑟 )
2𝐿
At 𝑡 = , the flow velocity in the entire pipe is . This causes
𝑎
−𝑣0 another discontinuity at the downstream valve, where the
velocity must be zero.
The change in velocity from −𝑣0 to zero, cause a sudden negative
change in pressure of −∆𝐻 . This low-pressure wave travels
upward as the fluid in the pipe again comes to rest, reaching the
reservoir at 𝑡 = 1.5 𝑇𝑟 .
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At 𝑡 = 1.5 𝑇𝑟 , the fluid in the pipe is at rest but there is a
discontinuity at the constant pressure reservoir boundary.
As the pressure resumes the reservoir pressure, a wave of
increased pressure originating from the reservoir travels back to
the valve as the flow velocity in the pipe changes to 𝑣0
At 𝑡 = 2𝑇, the conditions in the system are the same as 1, and the
whole process starts over again.
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Application of
Water Hammer
Hydraulic Ram works
with the principle of
Water hammer
The US Navy is Leaks can
conducting field sometimes be
trials for mine
clearing using a detected using a
water hammer. water hammer
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Effect of Water
Hammer
• Damage to pipes, fittings, valves, or any connected
equipment causing leaks and shortening the life of
the system
• The bursting of pipes can occur if the pressure is
high enough
• Water hammer can have devastating effects on
pump system
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Effect of Water Hammer
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Water
hammer
Effect
In June, 1950, errors in operation and malfunctioning of equipment resulted in water-
hammer surges which burst one 9-ft-diam penstock of the three-unit 68,200-kw Oigawa
Power Station in Japan. The accident resulted in the death of three workmen and about
one half million dollars in damage to equipment and powerhouse.
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How to mitigate?
Celerity
velocity
𝜕𝑃 𝜕𝑣
=𝜌𝑎
𝜕𝑡 𝑑𝑡
Change in time
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Decreasing the Velocity
𝑄 𝑄 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡
𝑑 Q
𝑣=
A
𝑄
𝐷
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Decreasing the
time
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Decreasing wave serenity
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Devices for Protection from Water Hammer
S oft Starters
Slow closing valves
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Variable Frequency Devices
Variable Frequency Devices
Water Hammer Arrestors
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Pressure Release Valves
Surge Tanks
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Thank you
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