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Water Hammer and pulsations
Because liquid is essentially incompressible any energyapplied to it is transmitted instantly.
If a moving column of liquid is slowed down suddenly by,for example, a quick-closing valve, the sudden change in
liquid velocity in the delivery line creates a pressurewave.
The pressure wave, travelling somewhere between 1000an 1300 m/s, travels backwards up the line to the end of
the pipe where it will reverse direction and travel backtowards the valve.
Depending upon valve size and system conditions, avalve closing in 1.5 s or less can produce a pressure
spike five times the system working pressure leadingto blown diaphragms, seals and gaskets and alsocatastrophic system component failure in transmitters,meters and gauges.
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Water Hammer Stopped Flow
If a moving column of liquid is
slowed down suddenly by, for
example, a quick-closing valve, the
sudden change in liquid velocity inthe delivery line creates a pressure
wave.
The pressure wave, travellingsomewhere between 1000 an 1300
m/s, travels backwards up the line
enlarging the pipe.
Large diameter riser
Branch
Quick closure
Valve closed
Shock
Large diameter riser
Branch
Normal flow
Open valve
Flow
Pressure wave enlarges pipe
Because liquid is essentially
incompressible any energy applied
to it is transmitted instantly.
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Water Hammer Stopped Flow
At the end of the pipe it reverses
direction and travels back towards
the valve.
A valve closing in 1.5 s or less can
produce a pressure spike five times
the system working pressure leading to blown diaphragms, seals
and gaskets and also catastrophic
system component failure in
transmitters, meters and gauges.
Reflected pressure wave
Pressure wave reaches valve
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Water Hammer from condensate
Condensing steam Heat loss
Steam
Sub-cooled condensate Bernoulli effectdraws up wave
Accumulation of condensate is trapped in a portion of
horizontal steam piping.
The velocity of the steam flowing over the condensate
causes ripples in the water.Turbulence builds up until the Bernoulli effect draws up a
wave.
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Water Hammer and pulsations
A general rule-of-thumb is:
where:
P = increase in pressure (bar)
v = flow velocity (m/s)
t = valve closing time (s)
L = upstream pipe length (m)
PI= inlet pressure (bar)
IP
t
Lv052.0P
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Water Hammer and pulsations
Example: assume a solenoid valve having a closure time
of approximately 40 ms, connected to a 15 m long
upstream pipe.
The water flow is 3 m/s and the inlet delivery pressure is4 bar.
What is the amplitude of the pressure spike?
IPt
Lv052.0P
41040
153052.0
P 3
= 62.5 bar
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The wave seals the pipe producing an isolated
pocket of steam.
Water Hammer from condensate
Isolated steam pocket Heat loss
Steam
Sub-cooled condensate Wave seals pipe
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The collapsing steam void produces a slug of
condensate that is carried along by the steam flow.
Water Hammer from condensate
Collapsing steam void
5 bar Steam
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The resulting implosion produces a slug of condensate
that can travel at the speed of the steam (up to 160
km/hr)
This will strike the first elbow in its path with a forcecomparable to a hammer blow.
Water Hammer from condensate
Rebounding wave
Implosion
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Water Hammer from condensate
NPS 24 line displaced,
supports damaged
NPS 8 branch ripped
from header
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Water Hammer solutions
In the previous example where the solenoid valve
closure time was 40 ms, increasing the time would have
a dramatic effect.
What is the amplitude of the pressure spike if the closuretime is increased to 1 s?
IPt
Lv052.0P
45.1
153052.0
P
= 5.6 bar
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Water Hammer solutions
Where water hammer and pulsations cannot be reduced,
use could be made of a snubber (either a sintered filter
or small-bore restriction) inside the pressure connection.
Although very effective in absorbing high frequencyshock pressures, there is a trade off with the dynamic
measurement response.
Furthermore, over time the snubber could become
blocked from a build up of contaminant particles.
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Water Hammer solutions
Use can also be made of a pulsation dampener or surge
suppressor.
Typically a pulsation dampener is a hydro-pneumatic
dampener comprising a pressure vessel containing acompressed gas, generally air or nitrogen, separated
from the process liquid by a bladder or diaphragm.
The dampener is installed as close as possible to the
pump or quick closing valve and is charged to 85% of
the liquid line pressure.
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Water Hammer solutions
Air/gas
Liquid
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