WATER HAMMER CLASS NOTES
FOR MECHANICAL ENGINEERING
Water hammer is also called a fluid
hammer or Hydraulic Shock’ or a water
blow. It largely takes place in liquids but
to a limited extent it can also be present in
gases. It is a very high pressure wave.
Water hammer is created due to sudden
change in momentum. Change of
momentum is due to sudden stop of a
moving fluid. This causes a very high
pressure wave. This pressure wave will
move backward and forward. High
pressure wave causes a big harm to the
equipment. It can cause bursting of pipes
or pipe fittings. It may be due an
obstruction such as a bend or a valve.
Sudden closure of valve can cause it
when liquid flowing at a high velocity. It is
a system as well as a safety issue.
GENERATION OF WATER HAMMER
Usually this occurs when a valve such as a tap or faucet closes quickly. Water hammer can generate very high pressure wave as explained below:
F = m (dv/dt)
In this dv is large and dt is small and hence the force is greatly magnified i.e. almost infinite value of force. But there is some safety the valve closure can be abrupt it may take some small time and hence it may not be so sudden for a valve closure. But it can be sudden from pumps point of view that the pump is being off may be due to disruption of power. The elasticity modulus of pipe as well as for fluid can play a significant role in managing the water hammer effect.
DIFFERENT POSSIBILITIES OF WATER HAMMER
It can be due to the following reasons:
Gradual closure of valve
Sudden closure of valve in a rigid pipe
Sudden closure of valve in an elastic pipe
FACTORS ON WHICH WATER HAMMER DEPENDS
Magnitude of flow velocity in the pipe
Time interval in closing the valve
length of the pipe
Pipe material elastic modulus
Presence of very High water pressure
Poor anchoring of pipes with the structure
Absence of any dampening system(springs) to absorb shock waves
EFFECTS OF WATER HAMMER
(i) If this wave reaches the velocity of sound, there will be a high knocking sound.
(ii) A very high pressure wave may burst the pipe line, pipe fittings, a valve or the pump.
THEORY OF WATER HAMMER
Let L be the length of pipe. Total length to and fro will be = 2L
C is the velocity of pressure wave normally called as CELERITY, in
(m/s) and is given as C = E/ρwater
Then time taken to travel pressure wave back and forth
t = 2L/C,
Now let the time to close the valve be tv
CASE 1: If tv > 2L/C then closing of valve is gradual and there is absolutely no chance of a water hammer.
CASE II: If tv < 2L/C then closure of the valve is sudden. There are prime chances of a water hammer being present.
The velocity of pressure C is given by
C = (E/ρwater)
Therefore Velocity C depends upon
Elasticity modulus of pipe material
Density of liquid
Method of fixing of pipe
Therefore as far as possible the time taken to close valve should be > 2L/C
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