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 created due to sudden change in momentum due to sudden stopping a moving fluid or due to a sudden change in flow direction. It produces a knocking sound when a valve is suddenly closed. When a flow is stopped suddenly, backward flow takes place with a very high pressure and this can cause a big harm to the equipment (pipe, valve, pump etc.).Since this change of momentum is sudden (very less time involved) there will be a momentum pulse giving rise to a very large force and hence a backwards very high pressure wave. It can burst pipes or pipe fittings. It may be due an obstruction, bend or valve or sudden closure of pump in the pipeline having liquid flow at high velocity. This pressure wave will move backward and forward. 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