WATER HAMMER CLASS NOTES FOR MECHANICAL ENGINEERING

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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:

  1. Gradual closure of valve

  2. Sudden closure of valve in a rigid pipe

  3. Sudden closure of valve in an elastic pipe

FACTORS ON WHICH WATER HAMMER DEPENDS

  1. Magnitude of flow velocity in the pipe

  2. Time interval in closing the valve

  3. length of the pipe

  4. Pipe material elastic modulus

  5. Presence of very High water pressure

  6. Poor anchoring of pipes with the structure

  7. 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 t< 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

https://www.mesubjects.net/wp-admin/post.php?post=7521&action=edit       MCQ water Hammer

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