(06 Marks) Dec-2018

Subject Fluid Mechanics 2

Topic Pipe Network and water hammer

Difficulty Medium

• Consider a long pipe AB connected at one end to a tank containing water to a height of H from the centre of pipe. At the outer end of the pipe, a valve to regulate the flow of water is provided.
  • When the valve is completely open, the water is flowing with a velocity, V in the pipe.
  • If now the valve is suddenly closed, then the momentum of flowing water get disturbed due to which a wave of high pressure will be set up.
  • This wave of high pressure will be transmitted along the pipe with a velocity equal to the velocity of sound wave and may create noise called as Knocking,
  • Also this wave of high pressure has the effect of hammering action on the walls of the pipe, hence it is also knowns as water hammer.

Control measures:

• Reduce the pressure of the water supply to the building by fitting a regulator.

• Lower fluid velocities. To keep water hammer low, pipe-sizing charts for some applications recommend flow velocity at or below 1.5 m/s (4.9 ft/s)

• Fit slowly closing valves. Toilet fill valves are available in a quiet fill type that closes quietly.

• High pipeline pressure rating (expensive).

• Good pipeline control (start-up and shut-down procedures).

• Shorter branch pipe lengths.

• Shorter lengths of straight pipe, i.e. add elbows, expansion loops. Water hammer is related to the speed of sound in the fluid, and elbows reduce the influences of pressure waves.