Water Hammer Effect and Cavitation in Hydro Electric Power Station

Water Hammer Effect and Cavitation in Hydro Electric Power Station

Q. Explain the water hammer effect and cavitation in hydro-electric power station and State the arrangement to reduce these.

Water Hammer Effect and Cavitation in Hydro-electric Power Station

Water hammer effect : Any change of the water flow velocity in a pipeline causes pressure fluctuation. This is called water hammer or surge. As the velocity change is larger and faster, so are the pressure changes. Fast stoppage of high velocity flow may cause dangerous high/low pressure oscillating waves, exceeding the safe operating limits of the pipeline, accessories, pump castings etc. Sub-atmospheric pressure, common phenomena in the pressure oscillation, may damage the pipeline by cavitation and collapsing of the pipe due to the external pressure.

Prevention of water hammer :

  1. By slowing down the flow variations in the network.
  2. By relieving returning flow, after pump’s shutdown, out of the pipe.
  3. By preventing fast filling of empty networks.
  4. By admitting air or water into the pipe, when sub-atmospheric pressure is expected.

Cavitation : Cavitation is the formation of vapour bubbles in the liquid flowing through any hydraulic turbine. It occurs when the pressure of the liquid falls below its vapour pressure. Cavitation is most likely to occur near the fast moving blades of the turbine and in the exit region of the turbines.

Avoiding cavitation : To avoid cavitation while operating hydraulic turbines, parameters should be set such that at any point of flow, static pressure may not fall below the vapour pressure of the liquid. The control parameters for cavitation free operation of hydraulic turbines can be obtained by conducting tests on model of the turbine under consideration. The parameters beyond which cavitation starts and turbine efficiency falls significantly should be avoided while operation of hydraulic turbines. Flow separation at the ext of the turbine in the draft tube causes vibrations which can damage the draft tube. To dampen the vibration and stabilize the flow, air is injected in the draft tube. To totally avoid the flow separation and cavitation in the draft tube, it is submerged below the level of the water in the tail race.

Leave a Comment