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18 Cards in this Set
- Front
- Back
- 3rd side (hint)
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Water hammer |
A pressure surge or wave caused when a fluid in motion is forced to stop or change direction suddenly, a momentum change. |
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Causes of water hammer |
* Unstable working condition - Change of the opening of the guide vane, - Flow of the turbine, - Head of the hydropower station - Rotating speed of the unit due to the change of the load |
CFHR |
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Conditions causing turbine flow change |
* Load change under normal operation of hydropower station: * Load change caused by hydropower station accident |
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Manifestation(Effects) of unstable working conditions of hydropower station |
* Large change of unit speed * Water hammer occurs in the pressure diversion pipeline When the guide vane is closed, the pressure will rise in the penstock and volute and drop in the draft tube.When the guide vane is opened, it is opposite. * Water level fluctuation in non-pressure diversion system |
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Process of water hammer in penstock |
* Water compressed * Pressure increase * Velocity change to zero throughout * Penstock diameter expand * Direction from gate to reservoir |
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Process of water hammer in penstock |
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The harm of water hammer |
•Fracture of pipeline •Internal pressure instability •Cause the pipe to collapse and implode •Influence generating efficiency |
FICI |
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Classification of water hammer |
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The first phase water hammer |
During the 1st phase, the water hammer pressure increases linearly. When the hammer pressure get to the reservoir, it carries -∆H. It begins to propagate to the gate as +∆H.
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Methods to reduce water hammer pressure |
• Increasing the cross-sectional area, F of the conduit.• Increasing the gate closure time Ts.• Changing the guide vane closure mode• Reducing the length of conduit • Construction of surge tanks |
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Calculation of maximum water hammer pressure |
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Surge tank |
A surge tank (surge chamber) is a standpipe or storage reservoir at the downstream end of a closed aqueduct, feeder, dam, barrage pipe to absorb sudden rise of pressure, as well as to quickly provide extra water during a brief drop in pressure. |
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Functions of surge tank |
* It mitigates pressure variations due to rapid changes in velocity of water. * When the load decreases, the water moves backwards and gets stored in it * When the load increase, additional supply of water will be provided by surge tank. * Reduce the length of penstock |
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The determining conditions to construct surge tank |
* The simplest mean of eliminating positive (or negative) water-hammer pressure and improving speed-regulation conditions is to provide a bypass to take the rejected flow. * This may be accomplished by what are known as surge tank.
* Surge tanks are generally installed either at the lower end of a pressure diversion conduit to link the diversion conduit to penstock or at the lower end of long penstocks leading from a dam to an isolated power house.
* Surge tank may also be provided at the upper end of a long tailrace diversion system.
* Whether or not conduit requires a surge tank depends on its Tw.As s rule, the need for a surge tank arises when: |
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The principle of operation of a surge tank |
* When the turbines operate at constant velocity and a uniform discharge, the free surface level of the tank is by lower than the static headwater level, where h(div)= av2/2g lower than the static headwater level, wherehdiv is the head lost in the diversion system.
* When the wicket gates are closed and the turbine is unloaded, the flow through the penstocks comes to standstill, and the water arriving with the initial velocity is rejected into the surge tank, thereby raising its level due to the principle of inertia. * As this take place, the kinetic energy of the flowing water is converted into the potential energy of water stored above the normal surge-tank level. |
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Water hammer and fluctuation in surge tank |
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Why different power station have different cross-sectional area of penstock? |
* Small power house has big water head loss than big power house therefore they have different cross-sectional area of penstock
* Therefore the small power house should hace big cross-sectional area penstock to reduce the big water head |
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Classification of surge tank |
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