The turbine components shall comprise of spiral case, runner, turbine shaft, guide vanes, stay vanes, draft tube bend, links and levers for guide vane operation, servomotor etc. The turbine components dimensions and weights worked out by the computation programs, however, these dimensions may vary slightly depending upon the selected manufacturer.
Nevertheless, due to these small variations, the powerhouse structures shall not be affected.
Performance of turbine runner, in all hydraulic turbines, has great importance because of silty water and high sediments flow during flood season. Their design is subjected to special site conditions, available repair facilities, and water quality of the stream. The runner shall be of stainless steel or shall be high strength steel, or bronze casting and plate shall be 13:4 chrome nickel alloy steel. All field and shop weld shall be stress relieved. The runner shall be made of a material, which meets virtually any kind of silty floodwater. Special packing shall be adopted for water sealing on the main shaft for water sealing on the turbine shaft to minimize water flow along the main shaft. Packing can easily be replaced merely by removing the water packing cover. The packing ensuring a high water sealing effect shall be provided with a stainless steel sleeve for shaft protection. The sleeve which is split into halves can be readily replaceable. A spear or deflector moving inside the nozzle shall achieve flow control. Closing the spear rapidly can cause damage to penstock resulting due to water hammer. This shall be prevented by spear closing time in the range of 30 to 40 sec. Deflector arrangement shall also be to direct the water away from the turbine buckets when there is load rejection to reduce the speed rise on generator.
Nozzles that are equipped to distributing pipes shall be comprised of nozzle pipe and nozzle tip. Nozzle tip shall be made of cast steel and shall be combined as one with a deflector bearing support. The nozzle tip liner that is contacted with needles shall be made of stain less steel and shall be replaceable.
Needles are an adjusting system for Pelton turbine’s discharge, as the guide vane for a reaction turbine. The opening closing motion of the needles shall be operated by oil pressure and at the time of oil pressure failures self-closing construction with the help of water pressure and spring force. Needle tip, which contracts with needle, shall also be made of stainless steel and replaceable.
Normally two types of needle servomotors are applied. Internal needle servomotor is placed inside the nozzle and the external servomotor is placed at out side of distributing pipe. Both servomotor arrangements have merits and demerits and meet the site-specific requirements. The construction of internal needle servomotor is complicated as the servomotors are placed inside the nozzle and it is necessary to take sufficient measures for preventing oil leakage, because the oil pressure regulation mechanism is placed under the water in the case of internal servomotor. For external servomotor, the same is placed outside a distributing pipe. External needle servomotor requires larger space in order to remove a needle rod from the servomotor but has some advantages of no oil leakage to the water and easy checking and maintenance as the oil pressure regulating mechanism is placed outside the nozzle. On the other hand internal needle servomotor required reduced dimensions of powerhouse building but for oil leakage problem and maintenance problem. Based on these at this stage of study external needle servomotor is recommended to avoid oil leakage and future maintenance problem.
The purpose of the deflectors is to rapidly reduce the amount of water striking the Pelton runner so that in the event of a trip condition, the turbine will shut down with minimum over speeding. The deflectors shall be keyed to a cross shafts which will be mounted on anti friction bearings bolted to the outside of the turbine casing, V ring seals shall be provided to retain the water within the casing.
A hydraulic ram operated by a poppet directional valve shall move the deflector out of the jet for normal operation. Drop weight fitted to the shaft of the deflectors shall be arranged in such a way that when the direction control valve is de-energized the deflectors shall move into the jets. Deflector is comprised of deflector tip that directly hits a jet and of deflector arm. Deflectors shall be made of stainless steel or cast steel or overlaid on cast steel and be easily replaceable.
Distributing pipe is the pipe, which leads the water flow from penstock to the nozzles. These shall be made of steel or cast steel and fulfill as applicable international standards.
Turbine housing has two main functions, one is to support the deflector shafts to its upper and lower parts and the other is to guide the discharge water from the runner to tailrace effectively without interference of water flow. The housing will be fully welded fabrication and split horizontally by a flanged joint. This specific feature will facilitate removal of the runner and allow access to other internal mechanisms. For the horizontal shaft turbine, the concrete protection is purposely eliminated for easy removal of runner, there fore sufficient rigidity is necessary against the expected vibration and the noise.
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