Sunday, April 24, 2016

Pelton turbine

Pelton Hydro Turbine  for high water head Project

The Pelton turbine is widely used in hydropower station with the heads from 80-1000m.
Pelton turbine is consisted of runner, turbine case, nozzles assembly, inlet pipe, deflector etc. The runner is assembled in the shaft of the generator, torque transmission by ping.
lt utilizes the dynamic power of high pressure water flow to rotate the runner and force the rotor work, The centre line of the jet is in the same plane as the runner while rotating.
Pelton turbine has the advantages of compact construction, stable running and easy operation. Normally, Small pelton turbine always arranged with horizontal shaft and one or two nozzles,   Medium size and larger Pelton turbines are most often arranged with vertical shaft, and equip four or six nozzles to obtain more power.

1. Runner

The runner is the principle part of the turbine. It has influence over the characteristic of the turbine. The runner consists of vanes, the hub and the outer ring of runner. The shape of vane is three dimension cambers. The outer ring applies to strengthen the runner and to reduce the loss of wind.

The runner disc and buckets are CNC machined from and integrally forged 13%Cr 4% Ni steel, the faces of buckets which are in contact with water must be CNC machined and ground to a smooth surface with a maximum surface roughness of 0.8 Microns. The buckets  must be controlled by a coordinate measuring machine via a certified measuring company and must be submitted for approval. The center line allowance between the buckets is 0.57mm

Additional tests for material must be completed according to specification for Inspection of steel castings for hydraulic machines CCH 70.3. A detailed chemical analysis, Ultrasonic tests UT70.3, magnetic particle tests MT 70.3, and dye penetrant tests PT70.3 must be performed via a certified company and results should be submitted for approval.

Runner must be statically balanced according to ISO 1940/1 class G6.3 and G2.5.

2. The case and the cover of the case

The case used is a fine gradewelding steel with a minimum yield strength of 550 Mpa (St52 or equivalent), it has enough intensity and is convenient for maintenance. There is trachea on the case, which should be connected to the ventilating pipes from the tailrace channel and shall not use air from the power house. it is for complementing the air the jet takes, avoid vacuum in the house, keep the circulation, and there is a gate in the unit for overhauling, between up cover and down seat there is o-washer for overhauling.

3. Jet deflectors
Deflectors be manufactured from a monolithic stainless steel. Deflectors be
designed to withstand maximum discharge from the nozzle without vibration. The deflectors
be controlled by a double acting servomotor located outside the turbine casing.

4. Nozzle

The nozzle is components of the distributor of the pelton turbine.The intake pipe conduits pressure water flow to the nozzle via the guide support which is used to support the needle stem to provide against a vortex. In practice the nozzle is just a needle valve; however the valve plays a role of shut off and of transform energy in impulse turbine. The pressure water flow enters the nozzle body. Then the velocity of water flow shall be increased rapidly at the nozzle tip and the water flow is formed a jet to inject to the vanes of runner.
 So pressure energy of the flow changes to velocity energy and the runner does a work. The opening of nozzle shall be adjusted by nozzle control mechanism which is consisted distributing valve, the servomotor of needle and by combination mechanism. The operation oil pressure is 6.3Mpa, it used internal control structure, the opening of the needle is 75%~85% on the full-load. The server-motor piston and the piston rod is made of 35 steel welding.

Allowance of the position of the jet center line and the rotor pitch circle is ±1.2mm. Allowance of the position of the jet center line and the pelton watershed edge is ±1.2mm as per the IEC standards.

5. Needle control mechanism

The needle control mechanism includes a jet deflector; the jet deflector is also called deviator. It is used to deflect the jet on a sudden load rejection in order to avoid run away of the unit and pressure over rise for excessive water hammer in the penstock.

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