Water Turbines Classification
Generally, the water turbine could be classified by Impulse type and Reaction type. Impulse type turbine (including Pelton, Turgo, Crossflow type) makes a strong jet stream by nozzle to drive turbine runner. Reaction type (including the Francis, and axial flow type) makes use of water pressure difference between front and reverse side of vanes. The pressure difference drives runner to apply work. Turbine plays a very important role in micro water power generator set. It should be chosen, installed, managed and maintained correctly.
Pelton type turbine consists of nozzle, runner, supporting parts and deflector
1). Runner: Runner of micro water turbine consists of turn plate and bucket vane. It is generally made by inblock cast or fixed by bolts and welding.
2). Nozzle: Nozzle consists of nozzle body and adjusting needle. Its main function is to make and shoot strong jet stream to water bucket from best direction. Adjusting needle is used to adjust flow rate. It has the function of water distributor. In general, only one nozzle is equipped for turbine, but when flow rate is enough, nozzle quantities can be increased.
Pelton type turbine is suitable for conditions of big water head and small flow rate.
Turgo turbine (also called inclined jet type turbine) consists of nozzle, runner, supporting part and base. The follows are the differences between Pelton type and inclined jet type turbine.
1). Nozzle and runner of Turgo turbine are not in the same plane, but come into a inclined jet angle. Jet stream impulses runner aslant. Water enters from one side of water bucket, and escape from another side. Inclined jet angle between nozzle and rotary plane of runner is generally 22.5°.
2). Runner consists of external runner ring, inner runner ring and bucket vane. External runner ring can strengthen runner and reduce wind resistance. Bucket vane has single-curved surface. Turgo turbine is simple in conformation, low in cost and high in efficiency. It is suitable for micro water power station with medium or high water head.
Crossflow type turbine
Crossflow type turbine consists of nozzle, runner and frame. Water from nozzle impacts runner vanes and flows to center of runner. Most hydraulic energy is transferred to mechanical energy in this stage. Then water flows to next stage and impacts runner again. Tail water outflows from tailrace. Water in crossflow type turbine impacts runner for two times. It is suitable for conditions of medium or low water head.
Francis type turbine
Francis type turbine is shown in the picture, Water enters runner radially, and leaves axially. Francis type turbine is divided into vertical type and horizontal type. In general, vertical type is often used in micro water power station. Francis type turbine is suitable for water power station with medium or low water head and flow rate. It consists of water guiding chamber, runner, water distributor and draft tube. Functions and conformations of these parts are introduced as follows.
1) Water guiding chamber: There are metal spiral case and metal tank two types of water guiding chamber are generally used. Its main function is to guarantee water enters water distributor evenly and symmetrically in order to reduce loss of water head. Spiral case is the best choice for turbine behavior. Advantages of metal tank type is simple in conformation and low in cost, but loss of water head is bigger than spiral case. So it is generally used in small turbine.
2) Runner: Runner is key part of turbine. Hydraulic energy is transferred to mechanical energy by runner. Runner of Francis turbine consists of vanes, runner hub and torus. Radial water flow is changed to axial flow by vanes. Quantity of vanes can be 10 to 24 pieces. Usual quantity is 14 to 17 pieces. Vanes, runner hub and torus can be made by inblock cast, or by separate cast and welding.
3) Water distributor: Water distributor has the following function.
Guide water enters runner from best direction. Adjust flow rate of water in runner to fit outside load change. For start or stop of generator set.
Axial Flow type turbine
Water enters runner of axial flow turbine axially, and leaves axially also. It is suitable for conditions of low water head and big flow rate. The same as Francis turbine, axial flow turbine also consists of water guiding chamber, runner, water distributor and draft tube. All parts are similar to those of Francis turbine except runner. In order to meet requests of big flux, there are only 3 to 8 vanes are equipped for runner of axial flow turbine. Vanes of small runner are generally fixed, but in order to improve efficiency and working conditions, sometimes adjustable vanes are equipped for turbine runner of some water power stations whose water head and flow rate vary seasonally.