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Additional information about Kaplan Turbines
As shown schematically in the drawing, the Kaplan turbine essentially consists of:
Kaplan runners are characterised by the possibility of being able to vary the blade inclination with consequent variation of the developed capacity; for this reason these machines can be built either with a mobile blade system or with a simple row of fixed guide blades. This machine gives a first class performance in both cases, even with partial capacities and water heads, even though it works more constantly with the double adjust-ment version. It is possible to vary the power produced by the machine by operating exclusively with the degree of opening of the runner blades or, in double regulating version, by opening at the same time the runner blades and wicket gate blades. The wicket gate blades are moved contemporarily by a system of connecting rods and levers (2) outside the turbine shell so that it is easy to reach them for maintenance purposes. This kind of machine is therefore particularly suitable for utilising in plants where the flow capacities vary even at elevated percentages, and also in installations with considerable changes in water heads, keeping up a highly efficient and reliable performance for the whole field of tolerated operations.
The distributor is made of high tensile carbon steel plating and the tubular kind consists of two concentric cones con-nected by a first series of 8 blades (6) deviating the flow, which make up the pre-distributor, and by a second series of 16 blades (2a-2b) in a row which make up the distributor. With double adjustment turbines, the distributor blades can rotate around their axis. Their move-ment is co-ordinated by a system of connecting rods and levers outside the turbine stator, easy to reach for maintenance works.
The draft tube (3) is made of steel plating and is sufficiently long to connect up with the end part, normally in plain concrete. The shape and length of the outlet pipeline plays a decisive role in the performance of the machine, particularly in the case of Kaplan turbines, as in some cases more than 50% of the total power may be developed inside this component.
Standard series of mechanical seals are used for making the main shaft watertight. A wear ring in two halves may be supplied with the seal for easier maintenance.
All structural components undergo a stress relieving heat treatment, and are then sandblasted until the metal is almost white to SA 2.5 degree then finished with one coat of inorganic zincate, or equivalent, by hand with about 75 micron thickness. Two coats of epoxy or acrylic paint, resistant to abrasion and without noxious sub-stances, with an average thickness of about 200 microns, are applied before the mechanical processing.
The wheel consists of a partly spherical shaped hub(5), holding the hydrodynamically profiled blades(1). The latter, from a minimum of 3 to a maximum of 6, can rotate around their axis, so that they vary the pitch of the propeller. The movement is kinematic connected to a hydraulic cylinder, normally located outside the turbine.
The blades are made by casting, normally in CrNi 13/4 type, or equivalent, high tensile stainless steel. The correct shape of the profiles, designed by computer with tridimensional analysis, is guaranteed by ceramic moulds and accurate dimensional controls both on the models and on the finished piece. After casting, the blades undergo an accurate quality control and then the hydraulic profiles are processed to a degree of roughness below 0,8 microns. All the wheel components are as-sembled and balanced, after completing all the necessary working processes.
The wheel shell is made to form a spherical housing so that the wheel can rotate freely inside it in any position of the wheel blades, thus avoiding excessive gaps which would considerably affect efficiency. The shell is normally made in two halves, united by flanges, to facilitate maintenance works on the wheel.
A water lubricated radial bearing (7) is located inside turbine casing, while axial thrust has to be supported by axial bearings, scaled to work also during transitional phases of commissioning or if it is out of turn. If the turbine is designed to be connected up directly to the generator, this work is done by generator or speed gear bearings, otherwise the turbine is supplied with its own shaft housing consisting of radial and axial bearings, scaled for a working life of more than 100,000 hours.
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