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Additional information about Francis Turbines
As shown in the drawing, the Francis turbine mainly consists of:
spiral casing with fixed vanes shields on outlet side (7) and high pressure side (12) wicket gate with mobile guide blades (14) and controlling mechanism shaft seal system (11) draft tube (8) runner (1) (3) (5)
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It is possible to vary the power produced by the machine by operating exclusively with the degree of opening of the wicket gate, which, in this machine, consists of a series of mobile blades (13), rotating on their own axis. The blades are moved simultaneously by a system of connecting rods (15) and levers (16) outside the turbine shell, so that it is easy to reach them for maintenance.
The spiral casing (17) may be either circular or rectangular, depending on the working pressure. It is made of electro-welded high tensile carbon steel plating. The casing undergoes a stress relieving heat treatment after welding and is then sandblasted until the metal is almost white to SA 2.5 degree. Then, one coat of inorganic zincate, or the like, is applied by hand for a thickness of about 75 microns. Two coats of epoxy or acrylic paint, resistant to abrasion and without toxic substances, with an average thickness of about 200 microns are applied, before mechanical processing. The spiral casing is always supplied with inspection holes for removing any foreign bodies which may be caught up by the fixed blades of the pre-distributor if they get into the machine.
Anchoring feet (18) are also part of the spiral casing, placed in such a way so that the machine can either be set up with vertical centre line or with a horizontal centre line. A housing for the generator or a speed gear support, if necessary, is machined on the upper part of the spiral casing, in small to medium sized machines with vertical centre lines.
This scheme makes it possible to start off assembly in the workshop, cutting down time and avoiding problems at the erection site. Beyond this, the machine is guaranteed to keep its alignment as time passes even if the civil engineering structure should start sagging.
The draft tube (8) is also made of electro-welded plating and may simply have a cone or elbow shape.
It can be seen in the figure, that the wicket gate blades (13) are housed between the shields on the outlet side (7) and the sealing side (12). The blades, normally made of stainless steel, rotate on standard bushings made of anti-friction material. The blades are moved by a connecting rod (15) mechanism fixed to the blade shaft by a cone clutch system. This is necessary for ensuring that the system will work even when one of the blades are blocked by foreign bodies working their way into the machine. A monolithic or two half connecting ring (16) is used to synchronise the blade movement and is designed to make it easy to adjust each single blade without having to dismantle the whole system.
Standard series of mecha-nical seals are used for making the motor shaft watertight (11). A wear ring in two halves may be supplied with the seals for easier maintenance.
The runner, heart of the turbine, is made of CrNi 13/4 or equivalent stainless steel with first class mechanical and precision characteristics, normally single-cast or, depending on the size, made by welding the blades (3) to the hub (1) or to the outside crown (5). The correct form of the profiles, computer designed with 3D analysis, is guaranteed by ceramic moulds and accurate dimensional controls both on the models and on the finished piece. After casting or welding, the wheel undergoes an accurate quality control and then the hydraulic profiles are processed to a degree of roughness below 0,8 microns.
The runner undergoes dynamic balancing in compliance with ISO 1940/1 with G 6.3 degree, under working conditions after all the work processes have been completed.
If the runner is not the double outlet type (see figure FDR) it will cause axial thrust whilst working under normal conditions. These will have to be supported by axial bearings, scaled to work also during transitional phases of start-up and runaway.
If the turbine is designed to be connected up directly to the generator, this work is done by generator bearings, the right size, 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.
For further information about this product, feel free to contact us.
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