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A Brief Description of Metal Spiral Case

Clicks:576  time:2018/12/29 11:39:15
The angle enclosed by the volute from the nose end to the entrance section is called the envelope angle of the volute. The metal spiral case is usually used in high head hydraulic turbines. According to its manufacturing method, there are three types of metal spiral case: welding, casting and welding and casting. The structure type of metal spiral case is closely related to the head and size of the turbine. Cast-welded and cast volutes are generally used for high head Francis turbines with diameter D1 < 3m. In order to save steel, the section of metal volute is circular, and the thickness of steel plate should be different according to the stress of the section. Usually, the thickness of the section at the volute inlet is larger, and the closer to the nose, the smaller the thickness.
In addition to the thin-walled stress caused by internal water pressure, there are also local stresses caused by different stiffness at the connection between the spiral case and the seat ring and the connection of steel plates with different thickness in the same axial section.
The strength of the volute must be calculated according to the internal water pressure. It is assumed that all the internal water pressure of the volute is borne by the volute itself, so as to determine the thickness of the steel plate of the volute and ensure its normal operation. In addition to the thin-walled stress, the spiral case can be considered to be rigidly connected to the seat ring due to the large stiffness and small deformation of the saucer-shaped edges of the seat ring (the outer edges of the upper and lower rings of the seat ring). This connection produces additional local stress in the spiral case steel plate. In addition, the stiffness varies with the thickness of the steel plate at the joints of different thicknesses of the same axial section, so additional local stresses will also occur at the joints, which is similar to that at the joints of volutes and seat rings. The strength calculation of this part can be done with reference to relevant data.
Steel plate welded structures are generally used in large-sized medium and low-head Francis turbines. The spiral case and the seat ring are also joined by welding. The number of welded spiral case should not be too small, otherwise the hydraulic performance of spiral case will be affected. However, it is difficult and uneconomical to use too many sections to make the spiral case as smooth as possible and improve its hydraulic performance.
The casting volute has a high stiffness and can withstand a certain external pressure. It is often used as the supporting point of the turbine and directly arranges the water guide mechanism and its transmission device on it. Cast spiral case is usually not all embedded in concrete. Different materials can be used to cast spiral case according to different water head. Small units with water head less than 120 m generally use cast iron; when water head is more than 120 m, cast steel is often used; when water head is high and there are more solid particles in water, stainless steel can also be used to cast spiral case.
The cast-welded volute is as suitable as the cast volute for the high head Francis turbine with small size. The shell of the cast-welded spiral case is pressed by steel plate. The fixed guide vanes and seat rings are usually cast and then joined together by welding. Necessary heat treatment is needed to eliminate welding stress after welding.
Concrete spiral case
Usually used in large and medium-sized low-head power plants with water head less than 40 meters, it is actually a spiral cavity made directly in the underwater part of the large volume concrete of the powerhouse. When the underwater part of the factory building is poured, the worm-shaped formwork is pre-installed, and the formwork is removed to form the worm case. In order to strengthen the strength of the spiral case, many steel bars are added to the concrete, so it is sometimes called reinforced concrete spiral case.
 
 
   

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