A Volute casing is commonly referred to as a-shaped water intake chamber, abbreviated as a "snail shell". To ensure uniform water supply to the guiding water mechanism, the cross-section of the it gradually decreases. Additionally, it can create necessary swirl to reduce the workload of the guiding water mechanism.
It should be sized appropriately to minimize hydraulic losses and reduce the dimensions of the plant and civil engineering investments. It is a closed arrangement made of reinforced concrete or metal, adaptable to various head and capacity requirements. The snail shell is the most commonly used intake chamber in reaction turbines.
Classification:
Turbine shells can be divided into metal and concrete .
Metal:
The angle enclosed by the from the nose end to the inlet section is called the included angle of the snail shell. High-head turbines mostly use metal. Metal can be classified into welded, cast-welded, and cast types according to their manufacturing methods. The structural type of metal is closely related to the head and size of the turbine. Cast-welded and cast are generally used for high-head mixed-flow turbines with a diameter D1＜3m. The cross-section of metal is usually circular to save steel, and the thickness of steel plates should vary according to the different stress conditions of the it section. Generally, the thickness of the inlet section of the snail shell is larger, and it decreases towards the nose end.
The stress situation of metal is complex, involving not only thin-wall stress caused by internal water pressure but also local stresses caused by differences in stiffness at the connection between the snail shell and the seat ring and at the connection between steel plates of different thicknesses within the same axial section.
The snail shell must be strength-calculated based on internal water pressure, assuming that the internal water pressure of the snail shell is entirely borne by the snail shell itself, to determine the thickness of the snail shell steel plate to ensure its normal operation. In addition to thin-wall stress, due to the large stiffness and small deformation of the seat ring disc edges, the snail shell can be considered rigidly connected to the seat ring, which generates additional local stresses in the snail shell steel plate. Furthermore, at the connection of steel plates of different thicknesses within the same axial section, additional local stresses will also be generated due to the different thicknesses of the steel plates, similar to the situation at the connection between the snail shell and the seat ring. The strength calculation of this part can refer to relevant materials.
Large and medium-head mixed-flow turbines generally use steel plate welded structures. The connection between the snail shell and the seat ring is also welded. The number of sections welded for welded snail shells should not be too few, as it will affect the hydraulic performance of the snail shell. However, using too many sections to make the snail shell profile as smooth as possible and improve its hydraulic performance will bring difficulties and uneconomical aspects to manufacturing and installation.
Cast snail shells have larger stiffness and can withstand certain external pressure, often serving as the support points of turbines and directly arranging the guiding mechanism and its transmission devices on them. Cast snail shells are generally not completely buried in concrete. According to different application heads, different materials can be used for cast snail shells. Cast iron is generally used for small units with heads less than 120m; cast steel is mostly used when the head exceeds 120m; and stainless steel can also be used for cast snail shells when the head is very high and the water contains a large number of solid particles.
Cast-welded snail shells are suitable for high-head mixed-flow turbines with small dimensions, similar to cast snail shells. The outer shell of cast-welded snail shells is pressed from steel plates, and the fixed guide vanes and seat rings are generally cast and then welded to form an integral part. Necessary heat treatment is required after welding to eliminate welding stress.
Concrete Snail Shells:
They are generally used for large and medium-sized power stations with heads below 40m, mainly created as a spiral cavity directly within a large volume of concrete in the underwater part of the power station. When pouring the underwater part of the power station, the spiral template is pre-installed, and after removing the template, it becomes the snail shell. To strengthen the snail shell, a large number of rebars are added to the concrete, so it is sometimes also called reinforced concrete snail shells.