A volute is a curved funnel that increases in area as it approaches the discharge port.[1] The volute of a centrifugal pump is the casing that receives the fluid being pumped by the impeller, maintaining the velocity of the fluid through to the diffuser. As liquid exits the impeller it has high kinetic energy and the volute directs this flow through to the discharge. As the fluid travels along the volute it is joined by more and more fluid exiting the impeller but, as the cross sectional area of the volute increases, the velocity is maintained if the pump is running close to the design point. If the pump has a low flow rate then the velocity will decrease across the volute leading to a pressure rise causing a cross thrust across the impeller that we see as vibration. If the pump flow is higher than design the velocity will increase across the volute and the pressure will decrease according to the first law of thermodynamics. This will cause a side thrust in the opposite direction to that caused by low flow but the result is the same – vibration with resultant short bearing and seal life.
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The volute does not convert kinetic energy into pressure – that is done at the diffuser by reducing liquid velocity while increasing pressure.[citation needed]
The name "volute" is inspired by the resemblance of this kind of casing to the scroll-like part near the top of an ionic order column in classical architecture, called a volute.
In a split volute or double volute pump, the path along the volute is partitioned, providing two distinct discharge paths. The streams start out 180 degrees from each other, and merge by the time they reach the discharge port. This arrangement helps to balance the radial force on the bearings.[2]
The volute casing pump is the most common type of centrifugal pump. Its characteristic feature is the volute-shaped pump casing which is typical for single-stage centrifugal pumps.
There are designs whose main dimensions are standardised in accordance with EN 733, ISO and ISO , but this standardisation still leaves ample room for many design variants. See Figs. 1 to 6, Volute casing pump
The volute casing pump is usually a single-stage design, but sometimes incorporates two stages (see Multistage pump).
See Fig. 2 Pipeline pump
Both single-suction and double-suction variants are common (see Double-suction pump). Impellers can be of either radial or mixed flow type, as defined by the specific speeds of approx. 12 to 70 rpm (in individual cases up to 100 rpm and higher).
The axial thrust can be balanced by
The pump casing can be
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A volute casing pump can be avertical or horizontal pump and the pump shaft can be supported by rolling element bearings or plain bearings on one side only or on either side of the impeller.
On a horizontal volute casing with an overhung impeller, the bearing unit is integrated:
The bearing bracket is designed for a pump in back pull-out design. The support bracket has the advantage of being able to directly transmit the radial and axial forces from the impeller via the shaft to thepump foundation and enables smaller baseplates to be used.
The pump discharge nozzle can be arranged either tangentially on the volute casing or radially in the shaft plane through the provision of a "gooseneck". It can be arranged at the top, bottom or sides of the volute casing, providing it does not interfere with the pump feet on the casing.
Thepump suction nozzle of volute casing pumps with an overhung impeller (cantilever pumps) is frequently arranged axially, and either radially or tangentially in the case of in-line pumps and volute casing pumps in between-bearings design.
Instead of radial or mixed flow impellers, special impellers such as single-channel or multi-channel impellers can be fitted if required. A diffuser is sometimes fitted between the impeller and volute of large volute casing pumps to improve the pump efficiency and balance the radial thrust.
A double volute, i.e. two axially symmetrical volutes that are offset by 180° but normally have a common discharge nozzle, also serves to balance the radial thrust.
See Fig. 6 Volute casing pump
Depending on the fluid handled or the degree of maintenance required, a variety of different shaft seals can be fitted. It is also possible to heat or cool the volute casing pump, e.g. when used for chemical processes. See Fig. 12 Pump casing
Volute casing pumps are designated according to their
The term volute casing pump can therefore only express one of several aspects.
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