Vane
The vanes of a centrifugal pump are either permanently fixed to or fitted in the impeller or diffuser so as to allow adjustment. They represent the most important structural element of a pump for the conversion of mechanical power (see Power input) into pump power output (also see Internal and hydraulic efficiency) or the conversion of velocity energy into pressure energy.
The vanes are confined in the direction of flow by the leading edge (inlet edge) and the trailing edge (outlet edge), and, at right angles to the direction of flow, by the hub on the inside (in the case of axial and mixed flow Impellers and diffusers) or by the rear (inner or hub) shroud (in the case of radial and mixed flow impellers), and by the pump casing, or by the outer (suction or front) shroud (in the case of closed impellers).
A vane is called adjustable if during pump assembly it can be inserted, have its pitch angle adjusted and then be fixed in position.
If the vane's pitch angle can be altered while the pump is running, then this vane is called a variable pitch vane (see Impeller blade pitch control). The external shape of the vane is usually given as a circular projection in the meridian section (longitudinal section along the pump's rotation axis).
Various types of impeller vane shapes are used in centrifugal pump engineering. They include the conventional axial, mixed flow and radial vane forms; however, a differentiation between different flow directions (i.e. from the inside to the outside or vice versa) is not made. Mixed flow tubular casing pumps (see Mixed flow pumps) for example, often incorporate onion-type diffusers with mixed flow diffuser vanes traversed by the flow from the inside to the outside at the diffuser entry and from the outside to the inside at the diffuser exit.
As no normal components of the relative velocity can occur perpendicularly to the vane on the impeller (or, in the case of a diffuser, no normal components of the absolute velocity the vane surfaces represent flow areas consisting of stream lines infinitely close to each other.
With regard to the hydrodynamic flow deflection (see Turbomachinery) the effective shape of a vane can only be determined along a stream line. However, this is often complicated because determining the precise stream line pattern in the impeller and diffuser requires a great deal of effort and is only possible if certain assumptions are made (see CFD).
The velocity triangles on a stream line at the vane inlet and the vane outlet essentially determine the vane's shape, taking into account the vane's thickness and possible vane cascade reactions. The vane centreline section (with half the vane thickness) between vane inlet and vane outlet is referred to as the median line (see Flow profile). It is frequently created by means of a circular arc (e.g. in the case of a circular arc vane), sometimes by means of a parabolic arc, an S-shaped curve or another analytical curve.
As a general rule, the vane inlet is designed to provide shock-free entry and ensure a vortex-free approach flow (see Vortex flow). Ther inducer represents a well-known exception.
The (impeller) vane outlet angle (β2) is more or less steep, depending among other factors on the head to be achieved (see Flow profile). On radial impeller vanes, it is usually less than 90° (from 17° to 40° approximately). In this case the vane is called a "backward curved" vane. A "radial end" vane is characterised by a vane angle of 90° and a "forward curved" vane (with extremely high pressure coefficients)by an angle larger than 90°.
The vane thickness is mainly governed by the centrifugal force stresses and the manufacturing method. In the case of profiled propeller vanes, the thickness distribution along the median line in accordance with hydrodynamic factors plays a major role.
The minimum vane thickness is approx. 3 mm for cast iron, 4 mm for cast steel, and in special cases (e.g. inserted or welded-on sheet steel vanes) it is possible to produce even thinner vanes.
