8.4 Component Shaping
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one more than the number of rotor blades. When operating in off-design conditions,
the flow angle at the inlet of the diffuser differs from the vane angle, with flow separation as a possible consequence. At a high flow rate the fluid must strongly accelerate in order to pass through the throat area B. So, a pressure drop occurs upstream
of position B. Its magnitude may exceed the pressure rise downstream of B. So, it
may occur that, from a certain flow rate onward, higher than the design flow rate,
the vaned diffuser generates a pressure drop instead of a pressure rise.
8.4.7 Volute
Figure 8.15(left) sketches a volute fitting immediately to the rotor. There is no diffuser ring in between the rotor outlet and the volute inlet. This type is frequently
used, mainly with smaller pumps. Velocity reduction is realised in a post-connected
linear diffuser. If an internal diffuser ring is mounted, the following discussion stays
valid, however.
A volute collects the liquid leaving the rotor and conveys it to the discharge pipe.
For the design flow rate, the volute imposes a uniform backpressure to the rotor. At
a flow rate below the design value, deceleration occurs within the volute and at a
Fig. 8.15 Single volute ( left) and double volute ( right) for attenuation of non-uniformity in offdesign conditions
Fig. 8.14 Vaned diffuser ring
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one more than the number of rotor blades. When operating in off-design conditions,
the flow angle at the inlet of the diffuser differs from the vane angle, with flow separation as a possible consequence. At a high flow rate the fluid must strongly accelerate in order to pass through the throat area B. So, a pressure drop occurs upstream
of position B. Its magnitude may exceed the pressure rise downstream of B. So, it
may occur that, from a certain flow rate onward, higher than the design flow rate,
the vaned diffuser generates a pressure drop instead of a pressure rise.
8.4.7 Volute
Figure 8.15(left) sketches a volute fitting immediately to the rotor. There is no diffuser ring in between the rotor outlet and the volute inlet. This type is frequently
used, mainly with smaller pumps. Velocity reduction is realised in a post-connected
linear diffuser. If an internal diffuser ring is mounted, the following discussion stays
valid, however.
A volute collects the liquid leaving the rotor and conveys it to the discharge pipe.
For the design flow rate, the volute imposes a uniform backpressure to the rotor. At
a flow rate below the design value, deceleration occurs within the volute and at a
Fig. 8.15 Single volute ( left) and double volute ( right) for attenuation of non-uniformity in offdesign conditions
Fig. 8.14 Vaned diffuser ring