Volutes & Diffusers
Most commercial pumps have diverging (Figure A) channels called volutes cast into the discharge portion of the casing. Liquid leaving the impeller at a high velocity enters the volute and has its velocity reduced causing a pressure rise. (Volutes & Diffusers)
Single volute designs are used in most pump designs.
The single volute design imposes a radial force on the impeller due to variations of pressure around the impeller periphery above and below best efficiency point flow.
In cases where this radial force results in excessive shaft deflection, a double volute design (Figure B) is used.
The twin channels tend to balance the radial forces at all flow rates.
Some process pump designs employ a vaned diffuser rather than volutes. (Figure C)
These include some in-line, some axial flow pumps, and some pumps designed for high head low flow applications.
The advantages are balanced radial force, higher efficiency, and, compact size.
Disadvantages are they are higher cost and more complicated to manufacture and repair and the stage performance curve is more likely to droop.
Efficiency also tends to drop more severely at flows below 50% of best efficiency point flow.