The Function of the Volute

Misunderstood pump element serves to minimize mixing loss

It is a common misconception in the U.S. pump industry that the function of the volute is that of a diffuser: to convert velocity into pressure. The McGraw-Hill scientific dictionary states that a volute is “a spiral casing for a centrifugal pump…designed so that speed will be converted to pressure.”


Function at Best Efficiency Point (BEP)

Figure 1. A single-volute casing maintains a constant velocity and uniform pressure around the impeller only at BEP.

Figure 2. Hydraulic radial thrust for volute casings.

It is understandable that such a concept has been adopted because the volute has an increasing flow area as it wraps around the impeller, similar to a diffuser, but it is not the purpose of the volute to be a diffuser. Its function—when the pump is operating at the best efficiency point (BEP)—is to keep the velocity constant around the impeller so that mixing losses are minimized. To achieve that function, the area increases so as to accept the additional flow exiting the impeller, which exits the impeller all around the outside diameter (OD)—360 degrees. The pressure surrounding the impeller is uniform, resulting in zero hydraulic radial thrust on the impeller.
 
Performance with Restricted Flow
When the flow from the pump is restricted, forcing the pump to operate at a reduced capacity, the flow from the impeller is reduced, and the volute does act as a diffuser, creating an increasing pressure from the cutwater all the way around to the casing throat. The maximum pressure rise occurs at shut-off (zero flow). As shown in Figure 1, this rise in pressure around the impeller creates a radial thrust on the impeller that pushes the impeller in a direction approximately 90 degrees downstream from the cutwater. As shown in Figure 2, the maximum thrust occurs at shut-off.

Performance with Excess Capacity
When the pump is allowed to operate at a capacity that exceeds the BEP, the result is just the opposite. The velocity around the impeller increases, from the cutwater to the throat, causing a drop in pressure. This results in a radial thrust that pushes the impeller in the opposite direction, approximately 270 degrees downstream from the cutwater, as shown in Figures 1 and 2.
 
Taken From Pump-Zone

10 Ways to Select A Happy Pump

1)  Only select pumps with suction specific speeds less than 11,000-less than 9,000 is even better.

2)  Select your pump so it never operates below 70 percent to 80 percent of its best efficiency point.

3)  Remember that 1800-rpm pumps are usually more reliable than 3600-rpm pumps

4)  Hydraulic efficiency peaks at specific speeds between 2000 and 3000 and drops dramatically below a specific speed of 500. Higher efficiency means less vibration and noise and lower energy bills.

5) Use double suction impeller sparingly. They are less stable at off-design conditions than single suction impellers.

6)  For single stage pumps never select a pump with a maximum diameter impeller. You may need to increase the impeller diameter in the future.

7)  Select a driver that allows you to operate to the end of the pump curve.

8)  Use hydraulic stability, not temperature rise, as criteria for setting the minimum acceptable pump flow.

9)  Incorporate a healthy NPSH margin or ratio, i.e. NPSH _r/NPSH_a into your selection. This ratio should be anywhere from 1.1 to 2.0 depending on the fluid, criticality and suction energy level. A higher value is always better.

10)  Consider fluid volatility when making your pump selection. Be more conservative when the fluid has a single boiling point, as opposed to a fluid with a wide boiling point range.

Tips From Robert X. Perez