The double volute pump 14-6

In all of my classes and writings I discuss the advantages of using a double volute centrifugal pump to eliminate radial shaft deflection caused by operating off the best efficiency point (BEP) of a single stage centrifugal pump.

In this paper I will answer those questions I get the most frequently asked about double volute designs

The single volute pump impeller will deflect either 60° or 240° from the cut water depending upon which side of the pump's best efficiency point (BEP) you are operating.

These numbers can change if you are using a low or high specific speed impeller, but they are good numbers for the high percentage of Francis vane impellers (SS 1500 to 4000) we find in industry. You can read about specific speed (SS) in my Paper 7-3 .

The double volute design is actually two single volute designs combined together.

Although this drawing does not show it clearly, the total throat area of the two volutes is the same as the single volute design.

Double volute pumps were created to eliminate most of the radial thrust caused by operating off the pump's best efficiency point (BEP).

In its simplest form the double volute design tricks the impeller into thinking that it is located in a circular casing. A circular casing does not generate any significant radial forces.

Let's take a look at a few of the specifics:

Why do we see so many end suction and smaller double ended pumps being supplied without this double volute? The answer is easy. The lower efficiency at the pump's BEP (best efficiency point) has just about eliminated the double volute as a design that will be quoted in this era of high efficiency. As a consumer you should be looking for three features from your purchased items: performance, reliability and efficiency in that order.

Unfortunately most purchasing decisions specify efficiency first, assuming that reliability and performance are inherent in the product. Unfortunately they are not!

 

 

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