Double volute pump
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:
- Testing has shown that the double volute does not entirely eliminate the radial forces, but they are reduced greatly. Although the volute is symmetrical around its centerline, the two passages directing the liquid to the discharge nozzle are not. This means that the radial forces do not exactly cancel and a slight radial force does exist.
- Testing has further shown that a double volute pump will be 1% to 2% less efficient at its best efficiency point (BEP), but 2% or more efficient on either side of the best efficiency point (BEP). This means that the double volute will have an overall higher efficiency than its single volute cousin. Unfortunately many pumps are purchased with the efficiency given at the best efficiency point not the actual operating point used to make the purchasing decision.
- Double volute pumps should never be specified for low flow (less than 400 gpm or 90 m3/hr.) operations especially if there are solids in the product. The narrow passages behind the dividing rib can easily clog with solids. They should, however, always be specified for larger volume applications.
- The rib can cause some production problems with the castings especially in small sizes.
- Testing has shown the minimal radial thrust was experienced when the dividing rib did not extend all the way to the volute discharge flange.
- On large pumps there has been some problems with the rib cracking at the center when the pump is subjected to high hydro test pressures. The reason for this is not really understood. Some manufacturers ignore this because in operation they know that the pressure will be the same on both sides of the rib. Other manufacturers leave a gap of 2-3 millimeters in the center to prevent the cracking.
- Triple volute casings have been tried, but haven’t proved to be effective enough to justify their high manufacturing cost.
- If you have an occasion to repair the double volute cutwaters (and you can with some of the newer metal repair compounds), be sure the cutwaters are located physically 180 degrees apart.
- Many large double ended pumps have atrocious L3/D4 shaft numbers and are therefore supplied with a double volute as a standard.
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!
- On February 18, 2018