How single volute casing pump different with double volute.?
Answer:
*Yes, single volutes have more radial load on the shaft but the advantage is that the load is very predictable. We may not like radial loads but hey, bearing technology is very well devellopped.
A dual volute is basically two pumps in one casing. At one point or another depending on the design and on the machining quality and precision one of these pumps will shut off the other and radial loads will become unpredictable.
However, if your operation point is well known you will be able to stay out of troublesome operation area's regardless of the type of design.
*A double volute scroll pump has almost no radial thrust at any point on the curve, from zero flow all the way out to well beyond the BEP. In other words, the double volute pump has little or no radial thrust reaction to operation above or below BEP.
But a single volute pump has a very pronounced radial thrust reaction when operated off BEP. If the bearings and shaft are insufficient to withstand the thrust, bearing and shaft life are shortened.
*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.
*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).
*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.
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.
The double volute design is actually two single volute designs combined together.
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).
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