Double Volute vs Single Volute - Eng-Tips

generally double volute pumps are designed to reduced radial load on the impeller, this means smaller diameter shafts and bearings.
You may find that in a pump of this size, some of the manufacturers will have double volute design as standard as the pump has been designed for a higher duty ie, designed to operate at 960 rpm (50hz) or rpm (60hz).
For this duty there doesn't seem to be any advantage / disadvantage in either single or double volute. The main concern would be efficiency, reliabilty and capital costs.


Naresuan University
Phitsanulok
Thailand I would agree with Artisi especially on the efficiency. A small change in efficiency can add up to big dollars on large pumps that run often. I would also like to add that it could lower the cost for the supplier to manufacture the pumps substantially. With the double volute you have lower shaft deflection which normally means lower vibration. This is especially true if you are running with a VFD or throttling through a control valve. My thoughts would be to get what they quoted unless money is an issue and they offer large savings that would not be offset by higher operating cost because of lower efficiency.

Regards checman
What will be the system resistive curve and what will be the new pump performance curve.

You have the advantage of knowing the system perfectly well and this seems to me an opportunity to make sure that you get a pump that is perfectly suited to your system.

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.

Best regards.

Scalleke This is a great forum, the guys have many excellent comments.
I will add that the radial thrust problem is extremely important unless the pump operates continuously at or near the BEP .

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.

Many, perhaps even most manufacturers have stopped making the double volutes in the smaller sizes, Goulds being one of them. I and others have suffered trying to find double volute scroll pumps. I personally have given up and have gone to concentric, modified concentric, and turbine pumps instead. Your vendor may not be able to easily find a double volute in the size pump you need.

If that pump is forced to operate well off BEP for any length of time, I would strongly recommend you not accept a single volute pump.

Lastly, it is sad, but many manufacturers feel as though double volute is unnecessary in the smaller sizes, but they are wrong, even small pumps have serious problems with radial thrust.

PUMPDESIGNER I agree with all the above; one point to consider if you are forced to a single volute design ( it works for double volute also ) is that some pump manufacturers offer a larger diameter shaft and bearings for a relatively small extra cost. Another option is to check the shaft material and go for a better alloy, though this doesn't do much for the bearings. Again, the cost increase should be minor, and insignificant compared with the headache of a sudden broken shaft ! There seems to be some confusion as to double volute and double suction impeller pumps in this thread and one other currently active thread 407-.

The following has been posted there for comment.

My understanding is and I would welcome other comments:
1. A double suction pump is usually an axially split case unit with a single inlet dividing into 2 branches to a single double suction impeller, ie, flow to both sides of the impeller.
2. A double volute pump is one in which there are 2 outlets diametrically opposed to each other in a single casing which discharge into a single outlet branch.
3. A double suction pump can be either double volute or single volute design.
4. A single suction pump could be either double volute or single volute design.
5. A unit with more than 1 volute would be designated a multi stage pump.

Naresuan University
Phitsanulok
Thailand @ PUMPDESIGNER: as I was mistaking and referring my post to double suction pumps ; the explanation would be that uneven flow distribution can create axial disbalance which bearings are not designed for, leading to premature failure. (assumption is that in normal conditions double suction pumps have no axial load on the shaft)

as I can remember, this was an experience and an advice of my senior collegue which I heard a long time ago while we were discussing pump selection for a huge WTP; and this is the only reasonable explanation I can offer with my limited knowledge.

By Allan R. Budris

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There are often multiple pump types (Table 1) that can be selected for the same water application, with each pump type having its own strengths and weaknesses. This column tries to help guide the reader in the selection of the best pump type that will yield the greatest reliability and lowest life cycle cost for a specific application.

End Suction Water Pumps

An end suction water pump would probably have the lowest initial cost for most applications, with reasonable efficiency. However, these pumps do not follow any standards, especially with regard to bearing life, shaft seal housings and dimensional interchangeability. They are also typically constructed with the lowest cost materials, such as cast iron casings with bronze or brass impellers. The impellers are typically of closed construction, without replaceable casing or impeller wearing rings. Further, there is typically more deviation from published performance, such as efficiency, for this pump type.

For non-critical, intermittent service applications these pumps may be the best choice. However, for critical applications, requiring long operating life, the cost of maintenance and down time may far exceed any initial cost savings.

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End Suction ANSI/ASME B-73 Pumps

Chemical pumps (figure 1), which can handle corrosive, and/or toxic liquids and slurries, are available in a variety of configurations and materials. Pumps used in this industry are different from those used in other industries, primarily in the materials of construction and the many mechanical shaft seal configurations available. These pumps must also meet the American Society of Mechanical Engineers ANSI B73 standards, which require dimensional interchangeability, minimum bearing life, and many other quality specifications. The minimum casing material is ductile iron, with stainless steel being quite common. Typical construction is an adjustable open impeller, which is also good at handling entrained air.

Want more information on Double Suction Pump Manufacturer? Feel free to contact us.

Because of these upgraded features, reliability focused users will typically select an ANSI/ASME B-73 pump over a lower cost water pump for other critical applications, including water services.

The principal reasons for the popularity of this between-bearing configuration include:

1. The rotor can be removed by just taking off the upper casing half, without disturbing the suction or discharge piping, or moving the motor

2. Less shaft deflection due to the between-bearing design.

3. Lower NPSH requirement due to the fact that each impeller eye only handles one half of the total pump flow rate.

4. Virtually no axial hydraulic thrust, due to the back-to-back / double suction impeller design.

5. Higher efficiency because power is not lost to balance the hydraulic thrust.

6. Relatively high allowable nozzle loads due to the rigidity of the lower portion of the casing.

Although split case, between-bearing models are quite popular, and have many advantages as listed above, the pumps are not without some drawbacks.

1. Due to the large flanges required for the split casing sealing joint, these pumps are normally heavier and cost more than comparable end suction pumps, especially in higher alloy and higher pressure applications.

2. While double-suction impeller pumps have lower NPSHr values then comparable end suction pumps, the through-shaft reduces the impeller eye area. This requires an increase in the suction eye diameter (higher eye tip speed and suction energy). Also, in order to keep the shaft bearing span to a minimum for critical speed purposes, the compressed inlet passages result in tight turns for the liquid entering the impeller eye. The gating Suction Energy and required NPSH margins tend to increase as a consequence. This phenomenon has caused field problems with high and very high suction energy pumps, when operating at low NPSH margins and/or when operating in suction recirculation (See October column). It should be noted that some of the newer split case pumps do have improved casing inlet designs (less inlet turbulence), which allows them to approach the higher suction energy performance (gating values) of end suction pumps.

3. Double-suction pumps are more sensitive to the orientation and geometry (radius) of elbows in front of the pump inlet (see April column). Suction elbows should be perpendicular to the plan view of the shaft. This reduces the tendency of unequal flow quantities reaching the two impeller eyes. Suction elbows in the plan of the shaft can cause uneven flow patterns which can upset the axial thrust balance, causing high bearing loads and shorter bearing life. Uneven flows to each impeller eye also tends to increase the NPSHR, and/or potentially put one half of the impeller into suction recirculation.

4. Between-bearing pumps need two shaft seals, whereas only one seal assembly is required for an end suction pump. This could increase both initial and maintenance costs in services that require expensive mechanical seals.

5. The axial split complicates the radial gasket sealing and can result in the mismatch of the casing halves at the joint.

6. This construction also limits the mechanical seal chamber options, since most split case pumps have the stuffing boxes integral with the casing halves.

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