Double Volute vs Single Volute
Double Volute vs Single Volute
(OP)
Hi Guys,
We plan to install 3 circulating cooling water pump (from cooling tower) for our cogen project.
Initial Spec
1) 4800 m3/hr
2) 47m
3) Double volute
4) 740 rpm
5) 680 kW
However, due to process optimization, the head was reduced to 36m. Because of this the supplier requested changes from double volute to single volute.
Is this a genuine request...??? What are the pros/cons of double vs single volute?
Thank you in advance for your response
norzul
We plan to install 3 circulating cooling water pump (from cooling tower) for our cogen project.
Initial Spec
1) 4800 m3/hr
2) 47m
3) Double volute
4) 740 rpm
5) 680 kW
However, due to process optimization, the head was reduced to 36m. Because of this the supplier requested changes from double volute to single volute.
Is this a genuine request...??? What are the pros/cons of double vs single volute?
Thank you in advance for your response
norzul





RE: Double Volute vs Single Volute
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 1160 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
RE: Double Volute vs Single Volute
Regards checman
RE: Double Volute vs Single Volute
Too mnay D&C contractors go for lowest capital cost and leave the owner with the burden of cost for the life of the pump.
RE: Double Volute vs Single Volute
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
RE: Double Volute vs Single Volute
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
RE: Double Volute vs Single Volute
RE: Double Volute vs Single Volute
double volute is basically two pumps, but connected IN PARALLEL, which affects the shape of the curve
RE: Double Volute vs Single Volute
I have never heard that comment about intake on a double volute pump.
Do you have an explanation or reference?
I will examine this myself, but would appreciate any info you have.
PUMPDESIGNER
RE: Double Volute vs Single Volute
RE: Double Volute vs Single Volute
besides all mentioned above, double volute pumps are more sensitive to uneven inflow pattern, so you need to completely straighten the flow; while single volute pumps can better stand uneven inflow .
double volute is basically two pumps, but connected IN PARALLEL, which affects the shape of the curve "
Is this reference to double volute pumps or double suction pumps as they are different animals.
Naresuan University
Phitsanulok
Thailand
RE: Double Volute vs Single Volute
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
RE: Double Volute vs Single Volute
sorry again!
RE: Double Volute vs Single Volute
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.
RE: Double Volute vs Single Volute
It is possible under some adverse inlet flow conditions for the flow to each side of the impeller to be unbalanced, this can result in uneven loading on the bearings, although I would think the condition would have to be extreme to cause any major bearing problems but it would depend entirely on the bearing configuration.
Naresuan University
Phitsanulok
Thailand
RE: Double Volute vs Single Volute
A couple of comments to add to the mix.
Going back to the original question, I would suggest that there is nothing wrong with a single volute pump, provided the mechanical design of the bearing housing assembly is based on the single volute. It probably won't help at this late date, however, I would be inclined to ask what the expected shaft deflection at the seal and impeller are going to be, and what k factor the deflection was based on. Accepted standard seems to be less than 0.002" at the seal - look at wear ring clearances at the impeller before you decide if deflection at the impeller is an issue.
Hydraulic Institute has some guidelines for k factors. Not all manufacturer's follow them. The only down side to the single volute is typically a larger shaft diameter and a larger (and more expensive) seal.