Double Volute vs Single Volute 3

[norzul]

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

 

[Artisi]

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 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

 

[checman]

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

 

[stanier]

I agree with the previous postings. you need to do a whole of life cycle costing. The capital cost of the pump only represents 12-15% of the whole of life cost.

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.

 

[scalleke]

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

 

[PUMPDESIGNER]

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

 

[macmil]

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 !

 

[Bjegovic]

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

 

[PUMPDESIGNER]

Bjegovic
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

 

[Bjegovic]

I will try to find out how I got that information, and provide the reference. I had a similar double vs single volute problem a long time ago and i remember this as an argument against double volute pumps, but don't remember the source....

 

[Artisi]

"Bjegovic (Mechanical)
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

 

[Artisi]

There seems to be some confusion as to double volute and double suction impeller pumps in this thread and one other currently active thread 407-144831.

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

 

[Bjegovic]

I am sorry that my posts (both here and in thread 407-144831) were confusing since I was reffernig to double suction pumps.

sorry again!

 

[Bjegovic]

@ 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.

 

[Artisi]

Bjegovic (Mechanical)

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

 

[Enkidu]

wow - lot's of good stuff here.
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.