Vertical Turbine Pumps - Number of Stages

[ctg1964]

Based on a given set of performance criteria (i.e. flow rate, head, etc.) one manufacturer is providing a pump with a single stage vertical turbine pump and another with a 2-stage turbiner pumps. Both are offering similar efficiency and pump motor size. Why? What is the significane of single versus 2-stage turbine pumps?

Thanks for your input.

 

[Artisi]

Without more detail re, impleller dia., pump speed etc the only detail I can offer is that the 1 stage unit is probably fitted with an impeller which is more radial flow than the 2 stage which is likely to be more axial flow.

As far as operation is concerned there shouldn't be any difference.

 

[ctg1964]

Are pumps with 2-stage more efficient than single stage turbine pumps?

 

[Artisi]

Cannot say yes or no in your particular case as it depends on the hydraulic design. The pump efficencies should be shown on the performance curves for each unit.

 

[ctg1964]

I looked at the pump curve. For the given flow rate and head, the pump efficiency is 83% with 2 stages. There is a note in the pump curve stating that deduct 2% efficiency for single stage turbine pump.

Can you make a general statement saying that higher the number of stages better the pump efficeincy or it varies from pump to pump based on specific hydraulic design?

Thanks.

 

[Artisi]

It is not possible to make a general statement regarding efficiency changes based on the number of stages - only test results / manufacturers data can give this information with any reliabilty.

To decide whether to use the single stage or the 2 stage should come down to an engineering study and evaluation, it should not only based on the efficiency but the overall pump system ie, what is the effect on the pump at a higher / lower total head - if the submergence changes or the NPSHa should change etc.

 

[TenPenny]

For a given pump size and impeller style, the impellers for a multistage pump are usually identical, so the efficiency is the same.

In your case, you are comparing a 1 stage offering to a 2 stage offering. The motor hp should be approximately the same, because the work being done is the same. There is a trade off in selections - the single stage pump must be a bigger pump, to get x head. The two stage pump would be a slightly smaller pump (assuming the speeds are the same).

The differences will probably show up in NPSHr, min level for vortexing, etc. You may prefer a smaller, 2 stage pump, due to physical size, or you may prefer a 1 stage. Depending on the manufacturer's costing, maybe a smaller 2 stage is less expensive than a bigger 1 stage, or maybe the reverse.

Depending on your application, look at the min operating level and NPSHr, then the physical space requirements, to see if either one will fit. One thing about VT pumps - there are so many possible combinations that often each application engineer will come up with a different solution to your problem.

 

[GaTechTheron]

I'd say TenPenny hit it on the button with the little information provided. Everything else kept constant, a 2-stage bowl assembly will have a lower NPSHr, but the extra stage will drive up the cost, somewhat.

 

[Artisi]

A couple of comments from my side regarding earlier posts;

the likelyhood of vorticies forming is independant of the number of stages and without having the pump curves to review, saying that the 2-stage pump will have lower NPSHr is a dangerous assumption.

 

[dabluffrat]

If amble NPHSa is provided, then go with which ever one has the least req BHP. I imagine the single stage pump is running a little fatser, so wear parts will degrade quicker but not by much. An additional stage will increase the cost of the spare parts required, but typically will have a better MTBR as the work is split btw two impellers and probably running at a slower RPM. Single stage pump can probably pass solids a little easier, so it does depend on what you're pumping.

 

[Artisi]

Without any detail from the OP I would think it is reasonable to assume the pumps are operating at the same speed, OP also advised that Eff. and motor size the same.

 

[dabluffrat]

I think the OP is confused by the OEM curve comment,

"There is a note in the pump curve stating that deduct 2% efficiency for single stage turbine pump"

That note is on the curve because that curve was developed based on a multi stage test curve. For a given OEM pump model and size, yes typically a two stage will be more efficienct then a single stage of the same model and size, but if your trying to compare two different models and sizes that both fit the rated condition then more info is needed.