Multistage vs Single Stage

[Glenfiddich]

Would there be any advantage to use single stage pumps or multistage pumps?

In my application, it would be cheaper to use a single multistage than two single stage in series. It would also take up a whole lot less room to house a single multistage with one backup.

One long time pump salesman told me to avoid using multistage pumps if I could. I think he meant that they are harder to maintain.

Do you see any merit to that arguement? In another similar application I know the other company used some multistage pumps in series and they had no issues whatsoever.

Thanks

 

[dcasto]

Two individual pumps to get two stages would be less cost effective that a single pump with two impellars. two pumps, doble the lines, double the seals, double everything.

Now then is a single impellar better or worse than two impellars. I'd go for two impellars normally. The way you can get the required head as per the fan laws is to increase the speed of a single impellar. To get double the head, you'll increase the speed by 1.414. (sqrt 2). But then you'll get 141% of the flow, so you'll need a smaller impellar and so on.

I'd rather have an 1750 RPM pump than a 3600 rpm pump, just call me old fashion, but as they say, speed kills or at least increases the risk. As for the salesman saying multi stage pumps being avoided, his company probabilly didn't make multistage pumps. In the pipeline industry we have no choice and we use as high as 16 stages in a single pump. If you really want to see some multistage, look at a REDA pump, they can have over 50 stages.

 

[Glenfiddich]

I spoke with a vendor and he said that a multistage would need more instrumentation and more auxiliaries (oil cooling etc). Keep in mind that the pumps I'm looking at are in the 5000 HP range.

The company I spoke with makes multistage pumps as well. These pumps are for a pipeline. He said that the trend for pipeline pumps is more towards having single stage in series rather than multistage pumps.

 

[Artisi]

Well, 5000hp is not a toy and need a full engineering / cost study to decide which is the best configuration.
In the size you are looking at I can't see that a multi stage will be any more of a maintenance problem than a single stage of this size, which ever way to go you are still talking about 5000hp. Although if you can place single stage pumps along the pipe line this is also a consideration but should form part of your study.

Does the company you are speaking with have standard design for your duty or will they need to design something?

 

[dcasto]

I can imagine more instruments, you do not measure the interstage on the pumps, just in and out. Either pump at that size will have the same number of bearings +/-1. Cooling capacity is the same, you'll remove the heat from the 5000 HP input that is generated in the bearings.

The trend is not more single stage in the pipeline industry. The trend is to adjust speed and VFD controls to adjust for different conditions. The trend is to build fewer pump stations and operate at higher pressures. This is a liquid and the pressure drop and HP is nearly linear.

 

[Glenfiddich]

there's a stigma of better reliability and simplicity attached to single stage pumps.

 

[Glenfiddich]

no?

 

[vpl]

glenfiddich

there's a stigma of better reliability and simplicity attached to single stage pumps.

never heard of that. Granted I'm in a totally different field, but nuclear plants use multi-stage pumps and they are required to be highly reliable. I also don't see why a system with multiple single stage pumps would be any simpler than one with a single multiple stage pump. IMO, having multiple pumps equals more chances for something to go wrong..

Patricia Lougheed

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[Glenfiddich]

The flows, heads, fluid density and viscosity vary from time to time.

Hopefully someone with some pipeline experience can chime in to give their 2 cents.

Boiler feedwater pumps or clean services which have constant flows and fluid properties would have luck with any pump, I'd say.

 

[sms]

I have always used multistage pumps for pipeline service. If it fits the service, I can't think of a good reason to used multiple single stage pumps instead.

"Why don't you knock it off with them negative waves? Why don't you dig how beautiful it is out here? Why don't you say something righteous and hopeful for a change?" Oddball, "Kelly's Heros" 1970

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[Artisi]

Cannot see the problem being created, this is a simple "best" hydraulic selection / engineering / cost benefit analysis.

Your final statement in the original posting seems to say a lot, "__________I know the other company used some multistage pumps in series and they had no issues whatsoever."

 

[dcasto]

Here's the only scerio that you could look at for you cost benefit study.

If you need redundant pumps, then you could in stall two two stage 5000 HP pumps or three one stage 2500 HP pumps. Now in this application, you'd be ahead.

 

[gr2vessels]

Glen,
Good name thou, hope you're over 25...Patricia, as always right, pointed out that a number of things that could go wrong with several pumps instead of several stages of one pump. For fluids with less viscosity, like liquid gases, you need anyway multiple stages to avoid cavitation, to overcome NPSH problems and not lastly, the lubrication of those bearings, using perhaps your process fluid to lubricate and cool the bearings. My 2 cents would be on the multistage when comes to large units and large power sources, one breaks down once, two break down twice.
cheers,
gr2vessels

 

[Artisi]

gr2vessels - would be interested in your thoughts as to why multistage pumps overcome cavitation and NPSH problems.

 

[HEC]

Glen,
Slurry pumping over long distances uses multiple single stage pumps, rather than single multi stage pump. Generally slurry pumps do not come in multi stage versions. For variable flow rates as the flow (head) requirements drop off the higher stage pumps are switched off and slurry is pushed through the pump. I would agree with Artsi look carefully at the specific requirements of the process, fluid characteristic range (viscosity/density), flow range and then do the value engineering on it and derive the best answer.
Good Luck, Cheers


Mark Hutton