What Is A Multistage Centrifugal Pump Used For?

Hi All,

I have been looking for information that would outline when it would be suitable to use a multistage pump versus a single or two stage pump. I have a scenario where I have two different manufacturer's for my Boiler feed Water pumps. The first set of pumps are Byron jackson, a two stage pump. The second pump is a goulds M, with four stages.

The operating conditions are as follows:
1. Suction Pressure 75 psig
2. Discharge pressure 610 psig
3. Flow 860 gpm
4. Temp approx 300 F

In my experience with these pumpsets, I have a higher failure rate (mostly bearing failures) with the two stage Byron Jackson. I believe it is less forgiving when the pump is operating away from the BEP as opposed to the multistage pumps. I also think that the axial load on the two stage is higher than the multistage pump due to the absence of a balance drum and of course there is a higher differential pressure across each impeller.

I am doing a report to replace these pumps and I would like to recommend going with a multistage. Can you let me know if I am on the right track and what documentation I can use to back up my conclusions?

Thanks in advance. Hi Guys,

Both pumps are operating at rpm.

@ Pumps Only

Both pumps are opposed impeller design. The control system for the pumps are different. They both have minimum re circulation lines but the multi stage pumps ones are controlled automatically while the 2 stage pumps are controlled (and I use the word control very loosely) manually.

There are inherent problems with the operational and control system. However, the question is, if I had an ideal control and operating system, with the specifications I have give, would it be better to have a multistage or a 2 stage pump in this service? Assuming that both pumps are API 610 compliant and are operating at the same speed.

@ Dub Mac

I have attached the two pumps curves.

Thanks

 http://files.engineering.com/getfile.aspx?folder=2a744c7e-e49b--be15-388b812f3e3d&file=.pdf
Just my opinion....
I don't know of any reason you could say that the 4 stage(Goulds), is a more preferable design for BF service than the 2 stage BJ pump (now a part of Flowserve). They are both robust designs and both are quite capable of handling your BF service as originally designed. In terms of design, the only big difference I see is that one is axially split and the other radially. They both have opposed impellers and I'm sure they have dual volutes for axial and radial balance, respectively.

Maybe I'm missing something, but the BJ pump actually looks like it is a better fit on the curve than the Goulds; closer to BEP (my eyes are old though).

Having said that, certainly these "well aged" pumps have been rebuilt/redesigned to differing extents over the years(e.g. your switching ring materials). Your preference for the Goulds may have more to do with how these pumps have taken to their respective rebuilds, and how they have been operated.

If I were to do a true objective comparison for these two designs, I think I would look at some of the construction differences, and see how those match up to your Maintenance Depts. level of sophistication and service conditions.

These might include:
-Shaft stiffness L3/D4 ratios...skinny shafts can cause all sorts of problems when operating off-point.
-How are the impellers secured axially? All stacked against the same surface? Pinched between lock nuts?
-Got pipe strain? Over the years, many pumps become "pipe hangers" and can distort cases/rotor fits. The 2-stage probably more susceptible with a top/top nozzle design.
- Always have to consider repair costs; 2 stage has less parts and pieces to buy and put in backwards, just sayin'.....

These are a few things that come to my old, worn out mind; would like to hear opinions from some of our esteemed colleagues.......

Do you run both pumps in parallel? The reason I ask this is because I noted the 2 pumps have different shut off head
The Gould pump S/F head is about ft where as the BJ is about ft.
There is a hand writing at the top of the BJ pump curve "min flow =870gpm" BEP flow is gpm.
You mentioned in your first post the flow rate is only 860gpm.

The Gould Pump is rated for lower flow and higher head than the BJ pump. If you run them in parallel, the BJ pump will be running at minimum flow set by the "manually control"most of the time, That could be the reason for bearing problem.