When is it suitable to use a multistage pump 2

[arm83]

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 M3300, 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.

 

[Pumpsonly]

You did not mention about what speed the pumps are operating. Assuming both type are running at same RPM.
The 2 stage pump would be using large dia impellers and would likely operate towards the far left hand side of BEP.
Operating on far left hand side of the the BEP will cause high radial load on the shaft bearings.
On the other hand,the 2 stage pump will be opposed impeller design with in inherent balance balanced axial load without the need of balancing drum.

Your problem with the BJ pump would most likely due to operating too far left of BEP.

 

[DubMac]

Pump curves available??

 

[arm83]

Hi Guys,

Both pumps are operating at 3500 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

 

[ApplicationENG]

I only find the Byron Jackson curve.

As a side note until you can upload the Goulds curve, i would consider reliability of utmost importance and if youve had success with Goulds and their "system" is more robust, i would stick with them.

lets see those curves.

 

[arm83]

Hi,

App Eng,

The curve for the Goulds should be uploaded. The major problems I had with the Goulds pump were the fusing of the wear rings but I have installed Vespel in the casing rings and that problem has been solved.

Thanks in advance for everyone's assistance.

 

[DubMac]

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.......

 

[Pumpsonly]

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 1600ft where as the BJ is about 1450 ft.
There is a hand writing at the top of the BJ pump curve "min flow =870gpm" BEP flow is 1400gpm.
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.

 

[ahika]

Not sure if this will help with determining the most suitable pump.

860gpm = 0.054cum/s is pretty low flow and doesnt really mention on this curve. But maybe useful anyway

 

[arm83]

Hi guys,

Thanks for all your input. Between the two pumps I have always gotten longer and more efficient service from the Goulds pump. As DubMac has pointed out there have always been factors such as:
1. Pipe strain (prominent and has been corrected many times. Hopefully this time around maintenance would get it right as they are changing out the complete Boiler Feed Water header)
2. Lack of control (thus the BJ pumps would tend to operate to the left of its BEP or run flat out)
3. Poor repair procedures

Perhaps I am approaching this incorrectly and should look at the correct application of the pump instead. As Pumps Only pointed out, the Goulds pump has a lower flow than the BJ pumps. These pumps have always had their fair share of problems and that could just be a simple case of incorrect pump size. Unfortunately it would take about two years to replace the pumps but I am hoping to systematically go through each problem and deal with it until the pumps can be replaced.

Ahika, Thanks much for that info.