Pump Designing

[neilsk]

Hi All,

I am going to replace an old constant speed pump with a bigger capacity VFD driven pump. This is a result of an increasing demand of potable water in the locality.

old pump has these numbers:

Size 10 x 12 x 13.34"
Motor HP 250
Capacity 4,150 GPM
TDH 150 ft.
RPM 1775

Please give me your opinions on what size is appropriate if my expected peak demand is 12000 GPM and minimum demand of 3,800 GPM. Will it make sense if i use the existing head of 150 ft. as the basis of my computation for HP requirement? You can ask me other data you may want.

Thanks for the helps...

 

[Artisi]

Have you thought of placing your inquiry with a pump suppler / manufacturer along with a little bit more information that would allow them to make a pump selection, along the lines of pump style (wet-pit vertical, horizontal split case, vertical split case) inlet conditions, power supply available etc etc.

Or employ an engineer experienced in this field.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[LittleInch]

Is that a multi-piston pump you have at the moment?

First off you need to try and figure out what your system curve is that you're pumping into. Probably quite difficult for a potable water supply, but if you can try plotting flow rate vs pressure to see what sort of curve it gives you and then try and extrapolote for your much higher flows you are looking at, but if this is a fixed speed piston pump I don't know how you can vary flow at the moment?

When you increase flowrate, your required head will increase by a different (higher) factor than flowrate depending on what your static head loss is.

Before you can work anything else out work this out.

Also think about having a mixture of fixed speed units to do your base load and then a VFD to work the extra load. Having a 3:1 turndown on a single pump will be quite ambitious and will be difficult to work efficiently or effectively. Read up on VFD pumps on this forum - there's lots of previous posts on things similar.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Artisi]

A fairly meaningless post devoid of anything worthwhile in offering any concrete assistance, other than pointing out that the flow increases by a factor of 3, the friction head component of the total head by a factor of 9 - Q2/Q1^2 if using the same pipeline and a massive increase in power.

More thought and data needed.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[LittleInch]

Fair enough, but my point really was that without a good idea of what the pump actually needs to do in terms of differential head then there's not much any of us can actually do to really assist the OP - though I'm all for employing competent and experienced engineers .

A three fold increase in flow should mean a whole heap more system resistance and power consumtion unless, as usual, there is a lot more information which we're not being told. A 3: 1 turndown a single unit is still a bit high IMHO.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[PumpSmart]

Goes from 4000 to 12000 GPM will impact in TDH (actual of 150 ft)
So GPM increase around 3 times, and it is huge change.
If static head is 0, then TDH will increase 9 times, about 1350ft of TDH!!!!
So you may take care about pipeline system too.
Maybe modify to try to not to go to 1350 ft.

If you like to keep 150 TDH constant, you have to calculate pressure losses on pipeline
for these new capacity to see with which pipeline diameter TDH will keep constant.

At 12000 GPM @ 150 ft, you will need a split case pump around 18x20x30 or 14x16x26.

I hope it helps.

 

[Softedge]

this is an extremely wide range of flows for a pump. at this flow rate and head i assume it's a centrifugal pump. also consider installing multiple parallel pumps, 2 or 3, of various sizes to meet the system demands. it is correct that the discharge pipeline will also cause you problems with high friction losses as you approach the 12000usgpm. consider a new or parallel pipelines also.
regards,
arthur stack
softedge=innovation