Partially closed pump discharge valve question

[jimmy001]

Hi all,

Just got a quick question here wondering if you guys can help me out.

Right now I have the yellow pump curve and the orange 2 inch system curve , producing around 120 gpm against around 110 ft of head. If I expand the piping from 2 inch to 3 inch I end up with the grey system curve. The grey system curve doesn't intersect with the pump curve so this way of operating is not possible in my understanding; However, using the grey system curve, if I partially close the pump discharge valve to artificially increase the head so that the total head becomes 110 ft, would the new operating point be the same as before (120 gpm against 110 ft of head)? Similarly, if I partially close the discharge valve so that the total head becomes say 100 ft, would the operating point be around 130 gpm against 100 ft of head?

Any help is appreciated, thank you guys.

 

[Artisi]

Who says it doesn't intersect the pump H/Q curve. You need the pump manufacturers published or test curve to know what is going on,not that it matters by the further comments of your post.

How was the grey system curve derived, using diameter ^5.

Throttling the discharge you can select what ever flow you want at the corresponding head - but why increase the pipe diameter and then throttle the pump?

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.)

 

[jimmy001]

@Artisi

Hi, sorry I didn't include these information in the post. Here are the background information.

1. The pump curve is the manufacturer pump curve, I just jotted down the points from the original graph and made a trend line in excel so I could play around with it haha
2. Both system curves were derived by calculating friction losses (fittings accounted for) on top of the static head.
3. I got two of these pumps here, currently we just run one pump and the orange system curve, but I'm trying to increase the flow. By increasing the pipe size to the grey system curve would intersect the pump curve with two of these pumps running in parallel and would get me increased flow and head that we need(I didn't show the parallel curve in the graph, but they intersect). However, just in case if we ever run just one pump for some reason, the curves won't intersect. I am doing this for a back-up option I guess.

You said "Throttling the discharge you can select what ever flow you want at the corresponding head," that means my assumptions in the original post would be correct right? By throttling the discharge valve is essentially just moving the system curve left-ward right?

Thank you

 

[Artisi]

Correct.

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.)

 

[lilliput1]

If you want to increase the flow without replacing the [pump increase the pump speed. You need VFD and new inverter duty type motor with the proper hp.

 

[LittleInch]

I often like to think that throttling the valve adds a variable length vertical connection between the pump curve and the system curve, but you could consider it as moving the system curve left as well.

The reality is the a single pump will meet the system curve, but you would be running off the published max flow and may get one of the following problems / issues

Excess power / amps on the motor
Decrease in efficiency
vibration
NPSHR increase and possible cavitation

Or for a basic pump it might just work ok, especially if this is only an occasional operation. That is a pretty steep pump curve though - you don't often see a 50% reduction in head over a usable flow range.

But otherwise artisis response is very good.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Yes, your assumptions are correct. And agreed, you'd be wasting some driver power as you throttle at the discharge valve in the long term at this operating point, so see if the CAPEX for a VFD speed reduction to achieve the same effect for the 3inch line option will pay out quickly enough. But note that at reduced speed ( approx. <50% of design case speed), there may be undesirable effects on shaft seal cooling, so seek advice from the pump / seal supplier if you choose to go this way.

 

[Artisi]

lets get back to the original post.
The larger pipe was installed to run 2 pumps.
Currently 1 pump runs to gain an slight increase in flow.
Throttling the discharge will move the system curve left until the required (increased)flow is achieved.

Will overall pump efficiency increase or decrease - no pump curve so its an unknown.
Will the pump be mechanically sound as this increased duty - without full curve detail efficiency / NPSHr etc again not clear, however for such a small increase I don't foresee and upsets.


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.)