System curve modification new flows introduced downstream of pump

[DS PUMPS]

An existing system is made up of a duty standby pumping arrangement that pumps out of a tank with a control volume of 5000 L. Pumping rate is fixed at 50 L/s at 40m head. Existing pressure main is approximately 7000 m in length puming into open tank.

A new project is looking to introduce 16 L/s at 3000 m downstream of discharge pumps. I know the new main must meet the pressure at the interface point of the existing main to avoid reverse flow but...

How do I plot the above system curve to determine the new duty point of the centrifugal pump. Assume medium is water.

Thanks!

 

[LittleInch]

Are both of these flow volumes fixed?

Then you only need one point, not a curve.

Divide to into two sections. Start at the end because you know what that pressure is. Your flow over 4000m is 66 l/ sec. Work out the head required. Then do the same for 3000m at 50l/sec now knowing what the pressure is at the 3000m point.

If you want to find out the flow based on your existing pump then you need to do some iterations on your 50l/sec flow, to create your curve, so start at 50, then 45, then 40....until the required head meets the pump curve.

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

 

[DS PUMPS]

Thanks for the response LittleInch!

Yes both flows are currently 'fixed' but I would like to find out what the existing pump will now be pumping at since downstream conditions have introduced additional headlosses due to increased flow in 4000 m length of pipe from newly introduced flows up to discharge point.

Splitting the system into
A - Pump station
B - New flow introduced (3000 m)
C - Discharge point (7000 m)

Headlosses to calculate:

A-B: 3000 m at 50 l/s = HL1
B-C: 4000 m at 66 l/s = HL2

Total head required = HL1 + HL2 + Static Head + Fitting losses (can I just read the new flow rate of the pump of its pump curve based on this required head?)

Regarding the iterations would the above calculations be modified as well? i.e

pumping at 45 l/s

A-B: 3000 m at 45 l/s = HL1
B-C: 4000 m at 61 l/s = HL2

little confused with how you stated I dont need a curve but then said to create a curve

Thanks again!

 

[LittleInch]

That's the picture.

When I said you don't need a curve I thought that the flows were the fixed element and you would need to change the pump to match or increase the pressure.

Then I read it a bit further and thought actually you're trying to find out what flow you will now get from the pump with this fixed extra flow part way down the line.

My guess is that you will get about 40l/sec out of the pump if nothing else changes.

"can I just read the new flow rate of the pump of its pump curve based on this required head?" No, because the flow won't be the same as you've used to calculate the head required, so you need to generate the curve to see where it intersects.

Your head from the pump isn't big so a few metres of losses or fixed elements could make a big difference to the flow rate. You'll only get an estimate really within about 10% of what you will get in reality.

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

 

[DS PUMPS]

Thanks LittleInch.

This has been a lot of help.

Just confirming that once Ive determined the new duty point of the existing pump this will now also dictate the required pressure/Head the newly introduced flow will have to meet. As I am likely going to need to design that system next.

 

[LittleInch]

Yes, that's correct.



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