combining flows and pressures at a single point

[cams1975]

If i have 2 seperate pipes combining at one common point in the system what will happen to the flow and pressure at that point

i have one pump on one line pumping at say 2l/s@ 2 bar and another pump in the second line pumping at 4l/s @ 1 bar, when the combine at the common point what will the resultant flow and pressure be?

Will one pump stop the other from pumping?

 

[katmar]

There is no easy answer to this. It depends on the pressure drop from the point where they combine to the point of known/fixed pressure. The point of known/fixed pressure would typically be the discharge into a tank, open pond etc.

You have to guess a combined flowrate and calculate the pressure drop from the point of combination to the point of known pressure. Now you know the pressure at the merge point. Check the flow each pump would give against that pressure and compare the total flow with your guessed flowrate. Adjust your guess and repeat until the guess and the calculation are equal.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[vpl]

Will one pump stop the other from pumping?

Without going through the maths for your particular situation (as there isn't enough information), it is quite possible for one pump to stop the other from pumping.



Patricia Lougheed

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[TenPenny]

There are so many details missing, the answer could be anything from absolutely yes to absolutely no.

What type of pumps are these? Centrifugal, or positive displacement?

 

[BigInch]

Or fight each other until they both die.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Artisi]

More detail needed - you need to know the pressure from each pump at the common point when operating separately. If one pump has a higher pressure at this point then the other pump will see this as an addition to the hear which it must overcome for flow to be maintained.

At this stage the crystal ball is foggy.

 

[MortenA]

Take both pump curves then calculate dP for each pipesigment for a number of flows. Calculate pressure out of the pumps aminus pressure loss in the segment for each pump. Then plot the two curves (out pressure - pressure loss) in the same graph and see if they cross. If they do then they will both operate if they dont then the lowest wont pump anything. If you downstream pressure is not 0 barg then you just need to superposition the graphs accordingly - if your downstream dP varies with the total flow (theres a downstream pipe with dP): Get a simulator ;-)

Best regards

Morten

 

[GaTechTheron]

It is my understanding that you could:

1) Calculate the pressure drops from the pumps to where they combine flow
2) Add the pressure drop to each pump curve, and lay both of them over the system curve.
3) Where the pump with the higher discharge (call it pump 1) pressure meets the system curve, note the capacity and pressure.
4) Retrace the curve on pump 2 until you reach the pressure noted from where pump 1 intersects the system curve.
5) Add these capacities, and you should have a good idea of flow conditions at the point where they connect.

However, there are so many models involved, I would not consider this anymore than an estimate.

Any thoughts?

 

[Artisi]

"However, there are so many models involved, I would not consider this anymore than an estimate.

Any thoughts? "

Yes, physically measure the pressure at the common point as noted in my earlier post.

 

[BigInch]

Here's a thought

cams1975, check this out

http://www.epanet.com

Software
• EPANET 2.0
• WaterCAD V8 XM
• More



**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)

http://virtualpipeline.spaces.live.com/