How to calculate flow rates 1

[sturmovik]

I have an hydraulic circuit where the main flow rate from the pump divides in 2 or more pipes, of different diameters. How can I calculate the exact flow rates in each part?

 

[Hydromechdude]

The pressure where the pipe runs meet up again will be some value. If you know the downstream pressure where the lines converge and you know the pump pressure, you can calculate flow rates for all branches with the delta pressure.

If the downstream pressure is not known, I believe the solution would involve iterations assuming a flow rates for the branches and then calculating delta P. Compare and then adjust flow rates so that the delta P converges to the same answer.

It also looks possible to have three equations and three unknowns (for a three branch system). Haven't run the math to determine if that's actually plausible, though.

 

[sturmovik]

The relative downstream pressure is almost zero , because the lines converge to the reservoir.

This would mean that delta P is the whole pressure I measure right after the pump, which is set by the pressure relief valve. And it would not depend on pipe diameter.

But if I reduce the pipe diameters, I would see a reduction of flow rate. And if I increase the diameters, delta P would decrease.

I'm a little bit confused....

 

[hydtools]

Try reading through the approach at the end of the paper:

http://www.wepapers.com/Papers/2979/Pipe_flow_in_parallel

Ted

 

[PNachtwey]

"But if I reduce the pipe diameters, I would see a reduction of flow rate. And if I increase the diameters, delta P would decrease."
You are right if your are using a centrigul or vane pump. You need to look at the pump and system curves.

http://www.engineeringtoolbox.com/pump-system-curves-d_635.html

Notice the operating point where the pump curve and system curve intersect. Now you know the operating pressure and flow you can figure out which pipe gets what percentage of the total.


Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

 

[micalbrch]

"But if I reduce the pipe diameters, I would see a reduction of flow rate. And if I increase the diameters, delta P would decrease."
Of course you see that because the friction losses are higher with smaller pipe diameters respectively lower with bigger pipe diameters.

What kind of pump do you use? You wrote that there is a pressure relief valve right after the pump. Do you have a positive displacement pump?

 

[sturmovik]

yes, I use a vane pump