Two parallel pipes both open to atmosphere 1

[Derek87]

I have two pumps that each supply a 10" HDPE pipe approximately 650 ft and discharge to open atmosphere. The pipes run side by side. I am considering connecting the two pipes so that one pump could supply both pipes. I am trying to figure out how to calcuate the pressure drop at the pump discharge if the pipes are connected. I already know what the head loss is across one pipe.

The distance between the pipes is negligible, and I am not expecting much additional head loss across this connection. Also, the elevation differences are negligible. Attached is a diagram.

In class, we always talked about parallel pipes that eventually joined back together. I am sort of thinking that there will be no change in pressure and the only effect will be a lower velocity in each pipe. However, I am not sure and would appreciate any input.

Thanks!

 

[BigInch]

No the pressures won't stay the same. Having two pipes will obviously flow more, but at what pressure loss is the question. Straight away you can see that you can't tell the pump's discharge pressure from its curve, because you don't know the combined flowrate at the pump's discharge, and you don't know the pressure drop in either one of the pipes either.

You'll have to make a pressure drop equation for each pipe, set the outlet pressures to atmospheric, choose flowrates in each pipe that give the same inlet pressure at the branch point, then check the pump curve to see if the discharge head at the combined flowrate, converted to pressure, equals the pressure at the branch. If it doesn't repeat the above sequence until it does.


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[rmw]

If each of your pumps is happy pumping through its respective individual pipe, unless each pump is pumping way back on its curve (to the left) - which I doubt - with an atmospheric exit, one pump probably won't like putting its flow through both pipes. That will lower the resistance (as BI has pointed out) overall and run the pump way out on its curve (to the right) possibly overloading the motor and getting into NPSH trouble.

rmw

 

[Artisi]

The simple method to check if this is within the bounds of the pumps hydraulics is:

1. From the pump curve, read off the flow at various heads, halve the flow and re-plot the half flow on the curve at the same head.
2. Draw a system curve for the friction loses etc for 1 pipeline at various flow rates across the pump curve and see where it intersects the half flow curve you have drawn, this will give you the flow rate for each pipeline and show on the full flow curve at the same head where the pump will in operate terms of power, NPSH etc when pumping on both pipelines.

I would neglect the few losses between the pipelines unless you wish to become very academic and split hairs.

 

[BigInch]

If you do connect the pipes together at any location, the pressures at those connection points, if they are not equal, will tend to equalize. The key is the diameter of each pipe. If they are equal, little difference in flow in each should be expected, so likewise connecting the two together will have minimal effect in setting up a cross flow, because there is no differential pressure available between one pipe to the other.

If you had pipes of different diameters, flows could be vastly different, but pressures at each cross-over would essentially be equal, provided the connecting pipe caused relatively resistance to flow.

A difference in pressures across the connections will cause some flow to be initiated through the connection from one pipe to the other, thereby trying to reduce any pressure difference that was there before they were connected. So, flows in each pipe will always be distributed between them such that the pressure difference between the two pipes at that point is the minimum possible.

In this situation, its a little more complicated than usual, because of the presence of the pump, where the pressure vs flow varies as indicated by the pump curve, so an additional unknown is added and you must solve for the point where flows in the pump = sum of the flows in pipe1 and pipe2 and the pump discharge pressure = the sum of the pressure drop along all pipes that can be included in any one-line diagram along any route you can go to reach atmospheric pressure.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[Derek87]

Thanks for the help, I made significant progress on the project today. I'll post again if any more problems come up with this pump.