Two parallel pipes both open to atmosphere
Two parallel pipes both open to atmosphere
(OP)
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!
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!





RE: Two parallel pipes both open to atmosphere
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.
"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
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Two parallel pipes both open to atmosphere
rmw
RE: Two parallel pipes both open to atmosphere
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.
RE: Two parallel pipes both open to atmosphere
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
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Two parallel pipes both open to atmosphere