Help with system curve/outlet pressure
Help with system curve/outlet pressure
(OP)
Hi
I have a fixed speed pump, and want to design a discharge pipe system such that the outlet pressure at the end of the pipe is 0.4 bar at 0.5 m3/hr. If the system curve and the pump curve intersected at 0.5 m3/hr, there would be no pressure left at the end of the pipe - is that right? Do I need to design the system curve such that when the system curve and pump curve are both at 0.5 m3/hr, the system curve's corresponding pressure is 0.4 bar lower than the pump curve's? I.e. the intersection of the curves would be further down the pump curve at a flow somewhere greater than 0.5 m3/hr? Please see the attached sketch.
Hope that makes sense.
Thanks very much
Jack
I have a fixed speed pump, and want to design a discharge pipe system such that the outlet pressure at the end of the pipe is 0.4 bar at 0.5 m3/hr. If the system curve and the pump curve intersected at 0.5 m3/hr, there would be no pressure left at the end of the pipe - is that right? Do I need to design the system curve such that when the system curve and pump curve are both at 0.5 m3/hr, the system curve's corresponding pressure is 0.4 bar lower than the pump curve's? I.e. the intersection of the curves would be further down the pump curve at a flow somewhere greater than 0.5 m3/hr? Please see the attached sketch.
Hope that makes sense.
Thanks very much
Jack





RE: Help with system curve/outlet pressure
RE: Help with system curve/outlet pressure
What's been confusing me is as the pumps are fixed speed centrifugal pumps, is it impossible to have any residual pressure at the outlet of the system as this would just have the effect of moving the pump down its curve and increasing the flow? I.e. will the pressure at the outlet always be pretty much atmospheric pressure?
RE: Help with system curve/outlet pressure
Bit hard to say much more without knowing what you really want to achieve, but as suggested a valve to throttle flow is one way to adjust the system resistance curve to what you require.
Cheers,
John
RE: Help with system curve/outlet pressure
Note that you have drawn the system curve starting at the same zero point as the pump curve. This you cannot do, since the pump curve is a differential head and the system curve is inlet head, so this diagram is only correct if your suction pressure happens to be 0 barg.
Now assuming that your suction pressure IS zero;
If you need 0.4 barg at the outlet of the pipe, I would suggest that you put a pressure control valve at the end of your pipe and design the pipe such that the pressure drop at 0.5 m3/hr flowrate is a maximum of what looks like 0.6 - 0.4 barg = 0.2 barg. 0.1 Barg would be good, so you can afford to burn another 0.1 Barg off when driving flow through it. Your pressure arriving at the end of the pipe will then be about 0.5 Barg. Set your pressure control valve at the end of the pipe at 0.4 barg (downstream outlet) and size it so that at 70% open it loses a differential pressure of 0.1 barg at a flow of 0.5 m3/hr and you'll have 0.4 Barg on the outlet.
That should do it.
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