Help with system curve/outlet pressure

[jack880]

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

 

[Scotsinst]

I think you are exactly correct, however, it is very difficult to design the discharge system to match the calculated frictional loss exactly (unless you can vary the elevation). Normally you would use a valve to trim the system pressure drop to match your desired outlet pressure/flow.

 

[jack880]

Hi, thanks very much for the reply. Yeah, I was planning to do as you suggest and use a valve to restrict the flow.

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?

 

[JohnGP]

If you are discharging to atmosphere at the end of the pipe, then yes, pressure at the outlet will be atmospheric. You lose one velocity head at the outlet, but that should figure in your system resistance. And if you require 0.4 bar at some point in your system, then figure that in as well.

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

 

[BigInch]

Well, what your diagram is probably showing is that at 0.5 m3/hr you will need a control valve with 0.4 bar differential pressure to control the pump flow to 0.5 m3/hr. You will need what looks like 0.2 bar (assume Gage) to move that 0.5 m3/hr into your system. The system curve tells us that there is no static head, because it's 0 barG at 0 m3/hr flow, but it doesn't tell us what the loss due to flow will be, so we can't tell you what the final discharge pressure will be, other than it's somewhere between 0 to 0.2 barg. You must size your pipe to have no more than a 0.2 barg loss at a flowrate of 0.5 m3/hr. If the pipe has more than a 0.2 barg loss, more than is available, then you will not be able to flow 0.5 m3/hr and the flowrate will be forced lower, or you will have to open your pump discharge control valve to give more inlet pressure to the pipe. If the pipe has less than a 0.2 barg loss, the flowrate in the pipe will tend to increase and if you don't open the discharge control valve to supply that higher flowrate, the pipe may run partially full at that 0.5 m3/hr flowrate.

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.

"The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward X-CEO BP
"Being GREEN isn't easy." Kermit

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

http://virtualpipeline.spaces.live.com