5 replies to this topic

[carbon60]

Hello,

I had a quick conceptual question about adding different pipe line pressures into the same header. I was wondering if I a system like the one in the attached paint file (sorry could not figure out how to paste the object) where I have a 50 PSI supply Line and a 80 PSI supply line joining at a common header all 2" line, the total line pressure would only be 80 PSI not 140 PSI correct (ignoring frictional losses)? So if i were to buy PVC pipe for this system I would only need it rated at 100 PSI not 150 correct? Also, would the water hammer from the valve cycling cause a significant increase in pressure where I would need to be concerned about the lines integrity?

Thanks in advance for all your help, I am not too good with pipe systems yet and am trying to get these basic concepts down.

Attached Files

•  pipe_system.bmp   1.15MB   39 downloads

[demank]

There are missing information to answer your question:
- liquid or gas system?
- continuous or batch system?
- Flowrate?

If continuous, the 80 psi stream may enter the 50 psi pipe. This part you need a check valve. And I think the total line pressure is 50 psi, not 80 psi.
If final condition is 80 psi, the 50 psi stream cannot flow to the header.

[carbon60]

Sorry about the missing information, I forgot to say that the fluid is water and that it is a continuous system. It is my understanding the the flow rates don't really matter if we are ignoring friction and assuming laminar flow, but if they do lets just say 25 GPM for the 80 PSI line and whatever a 50 PSI 2 inch makeup line will provide for the 50 PSI, I think 50 or so GPM would be reasonable here.

about the check valve, yes you are right there would be one in the 50 PSI line, but the concept that I am having trouble with is if the check valve were to get stuck open (with another check valve somewhere else upstream still functioning that is not shown), would the total line pressure be the 80 PSI, or is it actually 50 PSI? In any case, the total line pressure in a header can never be more than the highest single line pressure if it is all just regular (no restrictions of flow or anything like that) PVC pipe right?

Thanks for your quick response

[demank]

, YUPPE, the flowrate is independent of pressure if pressure drop is negligible.
For your question, The final pressure of the line is 50 psi. .
Like my previous posting in this forum. Please imagine:
if the header pressure is 80 psi, the 50 psi line cannot flow to the header. (Flow is from high pressure to lower pressure., )

[psychodan]

Yeah but for the scenario where the check valve fails that you were asking about, the 50 psi line between the closed isolation valve and the second upstream check valve you were referring to would be pressurized to 80 psi.

With the check valve working, the 80 psi line would have to be regulated to 50 psi or below downstream of the T junction if you wanted the two streams to mix. Otherwise if you set the system up as shown above all that would happen is the 80 psi line would continue to flow forward, and the check valve on the 50 psi line wouldn't open to allow this stream to mix with the 80 psi stream because the back pressure is stopping it from opening.

With regards to the design pressure of the pipe work, surge is only likely in this water system with sudden closure of the isolation valve and would not be a problem when the valve is already closed, but unless this is motor-operated even closure isn't a problem. As long as the design pressure of the pipe isn't going to be exceeded under any foreseeable phase of operation then it doesn't matter how far above the operating pressure the design pressure is. You just have to take into account every situation, for example, if the upstream pump on the 80 psi line was blocked-in would the pressure rise above 100 psi? You'd have to look at the pump curve for this obviously.

Hope some of this is of help anyway,

Dan

[wonder]

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