Line Pressure

[eric4solid]

In a bit of an arguement, one pump discharging into two lines, you cannot combine the ID of both to figure pressure. You must figure one line to get pressure of both??? Correct? Two 4.75 do not equal one 9.5, even though they are manifolded together.

 

[BigInch]

The pressure at the inlet to each, assuming the manifold is relatively short, will essentially be the same. Ideally at the header the flow in each pipe would be proportional to the ratio of the cross-sectional flow area of each pipe, although that result can be upset by the length of each pipe and the downstream pressures at the end of each pipe.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[eric4solid]

Length and all elevations are the same. Pipe should equalize with one pump discharging into both. To get the right pressure I should just figure ID of one pipe?

 

[SNORGY]

If I understand the question being asked...

If you rationalize that you have fully developed turbulent flow in each case so that the respective friction factors are almost equal, then by Darcy-Wiesbach, when you compare say 520 USGPM thru one 8" sch 40 line versus 260 USGPM each through two parallel 4" SCH 40 lines, you find that the latter case has about eight times the head loss of the former case.

Thus, doubling the diameter to figure pressure (if that is what was meant) would not be correct.

I checked my hydraulics via Cameron; dH for 520 USGPM in 8" SCH 40 = 0.449 ft/100 ft, whereas dH for 260 USGPM in 4" SCH 40 = 3.72 ft/100 ft; 3.72/0.449 = 8.29

 

[zdas04]

It is really quite simple--the fluid doesn't care a bit about diameter. It is flowing through an area and down a length.

When you do the area calcs you see that each standard size is in the universe of twice the flow area of the next smaller size, so:

- two 2" is about the same as a 3" (8*π/4 vs 9*π/4)
- two 3" is about the same as a 4" (18 vs. 16)
- two 4" is about the same as a 6" (32 vs. 36)
- two 6" is about the same as an 8" (72 vs 64)
- two 8" is about the same as 10" (128 vs 100)

If you are doing bigger pipe than that then you don't have any business going by this rule of thumb.

In your example if you have two 4.75 inch ID lines manifolded together, they should discharge into a mythical 6.7 inch ID line (call it 8").

Oh, by the way, two seemingly identical lines will never have exactly the same pressure drop. A bit of debris, some slime, a touch of corrosion will cause one line to be more resistant to flow at the microscopic level and cause flow inequities. I always figure that if two similar lines are withing 20% of each other I'm ahead of the game.

David Simpson, PE
MuleShoe Engineering

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"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.