Pipe Flow and Pressure

[Job314]

This may be an elementary question, but I am having the hardest time finding the answer.

I have a 2" pipe that is running water at 50 psi (city pressure) with a valve on the end. Total length from the city main to the valve is about 100 ft. If I open this pipe up, fully open, to fill up a tank, what will the flow rate (gpm) be?

Thanks in advance for any help.

 

[BigInch]

2" NS is 2.375" OD, assuming std schedule 40 w.t. = 0.154". Flat elevation profile, no fittings, wide open full port ball valve, no backpressue, 50 psi friction drop, Colebrook-White with a pipe roughness = 0.0018 gives
265 gpm

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[SNORGY]

I just used Darcy-Wiesbach with f~0.015 and included 1.5 velocity heads lost due to entrance and exit effects. I ended up at 259 USGPM.

Cameron's Hydraulic Data book, for 2" SCH 40 new commercial steel pipe, shows 260 USGPM corresponding to a head loss of 113 ft per 100 ft of pipe.

That is at *start* of tank fill...

Close enough for me.


Regards,

SNORGY.

 

[BigInch]

Oh drat. I had a lot of confidence in Cameron's book until you said that.

"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.

"Being GREEN isn't easy." Kermit

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

http://virtualpipeline.spaces.live.com

 

[SNORGY]

The O.P. - Job314 - said 50 psi and 100 ft of pipe with a valve on the end.

50/0.4338 = 115 ft = dH
100 + 10x2/12 = 102 ft (adding 10 diameters for unspecified valve)
Add "a bit" for entrance / exit losses, say 8 ft.
Thus L = 110 ft

then dH/L = 115 ft per 110 ft of pipe

Cameron says 113 ft per 100 ft of 2" SCH 40 pipe yields 260 USGPM.

My number was 259 USGPM.

BigInch's number was 265 USGPM.

cvg, 220 USGPM; katmar, 240 USGPM.

Results seem pretty good across the board.

Just to qualify my number a bit, I just "threw f=0.015" at it. Looking at the Moody charts in Crane, it probably should have been closer to f=0.02. Then my number works out to 228 USGPM - but remember that I am also carrying 1.5 velocity heads as additional losses for entrance and exit points. I am thinking that ~260 USGPM is a good number, depending on whatever assumptions are made.



Regards,

SNORGY.

 

[vpl]

I realize it's been a while, but I thought I'd pick at this some more.

If the stated objective is to figure out how much longer it would take to fill a tank with a 2" pipe instead of a 3" one, wouldn't it be easier to use the relative incompressibility of water and figure it out via a ratio:

V₁t₁=V₂t₂ where V is the volume and t is time?

If the pipes (or hoses or whatever) are pretty much the same length, this would roughly work out to the tank filling taking a bit more than twice as long.

Patricia Lougheed

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[katmar]

Patricia, as far as I understood the problem, the volume required would not change with the size of the pipe. Or maybe you are referring to the volume of the pipe? Except for the fact that for pipes of equal length the volume is proportional to the cross sectional area, the volume of the pipe is irrelevant.

If you adjust your approach to say the volumetric flow rate will be proportional to the cross sectional area then you are effectively assuming the velocity in the pipe is the same for the 2 different pipe sizes. This is a reasonable assumption for a rough estimate, but in fact the velocity (for a fixed pressure drop) will be higher in the 3" pipe than in the 2". The higher flow rate in the 3" pipe will lower the residual pressure in the main, making the assumption of equal velocity closer to the truth.

Overall, I think you are coming close to the right answer, but for the wrong reasons.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com