Pipe elbows less than 90 degrees.

[dbecker]

Hello,

I have a Crane's handbook here. I am looking for pressure drops across pipe elbows that are not 90 degrees in bend. Basically I am looking for a correlation between angle in bend and pressure drop. Page A-29 has something like that but only for Standard Elbows at 90 and 45 degrees.

Thanks!

 

[SNORGY]

I am thinking that the correlation in the second figure from the bottom on the left hand side of Page A-29 can be used for bends of any angle, can it not?

And yet, if that was true, then how would one reconcile or treat the term "n" in that expression? That's what escapes me...

Maybe a simpler approach is this:

I have data that suggests that the "L/D" ratio for a standard 90-degree elbow is 14, and the "L/D" ratio for a standard 45-degree elbow is 8. At some point, after factoring in the appropriate centreline arclengths, extrapolation of the bend angle to zero would produce an "L/D" ratio for straight pipe of a length equal to the arclength of the fitting under consideration, and presumably, a line or curve through those data points would produce a correlation for equivalent lengths of bends of any angle between 90 and 0.

You could do the same for other ratios of bend radius to diameter with the knowledge that k = f*L/D.



Regards,

SNORGY.

 

[dbecker]

Hi SNORGY,

Thank you for the prompt reply.

If we are looking at the same figure, second one from the bottom on the left of A-29. It reads that n= number of 90 degree bends. So I assume I cant use it for anything less than 90.

I will study what you wrote about extrapolating arclength. But I don't see how it will capture momentum loss due to direction change?

Thanks again.

 

[Zapster]

dbecker, are there a lot of these non standard bends in your project that significantly add to the system losses? Perhaps it does not make much difference regarding the loss coefficients if they only add up to a few percent of the system loss. How accurate do you think your calculations will be and how much will the uncertainty in the non standard bends add?

 

[dbecker]

Zapster good question.

The system has 2 bends that are shallow that I can consider straight (less than 15 degrees). The rest of the system is coils.

Some of the bends are deep or close to 90 degrees (about 80 degrees). So in actuality, if I assume the aforementioned bends are either straight or 90 degrees, I should be ok.

If I have some correlation for non standard bends, I can compare 15 degree bends with straight pipe to know for sure what the difference is.

 

[mauricestoker]

Attached site may be useful.

http://www.scribd.com/doc/9712922/Introduction-to-Fluid-Mechanics-Ch07

Pages 126/127 address different angle bends.

 

[vpl]

Is the formula you're questioning: K[sub]B[/sub]=(n-1)(0.25[π]*f[sub]T[/sub]*r/d +0.5K)+K?

where n=number of 90° bends and K = resistance coefficient for one 90° bend per table?

You can set n to zero and calculate a K value from the table based on the r/d value.

With n at 0, the formula becomes K[sub]B[/sub]= K-(0.25[π]*f[sub]T[/sub]*r/d +0.5K)

Patricia Lougheed

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[Zapster]

dbecker, "Handbook of Hydraulic Resistance" by I.E. Idelchik has a chapter, "Resistance To Flow With Changes Of The Stream Direction; Resistance Coefficients of Curved Segments -- Elbows, Bends, etc.

The above book should cover most of the info you are asking. From your response, your main resistance might be the "curved segment" (coil). As such, Idelchik might be a good reference.

 

[SNORGY]

Thanks vpl...

I wasn't sure that that would be legitimate. Then I got sidetracked before I reviewed it in greater detail. But yes, that is the equation in question.

Regards,

SNORGY.

 

[IFRs]

The results of your calculations will be approximate at best. For my money, it's not worth worrying about - use 45 for 45 or less and 90 for 45 to 90. My opinion only.

 

[MJCronin]

I second the remarks of IFR.....

Correcting a piping system pressure drop calculation for elbows other than 90 degrees is not worth it....

Your calculation is only within 5-10% accuracy at best...

 

[iainuts]

See attached equation.