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180¦ bend L/D

180¦ bend L/D

180¦ bend L/D

(OP)
Hi there, hope somebody can help me

I have a 180 bend and I'd like to find the value that allows me to find the equivalent length, that is
(L/D)eq = ?

I am looking for values for pipes having various radius.

when looking in previous posts, I found
thread378-8478: Where can I find the Loss Coefficient (k) for bends below 90 degrees?.
Nonetheless, I'd like to find the L/D value, since I'd like to obtain an equivalent length.

Many thanks

RE: 180¦ bend L/D

TeeAr
Crane 410 gives a value of K=50*ft for a Close Radius bend of 180 degrees. Where ft is the friction factor in turbulent flow.

Hope that helps.

Regards
StoneCold

RE: 180¦ bend L/D

The resistance of the bend depends quite strongly on the ratio of the bend radius to the pipe diameter - the r/d ratio. A table for this is in the Crane 410 manual on page A-29.

Crane gives formulas for calculating the K values for a variety of fittings for fully developed turbulent flow.  Most of the formuals are in the form of the equation given above by StoneCold, i.e. in terms of ƒT  which is the Moody friction factor under fully developed turbulent flow.

Since K=ƒT(L/D) once you know ƒT you can calculate the L/D ratio you require by dividing the calculated K value by ƒT. By comparing this formula with that given by StoneCold you can immediately see that the constant in the Crane formula is in fact the (L/D) ratio. An inportant factor to remember is that the Crane K values (and indeed most of the K values you will find tabulated in the literature) are for fully developed turbulent flow.

Once you have divided the K value by ƒT (which is for turbulent flow) and you have extracted the (L/D) for the bend you can use this equivalent length for laminar flow as well with much better accuracy than if you tried to apply a Crane K value to laminar flow. Using euivalent lengths you should still expect errors of up to a factor of 2 - i.e. you will calculate answers that are between 50% and 200% of the "correct" answer.

Katmar Software - Engineering & Risk Analysis Software
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

RE: 180¦ bend L/D

The equivalent straight length of a pipe or tube 180° bend is (1/2)•π•D, or π•r, where r=bend radius.

RE: 180¦ bend L/D

"The equivalent straight length of a pipe or tube 180° bend is (1/2)•π•D, or π•r, where r=bend radius."

Sure - That's the physical length of the centerline of a perfect arc.  

But that won't give you the weight (centerline length*weight/ft for a nominal pipe won't be exactly correct, you need to check the supplier's catalogs for actual weight for each vender); and the required length for fabricating the bend is not that length - since they require extra lengths to draw the pipe through the bending machine; and the equivalent hydraulic length needs to come from the Crane catalog charts - or something equal.    

RE: 180¦ bend L/D

TeeAr

Let me add to the voices suggesting that you obtain a copy of Crane Technical Paper 410 "Flow of Fluids through Valves, Fittings, and Pipe."  

If you ask around your office, you will probably find someone who has a copy.  However, it is a relatively inexpensive book to buy and will probably prove useful to have.  Here's a link to the Flow of Fluids website:  http://www.flowoffluids.com/

Patricia Lougheed

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