# Equivalent length of pipe bend

1. Nov 24, 2011

### smoque

can someone explain to me why it seems that the smaller the diameter of pipe bend (i.e. 90 degree elbow = 30D) the shorter its equivalent length becomes? for 1.5" diameter = 45" equivalent length while 2" diameter = 60" length. does this mean 1.5" diameter 90 degree elbow has less friction than the 2"? does not make much sense to me

thanks

2. Nov 24, 2011

### Travis_King

Larger pipe diameters have more flow going through it which hit the "wall" created by the 90. There is more mixing.
There are many reasons, and the fact is a lot of it is empirical data.

It has a bit to do with the amount of flow going around the bend that is laminar and the amount that is turbulent.

3. Nov 25, 2011

### smoque

i have read somewhere on the net that a 2.5" diameter of 90 degree elbow have equivalent flow of 1.6" straight pipe of same length as the bend.

any way to figure out how the statement above is derived?

4. Nov 25, 2011

### Travis_King

This is generally an empirical study. In carbon steel piping, a normal 90 elbow of 2.5" pipe is equivalent to 9.3 ft of straight pipe.

So if you have 40ft of pipe, then a 90, then 40 ft. You have an "equivalent length" of 40+40+9.3, or 89.3 ft of pipe. You use this along with the friction factor of the pipe material to determine the pressure drop.

Here is a website that shows you some Leq's (equivalent lengths) based on the steel pipe's nominal diameter:

http://www.delftaerospace.org/en/organisation/departments-and-chairs/space-engineering/space-systems-engineering/expertise-areas/space-propulsion/design-of-elements/feed-systems/feed-systems-b/ [Broken] is one that shows you the Leq/D so you can multiply the number by the pipe diameter (in feet) and get the equivalent length

Here Is a site that kind of shows you the math involved

Last edited by a moderator: May 5, 2017