Why Does Smaller Pipe Diameter Reduce Equivalent Length in Elbows?

[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

 

[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.

 

[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?

 

[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/ 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