How to convert one pipe sie to another in terms of equivelent length? 2

[GasDistEng]

Years ago I used a simple furmula to convert one pipe size to another in terms of equivelent length. I.E. 20ft of 2" SCED40 Steel is equivelent to 7.2 Ft of 3" Sched 40. I realize the I can do this with the K values if f is known. I think the equation came from the "Pipeline rules of thumb". I do not have this book in hand. Any help would be appreciated.

Thanks....

 

[katmar]

In turbulent flow the pressure drop is roughly proportional to the diameter ratio raised to the power of 5. So a pipe with a 1" ID would have a pressure drop of (2^5 =) 32 times that of a 2" ID line (everything else being equal).

The equivalent length of a 2" line, if calculated as a 1" line, would therefore be 1/32x its actual length.

In your example, 20 ft of 2" line would be equivalent to about 150 ft of 3" line.

Hope this helps.

Katmar

 

[BenThayer]

I concur with katmar. this is basically assuming that mass flow and friction factor are held constant.

 

[zdas04]

I've solved the AGA single-phase equation for length (it is really messy) and created a table that yeilds slightly different results than Katmar's. The term is dominated by the diameter to the fifth, but there is also a log(3.7*D/epsilon) term. With epsilon as small as it is, this gives a non-trival change. For your example of a 20 ft of Sched 40 2-inch (I.D. 2.067 inches, assume epsilon = 0.0015 ft/ft) gives you 158 ft of Sched 40 3-inch (I.D. 3.068 inches).

The ratio of the two ID's raised to the fifth power times the 20 ft gives you 144 ft. That's a 9% difference which is probably not within the accuracy of the calculation. Katmar's way is a lot easier.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GasDistEng]

Thanks for the information. I think I may have some bigger issues - see my post on sizing vent line and "Y" factor.

Thanks again!

 

[25362]

One simple way to do such a conversion would be to use the Hydraulic Institute Pipe Friction Manual on water.

As an example, the friction drops in ft/100 ft tabulated in the manual are:


2 in. sch 40 3 in. sch 40 4 in. sch 40
GPM e/D=0.00087 e/D=0.000587 e/D=0.00047

100 17.4 2.39 (7.3) 0.624 (27.9)
200 66.3 8.90 (7.4) 2.27 (29.2)
300 146 19.2 (7.6) 4.89 (29.9)

The conversion factors are parenthesized. So, 20 ft of a 2" pipe would be roughly equivalent to 20*7.5=150 ft of 3" pipe, and to 20*29=580 ft of 4" pipe.

 

[rmw]

25326, I am going to give you a star for that one, because unless I needed something super sophisticated, that is the method I would use. QED

rmw