Equivalent Diameter

[cr1973]

I am looking for a table or chart that shows equivalent diameter between 2 or 3 smaller diameter pipes and one larger diameter (for water main). For example, a 12" diameter main has the same capacity as two 8" mains.

I remember seeing some tables on the internet some time ago, but just can't find it now.

 

[vzeos]

cr1973,
See the parallel pipes calculator in the attached link.

http://kirkmansoftware.com/

 

[RWF7437]

You can probably create your own Table usng the Manning Equation and remembering that the slope "S" is the slope of the hydraulic grade line.

BUT caution. It is almost never economical to replace a single larger pipe with two or more smaller pipes. For example:
Flow = 1000 gpm
Length = 1000 ft

Capacity of 12" pipe @ 5 ft/sec = 1762 gpm
Capacity of (2) 8" pipes @ 5 ft/sec = 1567 gpm
Velocity of 12" pipe @ 1000 gpm = 2.84 ft/sec
Velocity of each 8" pipe @ 1000 gpm = 3.19 ft/sec
Head Loss of 12" pipe @ 1000 gpm = 14.04 ft per 1000 ft
Head Loss of (2) 8" pipes @ 1000 gpm = 38.48 ft per 1000 ft

Add to this the cost difference in installed cost and energy costs over the life of the pipeline and you'll find a huge advantage for the single larger pipe.

good luck

 

[cr1973]

Thanks for all the replies. We've got an old system with pipes in the 1880s and 1890s still in use. Then in the 1940s or 50s, a 2nd line was installed, so we've got some blocks have 2 mains running in parallel - which will be replaced with 1 single pipe when upgrades are made.

Thanks again

 

[rconner]

There is a chart of equivalent pipe diameters (of nominal size dimensions) that may be helpful for some purposes in Table No. 17-12 pg 17-10 at

http://www.acipco.com/adip/products/Sect17.pdf.

 

[RWF7437]

There is also a simple "rule of thumb" used for many years in the water supply field. It goes like this:

Decide on the flow you want to transmit; say 1000 gpm
Guess at a nominal pipe size in inches: say 8-inch
Square the pipe size in inches: 8x8 = 64
Multiply this number by 10; 10x64 = 640
This is the approximate capacity, in gpm of an 8-inch pipe flowing at about 3 to 6 feet per second.

That is: an 8-inch pipe can carry about 640 gpm.
Two 8-inch pipes can carry about 2x640 = 1280 gpm.

From this you can conclude that an 8-inch pipe is about the right size. This will get you within a pipe size of the "correct" answer.

You can easily use this kind of logic to arrive at any number of equivalent pipes without creating a table or doing a lot of complicated math.

good luck