On Jul 17, 9:31*am, Wayne Whitney wrote:
On 2010-07-17, DD_BobK wrote:

Wow, Wayne, that was the long answer. *

Yeah, I guess I was feeling verbose. *Happens sometimes when I'm
procrastinating a little. *:-)

And the absolutely "correct: way to calc oneself to the answer but
there is a way to get there with a lot less work and typically
fittings dont matter all that much.
http://www.ci.pleasanton.ca.us/pdf/bldg-gaspipe.pdf

I disagree about the fittings. *The chart in the table above is still
based on equivalent length, so you need to include the fittings. *And
the equivalent length for a 3/4" 90 degree bend is 2'. *So if you use
4 of them to jog around an obstacle, you need to count those fittings
as 8 extra feet.

As for the method in the above PDF, it is a good method. *It actually
addresses a different question: given the lengths and demands, how
should I choose the pipe sizes? *Whereas the method I was using is
more appropriate for the OP's question: will a particular choice of
pipe sizes be adequate?

BTW, the sizing method in the PDF is quite conservative. *In the
example given, it comes up with 1 1/4" pipe for the main branch. *But
I'm sure that if you changed that to 1" and checked the pressure
drops, it would still be adequate.

So what you can do is use the PDF sizing method, and then once you
have a choice of pipe sizes, calculate all the pressure drops (which
will be per force OK), and then see how much "safety factor" you have
left at each outlet. *Based on that you can try downsizing a few
branches and then recheck the pressure drops to verify they are still
OK.

Cheers, Wayne

I think you hit on the real answer.....


BTW, the sizing method in the PDF is quite conservative.

yes, four elbows add to effective length but since the table steps are
10' at a time AND the delivered BTU's are conservative

And the equivalent length for a 3/4" 90 degree bend is 2'.

Got a cite specifically for low pressure gas flow equivalent lengths?

Cutting the pipe size close (imo) is not worth the calc time, maybe as
an academic exercise (which I'm not adverse to)
but from a practical point of view the PDF is the way to go.

And the typical new install (usually what's being modified) doesn't
have that many elbows unless the installer was a hack.

Sometimes its just not worth the time to do a close calc when the
quick & dirty method will give you an adequate design, its not like
steel pipe is all that expensive.
And most people can follow the PDF method way easier than the calc's
method.

I disagree with respect which method suits the OP.
It the OP checks his current piping system and proposed mods against
the PDF method & the "checks ok"....he's done.
If it doesn't check ok....then it's time to "sharpen" the pencil.

Plus it all depends on what is really the easiest way to physically do
the job.

cheers
Bob