gas pipe sizing and pressure drop question

[StephenJcan]

I have a few questions on gas pipe sizing and pressure drop.
The total load is 20,000 CFH with an equivalent length of 400 feet, gas pressure is 14" W.C. and minimum operating pressure for the equipment is 10" W.C.
Using the formulae for gas pressure I find that with 0.5" W.C. drop, I would use 10" pipe at the main.
My questions are:
1) would it be ok to use 3" W.C. pressure drop in the formula so I can use 6" pipe instead of the 10"?
2) how would I calculate the pressure loss for the pipes?
3) the tables recommend 0.5" W.C. is there a reason for this? Seems like a rather tight restriction for pipe sizing.

Any help would be welcome as this has always been something I'm not sure of.

 

[kbs]

Assuming you are using natural gas, the piping codes state the allowable pressure drop for low pressure (<2lb) is 1"w.c. Therefore using the pipe sizing formula for low pressure gas, you would require 7.33" diameter pipe, so you require 8" pipe. Is there a reason you can't run 5lb gas to the appliance and reduce the pressure at that point? A 3" line at 5psi gas has a delivery of 23617.7cfh.

 

[StephenJcan]

First thanks for the reply.
1) Would you cite me the code that says 1" w.c. drop for under 2lb?
2) Going to 5psi line requires additional permits which the powers that be do not care to do so we're stuck with using 14" w.c.

Thanks again for looking at this.
As an aside, the gas tables all do 0.5" w.c. drop or 10% drop (for high pressure). Why are these values used instead of a higher drop of say 3" w.c.?

 

[kbs]

1) CSA B149 Natural Gas and Propane Installation Code
or NFPA 54-2002 National Fuel Gas Code.

The low pressure regulators are generally set up to deliver 7"w.c. or 14"w.c. When sizing from a table, you would use the .5" drop for 7" delivery and 1" drop for 14" delivery. When using the formula, you would use the same.

 

[StephenJcan]

Sweet. Thanks for the info!

 

[vzeos]

Does your set-up include a service regulator?

 

[StephenJcan]

Well as far as I know the service will be metered and the gas company will be providing the regulator for 14" w.c.
The equipment also has separate regulators.

 

[vzeos]

If you have a service regulator installed by the gas company and set at 14”wc, you should defer to the gas company for their design advice regarding the “house” piping. They would be the knowledgeable authority in your area and they should be able to tell you if NFPA 54 is invoked by code, or what other codes apply.

NFPA 54 is the guideline that would cover the installation of house piping for natural gas. NFPA 54.2.4.4 essentially says you can take as much pressure drop as you can tolerate. But bear in mind that if the service regulator is set at 14”wc, the flow will incur some additional dp through the meter set. The dp through the meter set could be from 0.5 – 3.0’wc depending on the meter and configuration. If you have filters or strainers they will cause some additional drop. You may actually not have more than 0.5”wc to take in your pipe run to the appliance.

You indicated that you had an equivalent length of 400 feet (of 10” pipe?). My calculations indicate you need a smaller pipe, maybe 8” or 6”. I suggest you re-evaluate the equivalent length based on a smaller pipe size and then recalculate the required pipe size. Also, examine your equivalent length values, if you are using short pattern plug valves or even regulator pattern plug valves, you might benefit by replacing them with full bore ball valves.