# Calculating gas capacity with mixed sizes of black iron pipe

If I have a gas meter connected to 50' of 1" pipe and then 30' of 3/4" pipe, it doesn't seem correct to determine the amount of gas available at the end of the run based upon a chart that considers the entire run to be 80' of 3/4".

Using this table

IFGC TABLE 402.4(2)/IRC TABLE G2413.4(1) SCHEDULE 40 METALLIC PIPE Gas Natural Inlet Pressure Less than 2 psi Pressure Drop 0.5 in. w.c. Specific Gravity 0.60

Using the accepted Longest Run Method, the capacity at then end of 80' of 3/4" pipe would be 117 cubic feet per hour (ft3/hr).

I am thinking that it should be calculated by first determining the capacity available at the end of 40' of 1" pipe, which is 320 ft3/hr. Then determining the percentage drop in capacity of 3/4" pipe during the first 30' of pipe. Since the table doesn't show a value for capacity at 0 ft, the best that can be done would be between 10' and 40' (so 30' total) [(170-360)/360] which gives a 52.78% decrease. Applying this decrease to the originally calculated 320 ft3/hr results in [(100%-52.78%)*320] 151 ft3/hr.

This 151 seems much more reasonable than 117 ft3/hr.

Interestingly enough, I calculated the percentage pressure drop of 3/4" pipe between 50' and 80' [(117-151)/151] which gives a 22.52% decrease. Applying this decrease to the originally calculated 320 ft3/hr results in [(100%-22.52%)*320] 247.9 ft3/hr.

I feel like I am certainly not understanding something. Any assistance is greatly appreciated.

• If it was 1" all the way, then you'd only have 220 ft3/hr, so the "interestingly enough" paragraph is clearly wrong. Commented Dec 2, 2023 at 17:30
• I agree. I'm trying to understand why it is so wrong. Thanks for your comment. Commented Dec 2, 2023 at 17:33
• The "first order" calcuation would be to calculate the achievable flow rate for (1) 50' of 1" pipe and (2) 30' of 3/4" pipe. The achievable flow rate for these two in series would be the lesser of the two achievable separate flow rates. No reasonable code or rating system would require you to treat the series combination as 80' of 3/4" pipe. Commented Dec 2, 2023 at 20:43

If you must follow code tables, you're unlikely to get cut any slack. If your applicable code considers it 80 feet of 3/4" pipe, that's what you have to deal with, or replace 30 feet of pipe with 1" pipe if it's not enough.

For informational purposes only, using the calculator at https://www.engineeringtoolbox.com/natural-gas-pipe-calculator-d_1042.html (which page also has the formulas laid out) I broke out your 50 foot run of 1" sch 40 (actual ID 1.049") and your 30 foot run of 3/4 sch 40 (actual ID 0.824") and then played with dividing the 0.5 PSI (total for the run) pressure drop between them to approximate equal flow.

50 feet of 1.049" at 0.15psi drop and 50 feet will flow 135 cfh

30 feet of 0.824" at 0.35psi drop and 30 feet will flow 131 cfh

50 feet of 1.049" at 0.14psi drop and 50 feet will flow 129 cfh

30 feet of 0.824" at 0.36psi drop and 30 feet will flow 133 cfh

So it's between those, (presumably about 132) and I can't be bothered fiddling about in the thousandths of a PSI range making them match exactly. This is ignoring losses at the reducer fitting, so that's an upper limit of your expected flow at 0.5PSI total drop for two pipe sizes.

• Thank you very much! Commented Dec 2, 2023 at 18:08