Gas pipe capacity with mixed size pipes

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Mr.ZippyTheSquirrel

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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.

Please help me understand what is correct.

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.
 

wwhitney

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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".
That's correct; it is a simplifying conservative estimate, but a more detailed analysis would give you a higher, more accurate result.

Your approach for interpolating doesn't work, because the table is based on two things: an allowable pressure drop P (0.5" wc for Table 2), and an equation for pressure drop of the form P = (some constants) * length * flow^A / diameter^B. The exponents on flow and diameter are not 1, so linear interpolation on flow or on diameter doesn't work. The table just calculates the maximum flow for a given length and pipe inside diameter.

But the exponent on length is 1, so you can linear interpolation based on length. What you really need is a table that lets you input the pipe diameter and flow rate, and then the entry is the allowable length for those two variables. I'm not aware of a standard source for such a table (my old Wardflex installation manual includes one for some reason), but you could fit the table data to determine the exponent A and then make your own.

One example of a simple inference you can draw from the given table is, say: 3/4" for 30' is good for 199 CFM, and 1" for 100' is good for 195 CFM. Those flow rates are almost the same, and we can round 199 down to 195 conservatively and with little loss of accuracy. So if you had an appliance that demanded 195 CFM, you'd be fine using 30' of 3/4"; or 100' of 1", or 15' of 3/4" and 50' of 1"; or 3' of 3/4" and 90' of 1"; or any other similar interpolation.

Here's another inference we can make that should suffice for your original question: 3/4" for 80' is good for 117 CFM, while 1' for 250' is good for 119 CFM. Ideally those flow rates would be the same, and then we could say 80' of 3/4" = 250' of 1", or 1' of 1" = (8/25)' of 3/4". But since the allowable flow rate for 1" is slightly greater than for 3/4", that approximation is conservative.

Then your 50' of 1" pipe plus 30' of 3/4" pipe is conservatively equivalent to 16' + 30' = 46' of 3/4" pipe. Now you can use the 50' row of the chart and the 3/4" column to get an allowable flow of at least 151 CFM.

Cheers, Wayne
 
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