End of Gas Piping Run Capacity Issue

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GaryMetro

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For 70' my iron natural gas piping is oversized, mostly 1-1/4" with 1" at the end. I want to extend 5' to a gas log burner which wants 110-130K btus. But the route is super tight (width-wise, no tight bends), and 1/2" CSST is the only reasonable logistical choice. Per sizing/length schedule I would need 3/4" pipe to provide the capacity at that length. But could the 1/2" coming off of 1" for just 5' be enough? Only a gas range and a 60k btu furnace will also be on the line.
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wwhitney

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A diagram of the whole gas pipe system with the meter, fittings, pipe types, lengths, and appliance demands is required.

Once you have that diagram, here's how you check if it complies:

1) A segment is all the piping between two nodes, where the nodes are the meter, the appliances and the tees (edit for the record: and change of size/material fittings). Label each segment with the total flow if all the appliances were running. Also label with the equivalent length, which is the sum of the actual straight pipe segment lengths and the equivalent lengths of the fittings in the segment. For a tee, there are different equivalent length for the straight through path and the 90 degree path; assign each equivalent length to the downstream segment.

2) For each segment, based on pipe type, size, and flow, look up the allowable length (you might need to interpolate if you have the type of table that gives you minimum pipe size for a given length). Divide the actual segment length by the allowable length. That's the fraction of the pressure drop budget that is used by that segment. Label the segment (if you like, you can turn it into a percentage).

3) Now for each appliance, add up the fractions on the path of segments that goes from the meter to the appliance. If the sum is not more than 1 (or 100%), then the appliance will have enough pressure. If all the appliances pass, you're good to go. If some sum exceeds 1, upsize a pipe segment as required, and check again.

Note that in step two, if you have something like a short segment of 1/2" CSST with a high flow, and the wrong sort of table, it may be hard to interpolate E.g. if the table says for 10' you can use 1/2" and for 20' you need 3/4", you really want to know if the transition point is 11' or 19'. So you'll either need the right type of table, or to use a formula for pressure drop and divide that by the allowable pressure drop (for a typical 7" w.c. system, the allowable pressure drop is 0.5" w.c., which is a pretty stringent standard).

Cheers, Wayne
 
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GaryMetro

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Thanks very much Wayne. After re-reading your post several times, if I understand it (big if) I have 3 segments. Adding up actual equivalent length & dividing it by max allowed length for that flow demand, with a little conservative interpolation when chart didn't match, gets me: 61/225 = .271, + 32/100 = .32, + 5/15 = .333. Sum is .924, which means I would likely be OK?
Can't upsize the pipe, would need to downsize the log burner if cutting too close.
 

wwhitney

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Yes, provided (a) you used each segment's individual flow (so if there's just one pipe leaving your meter, that pipe is carrying the gas range BTUs, the 60K furnace BTUs, and the 120K log burner BTUs) and (b) for the last fraction, 5' / 15', if that 15' is from interpolating 10' and 20', it would pay to investigate a little more. Versus if you have a table that tells you 15' is OK for 1/2" CSST, then 5'/15' is conservative.

(b) also shows you that it's a big plus if you can reduce the 1/2" segment from 5' to 4' or 3'.

Cheers, Wayne
 

GaryMetro

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I will look into it a bit more. I could chop a foot of length off the last bit I suppose. Sizing tables for the CSST I'm looking at are identical to iron pipe.
 

wwhitney

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Really, what brand of CSST is that? I thought they always had more pressure drop than iron pipe.

For iron pipe, you should be able to find a "maximum allowable length" table so you don't need to any interpolation. If you can't, let me know and I look for a reference for you.

Cheers, Wayne
 

GaryMetro

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Looks like the sizing section in the CSST install manual also included iron pipe, which I mistook for CSST. Max btu capacity will be under 100k.
Thanks for your help
 

wwhitney

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No problem. If you want a second set of eyes on your calculations, go ahead and post the details.

Cheers, Wayne
 
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