# Mixed gas pipe size and BTU/Hour

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#### TheOverThinker

##### Member
Is there an accepted/reliable method of calculating expected gas flow for mixed diameter gas pipe?
For example
* 40 feet of 1", then 40' of 3/4"
* 40 feet of 3/4" then 40' of 1"

Would the second be limited only by the first run, e.g. 170,000 BTU/Hour?

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#### wwhitney

If each appliance is served by a separate run from the meter (all branching is at the meter), then you can just consider each appliance separately. If one of those runs is 40' of 3/4" and 40' of 1", the order doesn't matter. You can determine the maximum allowable demand at the end of that run as follows (assuming you have the correct table for your pipe type and regulator pressure):

You have 80'. If it were all 3/4" it could supply 117 BTU/hr. If it were all 1" it could supply 220 BTU/hr. Since it's a mix, it will be somewhere in between.

To actually figure out the allowable demand at the end of the run, you'd need to do something like this: Guess that it could be 150 BTU/hr. Per the table, that would be fine with 50' of 3/4" (151 BTU/hr on the table), or with 150' of 1" (157 BTU/hr on the table).

So 40' of 3/4" uses up 40/50 = 80% of the allowable pressure drop. And 40' of 1" uses up 40/150 = 27% of the allowable pressure drop. So that makes 107%, meaning 150 BTU/hr is probably a bit too high. Except that 157 > 150, and so 27% errs on the high side.

The answer is going to be 145 - 150 BTU/hr.

Cheers, Wayne

#### TheOverThinker

##### Member
So 40' of 3/4" uses up 40/50 = 80% of the allowable pressure drop. And 40' of 1" uses up 40/150 = 27% of the allowable pressure drop. So that makes 107%, meaning 150 BTU/hr is probably a bit too high. Except that 157 > 150, and so 27% errs on the high side.
The answer is going to be 145 - 150 BTU/hr.
Cheers, Wayne
For sequential pipe, I think it's going to be a limit.

If 30' of 3/4 can only supply 199,000 BTUs, no matter what pipe I put after that can't do more. It seems
that the smaller pipe would control.

Thus it matters if the big pipe is first. I'm assuming single pipe mixed size to the meter, but in the real world all sorts of complications are possible.

Is it possible to MEASURE actual flow capacity without, say, a blowtorch? This is for Tankless capacity measurements.

#### wwhitney

A quick preliminary comment on how pipe sizing for gas works: Your gas regulator is set to give a pressure at the meter outlet of (typically) 7" w.c. when the flow rate is the full rating of the meter (which will be labeled on the meter). Then your pipe system is supposed to be sized so that when all the appliances are running simultaneously, the pressure at the shutoff valve for each appliance is at least 6.5" w.c. (that difference is the 0.5" w.c. drop spec on Table 1315.2(1) from the California Mechanical Code that you took an image of in the OP).

Fluid flow through a pipe causes a pressure loss due to friction, where the pressure loss increases linearly with pipe length, but faster than linearly with flow rate. When you have two pipe segments in a row, with different characteristics but the same flow rate (no branches), the pressure losses add up. That's why the order of the two pipe segments doesn't matter.

For sequential pipe, I think it's going to be a limit.
The limit for a given system of pipes is certainly going to be less than the limit you get if you delete some of the pipes (as that deletion removes some pressure loss). So the allowable flow for 40' of 3/4" plus 40' of 1" is going to be less than either just 40' of 3/4" or just 40' of 1". And as I commented it's going to be less than for 80' of 1" (since half the pipe has been shrunk), but higher than for 80' of 3/4" (since half the pipe has gotten larger).

Is it possible to MEASURE actual flow capacity without, say, a blowtorch? This is for Tankless capacity measurements.
Certainly you can after you install your appliance. You use a manometer to measure the pressure at the appliance both when nothing is running, and when everything is running. Not sure if there's an easy way to do so beforehand. E.g. if you can install a flow meter and then vent say 50,000 or 100,000 CFH outside, and measure both the flow rate and pressure drop (while everything else on the system is also running), then that one data point is enough to determine the maximum allowable flow (by extrapolating with the right equation for what flow rate would give a 0.5" w.c. drop).

Cheers, Wayne

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Hey, wait a minute.

This is awkward, but...

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