Gas pipe sizing question

[dannye]

I am planning to convert to a gas stovetop (from electric), and so I started reviewing my gas system to make sure my piping is adequate to add this additional appliance. I need help in figuring out the effect of a regulator on my pipe sizing. All charts that I've seen for gas pipe sizing have grids for distance from the meter (and primary regulator) based on the inlet pressure and pressure drop. But how is the system affected by a secondary regulator?

In my system, I have a regulator (marked 1.87 psig = approx 52"wc, I believe) at the meter, then a 20ft run of 1/2 copper, then a secondary regulator marked 5.5"-12" wc. After the secondary regulator, the line T's twice - one line goes immediately into the furnace, one line goes 20ft with 1/2" copper to my water heater, and the last one is capped off -- and that's the one I would use for my stovetop if I can.

So - do I take my system measurements and sizing requirements from the meter or from the secondary regulator? If I take it from the secondary regulator, my sizing appears to be in good shape. If I take it from my meter, the charts indicate my main feed is significantly undersized. Thoughts?

 

[havasu]

You need to run 3/4" black pipe. 1/2" is way too small for that long run.

 

[Diehard]

What size is the line coming out of the secondary regulator? I believe that the critical one. Do you know what your authority having jurisdiction requires for allowable pressure loss, etc?
Not the one between regulators since you have close to 2 psi and a lot more excessive pressure that can be lost. But that would have to be verified as well but not included in the "Longest Length Method" approach.

 

[dannye]

All of the piping in the whole system is 1/2” copper, except for a very short section of 1/2” black iron where the line goes into the furnace.
Using the longest length method from the second regulator, I should be good.
I’ll have to find out about allowable pressure drop from our city inspector to make sure that first section is ok as-is. Thanks for the tip!!

 

[Diehard]

It doesn't sound right to me.
What's your loads?
Longest length?
And what are you allowing for pressure drop?
Many places only allow 0.5 in. WC when dealing with under 1/2 psi (13.8 in. WC)
Even at 1 in. WC pressure drop, it wouldn't work out.
What chart are you using?
I was assuming average loads like 91 cfh(furnace), 32 cfh(water heater) and 59 cfh(stove top).
In fact it doesn't seem to work even for your existing setup, assuming your longest length is 20 feet plus.

 

[Rickyman]

Sounds like propane, not natural gas

 

[Diehard]

Wow...Never occurred to me. Could very well be. Hence the reason for primary and secondary regulators.

That would make it look much better. Depending on the actual loads on that first common line after the secondary regulator and the longest length used.

EDIT: I suppose you should double check the rated maximum capacity of the secondary regulator, also.

 

[dannye]

Thanks all!

It is a natural gas system, not propane; my apologies for not specifying in the first place.

Actual loads:
Furnace: 100,000 btu / 91 cfh
Water heater: 40,000 btu / 36 cfh (longest length, 20ft from secondary regulator)
Range (proposed): 54,000 btu / 49 cfh
Total: 194,000 btu / 176 cfh

The secondary regulator is a Maxitrol 325-3, 5.5"-12" wc. According to Maxitrol's website, it has a max inlet pressure of 2 psi and max load capacity of 250,000 btu/hr.

I'm in Minnesota, using the 2015 Minnesota Mechanical and Fuel Gas Code (https://codes.iccsafe.org/content/MMFGC2015/chapter-4-gas-piping-installations). I can't find any information from my local folks about allowable pressure drop (in terms of a maximum drop that they will allow in system design) and the Code itself simply states that the allowable pressure drop is such that the appliance has sufficient pressure to operate within its specified tolerances. 5" wc is my minimum rating on both the water heater and the yet-to-be-installed range.

Here's the way I'm seeing it now:
Potential Issue 1 - Sizing of pipe from primary regulator to secondary regulator. I can lose more than 1psi and still be above the pressure range of the secondary regulator. So I believe I can use table 402.4(11) - which indicates an allowable pressure drop of 17"wc. Using this table, my 20ft 1/2 copper line can still deliver more than enough volume to serve my entire system. Is my logic sound on this?

Potential Issue 2 - Sizing of entire system from my secondary regulator. With my second regulator having a range as low as 5.5"wc, that tells me I can only lose .5 wc across the remaining system (to serve my 5" minimum appliances). Using table 402.4(9) for the longest length method on the remainder of the system, I find that each of my individual runs are sufficient at 1/2 (remember that the lines are all separate after two T's immediately after the regulator). Sound logic on this as well?

Follow-up thought to my "Issue 2" logic:the T's that come immediately after the regulator are also 1/2, as is the roughly 8 inches of piping coming out of the regulator. Do I need to consider increasing that small section to 3/4, because the load for this short section is actually the full system load of 176cfh? Or because the section is so short, can it essentially be ignored? Strict interpretation would tell me it can't/shouldn't be ignored, but I'm curious about common practice in this type of situation.

Thanks again for the feedback.

 

[PlumbGate]

I believe that copper is not allowed for natural gas. I think it is against all codes national and local. Are you required to have an inspection of this line?

 

[dannye]

I'll need an inspection only if I run a new line to convert the range over to gas, as I plan to do. Haven't pulled a permit yet, though--wanted to make sure I know the full scope of the project before I dig in.

Copper is acceptable here in MN, as long as it is Type L or K.

 

[Diehard]

As far as that 8 inches of piping coming out of the regulator, which would be the only section to see that maximum "design" load.

As I recall, the "longest Length Method" is a rather conservative approach to sizing the piping. So in all likelihood you would be okay as far as it working okay unless the inspector is a hard ass and/or really doesn't know that.

Tell him/her that it's less of an impact then a fitting, which they don't even bother including due to that conservative approach.

Also, that secondary regulator is 5.5 to 12 " WC. I assume that's it's adjustable range. Knowing the actual pressure it's set at should be known.

 

[dannye]

Many thanks, Diehard! This is helping me become confident enough in the design that I'll go ahead and pull the permit and get the ball rolling. I might run the actual gas pipe sizing equations on the design (rather than just using the longest length tables) just to be totally sure I'm within appropriate specs.

Hugely appreciate this forum and the many replies. I've used it as a resource on a number of occasions, but this is the first time I've posted a question. Much appreciation!!