# Pressure drop & specific gravity for gas pipe installation



## Eddie77 (Nov 6, 2013)

Im taking my state master exam in 2 weeks & studying my tail off. Can anyone help me to understand what “specific gravity” & “pressure drop” are on all the gas charts please? This has never been laid out for me, I have been made to understand the pressures of systems & the practical ins & outs, but i dont understand these things on the tables.


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## ken53 (Mar 1, 2011)

I think specific gravity relates to how much it weighs per cubic foot, gasoline, diesel, propane, methane. which one is heavier per cubic foot. Pressure drop can be related to the speed it travels to the point of delivery, ie: friction and velocity, expressed in feet per second. The weight of what's moving, also changes of direction which cause turbulence. I have likely made it as clear as mud. I hope it helps anyone that can correct a mistake,please weigh in. 



Eddie77 said:


> Im taking my state master exam in 2 weeks & studying my tail off. Can anyone help me to understand what “specific gravity” & “pressure drop” are on all the gas charts please? This has never been laid out for me, I have been made to understand the pressures of systems & the practical ins & outs, but i dont understand these things on the tables.


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## Plumbus (Aug 4, 2008)

*Specific gravity* (*relative density*)
*The specific gravity of a gas*, γ, is *the* ratio *of the density of the gas* at standard pressure *and* temperature to *the density of* air at *the* same standard pressure *and*temperature. *The* standard temperature is usually 60°F, *and the* standard pressure is usually 14.696 psia.








Gas properties







petrowiki.spe.org





*Pressure drop* is the amount of line *pressure* that is permanently lost as *gas* passes through an instrument in the *gas* line. This *pressure loss* is due to the frictional resistance of the components the *gas* touches.


https://www.alicat.com/wp-content/uploads/2014/01/pressure-drop-P1000833_12001.jpg


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## Eddie77 (Nov 6, 2013)

So, this table has a pressure drop of a 6” water column. how does that effect the installation of fixtures within the property? Is there anything specific i have to account for when sizing the gas system When the pressure drop or specific gravity varies? Ive never had my master or journeymen cite any changes weve had to make during installation. None of them can elaborate on these things either. They all say its been too long since they took their tests for them to remember.


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## Eddie77 (Nov 6, 2013)

I understand that there are different tables for these different scenarios for specific gravity & pressure drop. But, when Im given the print for sizing a gas system, will they also give me the all these details as well so i can look up the proper table?


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## Plumbus (Aug 4, 2008)

Ask you GC. If the GC doesn't know, someone will have to contact the gas utility and ask. Or, ask the building official what table is commonly used in his/her jurisdiction.
The table you referenced shows a 0.6 in wc pressure drop. Where I am, we use the table with a pressure drop of 0.5 in wc.
As for what to account for: 
gas demand of the various appliances
length of run between branches
The quantities shown in the tables take into account variation in the energy of a cfh of a utility's delivered gas. In other words they are a little underestimated as a precaution against under sizing. 
Because the major gas utility in my area generally delivers a CFH of NG with about 1050 btu's of energy, I use 1000 as my devisor rather than 1100 as my code book recommends as a further precaution.


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## Venomthirst (Jun 20, 2018)

In Canada our tables list maximum capacity based on a rated pressure and a pressure drop across the system so for 7-14" w.c the ratings are based off a drop of 1" w.c across the system... if the system is huge and long you drop maybe higher than 1" so best bet would up size your gas line to the next pipe size.. lower the operating pressure the lower the pressure drop because less resistance forcing the gas down the pipe..

Our gas can be delivered at different btu ratings 1000 per cf as a general.. but best way is to dial it in by adjusting gas valve pressure with a manometer and a combustion analyzer... most residential appliances call for 3.5" w.c on the inlet minimum but the burner side maybe lower or raised to get that free air c.o levels as low as possible and your burner efficiency higher as well..

Im still learning myself.. In Canada we have 2 levels of gas liscences in Ontario we have 3 levels. I'm only rated to work on appliances up to 400 mbtu


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## Eddie77 (Nov 6, 2013)

So for my testing, they’ll give me the proper table to use so i can size the system properly. Generally speaking on site, Ill be informed of the information pertaining to the gas utility specifics & can then use the proper tables. In certain cases, Ill have to test for myself?


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## Eddie77 (Nov 6, 2013)

Also, thank you all so much for all the support. I really appreciate the forum & the knowledge brought by all the members. Along with my local people here, I’m getting a lot of clarification which helps a ton!!


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## Plumbus (Aug 4, 2008)

Eddie77 said:


> So for my testing, they’ll give me the proper table to use so i can size the system properly. Generally speaking on site, Ill be informed of the information pertaining to the gas utility specifics & can then use the proper tables. In certain cases, Ill have to test for myself?


Testing for yourself? Wouldn't that be after the fact, since they don't install meters and regulators until after installation and inspection?
You may be over thinking this a bit.


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## ken53 (Mar 1, 2011)

Did you not get training in pipe sizing for gas installation. Most of it is in the code book. The gas company sets the values of what they supply. Something like 1 cubic foot equals 10,000 btu.


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## Eddie77 (Nov 6, 2013)

Plumbus said:


> Testing for yourself? Wouldn't that be after the fact, since they don't install meters and regulators until after installation and inspection?
> You may be over thinking this a bit.


I would much prefer to be given all the info from a reliable source. Venom made reference to using a manometer to adjust the pressure if need be (if I understood that correctly). Just reiterating in case I missed something.


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## Eddie77 (Nov 6, 2013)

I have received training on sizing, yes. Just trying to nail down anything I haven’t gone over with my master or journeyman so I’m not missing anything that could come up on the test.


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## Plumbus (Aug 4, 2008)

_"Something like 1 cubic foot equals 10,000 btu."_
Close. Move the decimal one notch to the left. My book says 1100/1. However, I think my provider (PG&E) pushes it out at about 1,050, so I use 1000 as my divisor.


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## OpenSights (Mar 23, 2015)

There’s this...






In brewing you measure your OG, or original gravity and your FG, final gravity, to calculate your ABV. What you’re measuring is the sugar content of the wort to determine how much fermentable sugars has been eaten by the yeast, thus producing alcohol.



Edit: this post is for humerus purposes only.


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## ken53 (Mar 1, 2011)

Plumbus said:


> _"Something like 1 cubic foot equals 10,000 btu."_
> Close. Move the decimal one notch to the left. My book says 1100/1. However, I think my provider (PG&E) pushes it out at about 1,050, so I use 1000 as my divisor.


Here it's 1000 btu I was just trying to give an example. The utility says 980 plus or minus. Inspectors tell us to clock the meter at 1,000 per cubic foot. My insurance company wanted the meter clock rate on my invoices for every install.


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## Eddie77 (Nov 6, 2013)

I started another topic also gentlemen. Anybody weighing in would be greatly appreciated. It has to do with business questions on the master exam.


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## Plumbus (Aug 4, 2008)

https://www.plumbingzone.com/thread...-for-gas-pipe-installation.87017/post-1254957


In brewing you measure your OG, or original gravity and your FG, final gravity, to calculate your ABV. What you’re measuring is the sugar content of the wort to determine how much fermentable sugars has been eaten by the yeast, thus producing alcohol.

Not unlike the process bread dough goes through.


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## Plumbus (Aug 4, 2008)

_"My insurance company wanted the meter clock rate on my invoices for every install." _

What difference does it make to them?


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## ken53 (Mar 1, 2011)

Plumbus said:


> _"My insurance company wanted the meter clock rate on my invoices for every install." _
> 
> What difference does it make to them?


Supposed to show they weren't over fired. The Gas company does it now for every turn on. They use the one cubic foot dile and time it the same as I did. I think they still do the largest appliance. 
The built-in regulators in H-W gas valves weren't too well tested in the 90s. Burnt down some houses, where the furnaces over fired, it's surprising, but it happened at 40 below. Oversized furnace, ductwork too small. That's why we do temperature rise on the plenum too, and the manufactures want it for warranty.


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## skoronesa (Oct 27, 2015)

Plumbus said:


> _"My insurance company wanted the meter clock rate on my invoices for every install." _
> 
> What difference does it make to them?


Probably some lawsuit 45 years ago they had to pay out big time on so now they want it just to cover their azz.


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