# Gas Pipe Sizing?



## TXPlumbBob (Dec 13, 2013)

On the charts for sizing gas piping there is the inlet pressure 0.50 psi or less.. Then there is the "pressure drop" 0.30 in. w.c. or 0.50 in w.c., and they both have the specific gravity of 0.60. What I do not understand is with the higher pressure drop there is more gas available. 

Can someone explain this to me in super simple terms I can relate to? I have asked the gas suppliers, city inspectors and no one can tell me what it is. 

Also how is the pressure drop determined? Is it in the NFG Code and I am missing it? Which chart do you use and why? Do you use the smaller pressure drop chart to CYA and upsize the piping? 

Any help is greatly appreciated.


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## Plumbus (Aug 4, 2008)

Which code is enforced in your area? If it's UPC, and you're gas company is providing house pressure, (the downstream pressure in a houseline after the meter/regulator is in general in the range of _7 to 11 inches Water Column_, or about _1/4 psi_.) refer to Table 12-8 _because it's values, on average, are the closest to those you are dealing with._


The Specific Gravity - _SG_ - of a liquid is a dimensionless unit defined as the ratio of density of the liquid to the density of water at a specified temperature. Specific Gravity of a liquid can be expressed_SG = ρ / ρH2O__ (3)_
_where_
_SG__ = specific gravity_
_ρ__ = density of fluid or substance (kg/m3)_
_ρH2O__ = density of water (kg/m3)_​How to calculate pressure drop
The drop in pressure in natural gas piping is related to the density of the gas and the vertical distance difference of two locations in the pipe. Density measures the compactness of the atoms that compose a substance. Gravity pulls down the gas, causing pressure differences to increase as the distance between the two locations in the pipe gets larger. Pressure is measured in pascals, or pounds per square inch.
Measure the vertical distance, in inches, between the two points for which you wish to get a pressure difference.



Suppose, for this example, the vertical distance is 1 foot

 Multiply the vertical distance times density of the gas times the acceleration due to gravity on Earth. The acceleration due to gravity, 32 feet per second squared, is the acceleration of falling objects on Earth. Completing the example leads to 1 ft times 0.54 lbs per ft cubed times 32 feet per second squared, or 2.7 pascals.


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## TXPlumbBob (Dec 13, 2013)

you may have to draw a picture. Why does the higher the pressure drop mean there is more BTUs available? I no savvy.


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## phishfood (Nov 18, 2012)

Plumbus said:


> Which code is enforced in your area? If it's UPC, and you're gas company is providing house pressure, (the downstream pressure in a houseline after the meter/regulator is in general in the range of _7 to 11 inches Water Column_, or about _1/4 psi_.) refer to Table 12-8 _because it's values, on average, are the closest to those you are dealing with._
> 
> 
> The Specific Gravity - _SG_ - of a liquid is a dimensionless unit defined as the ratio of density of the liquid to the density of water at a specified temperature. Specific Gravity of a liquid can be expressed_SG = ρ / ρH2O__ (3)_
> ...


 :wallbash::wallbash::hang:

I just finished learning long division, and you throw this at me? :laughing:


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## love2surf927 (Dec 22, 2011)

Plumbus said:


> Which code is enforced in your area? If it's UPC, and you're gas company is providing house pressure, (the downstream pressure in a houseline after the meter/regulator is in general in the range of 7 to 11 inches Water Column, or about 1/4 psi.) refer to Table 12-8 because it's values, on average, are the closest to those you are dealing with.
> 
> The Specific Gravity - SG - of a liquid is a dimensionless unit defined as the ratio of density of the liquid to the density of water at a specified temperature. Specific Gravity of a liquid can be expressed SG = ρ / ρH2O (3)
> where
> ...


So how would you figure if the appliance is at the same height roughly as the meter but goes up 10 ft and down 10 ft would it be negligible since they are ultimately at the same height? I'm confused on this as well how are the btus being delivered higher with a higher pressure drop?


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## ]3ones (Jun 9, 2011)

TXPlumbBob said:


> you may have to draw a picture. Why does the higher the pressure drop mean there is more BTUs available? I no savvy.


 BTU's don't flow through gas pipe. They are used in your gas size tables to make it easier to size pipe for the installer to the appliances. Gas is measured in cubic feet per minute so that pressure drop is not related to BTU's. Take the gas code for what it is, a tool to help you do ur job not a book of science lol


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## BC73RS (Jan 25, 2014)

I've had my gas ticket since '83 and seen some strange charts. In this case, to play it safe, using the greater pressure drop would be the way to go.


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## TXPlumbBob (Dec 13, 2013)

To me the "safe" play would be to use the chart that has the lower pressure drop. Like in the chart labeled 0.50 psi or less, 0.30 in w.c. pressure drop and a specific gravity of 0.60, at 60' a 1" gas line will supply 195 CFH. The same pipe and length with a 0.50 in w.c. pressure drop will supply 260CFH. 

This is where I do not understand when you have more pressure drop but have more gas available. I dunno, I would have thought less pressure = less gas?


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## ]3ones (Jun 9, 2011)

TXPlumbBob said:


> To me the "safe" play would be to use the chart that has the lower pressure drop. Like in the chart labeled 0.50 psi or less, 0.30 in w.c. pressure drop and a specific gravity of 0.60, at 60' a 1" gas line will supply 195 CFH. The same pipe and length with a 0.50 in w.c. pressure drop will supply 260CFH. This is where I do not understand when you have more pressure drop but have more gas available. I dunno, I would have thought less pressure = less gas?


I think it might have a relation to volume. The higher the pressure drop the more volume build up.....this is a good question TX


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## dclark (Dec 12, 2010)

Basics- 
1 PSI = will push a column of air up a pipe 27.7 inches.

1 psi is about 28” w/c
.5 psi is about 14” w/c

Gas piping is normally sized in pressure zones, from meter (reg) to next reg in system, or from meter (regulator) to appliance gas valve, or from appliance reg to orifice.

Using the numbers from your post..

Assume a .5 psi system, with 100’ of 1” gas line from meter to a shut off valve with an orifice open to atmosphere on the outlet.
Install one manometer at the meter, and another manometer just before your valve.

With the valve in the closed position, you’ll agree that both manometers would read 14” w/c.

Open the valve until you read 13.7" at your test setup manometer (pressure drop of .3”).
The manometer at the meter will still 14”. You should now be flowing 196 CFH.

Now open the valve more, until you read 13.5” at your test setup manometer (pressure drop of .5”)
The manometer at the meter will still 14”. You should now be flowing 265 CFH.

Now open the valve all the way.
The manometer at the meter will still read 14” if you are really close to the regulator, and the manometer over by your test setup will read close to 0.0 inches pressure left (14” pressure drop), but you will be flowing a huge amount of gas.

No Smoking.

Regards,
Don


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## love2surf927 (Dec 22, 2011)

dclark said:


> Basics-
> 1 PSI = will push a column of air up a pipe 27.7 inches.
> 
> 1 psi is about 28&#148; w/c
> ...


Thank you for this explanation. How did you figure CFH in this example?


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## dclark (Dec 12, 2010)

I thought I was using the numbers from the previous post, but without my reading glasses.
Numbers (CFH) may not be exact or even close to correct, just for explanation.

It's great getting old.


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## love2surf927 (Dec 22, 2011)

Found this interesting excerpt that may help to understand the concept. I'm learning here too, I love the scientific/technical aspects of our trade, makes me wish I went to school for it.

It is not the “pressure” that counts . . . it’s the “pressure drop” that moves the gas inside the pipe. Piping systems in apartments are traditionally operated at 8 in. water column (WC) or 1/3 PSI. By raising the pressure to 2PSI, several benefits become available when constructing multi-family units.

The pipe size is determined by the amount of energy that can be lost (also called the pressure drop) to move the gas, while leaving enough pressure to meet the minimum requirements of the appliance controls (usually 4 to 6 in. WC). As the pressure goes up, so too does the allowable pressure drop that can be accommodated. The greater the pressure drop, the greater the amount of gas that can be “pushed” through the pipe for a given size. For a given amount of gas, the pipe size can be decreased as the allowable pressure drop is increased. As shown in the figure below, the amount of gas (934 cubic feet per hour or CFH) that can be moved through a 100 ft long - 3/4 in. pipe (operated at 2PSI) would require a 2 in. diameter pipe when the same system is operated at 8 in. WC.


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## love2surf927 (Dec 22, 2011)

This seems to be using the term "pressure drop" differently from what I think you may be thinking of it as (me too). Like in sizing water service pressure drop is caused by friction of the pipe walls and fittings and height. I believe (someone please correct me if I'm wrong) the "pressure drop" in the gas sizing charts is a different type of pressure drop which is the pressure being lost through an appliance for instance. Again Im just learning this too so I'm researching trying to figure it out.


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## TXPlumbBob (Dec 13, 2013)

Probably like Love2Surf927 I have always thought about pressure loss as a restriction or friction in a system. Is this not the case in the "pressure drop" in a gas system? 

Is the pressure drop a measurement of the amount of flow available at the outlet, via a half opened valve or an orifice? 

How do you determine, in a design stage, if the system has a 0.30 or a 0.50 in w.c. pressure drop? 

You guys have been great and I appreciate your patience with an old dog.


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## BC73RS (Jan 25, 2014)

OK, thread your manometer to the appliance, fire up the appliance.
If you read 0.03" on the gauge with a delivery of 14" then 13.97" is used up from friction loss and consumption. 
If you read 0.05" then 13.95" is used up in friction loss and consumption.
This seems backwards to you I'm sure.


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## Caduceus (Mar 1, 2012)

BC73RS said:


> OK, thread your manometer to the appliance, fire up the appliance.
> If you read 0.03" on the gauge with a delivery of 14" then 13.97" is used up from friction loss and consumption.
> If you read 0.05" then 13.95" is used up in friction loss and consumption.
> This seems backwards to you I'm sure.




Correct me if I'm wrong, but it sounds like the scenario that you described would show that the appliance is demanding the 13.97" or 13.95" wc, respectively. Not a matter of just friction loss, but pressure drop by demand. If the appliance is off, you would read 14". If the appliance demands 6" you would read 8", demands 10" you would read 4" and so forth. If the appliance demands 13.95" you would read 0.05" wc.


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## BC73RS (Jan 25, 2014)

Caduceus said:


> Correct me if I'm wrong, but it sounds like the scenario that you described would show that the appliance is demanding the 13.97" or 13.95" wc, respectively. Not a matter of just friction loss, but pressure drop by demand. If the appliance is off, you would read 14". If the appliance demands 6" you would read 8", demands 10" you would read 4" and so forth. If the appliance demands 13.95" you would read 0.05" wc.


Sort of, don't forget the pipe and fitting pressure drop that is in the 13.95 "
Lets say the appliance reg is factory set at 3.5". Then the "Actual" pressure drop then would be 10.45" when reading .0.05" on the gauge when it's firing.


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## wyrickmech (Mar 16, 2013)

BC73RS said:


> Sort of, don't forget the pipe and fitting pressure drop that is in the 13.95 " Lets say the appliance reg is factory set at 3.5". Then the "Actual" pressure drop then would be 10.45" when reading .0.05" on the gauge when it's firing.


if there is that big of a fluctuation when the gas is in use you are undersized. Delivery pressure should not fall that far from the regulated delivery pressure. Go back to the charts and resize.


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## TXPlumbBob (Dec 13, 2013)

BC73RS said:


> OK, thread your manometer to the appliance, fire up the appliance.
> If you read 0.03" on the gauge with a delivery of 14" then 13.97" is used up from friction loss and consumption.
> If you read 0.05" then 13.95" is used up in friction loss and consumption.
> This seems backwards to you I'm sure.


I appreciate your responses, but. What about in the system design/sizing stage? There are not ports or openings to connect a manometer to yet. How do you know you will have 0.30 or 0.50 pressure drop? 

We have not gotten out of the conceptual stage yet or have we ordered material because we do not know what size to order.


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## BC73RS (Jan 25, 2014)

Use the chart that shows the least amount of cfh in this case 0.03".


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## Plumbus (Aug 4, 2008)

love2surf927 said:


> Thank you for this explanation. How did you figure CFH in this example?


CFH's of gas depends on the heat content of gas delivered by the gas purveyor. See below;

http://www.eia.gov/dnav/ng/ng_cons_heat_a_epg0_vgth_btucf_a.htm


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## TXPlumbBob (Dec 13, 2013)

Plumbus said:


> CFH's of gas depends on the heat content of gas delivered by the gas purveyor. See below;
> 
> http://www.eia.gov/dnav/ng/ng_cons_heat_a_epg0_vgth_btucf_a.htm


We used to use 1100 BTUs of Natural Gas was equal to 1 CFH. They changed it several years ago to 1000 btu=1 cfh at 0.60 specific gravity. 

For LP Gas 2516 btu = 1cfh with a specific gravity of 1.52 (heavier than air and sinks to the ground). 



> "Use the chart that shows the least amount of cfh in this case 0.03"."


That may be ok in a residential system at a few hundred cfh but I am dealing with a system that has a developed legnth over 200' and over 2,000,000 btu in volume. Could be hundreds of dollars difference in material and labor and welders.


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## Plumbus (Aug 4, 2008)

TXPlumbBob said:


> We used to use 1100 BTUs of Natural Gas was equal to 1 CFH. They changed it several years ago to 1000 btu=1 cfh at 0.60 specific gravity.
> 
> For LP Gas 2516 btu = 1cfh with a specific gravity of 1.52 (heavier than air and sinks to the ground).
> 
> ...


If you want to cut it to the bone, get in touch with your gas utility and ask they the btu load per cfh. 

However, the quantities you give dictate a 3" line. And, the next step down (2-1/2") has a capacity quite a bit below 2000 mbtu, even figuring 1100 btu's per CFH. Besides, it's always best to error over rather than under. Since I assume the difference of " hundreds of dollars difference in material and labor and welders" isn't coming out of your pocket, why would you want to cut it so close in the first place?



If you're looking to save your client some $$$, requesting medium pressure from the gas co and placing a house pressure regulator at point of use might be an option.


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## TXPlumbBob (Dec 13, 2013)

You are right in that it is shown as a 3" line. It is not a T&M project therefore I will not be saving the client any money just me. I have requested to have the main line supplied with 5psi gas and run an 1 1/2" line instead and there are 5 branches that will be regulated on the roof. 

Now the original system was designed with the 200' column and using the longest branch method. When we change it we will be measuring from the regulator and not the meter so the distances will be significantly shorter and the pipe sizes shown would no longer be accurate. There comes in the question why with a larger pressure drop do you have more btu available. I have always wondered why but could never find anyone that could explain it. 

There have been some great responses but I still do not get how a pressure drop equals more volume of gas. Volume does equal more btu ?


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## wyrickmech (Mar 16, 2013)

TXPlumbBob said:


> You are right in that it is shown as a 3" line. It is not a T&M project therefore I will not be saving the client any money just me. I have requested to have the main line supplied with 5psi gas and run an 1 1/2" line instead and there are 5 branches that will be regulated on the roof. Now the original system was designed with the 200' column and using the longest branch method. When we change it we will be measuring from the regulator and not the meter so the distances will be significantly shorter and the pipe sizes shown would no longer be accurate. There comes in the question why with a larger pressure drop do you have more btu available. I have always wondered why but could never find anyone that could explain it. There have been some great responses but I still do not get how a pressure drop equals more volume of gas. Volume does equal more btu ?


 correct me if I'm wrong but wouldn't you just simply size each run after the regulator for that individual load?


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## love2surf927 (Dec 22, 2011)

wyrickmech said:


> correct me if I'm wrong but wouldn't you just simply size each run after the regulator for that individual load?


Yes, pretty sure that's what he's saying. TX I am curious as to the answer of your question as well. I have been doing quite a bit of research to find the answer but still can't really understand the concept. I have read some really informative stuff but still can't grasp the concept. Here's another good read.

http://www.rkstat.com/Downloads/article-FlowCalculationsforValveSizing.pdf


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## TXPlumbBob (Dec 13, 2013)

love2surf927 said:


> Yes, pretty sure that's what he's saying. TX I am curious as to the answer of your question as well. I have been doing quite a bit of research to find the answer but still can't really understand the concept. I have read some really informative stuff but still can't grasp the concept. Here's another good read.
> 
> http://www.rkstat.com/Downloads/article-FlowCalculationsforValveSizing.pdf


That is a pretty good read. I have not thought about that much math since HS. I need to break out the good ole slide rule. I need to "chew" on that one for a little while. I think we are on the right track. 

P.S. Yes, if I can get the Gas Company, the City Inspector, the Engineer, and the GC to all agree I will install medium pressure gas on the roof and install regulators at each branch off the main. From there at the regulator I will resize the piping for that branch.


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## Plumbus (Aug 4, 2008)

Learning the dynamics of fluid mechanics is a commendable endeavor. However, I fail to see how it directly relates to your specific installation. 
You've indicated that you have a handle on the performance of this project in a cost effective manner without compromising quality or system performance. I'd go with that and leave the book learning for your leisure time. Of course, when you have a complete understanding of the subject, I'm sure more than a few of us would like your version of the CliffsNotes.


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## Plumbus (Aug 4, 2008)

I just Google Mapped Alpine, Texas. Johnny, can you spell boondocks?
I'm happy to know the art of plumbing is practiced so conscientiously in the hinterlands.

BTW: If you can get 5psi from the gas co., the pressure drop from that table (3.5psi) ought to be a real head scratcher.


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## justme (Jul 4, 2012)

I've read through this thread and can't take it anymore. This explains pressure drop 

http://www.socalgas.com/for-your-business/builders-and-construction/elevated-pressure.shtml


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## wyrickmech (Mar 16, 2013)

justme said:


> I've read through this thread and can't take it anymore. This explains pressure drop http://www.socalgas.com/for-your-business/builders-and-construction/elevated-pressure.shtml


 if you can't take these two charts and figure it out you don't need to be running gas lines anyway.lol


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## love2surf927 (Dec 22, 2011)

justme said:


> I've read through this thread and can't take it anymore. This explains pressure drop
> 
> http://www.socalgas.com/for-your-business/builders-and-construction/elevated-pressure.shtml


Yes my excerpt on the 1st page is from this page. I believe he wants to know how to figure the pressure drop in a system that is not installed yet which has still not really been answered.


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## love2surf927 (Dec 22, 2011)

Would it just be the max flow through the highest demand appliance?


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## justme (Jul 4, 2012)

http://www.mycheme.com/calculating-pressure-drops-in-gas-pipelines/

knock yourself out.


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## love2surf927 (Dec 22, 2011)

justme said:


> http://www.mycheme.com/calculating-pressure-drops-in-gas-pipelines/
> 
> knock yourself out.


This is not same kind of "pressure drop" we're discussing, or the same as discussed in the article you linked to, unless I'm just confused.

Edit: excerpted from the article you linked to:
"the greater the pressure drop, the greater the amount of gas that can be "pushed" through the pipe for given size."

This doesn't make sense if we were talking pressure drop as in Darcy Weisbach or hazen Williams formulas. I am aware of how to figure pressure drop, I believe there are two types of pressure drops being discussed here.


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## love2surf927 (Dec 22, 2011)

Pressure drop caused by friction is different than pressure drop through an appliance, so you can loose the condescending tone (implied from your text of course)


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## Nlindbert (Sep 10, 2010)

Off topic but this happend last night in my hometown less than a half mile from my house it was a 1" poly gas line in the ground under the concrete in the parking garage. Three people sent to hospital for injurys.


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## justme (Jul 4, 2012)

love2surf927 said:


> Pressure drop caused by friction is different than pressure drop through an appliance, so you can loose the condescending tone (implied from your text of course)



I've posted the answer but it is necessary to do the reading and understanding yourself , I'll try one more time.

http://solve.nitk.ac.in/dmdocuments/Chemical/theory_pipes.pdf


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## justme (Jul 4, 2012)

love2surf927 said:


> This is not same kind of "pressure drop" we're discussing, or the same as discussed in the article you linked to, unless I'm just confused.
> 
> Edit: excerpted from the article you linked to:
> "the greater the pressure drop, the greater the amount of gas that can be "pushed" through the pipe for given size."
> ...


You are so close .


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## love2surf927 (Dec 22, 2011)

justme said:


> I've posted the answer but it is necessary to do the reading and understanding yourself , I'll try one more time.
> 
> http://solve.nitk.ac.in/dmdocuments/Chemical/theory_pipes.pdf


Ok I read that, but that is discussing frictional pressure drop, am I wrong in saying that frictional pressure drop is not what the charts in the fuel piping section of my code book are referring to?


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## love2surf927 (Dec 22, 2011)

justme said:


> You are so close .


Care to elaborate?


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## justme (Jul 4, 2012)

love2surf927 said:


> Care to elaborate?



"Pressure drop is the loss of pressure of the gas as it travels through the system. It is caused by friction on the interior surface of 
the pipe, fittings etc.. For instance the table header shown below has a pressure drop of 
0.5 inch water column. Since one inch of water column is equal to .036 psi or less than 
4% of a pound of pressure You see that this is a very small loss"


It's just that simple.:yes:


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## TXPlumbBob (Dec 13, 2013)

justme said:


> "Pressure drop is the loss of pressure of the gas as it travels through the system. It is caused by friction on the interior surface of
> the pipe, fittings etc.. For instance the table header shown below has a pressure drop of
> 0.5 inch water column. Since one inch of water column is equal to .036 psi or less than
> 4% of a pound of pressure You see that this is a very small loss"
> ...


So how does a greater pressure drop allow for more BTUs in any given size of pipe. I understand how in a system with water or even air, the friction on the pipe walls and fittings create a loss in both pressure and as a result volume. Also playing in here is atmospheric pressures and rise in elevation from one point to another. Does it come down to velocity vs friction?


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## love2surf927 (Dec 22, 2011)

justme said:


> "Pressure drop is the loss of pressure of the gas as it travels through the system. It is caused by friction on the interior surface of
> the pipe, fittings etc.. For instance the table header shown below has a pressure drop of
> 0.5 inch water column. Since one inch of water column is equal to .036 psi or less than
> 4% of a pound of pressure You see that this is a very small loss"
> ...


I disagree we are talking two different types of pressure drop. Pressure drop from friction etc. does NOT increase the CFH thus increasing amount of btus available. Like I said before I understand the concept you are explaining. However, in the charts in the fuel gas section of my code book, for example table 12-16 compared with 12-17 in the UPC the pressure drop is GREATER in table 12-17 however the available btus is MORE. This is NOT due to pressure drop caused by friction loss or else the available btus would be LESS. 

You claim it's "simple" but maybe you don't understand either. I think we all understand the concept and can figure for pressure drop due to friction, however I believe this is a different pressure drop being discussed here, or I'm just not understanding how more friction loss could make more CFH. It's okay to not understand something, I am mature enough to accept the fact I do not know it all. We understand what friction loss is but that does not explain why the greater the pressure drop the higher the CFH.


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## wyrickmech (Mar 16, 2013)

love2surf927 said:


> I disagree we are talking two different types of pressure drop. Pressure drop from friction etc. does NOT increase the CFH thus increasing amount of btus available. Like I said before I understand the concept you are explaining. However, in the charts in the fuel gas section of my code book, for example table 12-16 compared with 12-17 in the UPC the pressure drop is GREATER in table 12-17 however the available btus is MORE. This is NOT due to pressure drop caused by friction loss or else the available btus would be LESS. You claim it's "simple" but maybe you don't understand either. I think we all understand the concept and can figure for pressure drop due to friction, however I believe this is a different pressure drop being discussed here, or I'm just not understanding how more friction loss could make more CFH. It's okay to not understand something, I am mature enough to accept the fact I do not know it all. We understand what friction loss is but that does not explain why the greater the pressure drop the higher the CFH.


 correct me if I am wrong but if you are talking about a pressure drop at the regulator this is right. The higher the available gas pressure the more stored btu,s. So if you had 5 psi and you dropped it to 14in there would be more btu,s available than if you dropped 2psi to 14in.


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