# More gas stuff



## smellslike$tome (Jun 16, 2008)

Didn't want to highjack the thread so I'm starting another. 

I've found nobody locally who can answer this question.

I'm working off of some assumptions that I think are pretty good but I am not certain and that bothers me.

In the IFGC pipe sizing tables, in the upper right hand corner of each page you will find a list of parameters for that particular sizing table.

For example:

Gas Natural
Inlet Pressure 5.0 psi
Pressure Drop 3.5 psi
Specific Gravity 0.60

My question is with respect to the "Pressure Drop", specifically pressure drop over what? I am assuming that they are referring to the pressure drop across the meter/regulator. They couldn't possibly expect to be able to predict the exact pressure drop due to friction through a distribution system. They could know the pressure drop across a standard type meter/regulator though and so this is what I have always taken this to mean.

It bothered me so much though that I actually called IFGC one day to get some clarification. Man that was scary. I don't know if I just got the wrong person or what but they had no idea. I'm not even really sure that they had any real concept of what I was talking about.

Anyway, does anybody have a definitive answer for this. I'm pretty sure I'm right, I'd just like to be certain. 

Oh yeah when they say pressure drop, should we take that to mean the pressure across the meter drops to this new value or that the pressure drops by this amount? In the example above that could give you an actual pressure on the discharge side of the meter of either 3.5 psi or 1.5 psi depending on how you read it. I think it is to this new value but the whole thing really is not all that clear to me.

All of this is obviously important because otherwise you might not be in the correct sizing table at all.


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## SlickRick (Sep 3, 2009)

I have a call into a mech. eng. I talked to a eng. student and he could not answer the question. Big dawg will be in later.


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## SlickRick (Sep 3, 2009)

OK, got an answer from a mech. engineer. The pressure drop is calculated in the sizing charts from the meter to the most remote outlet. It is based on the Mueller formula which is the standards.


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## TheMaster (Jun 12, 2009)

I think they are assigning a value to the pressure drop based on pipe size and lenght of said pipe in the chart and still have enough gas to operate the appliance. They are Pre figuring pressure drop as a rule of thumb....its not a perfect number. My point is this.....if the gas co installs a 2lb gas regulator just before the gas meter......your not gonna have any real pressure drop on the outlet of the meter...its gonna be 2 lbs. That leaves the size pipe/lenght of pipe to de-value or figure a pressure loss for and still have adequate flow at the appliance.


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## Miguel (Aug 10, 2009)

I think you got it.
It's simply a number they plugged in to cover a variable in the equation when they calculated the tables. Closest real world approximation.

And, yeh, it's the pressure drop that you can expect under the pressure the table was made for to your most distant appliance.

my 2 cents :yes:


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## smellslike$tome (Jun 16, 2008)

But how can this be? How can they possibly predict what a given distribution system will look like, how much pipe, how many fittings. I mean all other things being equal every foot of pipe reduces pressure. Every fitting reduces pressure. Let's say that that is what it means, tell me what sort of gas appliance calls for an operating pressure anywhere near .3" wc (pg. 31 of 2006 IFGC)or for that matter 1 psi which equals 27.?" wc (pg 33 of same). Even if it is calculated on a fixed distance of pipe w/ an average allowance for fittings they still don't know what the demand for a given appliance is and it could range wildly. 

I'm sorry I just don't see how that could possibly be right. Don't they have to be referring to the pressure drop across the meter regulator. With standardized regulators this is not difficult to know, once you know what your downside pressure is then you can calculate pipe diameter based on load vs. distance. You will then know whether you or not you need additional line regulators. I certainly don't know the operating specs for every gas appliance in the world but it seems like everything, at least inside a single family dwelling is going to be between 4"wc and about 10.5"wc. Why such a range? Why not 7.5"wc or 5" wc? It's because, I believe, that manufacturers are accounting for variances in supply pressure within the distribution system based on distance and friction loss throughout the system. You can't design a system that delivers exactly 9" wc unless you are over pressured and butt a regulator right up to the appliance. Most older systems do not allow for this and even on newer systems in which this is done there may be other problems associated with having the regulator right on top of the appliance.


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## TheMaster (Jun 12, 2009)

smellslike$tome said:


> But how can this be? How can they possibly predict what a given distribution system will look like, how much pipe, how many fittings. I mean all other things being equal every foot of pipe reduces pressure. Every fitting reduces pressure. Let's say that that is what it means, tell me what sort of gas appliance calls for an operating pressure anywhere near .3" wc (pg. 31 of 2006 IFGC)or for that matter 1 psi which equals 27.?" wc (pg 33 of same). Even if it is calculated on a fixed distance of pipe w/ an average allowance for fittings they still don't know what the demand for a given appliance is and it could range wildly.
> 
> I'm sorry I just don't see how that could possibly be right. Don't they have to be referring to the pressure drop across the meter regulator. With standardized regulators this is not difficult to know, once you know what your downside pressure is then you can calculate pipe diameter based on load vs. distance. You will then know whether you or not you need additional line regulators. I certainly don't know the operating specs for every gas appliance in the world but it seems like everything, at least inside a single family dwelling is going to be between 4"wc and about 10.5"wc. Why such a range? Why not 7.5"wc or 5" wc? It's because, I believe, that manufacturers are accounting for variances in supply pressure within the distribution system based on distance and friction loss throughout the system. You can't design a system that delivers exactly 9" wc unless you are over pressured and butt a regulator right up to the appliance. Most older systems do not allow for this and even on newer systems in which this is done there may be other problems associated with having the regulator right on top of the appliance.


 Nope they cant predict what the system will look like but your chart should have the pipes lenght,type,size,starting pressure listed. Then they figure a pressure drop based off that info.


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## TheMaster (Jun 12, 2009)

The reason why the manufacturers put a wide range of inlet pressures is because different suppliers deliver at different pressures + gas regs are not perfect. Here its 6" w.c on a low pressure system. The minimum w.c listed is the minimum amount of pressure that the appliance requires to function at the inlet and the highest w.c. being listed before regulation is required. You cannot run a 2lb med pressure system without regulators at the appliance period here.....no matter what kinda pressure you have at the end of the line......it just dont work that way. Med and high pressure systems here require regulators AT the appliance period.


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## smellslike$tome (Jun 16, 2008)

I believe I understand what you are saying and if I'm correct then what you are saying defies common sense.

If I look at the table on page 33, first column is 1/2' sch 40 metallic pipe and the first row is 10 feet. The capacity in cubic feet of gas per hour for this diameter pipe, this length of pipe, and this pressure (?) is 1510 cfh or roughly 1.51 million btu's per hour. 

So do I look at this cfh capacity and say "ok that's plenty for my tankless water heater, I'm good", of course not, that's ridiculous because we no good and well that that capacity has to be at a much higher pressure than the called for operating pressure of the tankless.

So which chart do I need to be looking at?

If the pressure drop is either the terminal pressure for a given length and diameter pipe then there is only 1 chart out of 22 that lists a terminal pressure anywhere near a standard operating pressure for anything I am familiar with. That one chart would be for CSST, a product I don't even use.

I don't see how "pressure drop" can be referring to anything other than the pressure drop across the meter regulator.

I'm still listening to arguments and please don't misunderstand me, I am not simply arguing for the sake of arguing, I just want to know that I am doing this right. If you are in the wrong table then you will not get the right answer.


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## TheMaster (Jun 12, 2009)

smellslike$tome said:


> I believe I understand what you are saying and if I'm correct then what you are saying defies common sense.
> 
> If I look at the table on page 33, first column is 1/2' sch 40 metallic pipe and the first row is 10 feet. The capacity in cubic feet of gas per hour for this diameter pipe, this length of pipe, and this pressure (?) is 1510 cfh or roughly 1.51 million btu's per hour.
> 
> ...


 I cant see your chart so I dunno but I do know its not talking about the pressure drop across the meter/regulator. In different parts of the city here that pressure will vary. What will not vary is the oulet side of the regulator...the house side of the meter. Here the supplier might have 35/60/80 whatever psi and then the service regulator at the meter drops that to either 6" w.c. or 2psi. Here the highest pressure your we can request for a home is 2psi......and at the outlet of the meter you will have 2psi or very close to it.


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## TheMaster (Jun 12, 2009)

Your chart needs to match your starting pressure. What does your gas supplier offer as to house pressures? 6" wc? 2psi? 5psi? Now once you know that you can get you a chart that match's up with that and it will have a built in pressure drop allowance based on type of pipe,size,lenght


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## A Good Plumber (Jun 18, 2008)

1209.4.4 Allowable Pressure Drop. The design
pressure loss in any piping system under
maximum probable flow conditions, from the
point of delivery to the inlet connection of the
gas utilization equipment, shall be such that the
supply pressure at the equipment is greater than
the minimum pressure required for proper
equipment operation. [NFPA 54: 5.4.4]

In other words this means that when all appliances on the gas system are in use, you are allowing for a designed pressure drop, say .5", at any one appliance. By sizing the piping based on a larger pressure drop you are in effect assuring you will not starve the appliance.

If you look at the charts and compare their pipe sizes, based on the load, you will see that a system with a higer allowable pressure drop allows for fewer CU/FT per hour.

The pressure drop refers to the system not he regulator or meter and is used as a guide only. 

BTW some appliances work fine with a pressure drop of .5wc, some others will fail upon ignition.


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## RealLivePlumber (Jun 22, 2008)

This explains it pretty good... from Gastite. ​

With respect to gas pipe sizing, the intent of all model codes is to ensure that there is sufficient gas volume and pressure
supplied to the appliance for proper operation. Language from the International Fuel Gas Code clearly illustrates this point.​​​​​​_“Allowable Pressure Drop: The design pressure loss on any piping system under maximum probable flow_
_conditions, from point of delivery to the inlet connection of the equipment, shall be such that the supply_
_pressure at the equipment is greater than the minimum pressure required for proper equipment operation.”_​

The Gastite
® low-pressure tables are intended for use at a system pressure of 1/2 PSI or less, which encompasses the range of
commonly delivered utility pressures and equipment requirements. To determine which table to use determine the system’s
allowable pressure drop. The appropriate pressure drop can be calculated by subtracting the appliance inlet pressure (typically
5"WC for NG, 10.5"WC for LPG) from the gas source pressure (gas meter for NG, secondary regulator for LPG). Use
the Gastite​​
® capacity table labeled with the appropriate allowable pressure drop and gas type. Increasing the available pressure
drop will increase the available BTUHs, thus decreasing pipe sizes. It is advantageous to allow for a larger pressure drop.
The Summation Method of pipe sizing calculates the actual pressure loss through each section of pipe. The sum of all the
losses is subtracted from the starting supply pressure to determine the inlet pressure to each appliance. The appliance inlet
pressure must fall within the manufacturer's range for proper operation.
Note: Regardless of sizing method employed, the typical NG system should be sized for a minimum appliance inlet pressure
of 5"WC and 10.5"WC for LPG system. Pressures less than the typical minimums may be sufficient for proper appliance
operation but should be reviewed with the manufacturers' input rating and the local administering authority.
Conversely, some modern higher performance appliances require an inlet pressure greater than the typical minimums. Check​the manufacturers' input rating before sizing.
​


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## 1703 (Jul 21, 2009)

Kinda like 

If you have "x" starting pressure and you have "Y" amount of pressure loss at the appliance, you will get "z" CuFt.


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