Effective uses of the Pipe Sizing Charts from NFPA 54 for low pressure natural gas in NYC

[msm15]

I am trying to size up the gas lines for a bunch of 115k btu Furnaces - 115 CFH. and would like to know if the sizing charts in NFPA 54 can be used, and how the pressure loss due to elevation effects things if it does at all.

I will simplify the question as the details are not that important.

Incoming pressure is 6" of water column - roughly .22psi the pipe will be Sch 40 metal, will have run length straight up vertically of 80' with a 115kBTU of 115 CFH furnace at the top (6th floor).

According to the tables - 6.2(a) a 1" would be more then sufficient - being able to supply 119CFH to a distance of 150'

The Architect designing the plans says we cant use those tables and wanted a 2" line just for that heater - that sounds ridiculous and I am quite sure he doesn't know what he is talking about. I would like to do the calcs based off the Low pressure Gas Formula in 6.4.1 but I am not sure what the pressure loss is for Delta H.

Does the elevation really effect things that greatly ?

Thanks

 

[LittleInch]

If I've calculated this right that elevation in essentially atmospheric pressure natural gas equates to a pressure head of 0.022 psi, so 10% of your available head. Significant when you've only got 6" water column.

I've never used those tables, but don't forget that the burners need some positive pressure at the flange so that needs to be taken off your 0.22 psig s well as the static head. 1 1/2" sounds a bit more like it and even slightly bigger pipe has a much bigger capacity for the same pressure drop.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[msm15]

are you saying the pressure loss is 10% ?

 

[LittleInch]

No. I'm saying that the pressure you need to get the gas vertically up 80 feet is 0.022psi. On top of this you add the required end pressure and then the pipe friction losses.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[katmar]

If you search through the archives of this forum you will find references to the gas pressure increasing as you go higher in a building. Although this sounds counter intuitive it is actually correct and I will try to illustrate it with your numbers.

In my opinion, the NFPA Table 6.2(a) numbers are the correct ones if your natural gas has an SG close to 0.6 (or a molecular weight of about 18). If 119 CFH gives 0.3 inch WC over 150 ft in a 1" pipe then the pressure drop due to friction will be 0.16 inch WC over your 80 ft (= 0.3 x 80 / 150). I have cross checked these numbers and am happy that they are reasonable. The gas velocity in your pipe is 5.2 ft/s, which is also reasonable in my book.

As LittleInch has pointed out, as you rise 80 ft there is a static head of gas that causes the absolute pressure in the gas pipe to decrease. I calculate this as 0.027 psi - a bit higher than LittleInch's 0.022, but not enough to matter. However, and this is the crux of the matter, as you rise up the 80 ft the air pressure outside the pipe also decreases. Air has a higher density than Low Pressure Natural Gas and in rising the 80 ft you will find that the atmospheric pressure is 0.042 psi lower than it is at grade.

Now that we have all the numbers we can put them together. Let us assume that the atmospheric pressure at grade is 14.7 psia. The Natural Gas line is controlled at grade to 6 inch WC or 0.217 psig. The absolute pressure in the Natural Gas line is therefore 14.917 psig. The friction due to gas flow up the 80 ft of pipe is 0.16 inch WC or 0.0058 psi. Also we lose 0.0271 psi because of the static head of the gas. The absolute pressure in the gas line at the 80 ft level is therefore 14.917 - 0.0058 - 0.0271 = 14.884 psia.

The atmospheric pressure at the 80 ft level is 14.7 - 0.042 = 14.658 psia. The gauge pressure at the 80 ft level is the difference between the absolute pressures inside and outside the pipe, i.e. 14.884 - 14.658 = 0.226 psig (6.26 inch WC). So, although the gauge pressure was 0.217 psig (6 inch WC) at grade it has increased to 0.226 psi (6.26 inch WC) even though we have lost some pressure due to friction in the pipe. You should point this out to your furnace supplier, even if it is just to cover yourself.

Unless you have some very high surges in flow at start up of the furnace, or if you have other loads to consider, I would regard the 1" pipe to be adequate.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[LittleInch]

I was wondering how the difference in SG (lighter than air) bit worked out. I normally work on high pressure stuff where this is irreelvant. Also the OP missed out the bit about the capacity of the pipe being based on 0.3" WC. 1" looks more than adequate.

However if there is a "bunch" of heaters, why are thay all being supplied as a single pipe each? Can't you put one bigger pipe in (3" / 4"?) and then branch off from it?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way