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





RE: Effective uses of the Pipe Sizing Charts from NFPA 54 for low pressure natural gas in NYC
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
RE: Effective uses of the Pipe Sizing Charts from NFPA 54 for low pressure natural gas in NYC
RE: Effective uses of the Pipe Sizing Charts from NFPA 54 for low pressure natural gas in NYC
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Effective uses of the Pipe Sizing Charts from NFPA 54 for low pressure natural gas in NYC
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.
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RE: Effective uses of the Pipe Sizing Charts from NFPA 54 for low pressure natural gas in NYC
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