Fuel gas design
By Timothy Allinson, P.E.
Murray Company, Long Beach, Calif.
At the end of this month, ASPE will hold its biannual Technical Symposium in Dearborn, Mich. I will be teaching a class on Fuel Gas Design so I thought I would give a synopsis of the lecture as this month’s column for those of you who can’t attend.
Fuel gas design is governed by NFPA 54 and either the IFGC (International Fuel Gas Code), UPC (Uniform Plumbing Code), or other local jurisdictional Code, depending on your project’s location. Additional design information can be found in the ASPE Design Handbook #2, Chapter 7, and there are miscellaneous requirements dictated by the American Gas Association, insurance carriers like FM and IRI, and various ASTM and CSA publications for individual components.
The properties of natural gas are that it has a specific gravity of 0.6, meaning 60% the weight of air. It is colorless and odorless, but odor is added by the utility to make leaks apparent. The heat content is approximately 1,050 BTUs per cubic foot, and 10 cubic feet of air are required to burn one cubic foot of gas. Its flame temperature is 3,416ºF.
Most gas utilities offer gas service as either firm service or interruptible service, meaning that during peak gas demand periods your gas can be shut off by the utility and you must switch to an alternate fuel such as fuel oil or propane. This is done of course to save money. Utility pressure is most often low pressure, which is less than 2 psi but is often as low as 4" to 6" water column. The average is 6" to 14" w.c. Medium pressure service is sometimes available depending on the utility and the projects demand. When available it is generally 5 psi. High pressure gas (50 psi +/-) is generally only used for utility distribution and only in some areas.
Gas regulators are provided on all but low pressure utility distribution systems. The regulator that reduces utility pressure to service pressure is called a service regulator and is provided by the utility ahead of the meter. If your project has a hybrid pressure design — meaning portions of medium and low pressure, the regulator that reduces the medium pressure distribution to a low pressure zone is a zone pressure regulator or appliance regulator if it is only serving one piece of equipment.
Seismic areas frequently require the use of earthquake valves that close based on seismic movement to prevent fires. Some utilities require the use of excess flow valves that close in the event the gas service has a sudden and unusually large increase in flow, indicating a dangerous malfunction of some sort.
Gas piping is generally run in black steel with threaded malleable fittings for smaller diameters, while larger pipes are welded. My own firm usually transitions to weld at 2½", although some contractors start welding at 4". The larger the pipe, the harder it is to make a threaded joint that doesn’t leak. Medium pressure pipe is always welded.
Corrugated stainless steel tubing (CSST) has become popular as of late and can be run in spools of 3/8" up to 2". Copper pipe can only be used where the gas has been documented by the utility as non-corrosive (less than 0.3 grains of H2S per 100 cu. ft.). Plastic (PE, not PVC) can be used — buried outdoors.
Gas load is determined simply by totaling the demand of all the equipment and appliances on the system. Each appliance has a gas input rating listed by the manufacturer in BTUs. Since there are approximately 1,000 BTUs per cubic foot of gas, the BTUs of the appliance divided by 1,000 gives cubic feet per hour (CFH.) All of the gas sizing tables are listed in CFH. This system is inherently inefficient, as no diversities are taken, and it assumes all the appliances in the building are burning at full capacity at the same time, and this is absurd for a large building. In my opinion the gas sizing process could benefit from the equivalent of Hunter’s Curve, but no Code authorities have gone through the effort.
If the actual appliance load is not known because the owner has not made specific selections, they can be approximated using Tables contained in all the major Codes, and these Tables all closely approximate Table 5.4.2.1 in NFPA 54. A gas oven and range, for example, is listed as 65 CFH. This assumes 10 CFH for each of four burners, plus 25 CFH for the oven/broiler. So if your project has 100 ranges, the load would be 6,500 CFH, and you can surely see the absurdity in assuming that all 100 ranges have all four burners and oven burning at maximum capacity at the same time — even on Thanksgiving — but this is the way the Codes are all written.
Final pipe sizing is determined using two variables — the load, as described above, as well as the pipe length. The major Codes all recognize three different methods of determining pipe length — the longest length method, the branch length method, and the hybrid pressure method.
The longest length method is the most simplistic and generates the most generous pipe sizes. With this method you measure the length of the longest pipe run and allow for fittings (I usually use 20%) for the total developed length. With the system length known, the appropriate sizing chart is used to determine the various pipe sizes based on load. The most commonly used sizing table is the low pressure table (less than 2 psi in NFPA 54) with 0.5" w.c. pressure drop. Thus one row of one chart can be used to size the entire system. What could be easier?
The branch length method starts out the same as the longest length method, but for the branches and risers that are successively closer to the source (gas meter), the total length can be commensurately reduced by the amount the branch is closer to the meter. This means that the total developed length (TDL) for each major branch or riser will be reduced and might result in slightly smaller pipe sizes for each branch or riser. This method is more time consuming but also more efficient. It is particularly useful if you are adding a branch to an existing system.
The hybrid pressure method is best suited for large systems. This method employs the use of medium pressure gas (MPG) for major distribution. Pressure regulators are provided where required to reduce to low pressure, such as for rooftop equipment and low pressure distribution zones to appliances. In the hybrid method the MPG is sized based on the load and distance from the source to the regulator(s) using the 5 psi chart with a 3.5 psi pressure drop. The low pressure piping is sized based on cumulative load and distance from the regulator to the furthest appliance. Note that both the MPG and low pressure system can be sized using either the longest length or branch length methods.
While a picture is worth a thousand words, it is impossible to print all of the necessary images needed to thoroughly depict examples of this process. However, the NFPA, IFGC, and UPC all have sizing examples. Refer to NFPA 54 Annex C, IFGC Appendix A, and UPC Figure 21-2.
Note that if you are designing with CSST or underground PE there are dedicated tables in the Codes to address these systems at various pressures. CSST has lesser capacity that steel because of the corrugations, while PE has greater capacity because it is so smooth.
On rare occasion you might encounter a situation requiring a gas booster pump. This has only come up once in my 25 year a career, in NYC (which supplies only low pressure gas, 4"-6" w.c.) for a pair of rooftop chillers that required 14" of gas pressure. The key to gas boosters is that there are two means of control. Either direct actuation, such that when chiller #1 runs, gas booster #1 turns on, or automatic control that would be required to boost the pressure to multiple appliances. This control system is fairly complex, requiring a radiator to cool the gas to prevent it from overheating. ASPE has good information on this in Chapter 7 of Data Book 2.
There are other considerations in gas design that go beyond the scope of this article, such as combustion air, altitude correction, pressure testing of piping, and propane design, but the NFPA and related Codes are very good references for all of this information.
Timothy Allinson is a senior professional engineer with Murray Co., Mechanical Contractors, in Long Beach, Calif. He holds a bsme from Tufts University and an MBA from New York University. He is a professional engineer licensed in both mechanical and fire protection engineering in various states, and is a leed accredited professional. Allinson is a past-president of aspe, both the New York and Orange County Chapters.