Over the past 40 years, I have been involved with the design of fuel gas systems, inspection of fuel gas installations and investigation of fuel gas system explosions and fires that have led to the loss of life and property. Fuel gas systems can be a safe and economical choice for space heating, water heating or process applications if designed, installed and maintained properly.
Fuel gases include natural gas, propane, butane, methane, hydrogen and mixed gas es. For a given project, the design professional should first determine if one of the above fuel gases is available to the site from a local utility or gas service provider — several projects I have been involved with used methane gas captured from wells drilled in landfills to run electric generators. They produced power and sold it to nearby industrial facilities.
Other projects used captured methane gas from wastewater treatment plants to run engines that drove air compressors for the aeration process at the wastewater plant. These gases — when designed, installed and maintained properly — can provide a relatively clean-burning energy source for fuel-burning equipment such as generators, boilers, water heaters, furnaces, infrared heaters, cooking equipment, fireplaces and many other fuel-burning appliances.
When designing fuel gas systems, the design professional or installer should determine which code has jurisdiction in the municipality where the project will be built. In the United States, there are primarily two model fuel gas codes. Local jurisdictions may have adopted one of these model codes or provided their own code with a version of a model code with local amendments. The applicable codes should be identified on the project drawings or in the specification manual.
Some local jurisdictions do not automatically adopt the latest version of the model code. Some jurisdictions using very old codes had the fuel gas and venting systems covered under various chapters in the old BOCA or Uniform Plumbing Code or Uniform Mechanical Code.
History of Fuel Gas Code Development
The National Fuel Gas Code (NFPA 54) is the oldest fuel gas code in the United States. Before 1974, the contents of NFPA 54 appeared in several other documents; in 1974, they were combined into one document. In 1980, portions of the ASME B31.2 piping code were included. Over the years, many additions have been made to incorporate important safety requirements.
Shortly after the International Code Council was formed, it produced an International Fuel Gas Code. Around the time, the UPC and UMC had chapters covering fuel gas piping installations. The ICC continues to update its code in three-year cycles. The International Association of Plumbing & Mechanical Officials struck a deal with the National Fire Protection Association to use the NFPA building code; they have been harmonizing the uniform codes with the NFPA 54 national fuel gas code over the last few code cycles.
The fuel gas codes have a section on approved materials for fuel gas pipe, valves and fittings. Over the years, fuel gas systems have been installed in accordance with various model fuel gas codes and various editions of the model or local codes. The model codes have changed and evolved over the years.
Fuel Gas Types
The most common fuel gas is natural gas. Propane also is a common fuel gas in rural areas where the gas delivery is in liquid form; it evaporates into the piping system at lower pressures. Butane and mixed gases are less common fuel gases but found occasionally.
All fuel gases have a heat content or caloric value when burned. Check with the gas supplier or gas utility to confirm the caloric value of the fuel gas. The caloric value helps to determine the cubic ft./hr. of the fuel gas required for the application. Knowing the caloric value of the fuel gas can help with pipe sizing.
• Natural gas. Generally, natural gas has a caloric value of 1,000 British thermal units/cubic ft. The caloric value per cubic foot can range from about 950 BTUs/cubic ft. to 1,100 BTUs/cubic ft., depending on the gas utilities’ caloric value for their gas. The specific gravity of natural gas is 0.65, while air has a specific gravity of 1. That means natural gas is lighter than air and will dissipate when released into the atmosphere.
The flammability range of natural gas is from 3.9 percent to 15 percent volume in air. Any concentration of natural gas below 3.9 percent will not ignite because there is not enough fuel-to-air ratio for combustion or it is often said to be too lean for combustion. A concentration of gas above 15 percent will generally not burn because the fuel-to-air ratio is too rich for combustion. The amount of combustion air required for natural gas is 10 cubic ft. of air for every cubic ft. of gas or 1 cubic ft. of air for every 100 BTUs.
I experienced this in a forensic investigation where a furnace not maintained properly would not fire properly. The maintenance personnel had let the combustion air filter get clogged to the point where hardly any air could get into the combustion chamber. During a maintenance check to get the furnace running, they noticed the combustion air pressure switch was not letting fuel into the combustion chamber (because the inlet filter was clogged), so maintenance worker No. 1 put a jumper wire across the safety switch contacts to get fuel into the combustion chamber.
At this point, it was a rich, sooty flame starved for air. As the filter continued to clog, finally the fuel mixture was too rich to burn and the electronic ignition on the furnace would not light the burner. Maintenance man No. 2 was called to investigate when the furnace did not light and factory workers were cold. The second worker was severely burned by a ball of fire that emitted from the inspection port when he opened the inspection port sight glass door — blackened with soot because of the rich flame condition.
• Propane. Propane use is common in rural areas and has 2,500 BTUs/cubic ft. The specific gravity of propane is 1.52. It is heavier than air and will pool on the ground or in low places such as a basement when released into the atmosphere. Some local codes have addressed this by not allowing fuel gases that are heavier than air in basements. The flammability range of propane is from approximately 2.1 percent to I0.1 percent volume in air.
Many years ago, a propane tank from a food vendor leaked propane into a low area under some bleachers at the Indiana Fairgrounds. The gas got to an ignition source and erupted into a lake of fire under the bleachers full of spectators.
The amount of combustion air required for propane is 25 cubic ft. of air for every cubic ft. of gas or 1 cubic ft. of air for every 100 BTUs. If you are designing a building using propane as the fuel gas, be sure to let the equipment manufacturer know which fuel you will be using. The orifices in the burner will be smaller for equipment using propane and even smaller for butane.
• Butane. Butane is not as common as propane and is used mostly in rural areas. Butane has a BTU content of approximately 3,200 BTUs/cubic ft. The specific gravity of butane is 1.95, which is much heavier than air. Like propane, butane will pool on the ground in low places and is subject to restriction in some codes. The flammability range of butane is from approximately 1.9 percent to 8.6 percent volume in air. The amount of combustion air required for butane is 32 cubic ft. of air for every cubic foot of gas or 1 cubic ft. of air for every 100 BTUs.
When using natural gas, the designer should determine the BTU requirements for each section of piping and divide by 1,000 BTUs/cubic ft. of gas to get the cubic ft./hr. (CFH) gas flow for each section of pipe. For example, if a pipe had a connected load that required 200,000 BTUs per hour, it would be a demand of 200 CFH.
Similar conversions for propane require dividing the connected load in BTUs in a given section of pipe by 2,500 for propane and 3,200 for a butane system to determine the CFH of fuel gas. After determining the CFH of gas for a section of pipe, using the appropriate gas pressure table and overall piping system length, professionals can select the pipe size from the tables in the code.
Natural gas pressures are expressed in several different ways. High-pressure gas systems are expressed in pounds/square inch (psi). Low-pressure gas can be expressed in psi, inches of water column (wc) or ounces (oz) of pressure. One psi equals approximately 28 wc or 16 oz. of pressure. Therefore, 1/2 lb. pound equals 14 in. wc or 8 oz.
Fuel gas pressures vary by the type of gas being used. Natural gas is typically piped from the gas fields to utility companies in high-pressure piping mains that can see pressures in the thousands of psi. The pressures are high to move large quantities of gas through a relatively small pipe. The local utility company typically buys gas from the market through transmission mains with a meter and regulator assembly to record the amount flowing.
Propane and butane are often supplied in high-pressure liquid cylinders and at lower pressures will be in a gaseous state.
In older East Coast cities that have old cast-iron gas main piping, the gas utility cannot provide pressures above 1/4 psi to 1/2 psi to the gas regulator because the older cast-iron pipe mains could rupture, crack or leak fuel gas. Check the fuel gas appliance manufacturer’s recommendations for minimum and maximum gas pressures for the equipment to operate properly.
Fuel gas piping is generally not allowed to enter a building below grade. Gas piping installations where the pipe enters the building below grade have been responsible for many building explosions and fires. When soil around the fuel gas pipe shifts or settles, it creates a void along the pipe where gas leaks, far away from the building. It can travel along the pipe and enter the basement or crawl space if not properly sealed. Piping in concealed spaces or under floors should be double-wall piping with the outer pipe vented to the outside.
Typically, the utility distribution pressure is less than 100 psi but can be higher in areas that have seen a lot of growth. The utility company will typically provide a gas regulator with the gas meter assembly at each service connection to reduce the pressure to the customer's desired pressure. The pressure often is reduced to about 1/2 psi or 14 in. or 8 oz. of gas pressure.
Some industrial or heavy gas users may use up to 5 psi. Most mechanical codes have pressure limitations of 5 psi in a building. There can be variances if large industrial boilers, furnaces or other equipment require higher pressures to operate.
Piping connections to equipment should have a gas shut-off valve with a dirt leg and a union for removal of the equipment. If a gas appliance is not connected to the gas piping system, gas outlets that are not connected to appliances are required to be capped gas-tight. I have investigated many explosions and fires associated with uncapped gas lines that leaked fuel gas. A simple cap or plug can prevent a disaster.
Gas piping valves should be rated for the required pressure and have a fuel gas service rating. Typically, in low-pressure piping systems, a lubricated gas cock is used. Higher-pressure systems may use ball valves or other approved valves.
Sizing gas piping should be done in accordance with the sizing tables in the mechanical or gas code. Typically, the total developed length of pipe is determined from the gas piping regulator to the farthest piece of equipment. The column for the next highest length in the sizing table should then be used to size the piping from the farthest outlet back to the pressure regulator, increasing in size for the additional CFH at each branch as required.
Model Fuel Gas Codes
The two model codes in the United States are the International Fuel Gas Code, which is administered by the International Code Council, and the NFPA 54 —National Fuel Gas Code. The following is a review of both codes.
1. The 2018 International Fuel Gas Code (IFGC)
The format of the IFGC allows each chapter to be devoted to a particular subject, except for Chapter 3, which contains general subject matters not extensive enough to warrant their own independent chapter.
• IFGC Chapter 1 — Scope and Administration. This chapter establishes the limits of applicability of the fuel gas code and describes how the code is to be applied and enforced. A fuel gas code, like any other code, is intended to be adopted as a legally enforceable document and it cannot be effective without adequate provisions for its administration and enforcement.
The provisions of Chapter 1 establish the authority and duties of the code official appointed by the authority having jurisdiction. It also establishes the rights and privileges of the design professional, the contractor and the property owner.
• IFGC Chapter 2 — Definitions. Chapter 2 is the repository of the definitions of terms used in the body of the code. Codes are technical documents and every word, term and punctuation mark can impact the meaning of the code text and the intended results. The code often uses terms having a unique meaning; the meaning can differ substantially from the ordinarily understood meaning of the term as used outside of the code.
The user of the code should know that terms defined in the code are generally italicized in the code text. This italicization helps the user become familiar with and consult the definition’s chapter. The definitions are essential to the correct interpretation of the code and the user may not always be aware the terms are defined in this chapter when they are using the code.
• IFGC Chapter 3 — General Regulations. This chapter contains broadly applicable requirements related to appliance location and installation, appliance and systems access, protection of structural elements and clearances to combustibles, among others. It also covers combustion air provisions for gas-fired appliances.
• IFGC Chapter 4 — Gas Piping Installations. The chapter covers the allowable materials for gas piping systems and the sizing and installation of such systems. It also covers pressure regulators, appliance connections and overpressure protection devices. Gas piping systems are sized to supply the maximum demand while maintaining the supply pressure necessary for safe operation of the appliances served.
• IFGC Chapter 5 — Chimneys and Vents. This chapter regulates the design, construction, installation, maintenance, repair and approval of chimneys, vents, venting systems and their connections to gas-fired appliances. Properly designed chimneys, vents and venting systems are necessary to conduct to the outdoors the flue gases produced by the combustion of fuels in appliances. The provisions of this chapter are intended to minimize the hazards associated with high temperatures and potentially toxic and corrosive combustion gases.
It also addresses all factory-built and site-built chimneys, vents and venting systems used to vent all types and categories of appliances. It covers direct-vent appliances, integral-vent appliances, side-wall mechanically vented appliances and exhaust hoods that convey the combustion byproducts from cooking and other process appliances.
• IFGC Chapter 6 — Specific Appliances. This chapter addresses specific appliances the code intends to regulate. Each main section applies to a unique type of gas-fired appliance and specifies the product standards to which the appliance must be listed. The general requirements found in previous Chapters 1 through 5 also apply. The sections in Chapter 6 add the special requirements that are specific to each type of appliance.
• IFGC Chapter 7 — Gaseous Hydrogen Systems. This chapter is specific to gaseous hydrogen generation, storage, distribution and utilization systems, appliances and equipment. Note that hydrogen is not within the definition of “fuel gas,” but it is, nonetheless, commonly used as a fuel for fuel-cell power generation and fuel-cell-powered motor vehicles.
The scope of Chapter 7 is not limited to any particular use of hydrogen (see Sections 633 and 635). Hydrogen systems have unique potential hazards because of the specific gravity of the gas, its chemical effect on materials and the fact that it is not odorized.
• IFGC Chapter 8 — Referenced Standards. This chapter lists all the product and installation standards and codes referenced throughout Chapters 1 through 7. As stated in Section 102.8, these standards and codes become an enforceable part of the code (to the prescribed extent of the reference) as if printed in the body of the code. Chapter 8 provides the full title and edition year of the standards and codes, in addition to the address of the promulgators and the section numbers in which the standards and codes are referenced.
• IFGC Appendix A — Sizing and Capacities of Gas Piping. This appendix is informative and not part of the code. It provides design guidance, useful facts and data, and multiple examples of how to apply the sizing tables and sizing methodologies of Chapter 4.
• IFGC Appendix B — Sizing of Venting Systems Serving Appliances Equipped with Draft Hoods, Category I Appliances and Appliances Listed for Use with Type B Vents. This appendix is informative and not part of the code. It contains multiple examples of how to apply the vent and chimney tables and methodologies of Chapter 5.
• IFGC Appendix C — Exit Terminals of Mechanical Draft and Direct-Vent Venting Systems. This appendix is informative and not part of the code. It consists of a figure and notes that visually depict code requirements from Chapter 5 for vent terminals concerning the openings found in building exterior walls.
• IFGC Appendix D — Recommended Procedure for Safety Inspection of an Existing Appliance Installation. This appendix is informative and not part of the code. It provides recommended procedures for testing and inspecting an appliance installation to determine if the installation is operating safely and if the appliance is in a safe condition.
2. The 2018 NFPA 54 National Fuel Gas Code (NFPA 54)
The other model fuel gas code is the 2018 edition of the NFPA 54 National Fuel Gas Code, developed and maintained in a three-year revision cycle by the National Fire Protection Association (NFPA). The 2018 National Fuel Gas Code is commonly referred to as NFPA 54. As with all codes, it is important to determine which code edition applies in a given jurisdiction for a given time period.
The National Fuel Gas Code is the oldest model fuel gas code and it has undergone revisions and renumbering over many years. As you can see, there are several chapters in the NFPA 54 not found in the IFGC. The NFPA 54 is currently undergoing the final steps in a revision process for the 2021 edition. This column covers the 2018 edition.
• NFPA 54, Chapter 1 — Administration. This chapter covers scope, purpose, retroactivity, equivalency and enforcement.
• NFPA 54, Chapter 2 — Referenced Publications. This chapter covers general reference text, NFPA reference publications, other reference publications and references for extracts in mandatory sections.
• NFPA 54, Chapter 3 — Definitions.
• NFPA 54, Chapter 4 — General. This chapter covers qualified agencies, interruption of service, prevention of accidental ignition and noncombustible material.
• NFPA 54, Chapter 5 — Gas Piping System Design, Materials and Components. This chapter covers piping plans, a provision for location of the point of delivery, interconnections between gas piping systems, sizing of gas piping systems, operating pressure, acceptable piping materials and joining methods, gas meters, gas pressure regulators, overpressure protection devices, backpressure protection, low-pressure protection, shut-off valves, excess flow valves, and expansion and flexibility.
• NFPA 54, Chapter 6 — Pipe Sizing. This chapter covers pipe sizing methods, sizing of natural gas piping systems and sizing of propane piping systems. It also covers fuel gas sizing equations.
• NFPA 54, Chapter 7 — Gas Piping Installations. This chapter covers installation of underground piping, aboveground piping, concealed piping in buildings, piping in vertical chases, gas pipe turns, drips and sediment traps, outlets, manual gas shut-off valves, prohibited devices, systems containing gas-air mixtures outside the flammable range, systems containing flammable gas-air mixtures, electrical bonding and grounding, electrical circuits and electrical connections.
• NFPA 54, Chapter 8 — Inspecting, Testing and Purging. This chapter covers pressure testing and inspection, piping system leak check and purging requirements. The purging requirements are important for new installations that experience odor fade where the odorant (ethyl mercaptan) can dissipate to undetectable levels by smell.
• NFPA 54, Chapter 9 — Appliance, Equipment and Accessory Installation. This chapter covers some general equipment and accessory requirements, accessibility and clearance requirements, air for combustion and ventilation, appliances on roofs, appliances in attics, appliance and equipment connections to building piping, electrical requirements and room temperature thermostat requirements.
• NFPA 54, Chapter 10 — Installation of Specific Appliances. This chapter covers general appliance requirements, and requirements for a long list of different appliances. It also covers compressed natural gas (CNG) for vehicle fuel systems, appliances for installation in manufactured housing, fuel-cell power plants and outdoor flame decorative appliances.
• NFPA 54, Chapter 11 — Procedures to Be Followed to Place Appliance in Operation. This chapter covers adjusting the burner input, primary air adjustment, safety shut-off devices, automatic ignition, protective devices, checking the draft and operating instructions.
• NFPA 54, Chapter 12 — Venting of Appliances. This chapter covers minimum safe performance, general appliance venting requirements, specification of venting, design and construction, type of venting system to be used for positive or negative vent systems and for condensing vs. noncondensing vent systems.
It also covers masonry, metal and factory-built chimneys, gas vents, single-wall metal flue pipes, through-the-wall vent terminations, condensation drains for condensing flues, vent connectors for category 1 appliances, vent connections for category 2, 3 and 4 appliances, draft hoods and draft controls, manually operated dampers, automatically operated vent dampers and obstruction if appliance vents.
• NFPA 54, Chapter 13 — Sizing of Category 1 Venting Systems. This chapter covers additional requirements for single-appliance vents and multiple-appliance vents.
• NFPA 54, Annex A — Explanatory Material. For each chapter.
• NFPA 54, Annex B — Sizing Capacities of Gas Piping. Sizing tables.
• NFPA 54, Annex C — Suggested Method of Checking for Leakage.
• NFPA 54, Annex D — Suggested Emergency Procedure for Gas Leaks.
• NFPA 54, Annex E — Flow of Gas through Fixed Orifices.
• NFPA 54, Annex F — Sizing of Venting Systems Serving Appliances Equipped with Draft Hoods, Category 1 Appliances and Appliances. Listed for use with Type B vents.
• NFPA 54, Annex G — Recommended Procedure for Safety Inspection of an Existing Appliance Installation.
• NFPA 54, Annex H — Indoor Combustion Air Calculation Examples.
• NFPA 54, Annex J — Enforcement.
• NFPA 54, Annex K — Informational References.
Revisions to NFPA 54
Following is a summary of the major changes to the National Fuel Gas Code for 2018 edition:
1. An appliance’s electrical grounding connector is permitted to be used as the bonding means for a listed arc-resistant jacket or coated corrugated stainless-steel tubing (CSST), and stainless-steel smooth wall pipe and tubing products have been added as acceptable piping materials.
2. The minimum allowed wall thickness of carbon and stainless-steel pipe is revised to Schedule 10; however, joints on Schedule 10 pipe cannot be made with screwed fittings.
3. Press-connect fittings are now an acceptable joining method for fuel gas pipe.
4. Revisions to the venting requirements include requiring listing to the appropriate UL standards for plastic venting materials, factory-built chimneys, Type B and BW vents, chimney lining systems and special gas vents.
This code change has me concerned. Plastic flue gas venting materials were recently added to the fuel gas codes because many high-efficiency equipment manufacturers were utilizing plastic pipe instead of stainless-steel flue pipe. There was not an approved standard for plastic pipe in the United States, so some manufacturers of plastic pipe went to a standard developer and asked to have a standard developed to address plastic piping for flue gas venting.
The plastic pipe tests allow the same plastic pipe to be tested and listed for this application and the price is much higher for basically the same product except a sticker. They chose the developing a standard route rather than focusing on requiring temperature-limiting switches or flue gas temperature sensors for fuel gas appliances that want to use plastic flue pipe. The code does require components to be listed for the given application, so I understand how they chose this compliance path but the products are not any safer.
The problem is the flue gas temperatures can easily exceed the temperature rating of the plastic pipe materials if the heat exchanger fouls on the fuel burning equipment. Some equipment manufacturers have added flue gas temperature-limit switches but it is not a requirement, and not all fuel-burning equipment has the limit switches.
When an installation fails, the plastic pipe will melt or collapse and cause a leak of combustion products that have been known to lead to carbon monoxide incidents. They also can be a fire hazard. Plastic flue gas piping is now approved if it meets the tests in this new standard.
I do not recommend plastic piping to be used on fuel-burning equipment unless the fuel gas appliance has a thermal-limit switch or flue gas temperature sensor built into the outlet flue connection of the equipment. This will shut off the fuel when the flue gas temperature reaches the temperature limit of the connected plastic flue pipe material, whatever the temperature/material may be. Each plastic flue gas material has a different temperature rating or point where it starts to yield or melt.
If a manufacturer wants its fuel-burning equipment to be rated for PVC plastic flue pipe materials, it should have a lower thermal cut-out switch temperature setting than an appliance rated for CVPV or other higher-temperature-rated plastic flue pipe materials.
5. An existing gas appliance installation is required to be inspected for combustion air and venting code compliance when the building structure that it is installed in is modified with specific air infiltration-reducing changes. Older construction methods allowed more infiltration that was factored into combustion air requirements.
With newer, tighter construction, older appliances may need to have combustion air provided to assure proper combustion. The text of NFPA 54 should be consulted for the complete language of these and other requirements.
6. With many recent fuel gas explosions and fires associated with fuel gas purging operations, NFPA 54 requires combustible gas detectors or indicators to be used when purging fuel gas lines or locating a leak during pressure testing of new or modified gas piping systems. However, they are not required to be permanently located in any spaces.
I have consulted on many fuel gas explosions where purging operations were conducted on new piping that had experienced odor fade because the gas had been in the pipe for an extended period. The odorant added by the gas utility had faded away and was not readily detectable by smell.
7. Listed fuel-gas-burning appliances are required to have a label attached to the appliance describing required clearances. The manufacturer’s instructions should have the clearance information. Chapter 10 of NFPA 54 provides clearances for many types of unlisted appliances.
8. Gas piping does not require a separate bonding connection unless one of the following situations occurs:
• Where there are gas appliances with electrical connections that are connected to ungrounded wiring systems (i.e., two-pronged plugs).
• Where there are sources of high-voltage electricity outside the piping system that could energize the gas piping system (highly unusual or unlikely).
• Where the gas piping material is CSST, NFPA 54 does not apply to portable LP-gas appliances and equipment of all types not connected to: a fixed-fuel piping system; LP gas installations at utility gas plants; fuel gas piping in electric utility power plants; gas piping, meters, gas pressure regulators and other appurtenances used by the serving gas supplier in distribution of gas, other than undiluted LP gas; or construction of appliances. Refer to section 18.104.22.168 of the NFPA 54 Fuel Gas Code for a list of applications NFPA 54 does not cover.