I have been involved with designing, inspecting and investigating fuel gas system installations since 1978. Fuel gas systems can be safe and economical choices for space heating, water heating, energy generation or industrial process applications if they are designed, installed and maintained properly.

The United States uses two model fuel gas codes — the National Fuel Gas Code and the International Fuel Gas Code. They include sections 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. 

The model codes are updated every three years, although some local jurisdictions do not automatically adopt the latest version of the model code. It is important to determine the code enforced by the local jurisdiction. The applicable codes should be identified on the project drawings or in the specification manual, as well as the year edition of the code to be followed. 

Fuel Gas Types

Fuel gases include natural gas, propane, butane, methane, hydrogen, and mixed gases. For a given project, the design professional should first determine if one of these gases is available to the site from a local utility or gas service provider. 

Several projects I was involved with used methane gas captured from wells drilled in landfills to run nearby gas-powered generators for electric power generation. The methane gas produced power, and the power was sold to nearby industrial facilities. Other projects used captured methane gas from wastewater treatment plants to run engines that drove electric generators and 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: generators, boilers, water heaters, furnaces, infrared heaters, cooking equipment, fireplaces, and many other fuel-burning appliances. 

When designing fuel gas systems, the design professional and the installer should determine which code has jurisdiction in the municipality where the project is to be built. Local jurisdictions may have adopted one of the model fuel gas codes with local amendments, or they may provide their own code.

The most common fuel gas is natural gas. Propane also is a common fuel gas in rural areas where the gas is delivered in liquid form and evaporates into a fuel gas in the piping system at lower pressures. Butane and mixed gases are less-common fuel gases but are found occasionally, and hydrogen is being used in some areas where it is mixed with natural gas or methane in small percentages. 

Verify the fuel gas type and the caloric value and document them in your calculations. 

Heat Content for Fuel Gases

All fuel gases have a heat content or caloric value when they are burned. The caloric value is measured in British thermal units (BTUs), where one BTU is the amount of heat required to raise one pound of water one degree F. 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 feet per hour of the fuel gas required for an application. Knowing the caloric value of the fuel gas can help with pipe sizing. Generally, fuel gases have the following heat content in BTUs: 

• Natural gas has a caloric value of 1,000 BTUs/cubic foot. 
• Propane has a caloric value of 2,500 BTUs/cubic foot. 
• Butane has a caloric value of 3,200 BTUs/cubic foot. 
• Hydrogen has a caloric value of around 325 BTUs/cubic foot. 

Switching from one fuel gas to another requires changing or modifying the orifices or jets in the burner assembly. Since hydrogen is so low in BTUs per cubic foot, it is often blended with natural gas in amounts up to about 15%. If hydrogen fuel is burned at 100%, the burner jets would need to be much larger, and the pipe sizing would also need to be adjusted accordingly.

• Natural gas. When using natural gas, the designer should determine the BTU requirements for the fuel-burning equipment and divide by 1,000 BTUs/cubic foot of gas to get the cubic feet per hour (cfh) gas required for the equipment. Add the cfh required for each section of piping with no diversity. 

The caloric value or heat content of natural gas per cubic foot (cf) can range from about 950 BTUs/cf to 1,100 BTUs/cf, depending on the gas utility’s caloric value or blending with other gases listed for their gas. The specific gravity of natural gas is about 0.65, while air has a specific gravity of one. That means natural gas is lighter than air and dissipates when released into the atmosphere. 

The flammability range of natural gas is from 3.9% to 15% volume in air. Any concentration of natural gas below 3.9% will not ignite because there is not enough fuel-to-air ratio to support combustion. This is often said to be too lean for combustion. 

A concentration of gas above about 15% fuel gas-to-air ratio generally will not support combustion because the ratio is too rich for combustion. The amount of combustion air required for natural gas is 10 cubic feet of air for every cubic foot of gas. 

• Propane. Common in rural areas where it is delivered to remote locations by filling tanks on-site that are piped into the building, propane has a BTU content 2.5 times higher than natural gas. The specific gravity of propane is 1.52. Propane is heavier than air and pools on the ground or in low places, such as a basement, when released into the atmosphere. Some local codes addressed this by not allowing fuel gases heavier than air in basements. 

The flammability range of propane is from approximately 2.1% to 10.1% volume in air. 

• Butane. Not as common as propane, butane is used mostly in rural areas. It has a BTU content of approximately 3,200 BTUs/cf. The specific gravity of butane is 1.95, which is much heavier than air. Like propane, butane pools on the ground in low places and is subject to restriction in some codes. The flammability range of butane is from approximately 1.9% to 8.6% volume in air. 

Fuel Gas Pressure 

Fuel gas pressures vary depending on the type of gas being used. Natural gas is typically piped from the gas fields to utility companies in high-pressure-rated piping mains that can withstand pressures in the thousands of pounds per square inch (psi). The reason the pressures are so high is to move large quantities of gas through relatively small pipe. 

The local utility company typically buys gas from the market through transmission mains with control valves, gas compressors (pumps), meters to record the amount flowing, and regulators assemblies to maintain constant pressures downstream. 

Propane and butane are typically delivered by tank trucks to on-site tanks and sometimes supplied in high-pressure liquid form. When released to lower pressures, they boil off or evaporate into a gas as they flow through the pipe or through an evaporator. Propane can be a combustible refrigerant; new code requirements are being developed to deal with combustible refrigerants. 

Natural gas pressures can be expressed in several different ways, and I have seen people mix up these pressures in the past. High- and medium-pressure fuel gas systems are expressed in psi. Low-pressure gas can be expressed in several different ways. It is important when communicating gas pressures to be consistent with the units you describe. Fuel gas pressures are often given in one of the following:

1. Pressure units: 
   1. Pounds per square inch (psi)
   2. Inches of water column (wc) 
   3. Ounces of pressure
2. Pressure conversions:
   1. One psi equals 28 inches of wc, which equals 16 ounces of pressure 
   2. One-half psi equals 14 inches wc, which equals 8 ounces of pressure 

Fuel Gas Valves

Valves for fuel gas piping should be rated for the fuel gas application and pressure and listed or approved by a third-party testing agency. In low-pressure piping systems (1/2 psi and below), a lubricated gas cock is sometimes used. These older-style valves have performed adequately because lubricated plug valves use thick grease as a seal against gas pressure. 

If the pressure exceeds about 1/2 psi or if the temperature gets too high, the grease can be forced out of the annular space between the plug and the valve body, allowing a gas leak. In a fire, the grease in the lubricated plug valve melts and allows gas to escape. 

For these reasons, I try not to use lubricated plug valves on gas lines. Newer gas valves use seals designed for fuel gas service for leak-free operation. Higher-pressure systems (above 1/2 psi) should use ball valves or other approved valves rated for water, oil or gas service pressures. 

Installation Requirements

Some areas of older East Coast cities have very old cast-iron gas main piping, and the gas utility cannot raise pressures too high because the brittle cast-iron can rupture, releasing the gas into the soil where it makes its way to the point of relief. Generally, these older gas mains operate between about 1/2 psi to 1/4 psi. Every gas appliance includes an appliance label with the manufacturer’s maximum and minimum gas pressure required for the equipment to operate properly.

• Fuel gas building service entrance. Fuel gas piping is generally not allowed to enter a building belowgrade. Gas piping installations where the pipe enters the building belowgrade 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. Gas leaks far away from the building can travel along the void space between the pipe and expansive soil and enter the basement or crawl space if not properly sealed. Piping in concealed spaces or under floors should be double-walled piping, with the outer pipe vented to the outside.

• Gas pressure within buildings. Typically, the utility distribution pressure up to the building is less than 100 psi but can be higher in areas that have seen a lot of growth. The utility company typically provides a gas regulator with the gas meter assembly at each service connection to reduce the pressure to a lower pressure for customer use. 

The pressure after the utility service meter/regulator assembly is typically reduced to about 1/2 psi or 14 inches or 8 ounces of gas pressure. In some areas, it may be slightly higher or lower, so check with the utility for pipe sizing within the building. 

Some large commercial, industrial or heavy gas users may use pressures up to 5 or 10 psi. Most mechanical codes have pressure limitations of 5 psi inside a building. Variances can exist if large industrial boilers, furnaces or other process equipment require higher pressures to operate.

• Gas appliance connections. Piping connections to equipment should include a gas shut-off valve with a dirt leg and a union for the removal of the equipment. After the gas appliance shut-off valve, if a gas line is connecting to an appliance, an approved gas appliance connector should be used if the appliance includes a motor that vibrates or if the appliance, such as a gas stove, grill or dryer, will be moved for service. 

A cap or plug should be placed on the gas line if no gas appliance has been installed yet. If a gas appliance is not connected to the gas piping system, the code requires that gas piping outlets not connected to appliances are required to be capped or plugged gas-tight. This is because if the gas line is left open and someone inadvertently opens the gas appliance valve or an upstream isolation valve, gas can escape, causing an explosion and fire. 

I have investigated many explosions and fires associated with uncapped gas lines where moving an appliance, like a dryer, bumped the valve open or where brooms, mops or pets beside or behind appliances bumped the valve and caused it to partially open, allowing gas to escape from an uncapped or unplugged gas outlet. 

In other cases, the gas appliance valve was left open after pressure-testing equipment was removed; when an upstream gas valve was opened, gas flowed freely into the building. In each case, an explosion and fire injured or killed building occupants. A simple cap or plug on the pipe is required by code and will prevent such a disaster. 

• Purging air from new gas lines. Many large gas explosions and fires have been attributed to the purging of gas due to a phenomenon known as “odor fade.” When new gas piping is installed, it has a dry interior surface with dirt and iron oxide that will condense and soak up the odorant added to the gas. 

Natural gas generally has little to no odor, and because of early gas leaks and explosions, the fuel gas industry decided to add an odorant — ethyl mercaptan — to the gas, allowing it to be detected before reaching flammable limits. In new piping installations, the odorant condenses on the walls of the pipe, where the dirt and iron oxide soak it up until the pipe walls are saturated. 

When new pipes condense the ethyl mercaptan, the odor fades. Many new gas piping installations have had explosions and fires, where the odorant was not noticed before the explosion or flash fire.

Main Gas Pipe Sizing

Gas piping should be sized in accordance with the sizing tables in the mechanical or fuel gas code. Typically, for sizing the main gas pipeline, you should determine the total developed length of the main gas pipe and the equivalent length of fittings from the gas service regulator at the building service entrance or in-line regulator to the farthest piece of gas equipment. 

Then refer to the fuel gas or mechanical code for the gas sizing tables and find the gas pressure table corresponding to the gas pressure downstream of the gas pressure regulator in your project. Choose the column for the pipe length that meets or exceeds your total developed length to size the gas main from the farthest fixture to the gas service meter/regulator assembly. 

For each closer branch, you add the cfh to the main pipe and select the appropriate size pipe size for the cfh flowing through that section of pipe. 

For sizing the branch piping off the main, you can use the same column in the table to size all the branches. You also can use the total developed length from the meter/regulator assembly to the farthest branch fixture on that particular branch and size the branches on a column based on the shorter developed length. 

In a few cases, going this extra step might allow you to downsize a branch pipe by one pipe size. In a large building, that might make a financial difference. 

Please refer to the online version of this column for a review of the two model fuel gas codes: the National Fuel Gas Code (NFPA 54), administered by the National Fire Protection Association, and the International Fuel Gas Code, administered by the International Code Council.