The two classifications of fuel gas distribution systems that may be installed using copper tube are known as:
- Low pressure: Less than 14" wc (water column) or psig (pounds per square inch gauge).
- Elevated pressure: 14" wc ( psig) to 5 psig.
The system designer should consult with the gas company regarding available pressure and metering requirements for each installation.
Currently in the United States, 2 psig tends to be the most commonly used elevated pressure system for residential and light commercial installations. However, 2 psig is not the only pressure at which gas can be delivered, and the system pressure selected must be compatible with available delivery pressures and other system requirements. For some installations, the elevated pressure system may be impractical or unnecessary; whereas, a low pressure system may be quite adequate and acceptable. The lowest pressure system that will provide proper operation is preferred, but when sizing indicates tube sizes of 1" O.D. or greater, consideration should be given to using an elevated pressure system and smaller tube sizes.
Low pressure systems are used when the gas company serving the consumer either operates a low pressure (less than 14" wc) distribution system or sets its service regulator to reduce the gas pressure at the meter to this lower pressure. In an elevated pressure system, the gas pressure is reduced at the meter by the service regulator to to 5 psig. This pressure is further reduced to less than 14" wc by a line regulator located within the house piping system at the appliance or a distribution manifold.
In either of the above systems, the tube must be of sufficient size to provide a supply of gas to meet the requirements of volume and pressure at the point of use. The differences in the capacities of the low pressure and elevated pressure systems, measured in cubic feet per hour (CFH), may be noted by comparing the values in Tables 3, 4, 5 and 6. Table 3 is for low pressure systems, or that portion of an elevated pressure system operating at less than 14" wc, and is based on a pressure drop of 0.5" wc. Table 4 is also for low pressure systems or that portion of an elevated pressure system operating at less than 14" wc and is based on a pressure drop of 1.0" wc. Tables 5 and 6 are for elevated pressure systems and are based on delivery pressures (at sea level) of 2 psig and 5 psig and pressure drops of 1.0 psig and 3.5 psig, respectively.
Tables 3 through 6 include a 20% factor to allow for a reasonable number of fittings and are based on gas with a specific gravity of 0.60. See Table 7 for gas with a specific gravity other than 0.60. The capacities in these tables were conservatively determined based on the dimensions of Type K tube and are suitable for use with ASTM B 88 Types K and L tube, ASTM B 280 Type ACR tube and Type Gas tube made to ASTM B 837.
The capacities listed in Tables 3 through 6 are for a tube of the stated size and parameters only and do not consider the limitations or effects of regulators or other system components. Many of the values listed indicate large capacities that can be delivered using the stated tube size. Realistically however, other system components, such as regulators, may not be able to handle or deliver these quantities of gas. The system designer should recognize that the limitations of these components may govern the capacity of the system.
The pressure drop values used in Tables 3 through 6 are commonly used pressure drops for each type of system. However, the limit for pressure drop between the outlet of the meter and the regulator at a manifold or an appliance can be quite high, as long as the inlet pressure to the regulator is sufficient to allow the required flow and delivery pressure downstream of the regulator. Local codes should be consulted for limits on allowable pressure drop in the system to determine the applicability of Tables 3 through 6. For pressure drops other than those used in the tables, the capacity of the tube can be calculated using guidance from Annex C of the National Fuel Gas Code.
Specific identification requirements are dependent on the applicable local codes, however, it is recommended that each system be clearly marked with appropriate labels to identify the system as a gas system, the system type (low pressure, elevated pressure) and for elevated pressure systems, the system delivery pressure. The low pressure system can use the label shown in Figure 1, while the elevated pressure system can use markings similar to that shown in Figure 2. These labels should be affixed to the tube on both sides of a wall, floor or partition and at maximum six-foot intervals throughout the length of the tube runs.