The natural gas received and transported by the major intrastate and
interstate mainline transmission systems must meet the quality
standards specified by pipeline companies in the “General Terms
and Conditions (GTC)” section of their tariffs. These quality
standards vary from pipeline to pipeline and are usually a function
of a pipeline system’s design, its downstream interconnecting
pipelines, and its customer base. In general, these standards specify
that the natural gas:
• Be within a specific Btu content range (1,035 Btu per cubic
feet, +/- 50 Btu)
• Be delivered at a specified hydrocarbon dew point temperature
level (below which any vaporized gas liquid in the mix will
tend to condense at pipeline pressure)
• Contain no more than trace amounts of elements such as
hydrogen sulfide, carbon dioxide, nitrogen, water vapor, and
oxygen
• Be free of particulate solids and liquid water that could be
detrimental to the pipeline or its ancillary operating equipment.
Gas processing equipment, whether in the field or at
processing/treatment plants, assures that these tariff requirements
can be met. While in most cases processing facilities extract
contaminants and heavy hydrocarbons from the gas stream, in some
cases they instead blend some heavy hydrocarbons into the gas
stream in order to bring it within acceptable Btu levels. For
instance, in some areas coalbed methane production falls below the
pipeline’s Btu standard, in which case a blend of higher btu-content
natural gas or a propane-air mixture is injected to enrich its heat
content (Btu) prior for delivery to the pipeline. In other instances,
such as at LNG import facilities where the heat content of the
regasified gas may be too high for pipeline receipt, vaporized
nitrogen may be injected into the natural gas stream to lower its Btu
content.
In recent years, as natural gas pricing has transitioned from a
volume basis (per thousand cubic feet) to a heat-content basis (per
million Btu), producers have tended, for economic reasons, to
increase the Btu content of the gas delivered into the pipeline grid
while decreasing the amount of natural gas liquids extracted from
the natural gas stream. Consequently, interstate pipeline companies
have had to monitor and enforce their hydrocarbon dew point
temperature level restrictions more frequently to avoid any potential
liquid formation within the pipes that may occur as a result of
producers maximizing Btu content.
eia.doe.gov
