Sales gas specifications
The exact sales gas specification is specified by pipeline operators and
distributors. Typical standard sales gas requirements use the following
parameters:
Volume is measured in standard cubic meters (scm) defined as 1 m3
at 0 ºC
and 101.325 kPa or standard cubic feet (scf) as 1 ft3
at 60 °F (16 °C) and
14.73 PSIA.
Calorific value specifies the total amount of energy per unit generated
during combustion of the gas. The value is used to calculate the amount of
energy delivered. Several values are listed:
• Gross calorific value or gross heat of combustion is the heat
released when a specific quantity of fuel in mixture with air is ignited
and the end products have returned to the initial temperature,
normally 25 ºC. EU specifications are typically 38.8 MJ (10.8 kWh)
±5% per scm. In the US 1030 BTU ±5% per scf.
• Net calorific value or net heat of combustion is the net heat
generated when the water vapor in the gas does not condense
(water forms during combustion) and can be 10% lower.
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Wobbe index measures the heating effect that a burner is exposed to during
combustion. A higher value means a greater thermal load on the burner.
Different gases with the same Wobbe index will impose the same load on
the burner. An excessively high value is a safety hazard, as it can lead to
burner overheating and to excess production of carbon monoxide during
combustion.
Calorific value and Wobbe index can be adjusted by blending gas from
different sources as well as by addition or removal of nitrogen (N2).
Methane number is a value similar to octane value for gasoline, and is
important when the gas is used for internal combustion engines (as CNG).
Hydrogen sulfide and overall sulfur content: Both hydrogen sulfide (H2S)
and total sulfur must be reduced. H2S is toxic as well as corrosive for the
pipeline, as it forms sulfuric acid (H2SO4) and should be kept as low as
possible. Typical maximum values are 5 mg per scm of H2S and total sulfur
at 10 mg per scm.
Mercury should be kept below 0.001 ppb (parts-per-billion) which is its
detectable limit. The goal is to limit emissions and to prevent damage to
equipment and pipelines by mercury amalgamation, which makes aluminum
and other metals brittle.
Dew point is a temperature below which some of the hydrocarbons in the
gas can condense at pipeline pressure, forming liquid slugs that can damage
the pipeline. The gas must also be clear of all water vapor to prevent the
formation of methane hydrates within the gas processing plant or within the
sales gas transmission pipeline.
Particles and other substances must be free of particulate solids and all
liquids to prevent erosion, corrosion or other damage to the pipeline and
satisfy limits on carbon dioxide, nitrogen, mercaptans, etc.
Additives:
When the natural gas is intended for domestic use,
tetrahydrothiophene (THT) is added so that the otherwise odorless natural
gas can be detected in the event of a gas leak. The sulfurous-smelling
substance added is equal to a sulfur content of 4-7 mg per scm.
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5.4 Pipelines
Pipeline installations consist of driving compressors and pumps, valve
stations, pig receive/launch facilities, where the pig is used for cleaning or
inspecting the pipeline. A pipeline SCADA system and pipeline management
system is required to control and operate the pipeline.
5.4.1 Pipeline terminal
Pipelines transport gas or
liquid, and are fed from the
high pressure compressors or
pumps.
The pipeline terminal includes
termination systems for the
pipeline. A pig launcher and
receiver is a minimum
requirement, allowing insertion
of a pipeline pigging device
used to clean or inspect the
pipeline on the inside. Essentially, it is a large chamber that can be
pressurized and purged to insert and remove the pig or scraper without
depressurizing the pipeline.
The pig is often driven by pipeline flow, either directly or through a pump or
turbine arrangement that also drives wheels that roll against the walls and
mechanisms to rotate brushes and other scraping mechanisms. Intelligent
pigs also contain instrumentation for remote control and cameras, etc.
