Page 409 - Compression Machinery for Oil and Gas

388 SECTION III Applications

Legend
Interstate Pipelines
Intrastate Pipelines
Source:
Energy Information Administration, Office of Oil & Gas,
Natural Gas Division, Gas Transportation Information System
FIG. 9.1 Pipeline network in the United States, 2007 [1].

maximum gas flow velocity and allowable pressure drop given the volume and
thermophysical properties of the transported gas. Various guidelines are avail-
able to aid in designing and sizing a pipeline, including the Handbook of Natural
Gas Transmission and Processing [2], ASME B31.8 piping code [3], and the
NORSOK P-001 standard.
The composition of the gas used in midstream operation is typically a nat-
ural gas mixture composed primarily of methane and light hydrocarbons, nitro-
gen, and carbon dioxide with very small mole weight percentages of heavy
hydrocarbons and lighter gases. When operating with evaporated liquids, such
as condensates (natural gas liquids) or water, the gas is referred to as wet gas. If
significant amounts of hydrogen sulfide (H 2 S) are also present, the gas is
referred to as sour gas that can easily become corrosive when exposed to undis-
solved water and can also create sulfide stress cracking (a form of hydrogen
embrittlement). Therefore, most sour gas is treated, or sweetened, to remove
these components. Acid gas, of which sour gas is one type, contains significant
amounts of various acidifying components such as CO 2 or sulfur compounds. It
also becomes corrosive when exposed to free water and can form hydrates that
block pipelines or plug equipment. With regulatory pressure encouraging trans-
portation and storage of CO 2 rather than venting, a few midstream transmission
gas pipelines do operate with pure CO 2 . However, these pipelines must be
designed with consideration of the risks of acid gas operation.