Page 414 - Compression Machinery for Oil and Gas

Midstream Chapter 9 393

is usually governed by the ASME B31.8 code. See additional discussion in
Chapter 13.
Basic pipe sizing will be determined based on the maximum design pres-
sures of the system and the required flow rates. Different areas of the system
will have different piping diameters based on how the flow is split among
one or more compressors operating in parallel.
Piping diameters should be selected to keep flow velocities within a target
range. Within a compressor station, flow velocities in the main station piping
should typically be kept between 6 and 20m/s (assuming typical natural gas
pipeline pressures of 4–10MPa. Issues such as excessive pressure drop, noise,
flow-induced pulsation, flow turbulence-induced vibration, erosion, etc., can be
attributed to usually high flow velocities. Concerns related to liquid or settle-
ment buildup at low points, proper check valve operation, and higher installa-
tion costs could result from unusually low flow velocities.
In certain specific piping segments that are subject to only occasional use,
such as recycle/antisurge piping or blowdown piping, flow velocities up to
roughly 37m/s may be acceptable. For systems operating with lower gas den-
sities or lower pressures, even greater velocities might also be possible.
The wall thickness and material selection of the piping will depend on the
MAOP of the piping. Most piping design codes specify the hoop stress due to
internal pressure in the piping limited to a maximum valve somewhere between
40% and 72% of the specified minimum yield strength (SMYS) of the material.
Other derating factors may also be applied by the design code to account for
other as a rule of thumb, the basic hoop stress in a cylindrical pipe can be cal-
culated by.
Hoop stress ¼ Pressure Mean radius=Pipe wall thickness
The most common material grade for compressor station piping is A 106
Grade B with a SMYS of 240MPa. Higher grade materials can have yield
strengths up to 410MPa or greater. For piping with nominal diameters between
6 and 24in., wall thickness will commonly vary from 0.280 to 1.0in., depending
on the design pressures and piping material.
The MAOP of the overall system will be limited by the lowest rated com-
ponent. Every component in the system (flanges, valves, coolers, vessels, etc.)
will have a separate rating. The use of higher grade materials (with correspond-
ing thinner wall thicknesses) as the piping diameters get larger may have some
economic benefits. Piping material selection should also consider other factors
such as cost, availability, material suitability for certain fluid types or temper-
ature ranges, corrosion, etc.

Piping Supports, Thermal Stress, and Vibration Control
There are numerous different types of piping supports or restraints that can be
installed on a piping system and they each serve specialized functions. Funda-
mentally, it is essential to securely support the weight of the piping and