Surface Equipment
Three common types of surface equipment where corrosion can occur are storage
tanks, wellheads and flow lines.
Storage Tanks
Storage tanks can be damaged by both external and internal corrosion. External
corrosion is most often found at the bottom of the tank, or anywhere the tank
comes into contact with water (such as when contacting wet soil). type
of corrosion and link. Methods for controlling external corrosion on storage
tanks include cathodic protection, coatings and foundations. Internal corrosion
often occurs at the inside top of the tank or at the bottom of the tank. Condensation
of oxygen(O2) or hydrogen
sulfide (H2S) gas often causes the top of the tank to corrode,
while salt water or solids settling causes corrosion at the bottom of the tank.
The schematic below shows some of the interfaces where corrosion can occur
in a surface tank. Many tanks do not vent to the surface due to environmental
concerns, which reduces or eliminates the oxygen.
Wellheads
Corrosion is a problem in wellhead and Christmas tree equipment, especially
in high pressure, high velocity gas wells. The best practice in these cases
is to use equipment constructed out of corrosive resistant materials. The API
Specification 6A provides a guide for material requirements (see the references section).
Flow Lines
Flow lines at the surface can corrode due to the presence of water (settling
due to gravity to the bottom of the pipe) and/or debris. The layout and sizes
of the flow lines can be designed to minimize corrosion, and inhibitors or
coatings are also used to mitigate corrosion in these lines. An example of how flow line layout can help
prevent erosion corrosion is found in the erosion section.
For lines buried below the surface, differential aeration is also a source
of corrosion potential. Refer to the oxygen section for more information.
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