Page 232 - Pipeline Rules of Thumb Handbook

Corrosion/Coatings 219

How to control corrosion at compressor stations

Eight factors design engineers should consider
R. E. Hodge, Superintendent of Corrosion Control, Natural Gas Pipeline Co. of America

Corrosion failures at compressor stations result from care- Grounding
lessness on the part of the user, or poor choice of material/
configuration by the designer. Grounds should be of materials anodic to steel. These
Design engineers should consider the following: might include magnesium or zinc anodes, or galvanized
ground rods.
• Coat everything underground, except ground rods, with Copper ground rods should not be used. Copper is cathodic
a coating properly selected for the conditions to steel. Some compressor stations have been installed with
• Use proper coating application and inspection massive bare copper grounding systems. These are large gal-
• Use ground rods anodic to steel and insulated (coated) vanic cells with the steel gas piping as the anode (corroding
ground wires element). Bare ground wires and rods also take cathodic pro-
• Avoid shielding as much as possible and allow space for tection current. This bad situation can be avoided by using
corrosion control additions galvanized rods, for example, and using copper ground wire
• Use a minimum of insulating fittings with insulation. The junction of copper ground wire and gal-
• Protect against water system corrosion vanized ground rod must be coated. Grounding systems
• Select materials carefully, with corrosion in mind as well should be checked periodically for proper functioning.
as the more obvious considerations
• Do not retire facilities in place—take them out.
Shielding

Shielding poses a major problem at compressor stations. It
could be described as the blocking of cathodic protection due
Coatings to geometric configuration.
Congested areas, where buried metal concentration is high,
Coatings are one of the most important considerations for are difficult to cathodically protect. Piping, or other metal
control of underground corrosion. Generally, all underground structures, will tend to shield cathodic protection currents
metallic structures, except ground rods, should be coated. from reaching all of the buried structures to be protected.
This would include gas piping, control lines, tubing, water Cathodic protection ground beds of various styles have
lines, conduit, air lines, braces, etc. been used at compressor stations. Remote conventional beds
There is enough buried conduit around a compressor and deep beds are popular for many applications. But NGPL
station to soak up large amounts of cathodic protection if not has found that distributed anodes seem to be the best way to
coated. Non-metallic carriers can be used in some cases get adequate cathodic protection to all the shielded areas in
instead of coating. a station. This type bed can be described as a ground bed with
Proper selection and use of coatings are important. the anodes distributed in a nongeometrical pattern, inter-
The best of coatings are no good if used in the wrong spersed through with piping.
place or improperly applied. The emphasis here is on the Designers should allow space between lines where pos-
selection of coating for the temperature and environment sible; avoid congestion. This would cause less shielding
expected. and allow space for distributed anode installations where
For gas discharge lines, temperature becomes a dominant required. It would also allow space for access to work equip-
consideration. Be sure to get a coating that will withstand gas ment for anode installations.
discharge temperature. Be careful, sag temperatures listed in
coating literature are not maximum operating temperatures.
Very few coatings on the market can withstand operating tem- Insulation
peratures above 120° without damage. Any bare metal under-
ground becomes a drain on the cathodic protection system Use insulating fittings (flanges, couplings, unions, etc.) only
and a potential shielding problem. where absolutely necessary.
There are other areas for coating consideration besides Hold insulating fittings to minimum. These can often
underground. For instance, tank interiors and atmospheric cause more problems than they solve. You might want to
coatings. These must also be matched to the situation. insulate water wells from the adjacent piping for control of