Corrosion failures at compressor stations result from carelessness mostly on the part of the users or poor choices of material and/or configuration by the project.
The following factors should be considered when studying options for the project:
- Coat everything underground, except ground rods, with a coating properly selected for the conditions
- Use proper coating application and inspection
- Use ground rods anodic to steel and insulated (coated) ground wires
- Avoid shielding as much as possible and allow space for corrosion control additions
- Use a minimum of insulating fittings
- Protect against water system corrosion
- Select materials carefully, with corrosion in mind as well as the more obvious considerations
- Not just to retire facilities in place - take them out, demolish it, remove it
Coatings are one of the most important considerations for control of underground corrosion. Generally, all underground metallic structures, except ground rods, should be coated. This would include gas piping, control lines, tubing, water lines, conduit, air lines, braces, etc.
There is enough buried conduit around a compressor station to soak up large amounts of cathodic protection if not coated. Non-metallic carriers can be used in some cases instead of coating.
Proper selection and use of coatings are important. The best of coatings are no good if used in the wrong place or improperly applied. The emphasis here is on the selection of coating for the temperature and environment expected.
For gas discharge lines, temperature becomes a dominant consideration. Be sure to get a coating that will withstand gas discharge temperature. Be careful, sag temperatures listed in coating literature are not maximum operating temperatures. Very few coatings on the market can withstand operating temperatures above 120 degree Fahrenheit without damage. Any bare metal underground becomes a drain on the cathodic protection system and a potential shielding problem.
There are other areas for coating consideration besides underground. For instance, tank interiors and atmospheric coatings. These must also be matched to the situation.
Grounds should be of materials anodic to steel. These might include magnesium or zinc anodes, or galvanized ground rods.
Copper ground rods should not be used. Copper is cathodic to steel. Some compressor stations have been installed with massive bare copper grounding systems. These are large galvanic cells with the steel gas piping as the anode (corroding element). Bare ground wires and rods also take cathodic protection current. This bad situation can be avoided by using galvanized rods, for example, and using copper ground wire with insulation. The junction of copper ground wire and galvanized ground rod must be coated. Grounding systems should be checked periodically for proper functioning.
Shielding poses a major problem at compressor stations. It could be described as the blocking cathodic protection due to geometric configuration.
Congested areas, where buried metal concentration is high, are difficult to cathodically protect. Piping, or other metal structures, will tend to shield cathodic protection currents from reaching all of the buried structures to be protected.
Cathodic protection ground beds of various styles have been used at compressor stations. Remote conventional beds and deep beds are popular for many applications. But industry has found that distributed anodes seem to be the best way to get adequate cathodic protection to all the shielded areas in a station. This type bed can be described as a ground bed with the anodes distributed in a non-geometrical pattern, interspersed through the piping.
Project should allow space between lines where possible; avoid congestion. This would cause less shielding and allow space for distributed anode installations where required. It would also allow space for access to work equipment for anode installation.
Use insulating fittings (flanges, couplings, unions, etc.) only where absolutely necessary.
Hold insulating fittings to minimum. These can often cause more problems than they solve. You might want to insulate water wells from the adjacent piping for control of cathodic protection current to the wells. This may not even be necessary if distributed ground beds are used.
Some companies insulate compressor stations from the lines entering and leaving the station. Other companies do not. This is a matter of individual company preference.
Each water system (jacket water, piston water, boiler water, etc.) should have corrosion monitoring provisions designed into the system, such as coupons or corrosion rate probes. If there is a gas treating plant in conjunction with the compressor station, corrosion monitoring provisions should be designed into that system also.
Water treatment for corrosion control would have to be considered, depending on individual circumstances. Some treatment facilities may be designed into the systems.
Provisions should be considered for cathodic protection of the internal surfaces of storage tanks and water softeners. One problem often encountered on smaller tanks is adequate openings. Enough openings of sufficient size should be allowed on any tank for possible future work.
Water should be kept out of some systems. For instance, use air dryers on control lines.
Proper selection of materials is important throughout design.
Two key considerations from a corrosion standpoint are:
- avoid the use of dissimilar metals
- match the material to its environment
Examples of these would include the use of a brass valve in a buried steel water line; or use of mechanically strong bolts in the wrong electrolyte, resulting in subsequent failure from stress corrosion cracking.
Select materials carefully, with corrosion in mind as well as the more obvious considerations. If this design method is applied, many subsequent failures can be eliminated.
Many compressor station designs involve retiring something that is already there.
If something must be retired, especially underground, take it out (demolish and complete removal). Many times a company pays unnecessarily to continue protection of facilities retired in place (abandon in place).
Even if the retired facilities can be completely disconnected from the protective systems, shielding is still a problem. Shielding is a problem by foundations also, so if they are retired they should he removed.
Thus, attacking the corrosion problems requires a truly interdisciplinary effort. In fact, corrosion offers one of the richest fields for interdisciplinary activity.