Buried treasure

People have buried valuables in the ground since prehistoric times, but as any archeologist will tell you, that's not necessarily the optimum choice for long-term survival. Heat, cold, seismic forces, bugs, bacteria and who-knows-what-else can be more of a threat to the structural integrity of the buried item than any marauders. The same kinds of threats that eventually destroy the buried treasure of Discovery Channel fame will wreak havoc with your buried piping systems as well.

The solution to the problem is pipeline coating, which provides barrier protection, insulation or other required properties for your metal piping, but getting the right coating for the right application requires some planning.

Figure 1. Field repair of coating damaged during transportation to site.

Because organic coatings are seldom applied perfectly and always are subject to deterioration during service, buried and immersed pipelines are normally protected cathodically. Indeed, many countries, including the United States, require cathodic protection on buried and immersed gas and oil lines.

A pipeline coating with a high electrical resistance significantly reduces the electrical current required to achieve effective cathodic protection, which in turn protects any exposed pipeline metal from deterioration. If coatings are to perform well in conjunction with cathodic protection, the cathodic protection voltage must be maintained at some minimum level, and the coatings must be resistant to the alkalinity that cathodes produce.

To be considered effective, pipeline coatings must have the following physical and chemical properties:

Good electrical resistance. The coating must provide a barrier that protects the steel from aggressive electrolytes in the soil or water.

Good chemical resistance. The coating must be resistant to chemical attack by its environment.

Resistance to cathodic disbonding. Cathodic protection can ruin a coating by driving water through it or by producing hydrogen gas on the steel that separates it from the metal should the cathodic voltage exceed 1.1 volt.

Good adhesion to metal surface. Adhesion is required to minimize coating damage during shipment, handling and service.

Heat resistance. In services where the piping is heated, the coating system must be resistant to the prevailing temperatures.

Ease of application. The coating must be relatively easy to apply to form a quality film; thus, coatings normally perform better when shop-applied under controlled conditions than when field-applied.

Resistance to damage during handling, storage and installation. The coating must have sufficient resistance to impact and abrasion and have good ductile properties to resist mechanical damage during handling, storage and installation. Coatings subjected to extended exterior exposure also must be resistant to deterioration by the sun's ultraviolet light.

Ease of repair. It must be easy to repair damage from handling, storage, transport and installation (see Figure 1). Welded construction damages coated areas and necessitates repairs. Thus, common practice is to leave areas uncoated where welding is to occur. The cut-back distance necessary for preventing heat damage ranges from about six inches for a polyethylene coatings to one inch for a fusion-bonded epoxy coatings. Laser welding generates much less heat and causes much less coating damage that the usual welding equipment.

In addition, joint coatings used on weld areas of pipelines have a few special requirements:

The weld must be ground relatively smooth and any weld spatter removed (see Figure 2).

The joint coating must be compatible with the pipe coating.

The application temperature must not exceed the temperature limit of the pipe coating.

The joint coating must adhere well to both the cleaned metal and the pipe coating (see Figure 3).

Figure 2. Grinding of girth weld for coating.

Figure 3. Liquid coating applied to girth weld.

Coal tar can be used to coat joints if the coating on the rest of the pipe contains coal tar or asphalt. More commonly, though, joints are coated with cold-applied polyvinyl chloride or polyethylene tapes having a synthetic rubber adhesive, which may then further be protected with heat-shrink sleeves.