Guidance Notes On Buried Piping

Dear All,

Most process engineers should try to understand basic concepts of piping installation along with routine process design calculations related to piping such as pressure drop, velocity and erosion / corrosion limits for various pipe metallurgies.

Today's blog entry provides some guidance on buried or underground pipe installation.

Piping should not be buried or installed underground when it can be reasonably avoided.

Major applications for buried piping are generally cross-country pipelines where security and safety justify burial. Here security implies the difficulty of sabotage and intentional damage to a buried pipeline vis-a-vis an aboveground pipeline. Safety implies the safety of the surroundings (environment / population centers) due to unintentional or accidental damage of the buried pipeline vis-a-vis an aboveground pipeline.

Other major applications for buried piping are firewater piping, critical piping running across the entire plant installation with certain sections aboveground and certain sections underground. Underground sections would generally be routine movement areas including road and rail crossings.

Underground firewater piping has gained popularity in recent years as explained below:

- Above ground carbon steel or lined carbon steel pipes filled with untreated water are a source of corrosion and undermine the integrity of the fire water system and are slowly being replaced with plastic pipes (HDPE / GRE etc.) which are much more corrosion resistant.

-Plastic pipes although having higher corrosion resistance to untreated water have much lower mechanical strength compared to carbon steel pipes when subjected to forces such as impact and vibration.

-To maintain mechanical integrity of the plastic pipe, burying the firewater pipe and running across the installation becomes a logical option.

- Option of having firewater piping with exotic metallurgies (Stainless steels / Cupro-Nickel etc.) is not an economical choice. It is important to note that common austenitic stainless steels are susceptible to chloride stress corrosion cracking in water environment with high chloride content.

Problems Associated with Buried Piping:

- Buried Steel pipes are subjected to external corrosion despite mitigating measures such as external coating and cathodic protection

-Draining, cleaning of buried pipes is difficult compared to aboveground pipe.

-Leak detection and repair of of buried pipe is a difficult and expensive exercise. Although modern buried pipeline leak detection systems are available now, they are very expensive to install.

-Buried pipes are almost always subject to mechanical damage if being excavated or if any excavation work is being carried out in close vicinity.

-Buried pipes carrying hot fluids and subject to thermal expansion can cause pipe deformation and / or partial / total removal of the external protective coating if applied.

Corrosion Protection of Buried Pipes:

All buried steel piping with the possible exception of cast iron piping should be protected from soil corrsion with a suitable exteranl coating.

Following is the list of the most commonly used acceptable coatings and wrappings with approximate pipe surface temperature limitations:

-Fusion Bonded Epoxy (< 93°C)
-Liquid epoxies (< 107°C)
-Extruded Plastic (< 82°C)
-Tape Wraps (< 60°C) (higher temperatures applicable in case of high temperature thermosetting tape)
-Coal Tar Enamels (< 60°C)

When small quantities of buried piping require protection, tape wrap is often selected because it is relatively inexpensive and easy to apply in the field. However, it is often the least reliable coating, with poor performance in water- and oil-saturated soils, and in cyclic temperature service. It requires proper pipe surface preparation and is easily applied improperly.

Protection for coated pipe at weld joints and tie-ins is provided by field-applied fusion bonded epoxy, shrink sleeves of polyethylene, heat-cured liquid epoxy, or tape wrap.

Aside from the type of coating selected, proper application of the coating and maintenance of its integrity are required for proper installation of a protected line. Because success or failure cannot be determined for an extended time after installation, usually years, attention should be paid to:

-Correct surface preparation for the type of coating used
-Application of the coating to the specified consistency and thickness
-Care in handling and laying to avoid coating damage
-Proper cleaning, priming, and field coating of joints and fittings
-Inspection of coating for any damage and proper repair
-Backfilling and compacting to prevent contact with any material that could damage the coating

Cathodic Protection:

Cathodic protection (CP) can be roughly defined as retarding or preventing the corrosion of a metal by imposing an electrical current flowing to the metal through an electrolyte. In the case of buried piping, the pipe is the metal and the soil is the electrolyte.

Fore more details on cathodic protection refer the following link:

http://en.wikipedia....odic_protection

Cathodic protection is often used with coatings to protect piping. Regardless of the care used in coating and installing buried lines there will often be small pinholes in the coating. A cathodic protection system can protect against corrosion at these points and significantly extend the life of the piping.

Cathodic protection is applied to underground piping as a system. At every location where cathodically protected pipe leaves the soil (or water) it must be electrically isolated from the aboveground continuation of the line if the continuation is not part of the CP system. This is done with an insulating flange kit (or insulating union on small diameter pipe) that uses electrically insulating bolt sleeves, nut washers, and sealing gasket in a conventional flange makeup.

Cross country steel pipelines, and steel submarine piping are the principal users of cathodic protection.

This is all folks, as a brief synopsis on the subject of buried pipes and I look forward to comments on the subject from the members of Cheresources.

Regards,
Ankur.