Application of Buried Piping
The piping that is in contact with soil or runs below grade level is called buried or underground piping. In the oil and gas industry, we find frequent use of buried piping mainly for cross-country pipelines where security and safety justify the pipelines to be underground. Here security means the difficulty of sabotage and intentional damage to an underground pipeline. Safety means the safety of the environment and the surrounding population centers due to accidental damage to the underground pipeline. The firefighting network piping mostly runs below ground with certain sections above-ground. Above-ground pipelines are also intermittently buried in roads and railway crossings. Other major applications include Cooling water lines inside process plants, Oily Water Sewer lines, Contaminated Rain Water Sewer from the process catchment area, Sanitary systems, Equipment drainage to slop tank, Closed Blow Down system, Storm Water, Liquid Effluent to the Effluent Treatment Plant, etc.
However, note that Piping should not be buried or installed underground if it can be reasonably avoided.
A basic understanding of the concepts of buried piping installation along with routine process design calculations related to piping such as pressure drop, velocity and erosion/corrosion limits for various pipe metallurgies are always advantageous for engineers. This article will provide some important guidance on buried or underground pipe installation.
Over the past few years, the underground firewater piping has gained popularity because of the following:
- When untreated water comes in contact with above-ground carbon steel or lined carbon steel pipes, it becomes a source of corrosion and undermines the integrity of the firewater system. For this reason, corrosion-resistant HDPE or GRE plastic piping systems are more frequently used in recent times.
- Although plastic piping possesses higher corrosion resistance to untreated water, it has much lower mechanical strength compared to carbon steel pipes considering forces such as impact and vibration.
- So, to maintain the mechanical integrity of the plastic pipe, It is a logical option to bury the firewater plastic pipe and running across the installation.
- Another option is to use exotic metallurgies (Stainless steels / Cupro-Nickel etc.) for firewater networks but that is not an economical choice. At the same time, common austenitic stainless steels are susceptible to chloride stress corrosion cracking in a water environment with high chloride content.
Problems Associated with Buried Piping
- Even after applying mitigating measures such as external coating and cathodic protection, Buried Steel pipes are subjected to external corrosion.
- Draining, cleaning of buried pipes is difficult compared to an aboveground pipe.
- Leak detection and repair of buried pipe is a difficult and expensive exercise. Modern underground pipeline leak detection systems are available in recent times but they are very expensive to install.
- Buried pipes are subjected to mechanical damage when a soil 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 is applied.
Corrosion Protection of Buried Pipes
All buried steel piping with the possible exception of cast iron piping should be protected from soil corrosion with a suitable external 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)
Tape wrap is often selected when small quantities of buried piping require protection because it is relatively inexpensive and easy to apply in the field. However, it has very low reliability 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 the 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 is required for the 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:
- Proper surface preparation for the type of coating used
- Coating Application as per the specified consistency and thickness
- Proper Care during laying and handling to avoid coating damage
- Proper cleaning, priming, and field coating of joints and fittings
- Thorough Inspection of the applied coating for any damage and proper repair
- Finally, Backfilling and compacting to prevent contact with any material that could damage the coating
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.
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 normally applied to buried piping as a system. At every place, where the 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 must be done with an insulating flange gasket kit that uses electrically insulating bolt sleeves, nut washers, and sealing gasket in a conventional flange makeup.
The major users of cathodic protection are Cross country steel pipelines and steel submarine piping.
Few more useful resources for you..
Buried GRP/FRP pipe Laying and Installation Procedure
Basics for Stress Analysis of Underground Piping using Caesar II
Underground Piping Stress Analysis Procedure using Caesar II
What if Piping Continuation is Unknown? Part 2. Underground Piping
Various Analysis methods for Underground Piping Using Caesar II
Cathodic Protection Basic Principles and Practices
About the Author: This article is written by Mr. Ankur Srivastava, a Chemical Engineer with more than 33 years of process engineering experience in oil and gas, refining, polymers and petrochemical industries.