This is the second of two articles that first appeared in the May 2012 issue of Idaho Outdoor Journal.
by Joe Evancho
Pig in a pipe
A pig is a device inserted into a petroleum or natural gas pipeline that travels freely through it, driven by the product flow to perform a task within the pipeline such as cleaning or inspecting.
Petroleum and natural gas pipelines represent a considerable investment for the operators as well as countries and governments. Pipelines are considered the most efficient way to move fluids long distances so in order to protect these valuable investments, maintenance must be done and pigging is one maintenance tool.
Cleaning pigs remove debris, sludge, wax, etc. from a pipeline.
Inline inspection pigs are used to assess the wall thickness and extent of internal and external corrosion in the line, thus providing important information regarding the safety and condition of the line.
Rust never sleeps
Corrosion is a major problem associated with underground structures, specifically natural gas and petroleum pipelines. Corrosion failures are expensive and can cause shutdowns, hazardous conditions, occasional fires or even catastrophes.
What is corrosion? In its simplest terms, corrosion is an electrical reaction (electro) between a metal and its environment – soil or water (chemical).
Electrochemical corrosion is essentially the tendency of refined metal to return to its original ore by releasing the refining energy. Steel, for example, is refined from iron ore and when steel corrodes the rust that forms is nearly the same as the ore.
This phenomenon occurs naturally and involves the flow of direct-current electricity from one point to another on the metal’s surface. This naturally occurring current is generated by a difference in voltage between the two points and it forms rust, and rust never sleeps.
Successful corrosion protection relies on a number of procedures such as using the proper material in the proper place, coating with corrosion inhibitors, cathodic protection and careful construction practices. All corrosion control systems need to be properly maintained.
For above ground structures coatings are an effective method of corrosion control. Coatings such as epoxies, vinyl and galvanizing are often used.
For buried pipe lines, coatings by themselves are not considered adequate for corrosion control because all coatings develop small breaks or “holidays” from construction damage or soil stress. The most important function of coatings comes in relation to cathodic protection.
Cathodic (electrical) protection is widely used by utilities as well as industry to run direct current through the soil from special locations to the structure to be protected.
This current overcomes corrosion currents and protects the pipeline or other structure from corrosion. For a coated pipeline it is necessary to protect only the metal exposed at holidays, typically less than one-half of one percent of the pipe surface.
These protective measures maintain the health and integrity of pipelines, but neither can protect exposed pipelines from moving debris such as large rocks or submerged trees as the flow downstream.