Introduction to Pigging in Pipeline
Pigging in pipeline is a process in pipeline maintenance that involves the use of devices known as pigs, which clean pipelines and are capable of checking pipeline condition without necessarily stopping or interfering with the flow of product in the pipeline.
The primary purpose of pipeline pigs is to make sure that the pipe is clean and free from obstruction. The pig is usually cylindrical or spherical to aid movement and efficient cleaning. As the pig moves through the pipeline, it can possibly detect and remove any build-ups within the pipe which can often lead to reduced performance, increased energy costs and cause corrosion, which could lead to disastrous leaks and cracks in the pipeline.
Pipeline pigging is used in number of industries including oil & gas, lube oil, chemical plants and hygienic applications such as pharmaceutical or food.
Working Principle of Pigging
Pigging in pipeline starts with installation of a pig trap in the pipeline that includes a pig launcher and a pig receiver. Pig trap is typically an over-sized section in the pipeline, reducing to the normal diameter, to accommodate the tight-fitting pig. Without interrupting the flow, pig is inserted into the pipeline using the pig trap. As pig travel along the pipeline, there are a number functions the pig can do, from clearing the line to inspecting the interior. A similar pig trap is located at the end of the pipe to receive the pig once it has traveled the required length of the pipeline. The pig receiver acts as a point to remove the pig from the pipeline, as well as any debris the pig has knocked loose. In the pipeline, pig is either pushed by the line fluid flow pressure or pulled through by a cable.
Pipeline Pigging Applications
- Pigs are used to clean solids, scale, wax buildup, and other debris from the pipelines to keep the pipeline flow efficiency high.
- In natural-gas pipelines, pigs are used to manage liquid accumulation and keep the pipeline free of liquids. Water and natural-gas liquids can condense out of the gas stream as it cools, which affects flow efficiency and can lead to enhanced corrosion.
- Pigs are used to empty the pipelines into the product tanks.
- Pigs (also known as smart pigs or inspection pigs) are used to inspect pipelines for detecting leaks and buildups, which could be explosive and dangerous to the environment. Inspection pigs collect information about temperature and pressure, corrosion/metal loss, diameter, bends and curvature, cracks, weld defects, surface pitting and areas of crushing/deformation to verify pipeline integrity.
- Pigs are used during hydrostatic testing to allow the pipeline to be filled with water, or other test medium, without entrapping air. The pig is inserted ahead of the fill point, and water is pumped behind the pig to keep the pipe full of water and force air out ahead of the pig.
- Pigs are used to apply internal pipe coatings, such as epoxy coating materials, in operating pipelines.
- Some new, “smart” pigs have GPS capabilities that can assist in mapping a pipeline. This helps maintenance teams save time and money by pin-pointing exactly where is a pipeline is run, instead of having to excavate a large area to reach a specific location in the line.
Types of Pipeline Pigs
The purpose of pipeline pigging has evolved with the development of technologies ans thus the types of pigs. There are variety of types of pigs used today, including:
Utility pigs are used to clean the pipeline of debris or unwanted materials. Debris can be accumulated during pipeline construction and during operating phase. Types of utility pigs include mandrel pigs, foam pigs, solid cast pigs and spherical pigs.
Inspection pigs, also known as smart pigs, gather information about the pipeline from within. . The type of information gathered by inspection pigs includes the temperature and pressure, corrosion/metal loss, diameter, bends and curvature, cracks, weld defects, surface pitting and areas of crushing/deformation. Inspection pigs use two methods to gather information about the interior condition of the pipeline:
- Magnetic Flux Leakage (MFL) – inspects the pipeline by sending magnetic flux into the walls of the pipe, detecting leakage, corrosion, or flaws in the pipeline.
- Ultrasonic (UT) – inspects the pipeline by using ultrasonic sounds to measure the amount of time it takes an echo to return to the sensor.
Specialty pigs are used to isolate a section of the pipeline for maintenance work to be performed. The pig keeps the pipeline pressure in the line by stopping up the pipeline on either side of where the remedial work is being done.
Gel pigs (a combination of gelled liquids) can be used in conjunction with conventional pigs or by themselves. Pumped through the pipeline, there are a number of uses for gel pigs, including product separation, debris removal, hydrostatic testing, condensate removal, as well as removing a stuck pig.
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