How It Works: Pipeline Pigging

What is a pig?

A pipeline inspection gauge, commonly referred to as a ‘pig’, is a device used to clean and/or inspect a pipeline. The term originates from the initial pig devices, which were bundles of straw wrapped with wire or leather that were forced down the pipeline, resulting in a squealing sound. The term ‘pipeline inspection gauge’ is a backronym and helps to accurately represent the increased capabilities of these devices.

The spherical or cylindrical devices manufactured today vary greatly in construction material, size, color and accessory components. Common to all pigs, though, is that they closely conform in diameter to the pipe bore. This allows the device to knock away any accumulated debris or material and to closely inspect for cracks or flaws in the pipe.

There are a variety of types of pigs used today, including:

  • Utility pigs1: used to clear the line of debris or seal the line
  • Inspection pigs2: used to gather information about the pipeline, which can include temperature and pressure, corrosion/metal loss, diameter, bends and curvature. Two methods:
    • Magnetic flux leakage (MFL) – sends magnetic flux into the pipe walls for leak, corrosion and flaw detection
    • Ultrasonics (UT) – measure ultrasonic sound wave echoes to determine pipe wall thickness
  • Specialty pigs: one example of a specialty pig is a plug, used to block off a line so that maintenance can be performed
  • Gel pigs: gelled liquids that can be used alone or in conjunction with other types of pigs for various procedures including debris removal, hydrostatic testing, product separation, dewatering, condensate removal, removing a stuck pig and more

What is pigging?

Pigging a pipeline starts with a pig trap – a funnel shaped Y-section in the pipeline that includes a launcher and a receiver. Certain valves, like the GROVE G4N gate valve and WKM Saf-T-Seal gate valve are ideal valves for these pig traps because of their smooth, through-conduit design that does not obstruct the line bore. The launcher3 inserts the pig into the pipeline, where it is either pushed along by line pressure or pulled through the pipe by a cable. The receiver4 acts as a point to remove the pig from the line, as well as any debris the pig has knocked loose.

Why pig a pipeline?

There are various reasons to pig a pipeline, but the most common is to clear the line of debris. This process ensures that the product running through the line is flowing smoothly and reduces the likelihood of product contamination. There also are major safety advantages to clearing a line, as built up debris can degrade the integrity of the pipe wall over time.

Smart pigs also are commonly used today to detect various elements about the pipeline. As mentioned earlier, inspection pigs collect information regarding temperature and pressure, corrosion/metal loss, diameter, bends and curvature to verify pipeline integrity. Cracks, weld defects, surface pitting and areas of crushing/deformation are often measured through pigging as well.

Because of the variety of environments that pipelines are found in, including underground or underwater, and the construction materials of most pipelines, pigs must record their data internally until it can be retrieved by the user. Retrieving and monitoring where certain data was collected5 is done through surface instruments using audible, magnetic or gravimetric methods to record where the pig is at various points. The user then uses this information to map out defects to aid repair crews in maintenance.

Some new, “smart” pigs have GPS capabilities that can assist in mapping a pipeline. This helps maintenance crews 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.

What makes a valve piggable?

The valves in a pipeline that will be pigged must be through-conduit, otherwise the pig will not pass through them. Through-conduit gate valves and ball valves are piggable, such as DEMCO gate valves, WKM ball valves, and NEWCO gate valves and ball valves. In addition, some check valves are piggable, like TOM WHEATLEY, WHEATLEY, and AOP check valves. Care should be taken, however, when using smart pigs or pigs with sensors, as the clapper of the check valve can damage the pig, unless special design considerations are included.  The Cameron TOM WHEATLEY check valve can be supplied with a lock open lever, which allows the clapper to be locked in the open position and out of the flow of the pig so that it will not damage the sensors or valve body. Generally, globe valves, butterfly valves, choke valves and plug valves are not piggable, unless a special pigging method or pig is used6. One exception to this is the Cameron TEXSTEAM Super G plug valve, which is available in full opening with a round port that allows pig passage.

The process fluid and the type of pipeline largely determine pig type and whether or not pigging a pipeline is necessary. Proper valve selection is essential at the beginning of the pipeline construction process to ensure that pigging is possible if needed.

[1] Types include mandrel pigs, foam pigs, solid cast pigs and spherical pigs

[2] Also known as ‘in-line inspection pigs’ or ‘smart pigs’ 

[3] Also known as a ‘pig launcher’ or ‘launching station’

[4] Also known as a ‘pig catcher’ 

[5] Often called ‘location verification’

[6] See, gel pigs