Function of Pigging Launcher and Receiver

Pigging Launchers and receivers also commonly referred to as pig trap assemblies are used to introduce and remove in a safe and effective manner, scraper cleaning pigs, spheres and inspection tools (commonly referred to as Smart Pigs or Intelligent pigs) through liquid, gas and multi-phase hydrocarbon pipelines. The main purpose of a pipeline pig trap system thus is to provide, in a safe manner and without flow interruption, the means to either: 

  • Insert and launch a pig into a pipeline at Pig Launcher End
  • Receive and retrieve a pig from a pipeline at Pig Receiver End

The overall dimensions of the pig trap assembly is dependent on the type of pig to be handled. Pig Trap stations for intelligent pigging will occupy more space due to larger dimensions of the intelligent pigs.

Parts of a Pig Trap - Pig Launcher or Pig Receiver

A typical pig launcher or pig receiver also commonly referred to as pig trap comprises of the following components :

  • Major barrel
  • Minor barrel
  • End closure
  • Pig signaller
  • Kicker line connection
  • Pressure gauge connection
  • Vent and drain connection
  • Utility connections

Major and Minor barrel

As a rough guideline, for pipelines less than 20 inches diameter, the major barrel diameter is 2 inches greater than the pipeline diameter. For pipelines with a diameter of 20 inches up to and including 46 inches, the oversize is 4 inches. For pipelines 48 inches and above the oversize is 6 inches. This guideline may vary from Organization to Organization. The barrel is sloped towards the end closure for a launcher and towards the pipeline for a receiver. Typically a slope of 1:100 is maintained on the pig traps. For bi-directional pig launchers and receivers no slope is required on the barrel.

The barrel should be designed to accommodate the longest pig that may be used. If the pipeline is designed for intelligent pigging, the length of intelligent pig will dictate the barrel length. During the early stages of piping study the type of intelligent pigs that will be used are agreed with COMPANY operations and documented in the pipeline pigging philosophy. The most conservative dimensions of pig trap are selected to develop the plot plans and detailed piping layouts. The reducer connecting the major and minor barrel is of eccentric type for horizontal pig trap assemblies. This ensures that the pig trap bottom is maintained over its entire length. For vertical pig traps the reducer is normally of concentric type.

End Closure

End Closure
END CLOSURE

The pig launchers and receivers and equipped with doors which are called end closures. The end closure is welded to the major barrel and hence its material and end wall thickness should be compatible with the barrel material. End closures are generally of quick opening type and are fitted with safety interlocks to prevent opening when the barrel is pressurized. The safety mechanism on the end closure may be specified to comply with the requirements of ASME Section VIII Division 1, UG-35. The safety feature in the end closure is important because the launching and receiving operation is complex and inadvertent opening of closure door while the pig trap is in a pressurized condition is not uncommon. The hinge position of the end closure and the maximum swing position should be checked for any obstructions. The clearance envelope for the door should be either modelled in 3D plant design system or shown on 2D piping layouts to ensure clear area is available for opening of the end closure. All the hinges should be preferably oriented on the same side in a pig trap area. Some of the reputed end closure manufacturing companies are GD Engineering and SHOLTZ.

Pig signaller

The pig signalers are located on the minor barrel. Pig signalers can be of intrusive and non-intrusive type. The pig trap shall have one signaler located on the minor barrel. Additionally, one more signaller must be suitably positioned on the line to allow the longest specified pig to pass completely through the pig trap isolation valve before activating the signaller.

Kicker line

The kicker line tapping is taken from the bypass line of pig trap. During the launching or receiving operation the pipeline fluid is diverted through the barrel to launch or receive the pig. For a pig launcher the kicker line connection should be connected towards the end closure on the major barrel. For a pig receiver the kicker line connection should be as close as possible to the reducer (i.e. away from the end closure). For a bi-directional pig trap the kicker line should be located at the center of the major barrel. Alternatively, two kicker connections can be provided to facilitate launching and receiving of pigs. 

Typical details of Pig Launcher



Pig Launcher

Design code

The pig trap assembly may be designed to pressure vessel code or the relevant pipeline or piping code. Most companies prefer to design the pig trap system to the piping code. The end closure is designed to ASME section VIII, Div.1.

Pig Launcher and Pig Receiver Layout

  1. Pig Launchers and Pig Receivers are generally grouped together in a dedicated scraper trap or pig trap area close to the plant boundary and preferably lined up along the primary roads to allow easy maintenance and provide adequate access for fire fighting. The orientation of the end closure shall be such that any projectile of the pig from the end closure will not be directed towards critical plant facility which may affect the integrity of the installation in case of pig or scraper colliding with the installation. The end closure shall also be oriented away from main access ways or roads with heavy traffic. Some organizations allocate an overall 20 to 30 degree sector clear envelope from the center of the trap end closure extending over a length of 40-50m.
Clearance behind Pig Trap
Clerance behind Pig or Scraper Trap
  1. Pig Launchers and Pig Receivers are generally installed in horizontal direction. They may be installed in vertical orientation in offshore platforms where space limitations do not permit horizontal installation. When the pig trap is installed in vertical direction, there is a possibility of the debris falling and accumulating on the valve seat thereby damaging the valve.
  2. Adequate room shall be provided around the traps for lifting and handling of pigs. The lifting facilities are provided predominantly for handling of pigs and pigging tools.
  3. The clearance between the bottom of pig trap end closure and the finished grade is maintained between 700mm to 1000mm to facilitate easy handling of the pigging tools. The minimum elevation of the pig trap should allow effective draining of the hydrocarbon inventories without any pockets.
  4. Where pig launchers and receivers are located adjacent to each other a minimum access clearance should be maintained for operation of the valves around the pig trap assembly.
  5. A surface drainage system should be provided to collect any spills from the trap during pig receiving or launching and for collection of wash water.
  6. For large pipelines operating platforms will be required adjacent to the pipeline to conveniently access the valve actuators and the end closure. The layout should be arranged such that all the valves are accessible from the platform from one side of the pig trap.

Typical Configuration of Pig Trap

The following section provides a brief outline of the various connections provided on a pig trap at the scraper stations and their functions. Figure below shows a typical schematic of the various connections. The actual details will vary on a project depending on the adopted design philosophies, the service and other process design parameters. For example, the pig trap may be provided with two pipeline isolation valves depending on the design pressure of the pipeline. An Emergency shutdown (ESD) valve may be provided after the barred tee where the pipeline exits or enters the plant facilities to minimize hydrocarbon containment in the plant. 

Typical Configuration of Scraper Trap
Typical Configuration of Pig Trap

Barred Tee

Barred Tee is a special type of tee used on pipelines that are provided with bars across the internal bore of the branch to prevent the pig getting stuck in the branch or to prevent the possibility of pig entering a branch connection which is of the same size as the pipeline. The barred tee facilitates the straight travel of pig through the pipeline into the scraper trap. The barred tee permits the flow of service fluid but prevents the scraper from striking the edge of the pipeline at the branch connection and getting damaged or trapped at the branch.

Kicker Line

The kicker line is used to kick or propel the scraper or pig through the pig trap into the pipeline. The kicker line size could be approximately half the size of the pipeline. For a launcher the kicker line should be connected to the major barrel as close as possible to the end closure and for a receiver the connection should be as close as possible to the reducer. For bi-directional pig traps, two kicker lines may be provided or a single kicker line could be located approximately at the center of the major barrel  It is common practice to provide a single kicker line approximately at the center of major barrel for bi-directional scraper traps.

Balancing Line or Equalizing Line

The balancing line, also referred to as equalizing line allows the filling of the pig launcher from both sides of scaper or pig, thereby allowing the pressure to be equalized on either sides of the scraper or pig. This balancing of pressure on both sides of the pig eliminates the possibility of the pig hitting and potentially damaging the main pipeline valve or damaging the end closure. The balancing line connection should be made on the minor barrel as close as possible to the pig trap valve. Similar provision is required on pig receivers to prevent any possible unbalanced pressure across a received pig damaging the pipeline valve or end closure.

Pig Trap Isolation Valve

The pig trap isolation valve should be a full-bore tight shut-off ball or a through-conduit gate valve, to isolate the scaper trap from the pipeline. The minimum internal diameter of the valve should allow the passage of the pig through the pipeline into the scraper trap. During procurement stage, the minimum bore ID is specified for a pipeline isolation valve to ensure that it is suitable for pigging operations.

Why two pig signallers are provided at pig trap launching and receiving stations

Two pig signallers or detectors are located at scraper stations. The first pig detector provides indication that the scraper has exited the scraper trap. The second pig detector after the barred tee provides confirmation that the scraper has run past the bypass line and has entered the main pipeline ready to exit the plant facilities. Similarly at the receiving station, the first pig signaller confirms the receipt of pig at the other end and the second pig signaller provides indication that the pig has been received in the scraper trap.