Pigging: Types, Function and Operation

1.0       PIGGING

 

Pigs are used daily in the operation and construction of pipelines.  They may be used for a variety of reasons:

 

  • To clean and increase line efficiency
  • To locate obstructions and inspections of the line
  • To separate products
  • Corrosion control
  • Air purge and water removal
  • Meter proving

 

The frequency of pig runs and the number of pigs to be run will depend on the condition of the line and is usually found by trial and error.  The cost of running the pig can be calculated by the loss of throughput while the pig is running.

 

2.0       PURPOSE OF PIGGING

 

Pigging refers to the passage of a pig or scraper through the line along with the fluid flow.  The pig or scraper is moved through the line by the differential pressure across it, caused by the process fluid moving by it at a higher velocity.  The purpose is to remove foreign bodies.  For cleaning purposes, a fairly good seal is required between the pig and the pipeline wall.

 

2.1       Increase Pipeline Flow Efficiency

 

Pigs are run to maintain the line efficiency.  Any decrease in pipeline efficiency reduces the throughput of a pipeline.  It is impossible for a pipeline to be 100% efficient.  Friction and other factors obstruct flow.  In a natural gas transmission line, oil from compressor discharge and other liquids such as glycol carryover, can combine with dust, distillate and minute droplets of water in the gas stream and coat the inside of the line.

 

Paraffin and sand build-up in crude lines, liquid hold-up in multiphase gas lines, and dirt and water build-up in product or gathering lines, presents the same problem.  In all cases, the contamination build-up increase resistance to flow which in turn, decreases pipeline efficiency and increases costs of transmission.  A “scraper” pig is used to clean debris from the pipe wall.

 

Flow or pump efficiency tests can be performed to test the efficiency of the line and determine whether or not the line requires cleaning.  If the line requires cleaning, a pig is launched into the line to scrape or wipe clean the wall and increase efficiency by reducing the resistance to flow.

A 1% increase in efficiency of a natural gas pipeline transporting 100 MMCFD is an increase of 1 MMCFD.  A 1% increase in efficiency of a crude or product pipeline transporting 50 000 barrels a day can increase throughput by 500 barrels a day.

 

2.2       Locate Obstructions and Inspections of the Line

 

After a new pipeline is installed, it is necessary to ensure that the line has no objectionable deformations or obstructions, and the most construction debris has been removed.  Dents or buckles create localised stress areas, increase turbulence and can block the running of a pig.  To locate the obstruction, a “gauging” or “calliper” pig is placed in a new pipeline and propelled by pressure until it reaches the pig receiver at the other end of the line.  The information gathered is stored in the onboard computer of the intelligent pig and can be analysed after the pig is recovered so that location of problem areas can be ascertained.  This type of pig usually incorporates a “pinger” by means of which its location can be determined, should it meet an obstruction and become lodged in the line.

 

An “electronic geometry pig” (EGP) can be used to survey the pipe for even minor deformations.  This type of pig does not touch the pipe wall to identify deformities.  Any variation in ideal roundness is detected and measured by the EGP’s electric field distance sensor over 360 degrees.

 

An “intelligent” pig is also used to periodically inspect transmission lines in service for damage or evidence of corrosive processes.  This type of pig uses ultra sonic and magnetic flux principles to detect deformities with the information stored in an onboard computer.  Visual inspections are possible using a visual inspection pig.

 

2.3       Separate Products

 

The commingling between two products in the region of contact as they are pumped through the line leads to contamination and sloping of some of the product at the interface.  This downgrades product and is expensive.  There are at least three sources of contamination in a products pipeline. namely:

 

  • Pipeline design
  • Operating procedures
  • Interface of products

 

To prevent contamination, a “batching” pig is inserted into the line either at the interface or at the beginning and end of a buffer batch that has been inserted between the two products.  The pig maintains a high seal between the products with minimal contamination and sloping.

 

2.4       Corrosion Control

Pigs are used to improve and maintain internal pipe cleanliness by removing contaminates and deposits in the pipe.  Periodic line cleaning with pigs can be used in conjunction with corrosion chemical inhibition or dehydration.  Some corrosive situations that can be remedied at least in part by pigging include:

 

  • Water and other fluids which settle out of gas, crude, or products due to insufficient flow velocity, for entrainment, intermittent flow, pressure
  • Loose sediment, including corrosion products, scale, sand, dirt, etc., which usually promote the formation of local corrosion cells on the pipe’s bottom quadrant, especially in conjunction with the above.

 

Corrosion products, wax, or other solid deposits adhering to the pipe wall which can shield actively corroding areas, thereby limiting the effectiveness of other corrosion mitigation measures such as chemical inhibition.

 

2.5       Air Purge and Water Removal

 

Hydrostatic testing involves purging a section of pipeline of all air, filling it with water, and then pressuring the line to a specified test pressure for some period of time.  On completion of the test, the water is removed so that the line can go onstream.  “Batching and displacement” are used to displace air ahead of the hydrostatic test water, and to displace water after the hydrostatic test.  Pigs can be used along with some water absorption fluid such as methanol.

 

2.6       Meter Proving

 

A meter prover is a special length of pipe of known volume that is used to calibrate a flow meter using a pig.  The pig is made of elastomer and is spherical in shape to obtain a good seal.

FIGURE 1

UNIDIRECTIONAL METER PROVER

Fluid is simultaneously passed through a turbine meter and through the prover.  To make a “proving run” a sphere is launched into the prover barrel.  When the sphere contacts the first detector switch, an electronic counter begins to record the output of the turbine meter.  When the sphere contacts the second detector switch, the electronic counter is stopped.  Runs are continued until several repeats within a set tolerance are achieved.  Since the volume between the two detector switches is accurately known, it can be compared to the volume recorded by the turbine meter.  The volume as measured by the meter is divided by the known volume of the prover to determine the meter factor.  The factor then becomes the calibration of that meter for that fluid at those conditions.

  

3.0       LINE DESIGN CONSIDERATIONS

 

To obtain trouble-free pigging operations, many pipeline design characteristics must be considered.  Any obstruction can hinder the free running of the pig.

 

Length of Pig Run

 

There is no formula for determining the length of a pig run.  The life of the pig depends on such things as quality of the line construction, pig velocity, pig design, interior line conditions (rough, semi-rough, smooth) and the media in which the pig is running.  There are some general guidelines:

 

  • 160 kilometres for newly constructed gas pipelines
  • 240 kilometres for newly constructed product lines
  • 320 kilometres for newly constructed crude oil lines

 

The energy required to propel the pig depends on the velocity, pressure, temperature, viscosity, density, pipe pressure drop, length of pipeline, etc.

 

General formula for calculating Pig run time can be used as reference/guidance:

 

Pipeline Volume                  V( M3)

Pig run Time( T )  =                                       =

Volumetric flow rate            Q(M3/HR)

 

Bends

 

Each pig is designed to traverse a certain minimum radius bend.  One pig may traverse a 1 R  bend (1.5 times NPS) while another may only traverse a 3 R bend.  Rigid metal body pigs require a straight run of pipe between bends that is equal to at least two pipe diameters.

 

Valves

 

Full opening valves provide un-obstructive pig passage through the line.  The two types of valves used for pigging service are the ball and gate valve.  If gate valves are used, the seat ring must be large enough to allow un obstructive passage of a pig.

 

If check valves are used, the pig must be able to span the bowl length.  Spheres and short pigs will often stop in the bowl due to bypass around them in this oversized area.  All valves must be in the fully open position for pigging operations.

 

If motorised valves are used in station pigging, pneumatic or electric pig signal indicators should be used as switches to open and close valves as the pig traverses the main line piping.  Pig signal indicators can also be used as switches to automatically shut down or start up compressors as the pig passes through.

 

Tees

 

Tees with outlets that are 75% of the main line pipe I.D. or greater should be installed with guide bars across the branch.  This prevents the pig from trying to enter the branch outlet.  Hot tap holes should be designed and checked so that there are no sharp edges that could allow a pig brush from hanging on such an edge.  Adjacent tees that could cause a large volume bypass around the pig should be avoided.  This could cause the pig to stop in the line.  Lateral branch openings must not permit large volume bypass of the pig as this could cause the pig to stop in the line.

FIGURE 2

BRANCH CONNECTIONS

 

 

Pipe Inside Diameter

 

The diameter of the line must allow unrestricted movement of the pig within the line.  Although pigs can be manufactured for dual diameter lines, they are much more expensive.  From launcher to receiver, the pipeline should be of the same diameter.

  

4.0       TYPES OF PIGS

 

If the purpose of pigging is to ensure that the line has no dents and is free of obstructions, a gauging pig is used.  If the purpose is to clean rust, dirt, or mill scale, a standard cleaning pig can be used.  If the purpose is to displace air or water, a batching or displacement pig can be used.  If the purpose of pigging is for line inspection, an intelligent pig is used.

 

Gauging pigs

 

The gauging pig is to check the pipeline has a full round opening from one end to the other.  It can check for dents, wrinkles, buckles, out of roundness, flat spots and even partially closed valves.  It will also clear construction debris such as welding rods and skids.

 

The pig is “intelligent” meaning it has a built in recording or transmitting device.  Examples of gauging pigs calliper pigs, conventional gauging pigs and electronic geometry pigs.

 

Cleaning Pigs

 

Cleaning pigs employ two elements, brushes and urethane blades.  Brushes can be utilised if the line has scale or hard deposits sticking to the pipe wall.  Urethane blades can be used to cut and remove gummy or loose deposits from the pipe wall.  Bypass ports are usually incorporated into the cleaning pig so that part of the flow is diverted to create turbulence ahead of the pig.  This is to prevent dirt and scale accumulating in and around the pig as this could impede or stop it from running.

 

Batching Pigs

 

These pigs have multiple cups and disks that maintain a seal through bends and pipe branches.  The inflatable sphere is ideal for traversing short radius bends where the body type pig will not.  They are filled with a liquid such as glycol or water and give a good seal.  This type of gelled pig has a number of advantages over a mechanical pig:

  • Performs effectively in lines of varying sizes
  • Passes line restrictions, intrusions and probes
  • Maintains a good seal over long distances
  • Moves large amounts of solid debris without sticking or plugging

 

FIGURE 3

PIG TYPES

  

5.0       PIG LAUNCHERS AND RECEIVERS

 

The location of a pig launcher depends on the position of incoming and outgoing lines, and suction and discharge manifolds.  The trap should be located to give easy handling for pig loading.

A blow down line is located on top of the barrel near the end closure along with a drain line on the lower quadrant of the trap.  A pig signal indicator is installed downstream from the launcher to indicate the passage of the pig out of the launcher into the main line.

The mainline trap valve and bypass line valve are full opening valves, either gate valves or ball valves.  See Figure 4.

The receiver barrel is usually 1 to 2 sizes larger than the line pipe with a minimum barrel length of at least 2.5 times the length of the longest pig to be received.  The

barrel is equipped with a quick opening end closure with an O-ring seal to prevent gas leakage.

A bypass line leaves the trap at a point near the reducer or valve end of the barrel.  This location causes a decrease in the flow behind the pig, which reduces it speed as it enters the trap.

A blow down line and drain line are located near the end closure of the barrel.  A pig signal indicator is installed on the receiver barrel to signal the arrival of the pig.

FIGURE 4

PIG LAUNCHER

 

The mainline receiver valve and bypass line valve are full opening valves, either gate valves or ball valves.  See Figure 5.

FIGURE 5

PIG RECEIVER

 

6.0       LAUNCHING AND RECEIVING PIGS AND SPHERES

 

The operation sequence described herein is general information and it is not intended to nor should it be used for training a technician for a particular pigging system.

 

Pig Launching

 

Starting conditions assume that the trap is pressurised, valves A, B, and C are open and valves D and E are closed.

 

  1. Close valves “A “and “C”
  2. Depressurise the trap by opening valve “E” and vent to flare, check gauge
  3. Drain the trap by opening valve “D” and drain to closed drain system
  4. Purge trap where applicable
  5. When the trap is vented, drained and purged, open the closure door
  6. Insert the pig into the chamber so that the first cup fits tight at the reducer
  7. Close and secure the closure door
  8. Set pig signal indicator(s)
  9. Close drain valve “D”, Crack open vent valve “E”
  10. Crack open valve “C” and fill chamber controlling rate on vent valve “E”
  11. When the chamber is full (All Air/Gas is released ) close the vent valve ”E”
  12. Continue pressurise till the Line Pressure & chamber pressure is equalized.
  13. Close valve “C” and open main trap valve “A” fully
  14. Re-open kicker valve “C” and throttle bypass valve “B”
  15. Throttle valve “B” until the pig moves out of the trap
  16. When the pig leaves the trap and enters the main line, open valve “B” fully
  17. Close kicker valve “C” and trap valve “A”
  18. Check pig signal indicator(s) to ensure the pig has been launched
  19. Depressurise the chamber by opening vent valve “E”, check gauge
  20. Drain the closure by opening drain valve “D”

  

Pig Receiving

Starting conditions assume that the trap is de-pressured, valves A and C are closed and valves B, D and E are open.  The closure door is closed and secure.

 

  1. Close drain valve “D”
  2. Fill trap by opening valve “C” and vent through valve “E”
  3. Close vent valve “E” when trap is full (no air/gas)and allow trap to equalise via “C”
  4. With valve “C” open, fully open trap valve “A”
  5. Set pig signal indicator(s)
  6. The pig may stop in the trap between the tee and valve “A”
  7. Throttle valve “B” to force the pig into the trap
  8. Check pig signal to confirm the pig is in the trap
  9. When the pig is in the trap, open valve “B” and close valves “A” & “C”
  10. Depressurise the trap by opening valve “E” and vent to flare, check gauge
  11. Drain the trap by opening valve “D” and drain to closed drain system
  12. Purge trap where applicable
  13. When the trap is vented, drained and purged, open the closure door by wearing

Proper PPE & safety Precautions for any possibility presence of Pyrophoric Iron.

  1. Remove the pig from the chamber
  2. Close and secure the closure door
  3. Fill trap by opening valve “C” and vent through valve “E”
  4. Close vent valve “E” when trap is full and allow trap to equalise via “C”

FIGURE 6

PIG SIGNAL INDICATOR

 

 

 

FIGURE 7

PIG TRAP CLOSURES

 

The introduction of hydrocarbons to a pig launcher or receiver which has been previously been open, creates an explosive mixture.  Similarly, the release of hydrocarbons from a launcher or receiver when the end closure door is opened will create an explosive mixture and could activate gas detector(s) located in the vicinity.  In order to prevent this, purging with an inert medium should be carried out.

 

In pigging operations, the greatest opportunity for an unsafe incident to occur will be when the end closure door is open and the pig is being manhandled into or out of the launcher or receiver.  In order to minimise the risks, the following steps should be taken:

 

  • The launcher or receiver to be purged prior to opening the end closure door
  • A permit to be raised for all pigging operations
  • All welding and burning operations on the platform to cease for pig operations
  • All un-associated operations within a 15 metre (50 feet) radius to cease
  • Only personnel directly involved in being operations to remain in this area
  • Communication links established and confirmed

 

The end closure door on pig launchers and receivers is prone to leakage.  This often occurs as the pressure in the launcher or receiver is approaching full system pressure.  This could result in the pig being stopped in the pipeline or received in a potentially dangerous manner.

 

Inert gas can also be used to leak test the end closure door.  For pig receivers, the purge and leak test should be carried out before the pig is launched into the line.  For pig launchers, a leak test should be carried out once the pig has been loaded into the launcher.

 

Prior to, during, and on completion of pigging operations, good radio communication is essential.  This includes communication with the control room, the platform sending and/or receiving the pig and with other operators.  All involved in the pigging operation are to be advised when a pig is about to be launched and when the pig has been received.  The control room operator, in addition to being advised of pig launching and receiving, must be advised when a launcher or receiver is about to be purged and leak tested.  One cannot over-stress the importance of effective communication between technicians involved, the control room, other platforms involved, or shore based facilities when sending and receiving pigs.

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