Table of Contents
The article gives details of pig traps used for pigging operation of oil and gas pipelines.
The major focus of the article is the technical requirement for the successful design and construction of pig traps. The article’s key focus areas include the length of pig traps, length of minor and major barrel, the type of reducer, nozzle orientation and locations, supports requirements, etc.
1.2 Pipeline Pigging
A pipeline is the most efficient means of transporting hydrocarbon products through a long distance. Pipelines used to convey hydrocarbon requires cleaning and integrity checks after a designated period.
Cleaning is necessitated by debris resulting from contaminated products which may accumulate on the pipeline, corrosion etc. Pipeline cleaning is performed by sending a cleaning tool (cleaning pig) through the pipeline. The action of the flowing fluid propels the pig tool from the start of the pipeline to the end.
Integrity checks are required to validate the state of the materials used for the pipeline construction. These include estimation of pipeline corrosion rate etc.
Checks are also required to validate the bend radius of the pipelines.
Multi-product pipelines utilise batching pigs used as barriers to separate the different products.
Based on the facts above, pipeline pigging is a very important activity during the operational life of the pipeline.
Pipeline pigging entails using a device called Pig to perform tasks such as cleaning, inspection, maintenance, batching, and dimension check on a new or existing pipeline. During pigging operation of an existing pipeline, pipeline operations are not stopped meaning the pigging tool is introduced into the flow stream and travels along the pipeline utilising the flow and pressure of the pipeline. Note that the flow rate may be reduced to the maximum required for the safe transportation of the Pig during pigging.
A key mechanical equipment required for pipeline pigging is the pig trap (Pig Launcher and Pig Receiver).
The Pig traps are static equipment through which the pigging tool is introduced into a pipeline and received from the pipeline. It comprises barrels, end closure for introducing a pig into a pipeline or removing a pig from a pipeline, and nozzle connections. Note that two pig traps are required (Pig Launcher and Pig Receiver) for a successful pigging operation. The pig traps may be permanently installed or temporarily installed.
Temporary or mobile pig traps are only installed when pigging operation is performed.
1.3 Key Definitions Associated with Pig Traps
For Proper explanation, see below schematics of a pig launcher and a receiver.
1.3.1 Pig Launcher
The Pigging tool is fed into the pipeline through the pig launcher. It may be horizontal or vertical. Horizontal pig launchers are associated with onshore installations, while vertical installations are mostly found on offshore platforms.
1.3.2 Pig Receiver
The pigging tool is retrieved from the pipeline through the receiver. The debris is removed through the pig receiver when pipeline cleaning is performed. Most receivers, both onshore and offshore, are horizontally located.
1.3.3 Major Barrel
The Major Barrel is the enlarged pipe section of a pig trap used to load or retrieve pigs.
The reducer is a fitting between the major and minor barrels. It connects the larger diameter major barrel to the smaller diameter minor barrel.
1.3.5 Minor Barrel
The minor barrel is the section between the reducer and the pig trap valve. It has the same diameter as the pipeline
1.3.6 End Closure
The end closure is a fitting, including a removable part or assembly, which provides quick and easy access to the major barrel when open and seals the bore when closed.
These are flanged connections to the Pig trap. They are the mating connection points for piping and instruments installed on the pig trap
2 Technical Consideration for Designing A pig Trap
The key considerations when design are pig trap are highlighted below
2.1 Design Codes and Standards
Pig traps should be designed to the pipeline design code. If the pipeline is designed as per ASME B1.8, the pig trap should be designed based on the same code.
Although older pig traps (launchers and receivers) were designed based on ASME Section VIII Div. 1 pressure vessel code, however, these designs are not economical because given the same pipeline design parameters, a pig trap designed to ASME Section VIII Div. 1 will tend to be more expensive than that design to ASME B31.4 or B31.8 because of the resulting difference in material grades and thicknesses.
As stated in ASME B31.4 section 404.7.3 Closure heads such as flat, ellipsoidal (other than in para. 404.7.2), spherical, or conical heads are allowed for use under this Code. Such items shall be designed in accordance with Section VIII, Division 1, of the ASME Boiler and Pressure Vessel Code.
The traps shall also be fabricated as per the design code. All welds shall be subjected to NDT/NDE.
2.2 Material of Construction
The pig traps should be fabricated of materials compatible with the pipeline, usually carbon steel.
Usually, the same materials used for the pipeline is recommended for the pig traps. For a large diameter pipeline, the grade of the materials of the major barrel may be increased to achieve a reduced wall thickness.
2.3 Barrel Size (Diameter)
The size of the barrel is vital to a successful pigging operation.
2.3.1 Major Barrel
For Pipelines with a nominal diameter smaller than DN 500 (NPS 20), it is required that the major barrel’s nominal diameter should be 50 mm (2 in) larger than the pipeline’s nominal diameter.
The actual internal diameter for thick pipelines should also be considered in determining the oversize. Assuming a NPS 12 pipeline, the outside diameter is 12 – 3/4in this implies the actual barrel size should be NPS 16 rather than NPS 14. NPS 14 does not meet the required 2in oversize.
For pipelines with a nominal diameter of DN 500 (NPS 20) and larger, it is required that the barrel’s nominal diameter should be 100 mm (4 in) larger than the pipeline’s nominal diameter.
The table below shows typical sizes of barrel sizes.
2.3.2 Minor Barrel
The internal diameter of the main pipeline should be the same as that of the minor barrel.
If there is a variation in wall thickness greater than 2.4mm (0.10in), the transition in the internal diameter should be tapered to a maximum angle of 14 degrees to the pipe axis to permit smooth passage of pigs.
2.4 Length of Pig Trap
The pig trap (major barrel) should be designed for the maximum tool length utilised for the pipeline pigging (usually an intelligent pig). A margin of about 10% should be added to the maximum pigging tool length.
Usually, ultrasonic inspection tool lengths are considered conservative. If there are specific pig tool products to be used on the pipeline, the tool vendor should be contacted for the actual tool dimension to be used for pig trap design.
The table below shows the conservative dimensions of pig traps (Launcher and Receiver) and should be read in conjunction with the two figures below.
The reducers should be eccentric for horizontal pig traps with flat on bottom. For vertical traps, the reducer should be concentric. For details of reducers, refer to ASME B16.9.
2.6 Nozzle Connection Locations
There are various nozzles connections to the pig traps; they are the interface for piping or instrument connection. Each nozzle should be appropriately located for effective pigging operation.
See below description of the nozzles.
2.6.1 Nozzle Orientation
On horizontal pig traps, all drain nozzles shall be located at the bottom of the trap.
All vents, blowdown, purge thermal relief, pig signaller and pressure gauges nozzles shall be located on top of the trap.
The kicker line, bypass line, and balance line should be located at the pipe’s side or top.
Nozzles should be evenly distributed around vertical traps and not clustered in the same location.
2.7 Nozzle Location and Sizes
Nozzle locations are critical to a successful pigging operation. Installing a nozzle in the wrong location may hamper the pigging operation, while choosing the wrong nozzle size harms the pigging operation.
2.7.1 Pig Trap Valve Nozzle
The pig trap valve nozzle is welded to the minor barrel. Usually, its size is equal to the pipeline size, and its internal diameter should flush with the internal diameter of the valve and the pig trap. The nozzle is connected to the pig trap valve used to isolate the pig trap from the operational pipeline system. The pig trap valve nozzle is only not applicable when the pig trap is permanently welded to the pipeline valve or pipeline.
2.7.2 Kicker Line Nozzle
The kicker line connects the bypass line to the major barrel, thus enabling the fluid diversion through the barrel to launch or receive a pig.
The kicker line nozzle on a pig launcher should be close to the end closure as possible, while the nozzle connected to the kicker line on the pig receiver should be as close as possible to the reducer. The kicker line nozzle should be approximately the major barrel’s mid-length for bi-directional pig traps.
Typical sizes of a kicker are shown in the table below.
2.7.3 Drain Nozzle
Two drain nozzles should be provided for horizontal pig traps, one on the major barrel close to the end closure while the other on the minor barrel close to the pig trap valve.
For vertical pig traps; one drain nozzle shall be provided close to the pig trap valve
Recommended nominal diameter of the drain nozzle shall be at least 50 mm (2 in) for pipelines having a nominal diameter of less than 350 mm (14 in) and at least 100 mm (4 in) for larger size pipelines.
2.7.4 Equalisation Nozzle
Usually, two equalisation nozzles are provided on the pig trap, one on the major barrel and the other on the minor barrel. In some instances, only one is provided on the minor barrel while the other end of the equalisation line is connected to the bypass line.
The equalisation line is connected to these nozzles. It is used to achieve equal pressure upstream and downstream of the pigging tool.
2.7.5 Vent/ Flare/ Thermal Relief Nozzles
A minimum of two vent nozzles on a gas pipelines pig trap. One should be on the major barrel and the other on the minor barrel. Since only the barrel space will be vented, 2in size nozzle is suitable for this application.
2.7.6 Pressure Indicator Nozzles
Two pressure indicator nozzles should be welded on the pig trap, one on the major barrel close to the end closure and the other on the minor barrel close to the pig trap valve. 2in size nozzles are recommended for this application.
2.7.7 Purge Connection Nozzle
A Purge connection nozzle should be welded to the trap, preferably on the minor barrel. 2” size nozzle is suitable for this application.
2.7.8 Pig Signaller Nozzle
When intrusive type pig signallers are used, a nozzle should be provided. Nozzles for this application are usually 2” in size. Though only one pig trap nozzle is welded on the trap, there should be provided on the pipeline just after the pig trap valve to install another pig signaller.
For pig launchers, the nozzle should be located close to the pig trap valve on the minor barrel, while on receivers, the nozzle should be located close to the reducer on the minor barrel.
2.7.9 Bypass Line
Though the bypass line is not part of the pig trap, it is indirectly connected to the pig trap; therefore, it is worthy of giving details of this connection.
A bypass line connects the pipeline with the downstream or upstream facilities such as a tank farm, gas processing facilities etc.
The bypass from the main pipeline is achieved using a barred tee installed welded to the pipeline.
Typical sizes of bypass lines are given in the table below.
2.8 Nozzle Class Rating
All nozzle connections shall have a minimum rating corresponding to the pipeline design pressure and temperature. The nozzle rating shall also correspond to the mating nozzle for proper alignment.
For details of nozzle ratings, refer to ASME B16.5
2.9 Pig Trap Support
Usually, saddle type support is used on pig traps. One support is on the major barrel, and the other is on the minor barrel. The supports should be designed to carry the weight of the trap filled with water.
Suppose there is not enough flexibility on the pipeline. In that case, the supports under the barrel should be of the sliding/clamp type to compensate for the expansion of the unrestrained part of the pipeline however. Fixed type supports may be used if there is enough justifiable flexibility
2.10 Lifting Lug
Lifting lugs should be designed to lift at least the minimum empty weight of the pig trap. The lugs should be appropriately located on top of the barrel. One of the lugs should be on the major barrel and the other on the minor barrel.
ASME B16.5: Pipe Flanges and Flanged Fittings NPS 1/2 through NPS 24
ASME B16.47: Large Diameter Steel Flanges NPS 26 through NPS 60 Metric/Inch Standard
ASME B 16.9: Factory-Made Wrought Buttwelding Fittings
ASME B31.4: Pipeline Transportation Systems for Liquid Hydrocarbons and Other Liquids
ASME B31.8: Gas Transmission and Distribution Piping Systems
DEP 188.8.131.52-Gen: Design of Pipeline Pig Trap Systems