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In some cases such as High Pressure Gas Transmission Mains, Pigs will also be used to further clean and dry the pipeline to remove residue moisture, rust, scale and debris in order to meet the Pipeline Pigs are used during the testing phase to fill pipelines with water and then after completion of the successful hydrostatic test to de-water the pipeline.Īdditionally Pigs are used to remove construction debris that may have accumulated during the construction phase of the pipeline. In the case of new pipelines once construction is complete they must be hydrostatically tested to prove they will be capable of the meeting the designated MAOP (Maximum Allowable Operating Pressure). They represent a major financial, environmental and operational commitment by all stakeholders and in order to protect these valuable investments, ongoing maintenance must be regularly undertaken to ensure the pipeline continues to deliver optimum performance. Pipelines are generally accepted as being the most efficient method of transporting liquids and gases across various distances. The fact the pig-traps are not present means that this pipeline is not pigged regularly. After closing the valves for isolation, the pig can be removed via a quick opening closure (like a flange) on the end of the pig-trap. The pig-trap normally includes a section of larger diameter pipe to allow the pig to come to a gentle stop with flow passing around the pig. The pre-installed thrust blocks (those empty concrete rectangles in front of the blank flanges) are to support a (temporary) pig-trap which is bolted on to the pipeline in place of the blind flange. This station is designed to receive pigs from the upstream section and launch pigs into the downstream section, with a cross-over for the fluid to flow from one section to the next.
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The image above shows a station for loading or unloading pigs, the pigs are used for cleaning out or inspecting pipelines. Pig launcher/receiver, belonging to the natural gas pipeline in Zwitzerland. When the pig reaches the other end of the pipeline it is captured in a Pig Catcher (or 'Receiving Station') which is isolated via a shut-off valve, allowing the pig to be safely removed. The degree of effectiveness in cleaning or clearing a pipeline is determined by the type of pig employed along with other influencing factors such as flow rate, pig speed, pressure, temperature, volume of debris to be removed, length of the pipeline, number of pigging runs, number and type of bends, pipeline elevations.
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Generally the outside diameter of most pigs will be sized to be larger than the internal bore and the resultant 'interference' enables the pig to scrape and remove debris as it traverses the pipeline. The pig speed can be calculated by tracking the pig at various points along the pipeline and calculating the time it takes to arrive at each point against the input pressure and flow rate and then converting to velocity. Once a pig has launched and is moving through the pipeline the differential pressure can be calculated by subtracting the pressure in front of the pig from the pressure acting on the back of the pig. The force applied by a pig as it traverses a pipeline can be calculated by multiplying the cross sectional area of the back of the pig by the pressure applied to the rear of the pig. How does the Pigging process work.īy inserting the Pig into a Pig Launcher (or Launching Station) and then applying flow under pressure to the rear of the device it will move into the pipeline.