In the maintenance and cleaning process of long-distance natural gas pipelines, various types of pipeline pigs are used to ensure the pipeline’s efficient operation and safety. These devices can be categorized into the following types based on their function and design:
Pipeline Pigs’ Type
Foam Pigs
Foam pigs are simple cleaning tools typically made from neoprene rubber with a hollow structure, where the wall thickness is about 10% of the pipeline diameter. These pigs adjust their interference fit by injecting water into a one-way valve on the ball, primarily used for removing fluids within the pipeline and separating media, such as isolating different oil products in sequential oil transport or expelling air during pipeline hydrostatic testing. Their main drawbacks are difficulty in removing blockages and dust, and they are prone to getting stuck under certain conditions.
Polyurethane Foam Pigs
Made from highly elastic and tough polyurethane foam, these pigs are about 1.75 times the length of the pipeline diameter, with an interference fit typically 25mm or 5% to 10% of the pipeline diameter. They can effectively navigate through bends and, even in case of blockages, can be compressed or burst due to their high deformability to relieve the blockage. Foam pigs are particularly suitable for pipelines with internal coatings as they do not damage the pipeline walls. However, their capability to remove debris is generally limited, and they are for single use only.
Mechanical Pigs
Mechanical pigs can remove both fluids and solid impurities within the pipeline, and can be equipped with attachments like steel brushes and scrapers to enhance cleaning effectiveness. These pigs have a long service life, typically traveling between 800 to 1000 kilometers. They are suitable for straight pipelines, but their passability decreases in pipelines with multiple bends, valves, or significant deformations. Common types of mechanical pigs include disc cup pigs and bidirectional plate pigs, the latter being particularly efficient in short pipeline segments and capable of moving in both directions to address blockages.
Gauge pigs
Gauge pigs are used to measure parameters such as the geometric shape, ovality, surface unevenness, and weld penetration of pipelines, providing essential data for pipeline maintenance and repair. These pigs generally have a diameter about 60% of the pipeline, featuring good flexibility and the ability to pass through constrictions reduced by 15%.
Selection of Pipeline Pigs
The choice of Pipeline Pigs depends on the specific conditions of the pipeline and the required cleaning or inspection tasks, ensuring effective operation and maintenance of the natural gas pipeline system.
In the selection of pipeline pigs used for cleaning and maintaining pipelines, several multi-objective, multifactorial considerations are pivotal. The choice is influenced by the pig’s mechanical strength, wear resistance, adsorptivity, reactivity with the transported medium, and its ability to traverse the pipeline. Here are the key factors in selecting the appropriate pipeline pig:
Cleaning Objective: The type of debris—whether liquid or solid impurities—needs to be cleared, and the anticipated quantity of such debris. For instance, in natural gas pipelines where sediment is expected, pigs with discharge holes are preferred. These holes help in lifting and expelling sand through the gas flow, preventing clogs.
Pipeline Lifecycle Stage: For new pipelines, the focus is on residue cleaning, initial measurements, pressure testing, drying, and trial operations. Operational pipelines require cleaning of wax deposits and contaminants, media isolation, and corrosion inhibition. Decommissioned pipelines involve clearing remaining media and filling with inert gases.
Drive Medium Characteristics: The selection of the pig also depends on the nature of the drive medium, which could be oil, water, or gas.
Pig Interference Fit: Factors such as the pipeline’s minimum bend radius, bending angle, minimum and maximum internal diameter, connections to branch pipelines, changes in elevation along the pipeline, and the type, location, and size of inline valves, tees, and bends need to be considered.
Cleaning Process: The operational parameters such as the range of driving pressure (maximum allowable operating pressure, operational pressure, variations in temperature parameters), potential speed ranges of the pig, and the maximum distance the pig needs to cover are also crucial.
Dimensions of Brushes and Scrapers: The thickness of wax deposits inside the pipeline and the range of driving forces are considered to determine the size of brushes and scrapers on the pig.
Mechanical Strength, Wear Resistance, and Reactivity: The pig’s strength, durability, and its chemical reactivity with the pipeline’s transported medium are essential factors.
