November 1, 2023
Many years ago spherical pigs were commonly used in natural gas pipelines to remove liquids – mostly water and condensates. The launchers were designed to only handle spherical pigs, using special valves such as the Wheatley Launch Valve and the Fitzpatrick Launch Valve. These resembled a check valve and were designed to release one spherical pig at a time by movement clapper with a lever. Then, with the implementation of Pipeline and Hazardous Materials Safety Administration (PHMSA) requiring that high pressure transmission lines be routinely inspected to prove integrity with inline inspection (ILI) tools, and with the installation of process equipment to remove these liquids, the spherical pigs slowly disappeared from the market. Today, however, with the drilling and development of the shale plays where producers found rich gas – containing large amounts of liquids such as butane, propane and ethane – the need to frequently pig to remove liquids has reappeared. The producers have learned that these rich-gas gathering systems are very profitable, but need to be pigged often – as many as four times a day – to keep production at high levels while capturing these liquids. Another factor that determines the pigging frequency is the amount of liquid being delivered with each pig run, compared to the amount of liquid that could be handled with the liquid separation equipment. If 500 barrels of liquid was delivered with each pig run, and the separation equipment could only handle 500 barrels, it would be necessary to pig the line daily. So, what pigs can be loaded several (10-12) at a time and released automatically on a timed basis or remotely, one at time, several times a day or a week, that can remove liquids? Hence, the reemergence of the spherical pig. Spherical pigs are hollow round balls, most are made from polyurethane, others from rubber products like neoprene. Most are inflatable and equipped with two 180-degree opposed molded-in inflation valves assembly. Today, there are also non-inflatable spheres appearing on the market. These require less pressure and less flow to move through a pipeline because they are not filled with liquid, have a thinner wall and vent holes to equalize pressure. The inflatable spheres must be filled and inflated with a liquid in order to hold their shape when exposed to high external pressure. With the use of an inflation pump the sphere is sized using a sizing ring at the pipe ID or slightly larger. They tend to roll and/or rotate while traversing a pipeline, which prolongs their wear life. Their wiping surface is all that touches the pipe and thus displaces most of the liquid within the pipeline. As a result, the wear life of an inflatable sphere will approach 1,000 miles.