January 2018, Vol. 245, No. 1
Features
The Re-Emergence of Spherical Pigs
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 re-emergence 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.
One primary reason spherical pigs are used for this application is that they will roll into launch position. Therefore, in an oversize launch barrel inclined at a 10-15-degree angle, 6-12 spherical pigs, based on the operator’s preference, can be loaded, and with the use of two launch pins that can be either hydraulic, pneumatic, or electric actuated, one spherical pig can be released at any given time.
The reduction in manpower alone will pay for an automated sphere launcher system in less than a year. In addition, less gas is released to the atmosphere by multiple-loading spheres when compared to loading a single standard-type displacement pig one at a time. Less valve maintenance due to less cycling of valves and reduced safety hazards also helps to lower costs. These systems today are not restricted to just spherical pigs, but are designed to allow the launching of cleaning pigs and ILI tools.
A common problem with spherical pigs is that under low-flow conditions, they tend to stop in full-size branch connection tees, allowing the flow to bypass around the spherical pig through the branch connection. The non-inflatable spherical pig works best in low-flow conditions due to their flexibility, needing less pressure and lower flow to move them through the tee.
However, the operator still needs to understand that spherical pigs are good at removing most, but not all liquids, and are not good at cleaning. Being round and when filled with liquid, they can be heavy, requiring special handling equipment such as sphere-handing tongs or a scissor-lift trailer that allows spherical pigs to be moved.
Additionally, spherical pigs are used in metering proving skids and to separate product batches within a common line.
Spherical pigs have become a useful tool in today’s gathering systems for removing valuable liquids and keeping well production at high levels. The automated spherical pig launchers and receivers make it easy to launch and receive them. That’s why the return of spherical pigs is making a difference in today’s production of wet gas in the shale play markets.
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