Pigs Unlimited details the use of a progressive pigging method for lines with reduced internal diameters.
The development of pigging methods and technologies are a constantly evolving section of the industry and a crucial practice in pipeline maintenance operations.
As such, the pigging process chosen to solve any pipeline issue should be carefully selected to incur the most comprehensively successful results.
Progressive pigging is a special method for pigging lines with reduced internal diameters. This method is necessary because most pigs cannot accommodate extreme reductions.
Additionally, trying to remove a large amount of build-up from a line with a single pig can spell certain disaster by completely plugging the pipeline. The two types of reduced internal diameter lines are concentric and non-concentric, in which the build-up is on the bottom of the pipe only.
The following procedure is for progressive pigging lines with concentric build-up, and these are recommendations only and should not be considered absolute. In all cases, it is advisable to consult with knowledgeable pigging personnel for variations in the procedure.
This paper uses an extreme example of a 16 inch (407 mm) pipeline with a 6 inch (153 mm) concentric build-up of mineral scale, resulting in a 4 inch (102 mm) effective internal diameter, in order to demonstrate the full realm of variables for progressive pigging.
The first step involves ascertaining the internal diameter of the line. This can be achieved by viewing the inside of the pipe at either end or removing a fitting such as valve; using a light-density swab to prove the line (described below); or using an educated guess.
Because deposits can be heavier in some sections of the line and lighter in others due to piping and flow variations, it is recommended to use a line-size, light-density swab to prove the line. From the previous example, this would be a 16 inch swab.
The swab is able to negotiate the reduced diameter and will also be able expand up to its full shape in sections where there the build-up is lighter. It will also remove a layer of soft deposits or loose debris (if any), establish pig run time, pressure and velocity.
These need to be noted for future reference. When retrieved,
the swab’s outer diameter is measured. This process needs to be performed
quickly because the swab will soon expand to its original shape. The
measurement obtained is the smallest effective internal of the
of the line.
At this point, the progressive pigging method can now begin. Using the same example, there is an 8 inch (204 mm) effective internal diameter, therefore pigging begins with an 8 inch medium-density bare pig followed immediately with a linesize (16 inch) swab.
The swab will help keep the 8 inch pig moving in sections of the pipe where the internal diameter is larger than 8 inches. This step may need to be repeated until the 8 inch pig is retrieved with minimal wear. The next step is to run an 8 inch medium-density criss-cross pig, again followed immediately with a 16 inch swab. The criss-cross will remove more of the deposited build-up.
This step may also need to be repeated until the 8 inch pig is retrieved with minimal wear. Next, an 8 inch medium-density criss-cross wirebrush pig will be run, also followed immediately with a 16 inch swab. The wirebrush pig will remove even more of the build-up.
Depending on the length and type of build-up, this step may or may not need to be repeated. At this point, the first stage of progressive pigging has been successfully completed. By increasing the diameter of the pigs and repeating the above three steps, pigging continues until the full internal diameter of the pipe is reached.
Depending on the length of the line and type of build, the increasing diameters of the subsequent pig runs may be in increments as small as 0.25 inch (7 mm) and as large as 2 inches (51 mm), (or next nominal pig size – 10 inches for the example used here).
The last stage of progressive pigging is to run a line-size, medium-density bare pig to perform a final sweep of the line removing any loose debris.
By adopting a routine pigging program, progressive pigging should not be needed again to clean the line.
This article was featured in the Spring edition of Pipelines International. To view the magazine on your PC, Mac, tablet, or mobile device, click here.
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