Page 244 - Pipeline Pigging Technology

Pigging and chemical treatment

Low flow conditions, with more complete cooling, cause greater paraffin
deposition. Once deposited, however, paraffin will not redissolve when the
oil is below the cloud point, or solubility limit of paraffin in the oil. It must be
removed either by solvent-dispersant chemicals, or mechanical or thermal
methods. Generally, the solubility of paraffin in paraffin "solvents" is only a
few percent, and mechanical methods are preferred. Putting "hot oil" into a
line can dissolve paraffin deposits, but these are likely to re-deposit further
down the line as the oil cools, merely transferring the problem downstream.

Paraffin control using pigs

Pigs are routinely used to control paraffin formation on pipe surfaces.
There are many different pig designs used by the industry, such as Polly Pigs,
spheres, and mandrel pigs equipped with cups (scraper, conical), discs or a
combination of both. The function of any pig in this application is twofold; to
scrape the adhered wax from the pipe wall and to remove the deposits out of
the pipeline.
The interaction of a pig's surface bearing area against the pipe wall causes
a shearing or scraping effect. By-pass around the pig assists in suspending
debris in the oil in front of a pig to help carry it out of the line. The ability of
a pig to remove wax is not necessarily its tight sealing capability (as in a
batching operation) as much as it is its cutting, scraping or pushing character-
istics.

Combined pigging and chemical treatment

Theoretically, either a chemical-treatment programme or pigging alone
should be adequate in controlling paraffin formation. But in actual pipeline
operating conditions, neither method can offer a complete guarantee. This is
especially true in pipelines that carry oil with high cloud points, low flow
velocities, and high paraffinic or asphaltenic characteristics. The rate of build-
up can be so aggressive that the amount of chemicals necessary are cost
prohibitive, and some paraffins exist which are difficult to fully treat. As well,
the rate of deposition can be so rapid that pig runs are not run frequently
enough to keep up with growth. Hard wax deposits can be removed by pigs
equipped with wire brushes, scraping discs and other cleaning devices.
A better paraffin-control programme combines pigging with chemical
treatment, as neither treatment alone is likely to provide all the benefits of a
combination programme. The principles followed in paraffin-control pro-
grammes are:

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