Page 237 - Pipeline Pigging Technology

Pipeline Pigging Technology

mould. As the resins chemically react, the mixture rises like cake dough as gas
molecules are released. It is the combination of the material rise and gas
pockets that forms the open-cell structure. The "walls" of each cell are
flexible and give the finished foam its compressibility and memory. Polly pig
foam can be classified into three groups based on the density range:

Low density (swabs) 1-4 lb/ft 3
3
Medium density 5-7 lb/ft
High density 8-10 lb/ft 3

The numeric densities are calculated by a weight/volume ratio and can be
somewhat misleading. It is suggested that one looks at density in terms of
firmness. The lower the density, the softer the foam; the higher the density,
the firmer the foam.
Each of the density ranges offers a different flexibility and wear resistance,
the lower density being more flexible and subject to wear than the higher
density. The elastomeric coatings on the pig bodies are colour coded to help
distinguish between densities. Normally, either a blue or red/orange coloured
coating identifies the medium density foam and crimson or scarlet is used for
the higher density foam.
Normally, the coatings are made from 70-90 Shore A durometer urethane
elastomer and are applied by hand. The hardness of the coating will determine
its flexibility and wear resistance, and as with the foam, the more flexible it
is, the more it will wear. The thickness of the coating is usually 0.125-0.25in
depending on the pig's diameter.
Abrasive materials can be attached to the surface of the foam body by
means of the urethane resin. Wire brush (steel, brass or plastic) straps, silicon
carbide grit and other materials increase the scraping ability of the pig.

COMMON TYPES OF POLLY PIG

There are numerous designs of foam pigs available, but the most frequently
used are:

Swabs - low-density foam with base coated for a seal. Used for removal
of soft materials, drying, absorption of liquids (a swab can absorb up
to 75% of its volume in liquids, such as water);
Bare squeegees - medium- or high-density foam, coated base. Used for
drying, dewatering and light cleaning;

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