Page 236 - Pipeline Pigging Technology

Foam pigs

of spiral bands was applied in the opposing direction, forming a criss-cross
pattern. With the differential pressure pushing the oversized pig forward, the
opposing frictional drag caused the cleaner to "swell" slightly and increase its'
force on the pipe wall. (This relates to the Kellem Anchor theory, more
commonly known as the secret behind the ancient Chinese finger puzzle.)
In order to increase cleaning ability, the ends of toothbrushes, containing
the plastic bristles, were moulded along the exterior of the pig body and
provided us with the first abrasive polly pig. The second generation of brush
pigs used the strips of plastic bristles from "TOM" hair curlers. The addition
of these brushes helped the pig remove soft deposits on the pipe wall, but still
were not abrasive enough for harder scales. This lead to the addition of silicon
carbide grit and emery cloth embedded along the surface of the pig.
The polly pigs of the 1960s still were only considered for light cleaning and
drying. The ether-based foam systems of this era were structurally not very
strong, would not travel long distances and did not hold up well when used
with hydrocarbons. With the advances made in polyurethanes over the last
20 years, the polly pig has become a more durable cleaner with many uses.
Urethane foam bodies are now manufactured from ester and ether/ester
blends, giving them the ability to withstand most hydrocarbons and chemi-
cals, and to have exceptional tear strength properties while still maintaining
flexibility. Improvements in the cell structure have increased its ability to
"breath", allowing the foam to be used in higher pressure applications.
Additionally, the urethane coatings and abrasive coverings have become
stronger and more aggressive.

SPECIFICATION AND DESIGN

Polly pigs are manufactured in many designs and sizes. Most are in the
shape of a bullet with an elastomeric coating on the base to provide for
maximum seal against the propelling force. Some have coating on the surface
to enhance the sealing and wiping capability of the pig, and to increase its
wear resistance. Other styles have special abrasive materials to aid in cleaning
and scraping. Normally, the overall length of the pig is 1.75-2 times the pipe
diameter, with the base-to-shoulder (the point where the surface bearing area
begins to taper towards the nose) dimension measuring 1.5 times the
diameter. Polly pigs are currently manufactured in diameters from 0.25in to
108in, with increments of 0.125in available in diameters under 12in.
The foam body is made by mixing several urethane resin components
together, under controlled conditions; the mixture is then poured into a

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