Page 238 - Pipeline Pigging Technology

Foam pigs

Criss-cross - medium- or high-density foam with coating on the surface-
bearing area. Used for dewatering, product separation/evacuation,
cleaning and removal of solids (such as wax);
Silicon carbide - medium- or high-density foam, coating on the surface-
bearing area with silicon carbide/aluminium oxide grit or straps.
Used for scraping or cracking hard deposits such as oxides or
carbonates (normally for short runs);
Wire-brush - medium- or high-density foam. Coating on surface can be
incorporated with either criss-cross pattern or total coverage of the
surface-bearing area. Used for maximum scraping of materials such
as scales (e.g. mill scale, etc.).

ADVANTAGES OF THE POLLY PIG

There are many reasons why polly pigs should be considered when
developing a pigging programme. First, they can perform many of the same
operations as other conventional pigs, while offering some advantages that
can give the pipeline operator more control over what is to be accomplished
inside the pipe. This is important when one considers that it is nearly
impossible accurately to predict the internal condition of a pipeline that is not
routinely pigged.

Safety - Polly pigs reduce the possibility of damage to the pipe. If for some
reason a steel mandrel pig breaks apart, or if there is a cup or disc failure, the
operator may be faced with an unwanted piece of unprotected steel lying
somewhere in the system. Running another pig to remove the pig parts may
result in damage to valves and other fittings. If there is any evidence that an
obstruction possibly exits inside a line, then a foam pig should be run before
a pig with metal parts is used. They are acceptable for use both in lined and
non-ferrous pipe.

Flexibility- The compressibility of the polly pig allows it to negotiate short-
radius bends, reducing valves, dented pipe and other pipe size reductions.
Most medium-density foam pigs can take a 35% reduction in cross-sectional
area. This means that a 20-in polly pig could conform to 16-in pipe, and a 36-
in pig could conform to 30-in pipe. The special urethane foam has physical
characteristics known as memory and resilience, which allow it to return to
its original shape and diameter once it has passed through a reduction.

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