A
pig is also a trade term for any object sent deliberately through a pipeline (other than the intended
liquid or
gaseous cargo). There are many reasons this might be done, and as many if not more varieties of pigs. Some do
naught but
ram through to clear out buildup and obstruction. Some are used to check the inside of the pipeline, and emit
sonar to all sides. Some have lights and
cameras for inspection.
The etymology of this isn't something I know, although I can make a couple of guesses. First, since early pigs were naught more than chunks of shaped metal, their name might have ben taken from metal pigs, or rough ingots (see mine pig). I've also heard from a pipeline professional that they were named this due to the fact that they squeal loudly as they are forced through a pipe.
They can be self-propelled, but are usually sent through on a blast of compressed air or high-pressure water; in some cases, the pipeline's payload is used. The act or job of doing this is called pigging, naturally.