In message , at 09:04:39 on
Tue, 17 Sep 2013, Chris J Dixon remarked:
"Pigging in the context of pipelines refers to the practice of
using pipeline inspection gauges or 'pigs' to perform various
maintenance operations on a pipeline. This is done without
stopping the flow of the product in the pipeline.

Or in the case of town gas, having let the product escape, it seems.
--
Roland Perry