30 CFR 57.4604 - Preparation of pipelines or containers.
Before welding, cutting, or applying heat with an open flame to pipelines or containers that have contained flammable or combustible liquids, flammable gases, or explosive solids, the pipelines or containers shall be -
(a) Drained, ventilated, and thoroughly cleaned of any residue;
(b) Vented to prevent pressure build-up during the application of heat; and
(1) Filled with an inert gas or water, where compatible; or