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
(c)(1) Filled with an inert gas or water, where compatible; or
(2) Determined to be free of flammable gases by a
flammable gas detection device prior to and at frequent intervals during the
application of heat.