California Code of Regulations
Title 8 - Industrial Relations
Division 1 - Department of Industrial Relations
Chapter 4 - Division of Industrial Safety
Subchapter 7 - General Industry Safety Orders
Group 20 - Flammable Liquids, Gases and Vapors
Article 135 - General
Section 5420 - Tanks, Vats and Containers Containing Flammable Liquids

(1) Lines which may convey hazardous materials to the vessels shall be disconnected, or other positive means shall be used to prevent discharge of such material into the vessel.

(2) If work involving the use of flame, arc, spark, or other source of ignition is to be done, the vessel shall be emptied, flushed or otherwise purged of flammable vapors. A test for flammability of the vapors in the vessel shall be made, using an appropriate device for this purpose, and no source of ignition shall be permitted in or in contact with the vessel if the percentage of combustible vapor is greater than 25 percent of the lower explosive limit. As long as any source of ignition is used in or in contact with the vessel, frequent tests shall be made to determine the concentration of combustible vapors. If this concentration exceeds 25 percent of the lower explosive limit of the vapor present, sources of ignition shall be extinguished or removed until such concentration is reduced below 25 percent of the lower explosive limit.

NOTE: For suggested procedures, see NFPA 327-1982.

(3) Fire extinguishing equipment adequate to cope with the hazards which may be encountered shall be provided and maintained close at hand.

NOTE: See Article 108 for further instruction when working in confined spaces.