Recommendations for maintenance and operation of petroleum storage tanks, Part 1:Preparation of tanks for safe entry and work
1981 Edition

The procedures for safely gas freeing petroleum storage tanks before personnel entry include thoroughly replacing hydrocarbon vapors with fresh air via forced ventilation and ensuring that all sludge is removed before personnel remain inside. Continuous ventilation must be maintained, with the area evacuated except for brief vapor testing. Vapor concentrations at tank openings should be measured; entry is permitted without respiratory gear when vapor levels are below 5% of the Lower Flammable Limit (LFL), allowed with supplied air respiratory equipment for cold work between 5% and 20% LFL, and prohibited above 20% LFL. Vapor testing must be repeated after any extended work breaks to ensure safety.

Sludge containing leaded gasoline residues consists of insoluble solid deposits wetted with hydrocarbons containing organic lead. Handling requires protective clothing and canister respirators approved for tetraethyl lead, with canisters replaced after 100 hours of use. Disposal methods include controlled burial in designated sites, weathering by exposing sludge layers to sunlight for several weeks mixed with sand or earth to reduce lead content, and thermal incineration where feasible. Liquid fractions should be treated as wastewater following pollution control regulations to ensure environmental safety.

Workers entering tanks with toxic or flammable atmospheres must ensure oxygen concentration is at least 19.5% for entry without respiratory protection. When vapor levels are below 5% of the LFL, no respiratory equipment is necessary, but between 5% and 20% LFL, supplied-air respiratory apparatus is required, permitting only cold work. Entry is forbidden if vapor levels exceed 20% LFL. Canister-type masks are only suitable for use outside tanks due to limited protection. Additional personal protective equipment such as gloves, boots, and breathing apparatus must be inspected and verified before use, alongside continuous ventilation and gas monitoring.

Hydrogen sulphide (H₂S) presence during tank cleaning is detected using moistened lead acetate paper, which darkens in H₂S presence, and specialized gas detectors or monitors. H₂S is highly toxic and flammable, heavier than air, and has flammable limits between 4.3% and 45% volume in air. Tanks must be vapor-freed prior to opening, and workers should wear appropriate respiratory protection when working inside tanks that contained sour stocks. Continuous monitoring for H₂S and hydrocarbons is essential throughout vapor freeing to manage hazards safely.

To prevent ignition during hot work inside petroleum storage tanks, all flammable liquids must be removed and vapors purged until concentrations are well below 1% of the Lower Flammable Limit (LFL). Workers performing welding or flame cutting on tank bottoms or lower shell sections should use fresh air respiratory equipment. Areas for hot work above the first shell ring should be cleaned to bare metal to remove lead-containing particles or be performed with respiratory protection. For tanks containing sour stocks, interior surfaces must remain wet until gas-free to prevent spontaneous ignition of pyrophoric iron sulfides. Personal protective equipment should be thoroughly inspected, and ignition sources must be controlled with proper ventilation and vapor level monitoring.