08/06/2026
CARGO TANK GAS FREE
The cargo tank degassing process is carried out to remove flammable, toxic, or inert gases from a tank and replace them with fresh air so that the tank becomes safe for inspection, maintenance, repairs, or hot work.
Typical Cargo Tank Degassing Procedure
1. Cargo Discharge Completion
× Ensure all cargo has been discharged.
× Strip cargo lines and pumps.
× Drain remaining residues to slop tanks where applicable.
2. Tank Washing
- Wash cargo tanks using fixed or portable tank cleaning machines.
- Remove cargo residues from tank surfaces.
- Collect wash water in slop tanks.
3. Purging (If Tank Is Inerted)
> Replace inert gas with fresh air, or in some cases use an intermediate gas depending on cargo type.
> Reduce hydrocarbon concentration gradually.
4. Ventilation / Degassing
* Open designated tank openings and vents.
* Use fixed or portable fans/blowers.
* Introduce fresh air and exhaust contaminated atmosphere through venting systems.
* Continue until hydrocarbon levels are below required limits.
5. Atmosphere Testing
Test at:
● Tank top
● Mid-depth
● Tank bottom
● Pump room (if applicable)
Typical acceptance criteria:
Oxygen: approximately 20.9%
● Hydrocarbon concentration: below 1% LFL (or as company procedures require)
● Toxic gases (H₂S, benzene, etc.) below occupational exposure limits.
6. Gas-Free Certification
☆ Authorized officer or marine chemist conducts final testing.
☆ Issue Gas-Free Certificate if required.
7. Entry Permit
Before personnel entry:
■ Complete enclosed-space entry permit.
■ Establish continuous ventilation.
■ Prepare rescue equipment and standby personnel.
Example Sequence for a Crude Oil Tanker
》Discharge crude oil.
》Crude Oil Washing (COW).
》Water wash.
》Strip residues.
》Ventilate tanks using portable fans.
》Measure O₂ and hydrocarbon levels.
》Gas-free certification.
》Tank inspection or repair.
Safety Precautions
◇ No smoking or ignition sources.
◇ Use intrinsically safe equipment.
◇ Monitor atmosphere continuously.
◇ Follow enclosed-space entry procedures.
◇ Maintain communication with personnel inside tanks.
For oil tankers, chemical tankers, and LNG/LPG carriers, the detailed degassing procedure differs because of the cargo characteristics and regulatory requirements.
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Octrex Holdings