Cargo Tank Cleaning Procedures & Residue Management Flashcards
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What is the primary purpose of prewashing a cargo tank before gas-freeing operations on a chemical tanker?
Answer: To remove cargo residues that could produce flammable or toxic vapors during gas-freeing
Prewashing removes cargo residues that would otherwise continue to off-gas hazardous vapors during gas-freeing, making the process safer and more effective.
According to MARPOL Annex II, which types of chemical tanker cargoes require mandatory prewashing before washings may be discharged to the sea?
Answer: High-viscosity substances and solidifying substances that cannot be effectively stripped
MARPOL Annex II mandates prewashing for high-viscosity and solidifying substances because residues cling to tank surfaces and cannot be removed by pumping alone.
When a cargo tank has carried a Category X substance under MARPOL Annex II, what must occur before the vessel may proceed to sea?
Answer: The tank must be prewashed and the washings discharged to a shore reception facility
Category X substances are most harmful to the marine environment; MARPOL requires prewashing and discharge of washings to a reception facility before the vessel departs.
What is 'clingage' in the context of cargo tank cleaning operations?
Answer: Cargo residue that adheres to tank surfaces and cannot be removed by pumping alone
Clingage is cargo residue that sticks to tank surfaces after pumping is complete; it must be removed by washing before the tank atmosphere can be made safe.
Which type of tank cleaning method is most appropriate when switching between cargoes that are incompatible with water?
Answer: Solvent washing or dry cleaning methods compatible with both cargoes
When cargoes are incompatible with water or the next cargo cannot tolerate water contamination, solvent washing or dry cleaning methods must be used to avoid product damage or dangerous reactions.
Before personnel enter a tank that has been cleaned with a solvent, which atmospheric conditions must be verified to be within safe limits?
Answer: Oxygen level, flammability (% LEL), and toxic vapor concentrations must all be within acceptable limits
NFPA 306 requires all three parameters—oxygen content, flammability, and toxic gas concentrations—to be within safe limits before any personnel entry into a tank.
In cargo tank cleaning operations, what does 'tank stripping' refer to?
Answer: Pumping out the maximum possible cargo residue before washing begins
Tank stripping is the process of removing as much cargo as possible using stripping pumps before washing, reducing the volume of contaminated washings generated.