CMC Cargo Tank Cleaning Procedures & Residue Management 1 â Questions and Answers
Question 1: What is the primary purpose of prewashing a cargo tank before gas-freeing operations on a chemical tanker?
- To cool the tank surfaces before ventilation begins
- To remove cargo residues that could produce flammable or toxic vapors during gas-freeing (Correct answer)
- To test the structural integrity of the tank before personnel entry
- To calibrate atmospheric testing equipment against known concentrations
Correct 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.
Question 2: According to MARPOL Annex II, which types of chemical tanker cargoes require mandatory prewashing before washings may be discharged to the sea?
- All petroleum products regardless of viscosity
- High-viscosity substances and solidifying substances that cannot be effectively stripped (Correct answer)
- Only cargoes with flash points below 60°C
- Only water-miscible substances listed in Category Z
Correct 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.
Question 3: When a cargo tank has carried a Category X substance under MARPOL Annex II, what must occur before the vessel may proceed to sea?
- The tank must be inspected by a port state official and sealed
- The tank must be prewashed and the washings discharged to a shore reception facility (Correct answer)
- The tank must be purged with inert gas to below 2% oxygen
- Atmospheric testing must confirm LEL below 1% throughout the tank
Correct 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.
Question 4: What is 'clingage' in the context of cargo tank cleaning operations?
- Static electricity buildup on tank walls due to liquid flow
- Cargo residue that adheres to tank surfaces and cannot be removed by pumping alone (Correct answer)
- The attachment mechanism of rotating tank cleaning machines
- Corrosion products that bond to tank coating after exposure to cargo
Correct 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.
Question 5: Which type of tank cleaning method is most appropriate when switching between cargoes that are incompatible with water?
- Hot seawater washing at maximum pressure
- Cold freshwater washing followed by ventilation
- Solvent washing or dry cleaning methods compatible with both cargoes (Correct answer)
- Steam cleaning to volatilize remaining residue
Correct 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.
Question 6: Before personnel enter a tank that has been cleaned with a solvent, which atmospheric conditions must be verified to be within safe limits?
- Only oxygen level, since solvent washing eliminates flammable vapors
- Oxygen level, flammability (% LEL), and toxic vapor concentrations must all be within acceptable limits (Correct answer)
- Only flammability must be below 10% LEL for solvent-cleaned tanks
- Only toxic vapor concentration below the TLV-C is required
Correct answer: Oxygen level, flammability (% LEL), and toxic vapor concentrations must all be within acceptable limits
NFPA 306 requires all three parametersâoxygen content, flammability, and toxic gas concentrationsâto be within safe limits before any personnel entry into a tank.
Question 7: In cargo tank cleaning operations, what does 'tank stripping' refer to?
- Removing the tank coating for inspection and recoating
- Pumping out the maximum possible cargo residue before washing begins (Correct answer)
- Stripping paint from tank surfaces using chemical agents
- Removing removable tank internals such as suction bell mouths for cleaning
Correct 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.
CMC Cargo Tank Cleaning Procedures & Residue Management 1
What is the primary purpose of prewashing a cargo tank before gas-freeing operations on a chemical tanker?