CMC Cargo Tank Cleaning Procedures & Residue Management

Question 1

What is the primary purpose of prewashing a cargo tank before gas-freeing operations on a chemical tanker?

Accepted Answer

To remove cargo residues that could produce flammable or toxic vapors during gas-freeing

Answer

To cool the tank surfaces before ventilation begins

Answer

To test the structural integrity of the tank before personnel entry

Answer

To calibrate atmospheric testing equipment against known concentrations

Question 2

According to MARPOL Annex II, which types of chemical tanker cargoes require mandatory prewashing before washings may be discharged to the sea?

Accepted Answer

High-viscosity substances and solidifying substances that cannot be effectively stripped

Answer

All petroleum products regardless of viscosity

Answer

Only cargoes with flash points below 60°C

Answer

Only water-miscible substances listed in Category Z

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?

Accepted Answer

The tank must be prewashed and the washings discharged to a shore reception facility

Answer

The tank must be inspected by a port state official and sealed

Answer

The tank must be purged with inert gas to below 2% oxygen

Answer

Atmospheric testing must confirm LEL below 1% throughout the tank

Question 4

What is 'clingage' in the context of cargo tank cleaning operations?

Accepted Answer

Cargo residue that adheres to tank surfaces and cannot be removed by pumping alone

Answer

Static electricity buildup on tank walls due to liquid flow

Answer

The attachment mechanism of rotating tank cleaning machines

Answer

Corrosion products that bond to tank coating after exposure to cargo

Question 5

Which type of tank cleaning method is most appropriate when switching between cargoes that are incompatible with water?

Accepted Answer

Solvent washing or dry cleaning methods compatible with both cargoes

Answer

Hot seawater washing at maximum pressure

Answer

Cold freshwater washing followed by ventilation

Answer

Steam cleaning to volatilize remaining residue

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?

Accepted Answer

Oxygen level, flammability (% LEL), and toxic vapor concentrations must all be within acceptable limits

Answer

Only oxygen level, since solvent washing eliminates flammable vapors

Answer

Only flammability must be below 10% LEL for solvent-cleaned tanks

Answer

Only toxic vapor concentration below the TLV-C is required

Question 7

In cargo tank cleaning operations, what does 'tank stripping' refer to?

Accepted Answer

Pumping out the maximum possible cargo residue before washing begins

Answer

Removing the tank coating for inspection and recoating

Answer

Stripping paint from tank surfaces using chemical agents

Answer

Removing removable tank internals such as suction bell mouths for cleaning