Answer
Objective
A Wall Wash is a precise laboratory-grade analysis performed on chemical tankers to verify that the cargo tank surfaces are completely free from microscopic contamination (hydrocarbons, chlorides, or previous cargo residues) prior to loading high-purity or sensitive chemical cargoes.
The Step-by-Step Procedure
Safety and Tank Entry: Ensure the cargo tank is fully ventilated, gas-free, certified safe for entry, and isolated as per the enclosed space entry permit.
Equipment Preparation: Gather laboratory-grade, lint-free collection funnels, clean glass sample bottles (amber or clear, sealed with Teflon-lined caps), and a clean, laboratory-grade plastic squeeze bottle containing the analytical solvent.
Selecting the Testing Area: Choose multiple representative, high-stress locations inside the tank: bulkheads, corners, vertical frames, and areas near the bottom sumps. The surface must be completely dry before beginning.
Washing the Bulkhead: Hold the collection funnel tightly against the vertical bulkhead plate. Squeeze the analytical solvent onto the bulkhead roughly 1 meter above the funnel, allowing it to run down the steel surface over an area of approximately $1\text{ m}^2$ into the funnel.
Collection & Blank Sample: Direct the solvent running through the funnel straight into the sample bottle. Seal it immediately and label it with the tank number and sampling location. Crucial Step: Collect a "Blank Sample" directly from the solvent container without touching the bulkhead to serve as a laboratory control reference.
Chemical Solvents Utilized
Methanol (Methyl Alcohol): Most common. Industrial laboratory-grade Methanol ($>99.9\%$ purity) is the standard solvent used for most chemical cargoes because it acts as an excellent solvent for both hydrocarbons and organic compounds.
Acetone: Utilized if the next cargo is highly sensitive to trace polymers or when testing for tough resinous or hydrocarbon residues that Methanol cannot easily dissolve.
De-ionized Water (DI Water): Used specifically when testing for Chlorides (saltwater residues) or when loading chemicals that are highly reactive with alcohol solvents.