Answer
A Wall Wash Test (WWT) is a precise quality control procedure executed by the vessel's crew or an independent surveyor. Its purpose is to chemically verify that a cargo tank's bulkheads are totally free from previous cargo residues, tank cleaning detergents, or moisture before loading ultra-high-purity, sensitive chemical cargoes (such as Methanol or Glycols).
A crew member collecting a wall wash sample inside a chemical carrier tank.
Basic Procedure
After the tank is washed, drained, dried, and well-ventilated, a crew member enters wearing clean, lint-free PPE and rubber gloves.
A lab-grade, high-purity solvent (typically Methanol or Acetone) is sprayed or squirted onto a clean section of the bulkhead at multiple heights (upper, middle, lower).
As the solvent drains down the vertical surface, it dissolves any micro-residues. It is caught directly into a pristine glass sample bottle and sealed for analysis.
Main Laboratory Test Parameters
| Test Name | What it Detects | How it is Evaluated |
|---|---|---|
| Hydrocarbon Test (HC) | Traces of oil, fats, or previous petroleum cargoes. | Distilled water is added to the sample. If hydrocarbons are present, the mixture becomes turbid (cloudy). |
| Chloride Test | Saltwater or marine atmosphere contamination. | Silver Nitrate ($AgNO_3$) is added. Any white precipitation indicates the presence of chlorides. |
| Permanganate Time Test (PTT) | Presence of oxidizable organic impurities. | Potassium Permanganate is added. The crew tracks how many minutes it takes for the pink color to fade. Shorter times mean more impurities. |
| Color / Acid Wash Color | Visual discoloration or chemical reaction traces. | Compared against standard platinum-cobalt scale color disks to ensure absolute transparency. |