When a shore tank or vessel/barge switches from storing one grade of product to another, washing walls and a wall wash may be required.

Washing walls, wall wash…sounds the same? Yes, they do, but they couldn’t be more

different.

Washing Walls

Washing walls is the actual preparation and cleaning of interior bulkheads and tank bottoms, performed by cleaning companies specializing in the process. It is done to prepare tanks for a change in product service and for the Wall Wash test.

When to Wash Walls

Oil & Chemical companies, as well as vessel/barge owners and operators, have their own tank cleaning guides and references. One of the most widely used industry reference guides is the Dr. Verwey Tank Cleaning Guide, which helps determine whether wall washing is required for specific grades of service.

These guides are filled with matrices that compare the previous cargo and the cargo to be loaded and determine whether:

  • No wash is required
  • Wash with COLD or HOT sea water
  • Wash required with a specific solvent

Other considerations that should be taken into account include, but are not limited to:

  • Volumes remaining on board (ROB)
  • Tank wall clingage
  • The physical and chemical characteristics of the last cargo and of that to be loaded.

Confined Space Inspections

One of the most dangerous tasks inspectors encounter is tank entry and confined space inspections, where safety and health procedures must be given careful consideration.

Considerations include possible electrostatic and other fire and explosion hazards, protection against potential dangers to personnel, and potential explosive, fire, toxic, and oxygen-deficiency/enriched hazards associated with a cargo tank’s environment. Also, extra care should be taken when moving around inside tanks, as surfaces may be slippery and lighting may be poor.

Wall Wash Test

Once wall washing is complete, the Wall Wash test generally follows.

Commonly referred to as a wall wash, it is a procedure for ‘washing’ selected areas, such as interior bulkheads and tank bottoms, with an appropriate wash liquid and subsequently testing the wash for the presence of material that might contaminate the cargo to be loaded. An inspection company performs this test.

The detailed Wall Wash Test procedure can be found in API MPMS Chapter 17, Sec 8, Guidelines for Preloading Inspection of Marine Vessel Cargo Tanks.

General Wall Wash Procedures

Reagent Selection

Reagent-grade chemicals should be used in all tests unless otherwise indicated. It is

intended that all reagents conform to the specifications of the Committee on Analytical

Reagents of the American Chemical Society (ASC) or equivalent.

Typical reagents include

  • Methanol – ACS reagent grade with very low concentrations of chlorides
  • Acetone – ACS reagent grade
  • High-purity Water – Unless otherwise indicated, references to water shall be understood to mean reagent water as defined by ASTM D1193

Other reagents or products may be used with the agreement of all parties, provided it is first ascertained that the material is of sufficiently high purity to permit its use without lessening the accuracy of the determination. Also, keep in mind that the reagents used for wall washing may adversely affect the tank’s atmosphere. There should be a safety review prior to handing in the reagent to determine the appropriate personal protective equipment to be used.

Funnel Wall Wash Method

This wall wash method is the most common method for identifying contamination on the walls of a cargo tank.

To conduct a wall wash test, begin by pouring approximately 1 liter of reagent into an HDPE squeeze bottle. Ensure the tank is dry before selecting multiple sites on each tank wall for testing. Apply around 100 mL of the reagent at each chosen site, positioning the squeeze bottle about 6 inches from the wall. Use a tight-fitting funnel to minimize reagent bypass and collect the reagent into a clean glass bottle from about 1 meter below the stream, avoiding contact with the tank wall to prevent contamination.

Repeat this process at all designated sites. In cases where discolored areas, lining breaks, or exposed metal cover less than 20% of the tank surface, include these in the same sample. If these areas exceed 20%, wash them separately and test them in a different bottle.

Include a sample of the reagent used in the wall wash procedure. This is the analytical “blank”. In the event that the wall wash sample fails, this reagent will be analyzed, and the results of the “blank” will be subtracted from each tank’s wall wash sample results.

Potential Sources of Contamination

  • Contaminants in vessel lines and cargo handling systems
  • Prior cargoes or prior cargo residues
  • Cleaning material residues
  • Pump suctions, including deep well pump casings
  • Vent and inert gas lines
  • Residue on heating coils
  • Unbroken blisters in coated tanks
  • Flaking or broken blisters of the tank coating
  • Discoloration of tank coating
  • Sea water residues
  • Condensation
  • Water
  • Odor
  • Polymerized materials
  • Rust
  • Debris/particulate matter
  • Additive residue

Typical Wall Wash Analysis

  • Visual appearance
  • Color ASTM D1209
  • Hydrocarbons ASTM D1722
  • Chlorides SMA 68-62
  • Permanganate time ASTM D1363
  • Odor ASTM D1296