For chemical tankers and some storage tanks it is a common standard to clean the tanks after discharge of the chemical, irrespective of its type.
The purpose is to minimize contamination of the following chemical by residues of the previous one. Besides, the cleaning will also prolong the service life of the tank lining, provided that proper precautions are taken not to damage the lining mechanically or chemically.
In general, one or more of the following operations are involved:
- Ventilation to "gas free"
- Wash for debris using seawater and fresh water, or fresh water only
- Wash for residues of previous commodity using detergents and/or chemicals
- Ventilation to "dry".
The actually employed detailed procedure depends on the commodity sequence in question, and is based on:
- Instructions from client/owner/charterer
- Guidelines from independent tank cleaning guides (e.g. Dr. Verwey).
- Recommendations by Specific Notes to the Resistance Table.
Some general remarks to the procedures are given below:
Ventilation to "gas free" aims at lowering the concentration of the chemical vapour to a predefined, acceptable level. It is most effectively done by suction from the bottom of the tank, while introducing fresh air at the top at a relatively low flow rate.
Ventilation to "visibly dry" aims at removing water adhering to the tank surfaces. Possible pools of water on the bottom are removed by mopping. It is most effectively done by suction from the top while introducing fresh air at the bottom at a relatively high flow rate. Drying will be facilitated by the use of heat (from heating coils) and by the use of dry air for replacement.
Ventilation to "completely dry" aims at removing all chemical absorbed in the tank lining, and is arranged as for ventilation to "visibly dry". For zinc silicate, which releases the absorbed chemical quickly, the high flow rate itself will affect the drying. For epoxies, which release the absorbed chemical through diffusion, heat is the determining factor, and the flow rate could be kept relatively low. The rate of diffusion is first and foremost controlled by temperature, and the effect of ventilation is mainly to distribute heat.
For some commodity sequences washing with water can be done without any implications for the tank lining, whereas for others it may have a detrimental effect. The use of chemicals and cleaning agents must be in accordance with the Chemical Protection Guide. If in doubt, please obtain Product Data Sheet and Material Safety Data Sheet and consult Hempel.
It is generally recommended to keep the washing time as short as possible and the temperature of the wash water as low as possible. As a rule of thumb, a high temperature is only justified if the cleaning efficiency is doubled by a 10°C/18°F increase in temperature.