REMOVING CHLORIDE

Quality standards of bulk chemical cargoes have increased dramatically over the past 5 years. It is reflected in the increasingly demanding inspection, loading and unloading quality control specification. In line with the above some of our charterers require more strict wall wash standard requirement,e.g:
1. Nil hydro carbon
2. Max 0.2 ppm of chloride
3. Pass visual inspection of all lines and connection
4. Suspended matter 12-16 spec

As we can see on the above chloride requirement, 0.2 ppm. It has increased from the previous requirement, 2-5 ppm. It is not so easy to achieve such a very low limit of chloride but of course it is achievable. Recently we have experienced many wwt failed due to chloride. Having a lot discussion with experienced Supercargo / Inspector and also some our chief officer. All agred that to meet above wall wash standard vessel has to conduct more intensive, effective, timely and proper tank cleaning.

As we know chlorides is most soluble in water and no chemical will help more than water. Earlier some steamed with Toluene not knowing that it was the steam itself and not the toluene removing the Chlorides. The best way to move chloride is by:
• Flushing/rinsing with freshwater
• Steaming
• Methanol/distillated water spray

Flushing with fresh water:
The most effective way is manually flushing/rinsing the tank with a hand held hose and freshwater. We have to start at the deckhead and ensure that the water flows down over all the construction. It is also essential to flush under the construction such as longitudinal and platforms. The final flushing should be the tank top (tank floor including the heating coils). The flushing should be from fore to aft and if possible the pump should be running to remove the water and contaminates.

Steaming
The steaming should be carried out in line and within the limits of the tank coating. It is essential that the steaming time be calculated to take into consideration the volume of the tank and the surface area. The condensate should be pumped out on a continuous basis if possible and if not then on a frequent basis.

This ensures that the condensate which contains the chlorides and other possible contaminates does not recirculate within the tank. Towards the end of the steaming time taking sample of the pump out condensate is very important. Test the sample to see if the level of chloride is high or low. By this testing, it is possible to find that the tank may need more steaming time. At the end of steaming vent the tank by mechanical means to extract the steam/vapour. Once this is removed stop the fan and let the tank cool down. The bulkheads will dry quickly since the coating is hot. The remaining condensate will collect at the pump and should be removed as soon as possible. Less water in the tank means less humidity and the tank will dry quicker.

Methanol/Distilated Water Spray
It is very useful especially if we have some wet/damp areas. Spray the tanks with Methanol and vent quickly. One point to note is that spraying a bulkhead with a high pressure spray and distilled water will not be so effective unless the water is actually flowing over the surface and carrying the chloride with it. We also have the added problem of drying the washed area especially in cold and damp areas.

Once the tanks are dry and it is time to wall wash and visually check the tanks it is essential to wear clean clothes, shoe covers and disposable gloves. The shoe covers and gloves should be replaced for each tanks. Do not sit or lean on the bulkheads as contaminate carry over can fail a tank if surveyor wall washes that particular area.

Beside the above method, definitely vessels crews and Master have to check and ensure the quality of fresh water, distillated water and methanol as well as to check whether the steam generator can produce chloride free steam.
Hope this is of some use