TANKCLEANING PROCEDURES, ON CHEMICAL TANKERS- CAPT AJIT VADAKAYIL
Tank cleaning is usually done with water, unless the cargo ROB in tank creates a stubborn residue like Isocyanates, in which case a non-reactive solvent must be used. Isocyantes will form stubborn Urethane and CO2 gas in large quantities with water.
Esters and chlorinated HC will slowly hydrolyse to acids. This is allright in epoxy tanks if the tank cleaning is uninterrupted at the prewash stage . SS can handle the pH of the acid.
It is most important to know the next cargo planned to be loaded . Washing to WW standard after a particular cargo is not as critical.
No cleaning can take place unless the mandatory prewash as required by MARPOL is done.
Substances with high VP can be removed from tank by evaporation, unless they leave a stubborn residue or contain inhibitors. The NVM content of the chemical data sheet must be consulted. Any inhibitor not washed off will be difficult to remove and will have a bad influence on the next cargo or fail the WWT. Most inhibitors are fully soluble in water.
The master must enforce precautions like “ no smoking “ and “ AC on recirc “. It is important for all on a chemical tanker to know the location of AC fresh air intake, and the Anemometer , to use the relative wind direction to advantage. Bioaccumulative vapours and carcinogenic fumes can enter the engine room via intake vents and cause health problems for the engine staff. It is not all right to say that just the deck crew are exposed to toxic vapors.
Water washing can be done even if the solubility of the chemical in water is low to as much as 0.3%. such solubility always increases if the water temperature is higher. Hence always try to use wash water at high temperature ( about 25 deg higher than MP after removing cold ballast interface ) unless the cargo being cleaned does not allow it, like drying oils and mineral oils with high paraffin content. Solidified matter when melted flows away with the stripped out water, it need not be soluble.
Mineral oils with high paraffin content and certain crude oils which require heating during transportation should always be cleaned with ambient wash to prevent evaporation of lighter fractions which would leave a waxy residue on tank bulkheads.
Drying oils if prewashed with hot water will polymerize. Drying oils must be washed immediately after dischg with ambient water. If they are left to dry polymerization takes place due to reaction with oxygen, and heat increases the reaction speed. This means by removing the air form the tank using Nitrogen this process can be slowed down. By the way, Ambient means upto 35 deg C. Moderate means upto 60 deg. Hot means >60 deg C.
Certain cargoes like acetic acid, benzene, luboils, caustic soda, paraffin, molasses, phenol, DINP, fatty alcohols, HMD , Hitec, WPAC, butyl acrylate , creosote etc can be hot pre-washed.
However if you hot prewash Styrene monomer or Acrylic acid polymerization will happen. The hotter the water, the faster the polymerization. Due to condensation of vapour , inhibitor free liquid is formed, as the inhibitor is not volatile. Inhibitor if not removed will affect the PTT test.
When a cargo is fully soluble in water, using tank cleaning chemicals in a mindless manner does more harm than good.
Cleaning the tanks is just not enough. Most often it is the tank appendages which cause huge cargo claims or tank rejection. PV valves, vent lines, fixed pipelines , portable manifold hoses, superstrip lines, sampling and drain cocks, air/ nitrogen/ steam portable connection stubs, pump internals , cofferdams –all can hold contaminant matter. With certain cargoes like LSHW FO even the butterfly valve Teflon seals can trap sludge and discolor the next cargo.
Washing tanks with portable/ fixed Butterworth machines from designated areas may not clean everything. This is why it is important to enter the tank and have a visual check before wasting fresh water and expensive tank cleaning chemicals after high MP cargoes like palm oil fatty acids. As soon as you open the tank dome you get a general idea of shadow areas.
The chief officer must know if the tank corrugations are horizontal or vertical. Horizontal corrugations mean that fixed tank cleaning machines which cannot be given drops are ineffective. Vertical corrugations mean that the position of the butterworth port is important.
Some of the butterworth heaters may not be able to deliver the correct temperature and pressure in cold winter. In such case use lesser tank cleaning machines to get more temperature with correct pressure.
If the previous cargo is strong smelling like Acrylates or Crude Turpentine or MTBE a smell killer can be used. Gaskets emit smell, hence they must be flushed with methanol.
STAGES OF TANKCLEANING:--
1) Precleaning with sea water.Pre cleaning is different from MARPOL mandatory Prewash. Tank cleaning machines may have shadow sectors , and this can be rectified for main wash. Non drying oils and fats can be steamed before the hot water precleaning.
Pre-cleaning is the first cleaning step, without cleaning agents in order to remove the majority of product residue. The cleaning temperature andthe temperature of the adjacent tanks are important parameters for successful cleaning. When water is not allowed , pre- cleaning is carried out with a suitable solvent. Pre-cleaning generally takes several hours and the sooner pre-cleaning is done after discharge, the easier itis to remove the product residue.Pre-cleaning is very important, becauseit is very difficult to obtaina satisfactory result following an initial mistake
Do not attempt to use tank cleaning chemicals unless the cargo clingage is removed.
If cargo is not water soluble or residues remain , the use of tank cleaning chemicals is justified. If the previous cargo is not water soluble using a 0.04% detergent wash will be good enough for WW standard ( this is not WALL WASH ). Graco barrel pump injecting into the tankcleaning line , is the best as the bottom can be kept stripped and tank cleaning chemical is not reused. Inject at the rate of 2 litres chemical per cubic meter of wash water. Discharge from both sides of the manifold to Annex 2 UW overboard.
Recirc using spider / octopus will clean dirty areas, but they will also dirty clean areas. Also if the solvent is volatile it will evaporate. Recirc is more effective after a Graco injection wash. Prevent static charge dangers. Annex 1 mineral oils which are not soluble in water requires a emulsifier/ degreaser wash. Handspray with undiluted chemicals is only for local application , sufficient contact time must be allowed.
Most cleaning agents are additives which are used in combination withwater to improve thewater solubility of the cargo to be cleaned. Only very few cargoes which cannot be cleaned with water-based systems require a non-water-based solvent as a cleaner (sometimes in combination with an emulsifier).
To neutralize the odor of some chemicals, the use of an odor remover may be recommended in combination with an emulsifier. For most cargoes a variety of cleaning agents are available. Cleaning agents must be IMO-approved. Cleaning concentrations, times and temperatures in the final cleaning steps are recommended in the cleaningguides to achieve a satisfactory result.Cleaning generally takes several hours.
This is done with tank cleaning machines, the main purpose is to get rid of the residues loosened up by the tank cleaning chemicals.
This is done with tank cleaning machine using a low throughput, to remove the salt before they dry up . If you anticipate a delay , as your ship does not have a dedicated FW pump or line prevent the salt from drying up by steaming, more so on zinc adsorbent porous surface.
Steaming is the introduction of saturated steam into the tank to evaporate volatile residue (odour removal). The steam will condense on the tank surfaces. The temperature should normally be as high as possible during steaming. This is enhanced if the adjacent tanks (including ballast tanks) are empty Steaming removes traces of volatile substances. The steam must hence be allowed to escape constantly via the PV valve.
If the chloride level of the wash water is too high, the use of steam forremoval of chloride is often the only feasible option. Clearly the steam quality depends on the construction of the boiler. If the steam is used to remove chloride, the wall temperatures should be cool (in contrast to the evaporation method described above) – this results in condensation and water film running down the tank walls to wash the chlorides off.
In case of Wall wash , with low chloride specs, for optimal results the chloride content of the water must be less than 0.1 mg/ litre (distilled water, deionized water, demineralized water by microfiltration).
Strip out the tank and the pump stack. Sometimes it will be necessary to use a Wilden pump and sponges to save time. Mopping reduces drying time if there are water pools on the tank bottom. Make sure no lint is left .
While venting please remember that warm moist air condenses on a cold surface. Ventilationremoves water, moisture and odor, which is usually done by forced air circulation.
CAPT AJIT VADAKAYIL
29 YEARS IN COMMAND
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