Tank cleaning – The process of removing hydrocarbon vapours, liquids or residues. Tank cleaning may be required for one or more of the following reasons:
- To carry clean ballast.
- To gas free tanks for internal inspections, repairs or prior to entering dry dock.
- To remove sediments from tank top plating. This may be required if the vessel is engaged in the repetitive carriage of fuel oil or similar sediment settling cargoes. Although washing may not be necessary in between consecutive voyages, assuming the cargoes are compatible, many Ship Owners have found it prudent to water wash a small group of tanks on a rotation basis between voyages, thus preventing any large accumulation of sediments.
- To load a different and not compatible grade of cargo. Washing in between carrying different grades of cargo is the most common reason for tank cleaning. In most cargo sequences on product tankers, the cleaning may consist of no more than a simple hot or cold seawater wash.
A simple water wash will disperse many types of chemicals and has been found effective between clean petroleum products such as gasoil and kerosene. However, it should be noted that there is a number of grade sequences, particularly in the petroleum product trade, where no washing at all needs to be be carried out. Thus the decision for necessary tank cleaning required in such trades is often made only when knowledge of the next grade to be loaded is obtained.
Responsibility : The Chief Officer is in charge of and shall supervise as the person in charge of the Tank Cleaning, Hydrocarbon Gas (HC) Purging, Gas Freeing & Re-Inerting operations. He shall ensure that all activities carried out during such operations are in compliance with the latest edition ICS/OCIMF International Safety Guide for Oil Tankers and Terminals (ISGOTT).
Safety Precautions during tank cleaning :
Non Flammable Atmosphere
On Tankers using the inert gas systems, the Chief Officer shall carry out the operations to maintain the cargo tanks in a “Non Flammable” condition at all times.
At no time should the atmosphere in the tank be allowed to enter the flammable range.
It has been demonstrated that electrostatic charging to the water mist, present in the tanks, exists under any cleaning condition, and that when washing operations are stopped the level of charge decreases only slowly in the tank, and can remain present for a long time, especially in the absence of ventilation.
The following precautions are required to prevent the foregoing hazards:
- Not to use sounding rods through any deck opening other than the sounding pipe, either during tank cleaning, or for one hour after cessation of washing if the tank is being blown, or five hours if the tank is not being blown.
- To carry out checks on the electrical continuity of bonding wires on the tank cleaning hoses before each use.
- To keep the hoses connected to the hydrants until the machines are out of the tank. The draining of the hose can be done by loosening the coupling of the hose carefully to let the air in and by tightening the coupling again.
Atmosphere Control during Tank Cleaning Operations
Tank atmospheres can be any of the following, However, ships fitted with an inert gas system, shall carry out the operations under the Inerted Condition, unless otherwise as instructed: It should be met with atmosphere containing less than 8% oxygen, and tank pressure of minimum 200 mmAq.
An atmosphere made incapable of burning by the introduction of inert gas and the resultant reduction of the overall oxygen content. For the purposes of this procedure, the oxygen content of the tank atmosphere should not exceed 8% by volume.
This is a condition where the tank atmosphere is known to be at it’s the lowest risk of explosion by virtue of its atmosphere being maintained at all times Non-Flammable through the introduction of inert gas and the resultant reduction of the overall oxygen content in any part of any cargo tank to a level not exceeding 8% by Volume, while being under positive pressure at all times.
Gas Freeing of the tank
The procedure of removing dangerous and explosive gases from the interior of tanks. Gas freeing consists of a series of operations in which cargo vapour is replaced with inert gas which, in turn is purged with air to prevent explosion hazard.
Inerting of the Tanks
After cargo discharge / tank cleaning, whenever it is necessary to gas free an empty tank containing hydrocarbon gas mixtures or a mixture of IG + HC gases, it shall first be purged, using inert gas, until the HC (hydrocarbon) content reaches to below the critical dilution line or HC concentration in the tank atmosphere is less than 2% by volume.
This is done so that during the subsequent gas freeing no portion of the tank atmosphere is brought within the flammable range.
This inert gas used for purging shall contain Oxygen, less than 5% by volume, to ensure the above.
The replacement of a tank atmosphere by inert gas can be achieved by either Inerting or Purging. In each of these methods one of two distinct processes, Dilution or displacement, will predominate.
1) Dilution: It takes place when the incoming inert gas mixes with the original tank atmosphere to form a homogeneous mixture through the tank so that, as the process continues, the concentration of the original gas decreases progressively.
It is important that the incoming inert gas has sufficient entry velocity to penetrate to the bottom of the tank. To ensure this a limit must be placed on the number of tanks which can be inerted simultaneously.
If dilution method of purging is used, it should be carried out with the inert gas system set for Maximum capacity to give the maximum turbulence in the atmosphere, within the tank.
2) Displacement : It depends on the fact that inert gas is slightly lighter than hydrocarbon gas so that, while the inert gas enters at the top of the tank, the heavier hydrocarbon gas escapes from the bottom through suitable piping.
When using this method it is important that the inert gas has a very low velocity to enable a stable horizontal interface to be developed between the incoming and escaping gas although, in practice, some dilution inevitably takes place owing to the turbulence caused in the inert gas flow. This system generally allows several tanks to be inerted or purged simultaneously. If displacement method is used, the gas inlet velocity should be lower, to prevent undue turbulence.
A mixture of inert gas and petroleum gas when vented and mixed with air can become flammable. The normal safety precautions required as described under “Procedures for Cargo Oil Operations” shall be followed.
Forced Air Ventilation / Aeration
i) Before starting to Gas free, the tank should be isolated from other tanks.
ii) Do not commence forced air ventilation (Gas free) until it has been confirmed that the oxygen level is less than 8% and the hydrocarbon vapor content is less than 2% by Volume.
iii) To ensure the dilution of the toxic components of inert gas to below their Threshold Limit Values (TLV), Gas freeing should continue until tests with an oxygen analyzer show a steady oxygen reading of 21% by volume and tests with a flammable gas indicator show not more than 1% LFL.
iv) If the presence of a toxic gas such as benzene or hydrogen sulfide is suspected, Gas freeing should be continued until tests indicate that its concentration is below its TLV.