Standard Procedures for Tank Cleaning, Purging and Gas free Operation for Oil tankers
Tank cleaning is the process of removing hydrocarbon vapors, liquids, or residues from cargo tanks onboard a tanker. 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 before entering the 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 for 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 (H.C.) Purging, Gas Freeing & Re-Inerting operations. He shall ensure that all activities carried out during such operations comply with the latest edition ICS/OCIMF International Safety Guide for Oil Tankers and Terminals (ISGOTT).
Gas-Freeing for Cargo Tank entry:
Cargo Tank entry shall not be permitted unless the Oxygen Content is 21%, and the hydrocarbon vapor content is less than 1% of the Lower Flammable Level (LFL).
Follow the company's "Procedure for Entry into Enclosed Spaces" with related permits.
If the previous cargo contains Hydrogen Sulfide (H2S) or other toxic contaminants that could evolve poisonous gases (e.g., benzene, toluene, Mercaptans, etc.), the tank should be checked for such gases.
Carrying out "Hot Work" inside Tanks within the 'Dangerous Area' need special caution as per "Procedures for Hot Work" and carry out preparation accordingly.
Gas-Freeing or Purging for the Reception of Cargo:
If the intention of Gas-Freeing or Purging operations is to prevent the next cargo from being loaded from contamination due to the previous cargo oil hydrocarbon gas, use the gas content indicated by the Charterer as standard, but go on with the operations mentioned in (2) of Article 1 until the LFL decreases down to 40% or under.
Safety Precautions:
For the operations to be followed, (Tank cleaning, H.C. Gas Purging, Gas Freeing, and Re-Inerting), the Chief Officer shall carry out the following precautions.
Detailed guidance on preparations and safety precautions are also described within relevant sections of ISGOTT.
Have persons engaged in the operations observe the necessary precautions described in this section and the "Precautions during Gas-freeing Operations."
Complete the necessary sections of "Tank Cleaning, Purging and Gas Freeing Checklist" to strictly confirm safety.
Fig: Oil tanker QUDS underway
Tank Preparation And Atmosphere Control During Operations
Non Flammable Atmosphere
On Tankers using the inert gas systems, the Chief Officer shall carry out the operations mentioned in Article 1 and maintain the cargo tanks in a "Non-Flammable condition at all times.
Refer to the "Flammability composition diagram- Hydrocarbon Gas/Inert/Air Gas Mixtures" from the ISGOTT. i.e., at no time should the atmosphere in the tank be allowed to enter the flammable range, as mentioned therein.
Pyrophoric hazards on a chemical reaction with Hydrogen Sulfide Gas
Pyrophoric Iron Sulphide, forms when Hydrogen Sulfide Gas (usually present in most crude) reacts with rusted surfaces in the absence of Oxygen (Inert conditions) inside cargo tanks.
These substances can heat to incandescence in contact with air. This risk is minimized by following the correct purging procedure.
Such procedures serve as general guidance for the preparation procedures required and may differ as per ship type.
Tanker Explosions
Investigations into electrostatics, certain new factors have emerged, relevant to the safety of tank cleaning operations, not only in huge tankers but also in tankers of all tonnages. Electrostatic charging to the water mist, present in the tanks, exists under any cleaning condition. When washing operations have 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 factors, therefore, need to be considered:-
In studying "charge concentration mechanisms", it has been found that insulated objects are not normally present in ship's tanks, but the use of sounding rods might present such a hazard.
When a sounding rod is lowered into a tank filled with charged mist, a high static charge can be fed into the operator via the wet suspension rope if the deck paint or his footwear insulates the operator. Such a charge accumulation is sufficient to cause incendive sparking when the operator, the sounding rod, or rope, comes into contact with the ship's structure. Of course, it does not apply if the rod is lowered into a sounding pipe extending to the bottom of the tank.
Portable tank cleaning machines are normally bonded to the ship's structure, and when in operation, the washing water provides a path for the electrostatic charge to dissipate. However, these machines may sometimes become insulated objects, and a potentially hazardous situation may exist in at least the two following cases;-
Where the bonding wire is defective.
Where the hose is disconnected from the hydrant before lifting the machine out of the tank.
a) Such disconnection, prior to lifting, is commonly done in order to drain water from the hose. A coat of paint in good condition is sufficient to insulate the hose flange from the deck's steel. Under these conditions, when machine lifted, an incendive spark may jump to lip of the tank cleaning hole, either from machine, or from securing rope, or from the operator who guides the machine through the hole.
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.
It is emphasized that while the likelihood of all the factors necessary to lead to an incendive spark being present at any one time is remote, the fact that neglect of the preceding precautions might lead to an explosion remains a possibility.
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 instructed:
It should be met with an atmosphere containing less than 8% oxygen and tank pressure of a minimum of 200 mmAq.
Refer details to “ISGOTT”
Inerted Tanks
An atmosphere made incapable of burning by the introduction of inert Gas and the resultant reduction of the overall oxygen content. For 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 under 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.
Purging with Inert Gas (IG)
(a) For reduction in hydrocarbon (HC) content in tank atmosphere for Cargo / Vapor contamination reasons:
After tank cleaning operations, the cargo tanks may be purged with inert Gas to reduce the hydrocarbon gas concentration inside the tank atmosphere.
Follow the procedures as laid out in the operation and equipment manual.
Purge pipes, with proper flame screens, shall be fitted, where provided.
Carry out the operations of replacing the tank atmosphere by introducing I.G. of which oxygen content is 5% by Volume or less into the tanks.
Go on with purging by IG until the hydrocarbon content reduces to the required /
desired level.
Oxygen content in Inert Gas for purging
Since the main purpose of H.C. gas purging is displacement H.C. gas with I.G., the procedure priority shall be supply I.G. with the full capacity of IG Blowers.
Under the procedure, Oxygen content in Inert Gas for purging may be permitted by 8% by Volume or less.
For carrying out Gas Freeing of the tank
After cargo discharge/tank cleaning, whenever it is necessary to Gas free an empty tank containing hydrocarbon gas mixtures or a mixture of I.G. + H.C. gases, it shall first be purged, using inert Gas, until the H.C. (hydrocarbon) content reaches to below the critical dilution line or H.C. concentration in the tank atmosphere is less than 2% by Volume.
It 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.
Either Inerting or Purging can achieve the replacement of a tank atmosphere by inert Gas.
In each of these methods one of two distinct processes, Dilution or displacement,
will predominate.
For more details on gas evolution, venting and dispersion criteria and precautions,
refer to the relevant ISGOTT chapters.
Distinct process
1) Dilution:
It takes place when the incoming inert Gas mixes with the original tank atmosphere to form a homogeneous mixture through the tank. As the process continues, the concentration of the original Gas decreases progressively.
The incoming Inert Gas must have 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 the 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 essential that the inert Gas has a superficial velocity to enable a stable horizontal interface to be developed between the incoming and escaping Gas. However, in practice, some dilution inevitably occurs due to the turbulence caused in the inert gas flow. This system generally allows several tanks to be inerted or purged simultaneously.
If the 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 standard safety precautions required, as described under "Procedures for Cargo Oil Operations," shall be followed.
Forced Air Ventilation
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.
Completion of work & Inerting Cargo tanks
After completion of man entry or repair work (in dry docks/lay-up berth) Cargo tanks shall be prepared for Loading as follows:
i) An Officer shall confirm each tank free of waste & material used in maintenance & inspection. Related pipelines and supports, including hydraulically operated valves, H.P. pipes, and flanges are all in place and tightly secured.
ii) All personnel out of tank & close tank dome or access, only keep designated vent ports open. Inert tanks to 8% of Oxygen level.
iii) Replace the tank's atmosphere by an inerted atmosphere, using I.G. with less than 5% of VolumeVolume's oxygen content. This gas replacement should continue until the average measured oxygen content in the tanks drops to below 8% by Volume.
Measures against Inert Gas System in Troubles
In case that proper I.G. can not be supplied, which could cause the oxygen content in tanks to exceeds 8% by Volume, or making it difficult to keep the internal pressure of tanks positive due to troubles in the I.G. system or other reasons during tank cleaning or hydrocarbon gas purging operations, suspend the services immediately, and do not restart the transactions until the proper supply of I.G. is secured.
Provided that the atmosphere in tanks is not under control, do not put improper I.G. (the oxygen content of which exceeds 8%).
If the recovery of the I.G. system is complicated, notify the Technical Superintendent in charge of consultation.
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