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Standard Procedures for Tank Cleaning, Purging and Gas free Operation for Oil tankers

Tank cleaning – The process of removing hydrocarbon vapours, liquids or residues. Tank cleaning may be required for one or more of the following reasons:
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).

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 company’s “Procedure for Entry into Enclosed Spaces” with related permits.

If the previous cargo contains Hydrogen Sulfide (H2S) or other toxic contaminants which could evolve toxic gases (eg benzene, toluene, Mercaptans, etc), the tank should be checked for such gases. Refer to “Guidelines for Toxic Gases Hazards”

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 to be 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, HC 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 as described in this section and the “Precautions during Gas-freeing Operations”

Complete the necessary sections of “Tank Cleaning, Purging and Gas Freeing Checklist” to confirm safety strictly at the appropriate time.
Oil tanker QUDS underway
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 should 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 chemical reaction with Hydrogen Sulfide Gas Pyrophoric Iron Sulphide, forms when Hydrogen Sulfide Gas (normally present in most crude) reacts with rusted surfaces in the absence of oxygen (Inert conditions) inside cargo tanks.

These substances, can heat to incandescence on contact with air. This risk is minimized, by following the correct purging procedure.

Such procedures serve as a general guidance for the preparation procedures required and may differ as per ship type.

Tanker Explosions

In the course of investigations into electrostatics, certain new factors have emerged which are relevant to the safety of tank cleaning operations, not only in very large tankers but in tankers of all tonnage’s. 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 factors therefore need to be considered:-

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. 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 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.

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 concentration of the hydrocarbon gas 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 IG 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 HC gas purging is displacement HC gas with IG, the procedure first priority shall be supply IG with 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 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.

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 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

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 IG with the oxygen content of less than 5% by Volume. 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 IG 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 IG system or other reasons during tank cleaning or hydrocarbon gas purging operations, suspend the operations immediately, and do not restart the operations until proper supply of IG is secured.

Provided that the atmosphere in tanks is not under control, do not put improper IG (the oxygen content of which exceeds 8%).

If the recovery of the IG system is difficult, notify the Technical Superintendent in charge for consultation.

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