Purging / Gas Freeing / Inerting Procedure
Doc No:
SP0250
Version:
2
Standard Operating Practices
Refer to Tank Cleaning - Tanker Op (RF0045) and Mitigating Risks Associated with Electrical
Storms / Mast Riser Fires (SP0671).
Purpose
To assist the ship staff in conducting safe and efficient “Gas freeing” Operations. Refer to List of
Definitions (SP0003).
Scope
This applies to all vessels that are certified under Teekay Shipping Limited’s Document of
Compliance.
Responsibilities
Master
•
Ensuring that gasfreeing operations including associated purging, or inerting are carried out in a
safe and efficient manner.
Chief Officer
•
Planning and carrying out purging, gasfreeing and inerting operations in a safe and efficient
manner.
Procedure
General
•
Be aware of and follow the requirements of the OCIMF publication, and ISGOTT (International
Safety Guide for Oil Tankers and Terminals)
•
When there is an electrical storm in the vicinity of the vessel, cargo operations including purging,
gas freeing, inerting or tank cleaning operations must be suspended. All tank openings and
vents must be secured.
•
During loading, when there is no wind or light winds, gasses coming from tank openings or vents
tend to form a cloud over the deck. It may be necessary to control the number of openings,
vents in use or restrict the vent valves as necessary in order to ensure that the escaping vented
gas has sufficient velocity to clear the decks. If necessary, to avoid accumulation of gases
including H2S at deck level, suspend the operation.
•
If, at any time, it is suspected that gas is being drawn into the accommodation or other service
spaces, stop the operation till the central air conditioning and mechanical ventilating systems
can be adjusted to prevent such ingress. If in doubt, test the atmosphere in such spaces in order
to confirm that the gas concentration levels are within acceptable limits (In the case of H2S, the
acceptable level is less than 10 ppm.)
•
On the apparent completion of any stage allow a period of about 10 minutes to elapse before
taking final gas measurements.
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Purging / Gas Freeing / Inerting Procedure
Doc No:
SP0250
Version:
2
•
Perform tests at several levels and in each compartment of the tank. In large compartments and
when it is subdivided by a wash bulkhead, ensure that tests are performed at widely separate
positions.
•
Use of one or more permanently installed fans (i.e. I.G. Fans).
! From the venting system:
! Through the cargo piping:
! Before putting such a system into service, where possible, flush the cargo piping system
through with sea water, including crossovers and discharge lines, and strip the lines and
tanks. Close all valves on the system, with the exception of those required for ventilation.
! If the tanks are connected by a common venting system, isolate each tank to prevent the
transfer of gas to or from other tanks.
Purging
•
Carry out all actions laid down in the Purging/Inerting/Gas Freeing (FM0025)
•
If the tank atmosphere represents a health hazard, conduct the purging through the mast riser, if
fitted. The covers of all tank openings should then be kept closed.
•
Carry out gas measuring during the purging operation to determine if there are any harmful
substances to the crew exceeding the occupational exposure limits.
•
Carry out purging using the most efficient method:
! Displacement or
! Dilution
•
After purging, check the pressure / vacuum valves, mast riser screen, and any high velocity vent
valves for cargo residue.
Gasfreeing
•
Before commencing the gasfreeing process, purge the tanks with inert gas until the hydrocarbon
content is less than 2% by volume.
•
Carry out gasfreeing for entry and for cold work and hot work until the tank atmosphere tests
confirm that hydrocarbon gas concentration through the compartment is less than 1% of the
lower flammable level (LFL). List of Definitions (SP0003).
•
Perform additional tests to check for 20.8% Oxygen content, and to ensure the absence of
hydrogen sulphide, benzene and other toxic gases as appropriate, in case entry is to be
effected. Entry into Enclosed Space Procedure (SP0411)
•
Use of portable fans or blowers
! Before starting, connect portable fans to the deck so that an effective electrical bond
exists between the fan and the deck.
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Purging / Gas Freeing / Inerting Procedure
Doc No:
SP0250
Version:
2
! Ensure that the penetration and capacity of the approved portable fans are such that the
entire atmosphere of the tank on which the fan is employed can be made gasfree in the
shortest possible time.
! Place portable fans in such positions, relative to the ventilation openings, that all parts of
the tank are equally and effectively gas freed.
! Ensure that ventilation outlets are as remote as possible from the fans.
! Control the number of tank ventilation openings in order to produce a gas velocity
sufficient to carry the gas clear of the deck.
Inerting
•
Carry out all actions laid down in the Purging/Inerting/Gas Freeing (FM0025)
•
Inerting is to be carried out by the most efficient method:
! Displacement or
! Dilution
•
Control the number of tank ventilation openings in order to produce a gas velocity sufficient to
carry the gas clear of the deck.
•
On completion of inerting, close all openings.
•
On completion of all inerting, check the pressure/vacuum valves, mast riser screen, and any
high velocity vent valves for cargo residue.
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