Fixed inert gas systems for oil tanker operation
Inert gas installation onboard oil tankers - functional requirement
To maintain a safe atmosphere within ships cargo tanks a fixed inert gas system is used. Reference should be made to the ships operations manual, the manufacturers instructions and installation drawings, as appropriate, for details on the operation of a particular system.
Hydrocarbon gas normally encountered in petroleum tankers cannot burn in an atmosphere containing less than approximately 11% oxygen by volume. Accordingly, one way to provide protection against fire or explosion in the vapour space of cargo tanks is to keep the oxygen level below that figure. This is usually achieved by using a fixed piping arrangement to blow inert gas into each cargo tank in order to reduce the air content, and hence the oxygen content, and render the tank atmosphere non-flammable.
The flammable limits vary for different pure hydrocarbon gases and for mixtures derived from different petroleum liquids. For practical purposes, the lower and upper flammable limits (LFL and UFL) of crude oil vapours are taken to be 1% and 10% respectively by volume.
As inert gas is added to the hydrocarbon gas/air mixture, the flammable range decreases until a point, is reached where the LFL and UFL coincide. This point corresponds to an oxygen content of approximately 11%. No hydrocarbon gas/air mixture can burn at this oxygen level. For practical purposes and to allow a safety margin, 8% is taken as the level of oxygen at which no hydrocarbon gas/air mixture can burn under any circumstances. To prevent fire or explosion in a tank containing a hydrocarbon gas/air mixture, it is therefore necessary to produce and supply inert gas having an oxygen content not normally exceeding 5% and to displace the existing air in the tank until the resultant oxygen level throughout the tank does not exceed 8% by volume.
The International Convention for the Safety of Life at Sea (SOLAS 1974), as amended, requires that inert gas systems be capable of delivering inert gas with an oxygen content in the inert gas main of not more than 5% by volume at any required rate of flow; and of maintaining a positive pressure in the cargo tanks at all times with an atmosphere having an oxygen content of not more than 8% by volume except when it is necessary for the tank to be gas free. Existing systems are only required to be capable of producing inert gas with an oxygen content not normally exceeding 5% by volume, and of maintaining the tank inerted at all times except when it is necessary for the tank to be gas free.
Sources of inert gas
Possible sources of inert gas on tankers and combination carriers are:
Composition and quality of inert gas
Inert gas must be delivered in the gas main with a maximum oxygen content of 5%. The oxygen content of inert gas in the cargo tanks must not exceed 8%. When using flue gas from a main or auxiliary boiler, an oxygen level of less than 5% can generally be obtained, depending on the quality of combustion control and the load on the boiler.
When an independent inert gas generator or a gas turbine plant with afterburner is fitted, the oxygen content can be automatically controlled within finer limits, usually within the range 1.5% to 2.5% by volume, and not normally exceeding 5%.
In certain ports, the maximum oxygen content of inert gas in the cargo tanks may be set at 5% to meet particular safety requirements, such as the operation of a vapour emission control system. Where such a limitation is in place, the vessel is usually advised of the requirements in the pre-arrival information exchange.
Efficient scrubbing of the inert gas is essential, particularly for the reduction of the sulphur dioxide content. High levels of sulphur dioxide increase the acidic characteristic of the inert gas, which is harmful for personnel and may cause accelerated corrosion to the structure of a vessel.
Below data provides an indication of the typical composition of inert gas generated from boiler flue gas, expressed as a percentage by volume.
Inert gas system maintenance
There should be close co-operation between the deck and engine departments to ensure the proper maintenance and operation of the inert gas system. It is particularly important to ensure that non-return barriers function correctly, especially the deck water seal or block and bleed valves, so that there is no possibility of petroleum gas or liquid petroleum passing back to the machinery spaces.
To demonstrate that the inert gas plant is fully operational and in good working order, a record of inspection of the inert gas plant, including defects and their rectification, should be maintained on board.
Degradation of inert gas quality
Tanker personnel should be alert to the possible degradation of inert gas quality as a result of air being drawn into the tanks due to inappropriate operation of the inert gas or cargo systems, for example, by:
During tank entry operations, any draining of water from a non-inerted tank into the slop tank will result in entrainment of air into the drainings and, ultimately, into the inerted tank atmospheres via the slop tank. The volume of air entrained in this manner can be particularly high if an eductor is used on recirculation to the slop tank. If the liquid is to be drained to the slop tank, the inert gas quality in all tanks should be closely monitored.
Every inert gas system is required to be fitted with one or more pressure/vacuum breakers or other approved devices. These are designed to protect the cargo tanks against excessive pressure or vacuum and must therefore be kept in good working order by regular maintenance in accordance with the manufacturer’s instructions.
When these breakers are liquid filled, it is important to ensure that the correct fluid is used and the correct level is maintained. The level can normally only be checked when there is no pressure in the inert gas main line. Evaporation, condensation and possible ingress of sea water should be taken into consideration when checking the liquid condition and level. In heavy weather, the pressure surge caused by the motion of liquid in the cargo tanks may cause the liquid in the pressure/vacuum breaker to be blown out. This may be more liable to happen on combination carriers than on tankers.
These are designed to provide for the flow of the small volumes of tank atmosphere caused by thermal variations in a cargo tank and should operate in advance of the pressure/vacuum breakers. To avoid unnecessary operation of the pressure/vacuum breaker, the pressure/vacuum valves should be kept in good working order by regular inspection and cleaning.
Full flow pressure/vacuum venting arrangements
In inert gas systems fitted with tank isolating valves, protection from over and under pressurisation of the cargo tanks may be provided by using high velocity vent and vacuum valves as the full flow protection device. Where this is the case, particular attention should be paid to ensuring that the valves operate at the required pressure and vacuum settings. Planned maintenance procedures should be established to maintain and test these safety devices.
Individual tank pressure monitoring and alarm systems
In inert gas systems fitted with tank isolating valves, indication of the possible over and under pressurisation of the cargo tank is provided by using individual tank pressure sensors connected to an alarm system. Where such systems are used, planned maintenance procedures should be established to maintain and test these sensors and confirm that they are providing accurate readings.
Emergency inert gas supply
The International Convention for the Safety of Life at Sea (SOLAS 1974), as amended, requires that suitable arrangements are provided to enable the inert gas system to be connected to an external supply of inert gas.
These arrangements should consist of a 250 mm nominal pipe size bolted flange, isolated from the inert gas main by a valve and located forward of the non-return valve. The design of the flange should be compatible with the design of other connections in the ship’s cargo piping system.
As inert gas causes asphyxiation, great care must be taken to avoid the escape of inert gas into any enclosed or partly enclosed space.
Before opening the system, if possible, it should be gas freed and any enclosed space in which the system is opened up should be ventilated to avoid any risk of oxygen deficiency.
Continuous positive ventilation must be maintained before and during the work. No one should be allowed inside the scrubber or deck water seal until the atmosphere has first been tested and an oxygen level of 21% by volume obtained. In addition, while personnel are working inside the scrubber tower, the atmosphere must be continuously monitored for oxygen content and the personnel should be under constant supervision.
The IMO publication *Guidelines for Inert Gas Systems* provides a comprehensive explanation of the design and operating principles and practices of typical inert gas systems.
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