Hydrocarbon gas normally encountered in petroleum tankers cannot burn in an atmosphere containing less than approximately 11% oxygen by volume. 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 oxygen content and render the tank atmosphere non-flammable.
Sources of Inert Gas
Possible sources of IG on tankers and combination carriers are:
- Uptake gas from the ship’s main auxiliary boilers;
- An independent IG Generator and
- A gas turbine plant when equipped with an afterburner.
Composition and Quality of Inert Gas
The International Convention for the Safety of Life at Sea (SOLAS) as amended, requires that IG systems be capable of delivering IG with an oxygen content of the IG main not more than 5% by volume at any required rate of flow; and of maintaining a positive pressure in the cargo tanks 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.
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 IG 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%-2.5% by volume.
In certain ports, the maximum oxygen content of IG 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 ship should be advised of the requirements in the pre-arrival information exchange.
Efficient scrubbing of the gas is essential, particularly for the reduction of the sulphur dioxide content. High levels of sulphur dioxide increase the acidic characteristic of the IG, which is harmful to personnel, and may cause accelerated corrosion to the structure of a ship.
The table provides an indication of the typical composition of inert gas generated from boiler flue gas, expressed as a percentage by volume.
|Water Vapour||H2O||Trace (High if not dried)|
|Ash and Soot||(C)||Traces|
Methods of Replacing Tank Atmospheres
If the entire tank atmosphere could be replaced by an equal volume of inert gas, the resulting tank atmosphere would have the same oxygen level as the incoming inert gas. In practice, this is impossible to achieve and a volume of inert gas equal to several tank volumes must be introduced into the tank before the desired result can be achieved.
The replacement of a tank atmosphere by inert gas can be achieved by either inerting or purging. In each of this methods, one of two distinct processes, dilution or displacement, will predominate.
Dilution takes place when the incoming inert gas mixes with the original tank atmosphere to form a homogeneous mixture throughout 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 that can be inerted simultaneously. Where this limit is not clearly stipulated in the operations manual, only one tank should be inerted or purged at a time when using the dilution method. Below is a representation of Dilution method onboard.
Displacement 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. However, in practice, some dilution inevitably takes place owing to the turbulence caused by the inert gas flow. Displacement generally allows several tanks to be inerted or purged simultaneously. Below is a representation of Displacement method achieved onboard.
Whichever method is employed, and whether inerting or purging it is vital that oxygen or gas measurements are taken at several heights and horizontal positions within the tank to check the efficiency of the operation. A mixture of inert gas and petroleum gas, when vented and mixed with air, can become flammable. The normal safety precautions are taken when petroleum gas is vented from a tank, therefore, should not be relaxed.
*Read about Flammability Diagram
Inert Gas Operations
Tankers using the inert gas system should maintain their cargo tanks in a non-flammable condition at all times. It follows that:
- Tanks should be kept in an inert condition at all times, except when it is necessary for them to be gas free for inspection or work. The oxygen content should not be more than 8% by volume and the atmosphere should be maintained at a positive pressure.
- The atmosphere within the tank should make the transition from the inert condition to the gas-free condition without passing through the flammable condition. In practice, this means that, before any tank is gas freed, it should be purged with inert gas until the hydrocarbon content of the tank atmosphere is below the critical dilution line. Check Flammability Diagram (Line GA)
- When a ship is in a gas-free condition before arrival at a loading port, the tanks must be inerted prior to loading.
In order to maintain cargo tanks in a non-flammable condition, the inert gas plant will be required to:
- Inert empty cargo tanks
- Be in operation, or be ready for immediate operation, during cargo discharge, deballasting, COW and tank cleaning
- Purge tanks prior to gas freeing
- Top up the pressure in the cargo tanks when necessary during other stages of the voyage
It must be emphasised that the protection provided by an inert gas system depends on the proper operation and maintenance of the entire system.
Inert Gas System Maintenance
There should be close co-operation between the deck and engine departments to ensure 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 within tanks as a result of air being drawn into the tanks due to inappropriate operation of the inert gas or cargo systems. For instance:
- Not topping up the inert gas promptly if the pressure in the system falls, due to temperature changes at night.
- Prolonged opening of tank apertures for tank gauging, sampling and dipping.
When water is drained from a non-inerted tank, air will be entered into the drainings delivered to the slop tank and may ultimately enter into inerted tank atmospheres. The volume of air entrained in this manner can be particularly high if an eductor is used on recirculation to the slop tank. Therefore, when liquid is to be drained to the slop tank, the inert gas quality in all tanks should be closely monitored.
Application to Cargo Tank Operations
Before the Inert gas system Is put Into service, the tests required by the operations manual or manufacturer’s Instructions should be carried out. The feed oxygen analyser and recorder should be tested and proved to be In good order. Portable oxygen and hydrocarbon meters should also be prepared and tested.
Inerting of Empty Tanks
When Inerting empty tanks that are gas free, for example following a dry docking or tank entry, inert gas should be introduced into the distribution system while venting the air in the tank to the atmosphere. This operation should continue until the oxygen content throughout the tank is not more than 8% by volume. Thereafter, the oxygen level will not increase if a positive pressure is maintained by using the Inert gas system to introduce additional inert gas when necessary. If the tank is not gas free, the precautions against static electricity should be taken until the oxygen content of the tank has been reduced to 8% by volume. When all tanks have been inerted, they should be kept common with the inert gas main and the system pressurised with a minimum positive pressure of at least 1 DO mm water gauge. If Individual tanks have to be segregated from a common line (e.g for product Integrity}, the segregated tanks should be provided with an alternative means of maintaining an inert gas blanket.
Loading Cargo or Ballast into Tanks in an Inert Condition
When loading cargo or ballast, the Inert gas plant should be shut down and the tanks vented through the appropriate venting system. On completion of loading or ballasting, and when all ullaging is completed, the tanks should be closed and the Inert gas system restarted and re-pressurised. The system should then be shut down and all safety isolating valves secured. Local regulations may prohibit venting after crude oil washing.
Discharge of Cargo or Ballast from Tanks in an Inert Condition
The Inert gas supply must be maintained throughout cargo or ballast discharge operations to prevent air entering the tanks. If a satisfactory positive inert gas pressure can be safely maintained without a continuous supply of Inert gas, then it’s acceptable to re-circulate or stop the supply of Inert gas provided that the Inert gas plant is kept ready for immediate operation. If on arrival in port the inert gas has to be de-pressurised in order to measure or sample the cargo, it may be difficult, because of the low boiler load, to re-pressurise with an Inert gas having sufficiently low oxygen content. In this situation, it may be necessary to create a load on the boiler by using the main cargo pumps to circulate the cargo around the ship’s pipelines until the Inert gas quality Is satisfactory. Great care is necessary to ensure that the pumping arrangements used for circulating cargo do not give rise to an overflow. Throughout the discharge of cargo, particularly when the boiler load is low or fluctuating, the oxygen content of the Inert gas supply must be carefully monitored. Additionally, both the oxygen content and pressure of the Inert gas main should be continuously recorded during discharge, for action to be taken in the event of failure of the Inert gas plant during discharge from Inerted tanks. If hand dipping of a tank Is necessary, pressure may be reduced while dipping ports are open, but care must be taken not to allow a vacuum to develop since this would pull air Into the tank. To prevent this, It may be necessary to reduce the cargo pumping rate, and discharge should be stopped immediately if there is a danger of the tanks coming under vacuum.
A positive pressure of Inert gas should be maintained in the ullage space at all times during the loaded passage In order to prevent the possible ingress of air. If the pressure falls below the low-pressure alarm level, it will be necessary to start the Inert gas plant to restore an adequate pressure in the system. Loss of pressure is normally associated with leakages from tank openings and falling air and sea temperatures. In the latter cases, It is all the more important to ensure that the tanks are gas tight. Gas leaks are usually easily detected by their noise and every effort must be made to eliminate leaks at tank hatches, ullage lids, tank washing machine openings, valves etc. Leaks that cannot be eliminated should be marked and recorded for sealing during the next ballast passage or at another suitable opportunity. Certain all products, principally aviation turbine kerosene and diesel oil, can absorb oxygen during the refining and storage process. This oxygen can later be liberated into an oxygen deficient atmosphere such as the ullage space of an Inerted cargo tank. Although the recorded Incidence of oxygen liberation is low, cargo tank oxygen levels should be monitored so that any necessary precautionary measures can be taken prior to the commencement of discharge.
Static Electricity Precautions
In normal operations, the presence of Inert gas prevents the existence of flammable gas mixtures inside cargo tanks. Hazards due to static electricity may arise, however; mainly in the case of a failure of the Inert gas system. To avoid these hazards, the following procedures are recommended:
- If the Inert gas plant breaks down during discharge, operations should be suspended. If air has entered the tank, no dipping, ullaging, sampling or other equipment should be introduced into the tank until at least 30 minutes have elapsed since the injection of inert gas ceased. After this period, equipment may be introduced provided that all metallic components are securely earthed. This requirement for earthing should be applied until a period of five hours has elapsed since the Injection of Inert gas ceased.
- During any necessary re-inerting of a tank following a failure and repair of the Inert gas system, or during Initial Inerting of a non-gas free tank, no dipping, ullaging, sampling or other equipment should be inserted until the tank is in an Inert condition, as established by monitoring the gas vented from the tank being Inerted. However, should It be necessary to Introduce a gas sampling system Into the tank to establish its condition, at least 30 minutes should elapse after stopping the injection of Inert gas before inserting the sampling system. Metallic components of the sampling system should be electrically continuous and securely earthed.
When it Is required to gas free a tank after washing, the tank should first be purged with Inert gas to reduce the hydrocarbon content to 2% or less by volume. This is to ensure that, during the subsequent gas freeing operation, no portion of the tank atmosphere Is brought within the flammable range. The hydrocarbon content must be measured with an appropriate meter designed to measure the percentage of hydrocarbon gas in an oxygen deficient atmosphere. The usual flammable gas Indicator Is not suitable for this purpose. If the dilution method of purging Is used, It should be carried out with the inert gas system set for maximum capacity to give maximum turbulence within the tank. If the displacement method Is used, the gas Inlet velocity should be lower to prevent undue turbulence.
Before starting gas freeing, the tank should be isolated from other tanks by means of closing valves or blanking off associated pipelines. When either portable fans or fixed fans connected to the cargo pipeline system are used to introduce air into the tank, the Inert gas inlet should be isolated. If the Inert gas system fan Is employed to draw air into the tank, both the line back to the Inert gas source and the Inert gas Inlet into each tank that Is being kept Inerted should be isolated.
Preparation for Tank Entry
To ensure the dilution of the toxic components of Inert gas to below their Threshold Limit Values (TLVs), gas freeing should continue until lasts with an oxygen analyser show a steady oxygen reading of 20.9% by volume and tests with a flammable gas Indicator show not more than 1% LFL. If the presence of a toxic gas such as benzene or hydrogen sulphide is suspected, gas freeing should be continued until tests Indicate that its concentration Is below its TLV-TWA. Positive fresh air ventilation should be maintained throughout the period that personnel are In a tank, and frequent tests should be made of both oxygen and hydrocarbon content of the tank atmosphere. When other tanks in an Inert condition are either adjacent or Interconnected (e.g. by a pipeline) to the tank being entered, personnel should be alert to the possibility of Inert gas leaking Into the gas free tank through, for example, bulkhead fractures or defective valves. The risk of this occurring can be minimised by maintaining a small but positive inert gas pressure. When a gas free tank is re-connected to the inert gas main, it should immediately be re-inerted.
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, he 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 the 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.
* Read about Deck Water Seals
IGS Requirement Table (DNV)
IG Systems – Amendments & New Requirements
The IMO’S Maritime Safety Committee (MSC) has worked closely on the matter and following their 93rd session have introduced new requirements and amendments to the IG Systems;
- Apply to oil and chemical tankers constructed on or after 1st January 2016.
- Do not apply retrospectively to existing tankers constructed before 1st January 2016.
- Have been included in SOLAS Ch.II-2, the IBC code and the FSS code Ch.15
- The fitting of a fixed inert gas system will be required for tankers of 8,000 tonnes deadweight (dwt) and over, constructed (keel laid) on or after 1 January 2016. Previously, this applied only to tankers of 20,000 tonnes dwt and over.
- Tankers 8,000 dwt and over, carrying low-flashpoint cargoes, and constructed (keel laid) on or after 1 January 2016, must be provided with a fixed inert gas system complying with Chapter 15 of the amended FSS Code (or an equivalent system – subject to acceptance by the flag administration).
- The existing clause in SOLAS Regulation II-2/188.8.131.52 for waiving the requirements for a fixed inert gas system still applies to all gas carriers, but for chemical tankers, it now only applies to those constructed before 1 January 2016. This means that chemical tankers constructed (keel laid) on or after 1 January 2016, and carrying flammable cargoes such as those listed in the IBC Code chapters 17 and 18, will be required to have a fixed inert gas system, regardless of cargo tank size and tank washing machine capacities.
- The amendments also require the oxygen content supplied to the cargo tanks by the inert gas system to be reduced to 5%. The earlier limit was 8%.
Amendments for Oil tankers:
Tankers fitted with exhaust gas type inerting system will require inerting to be carried out during loading, on the voyage, during unloading, tank cleaning and for purging prior to gas freeing with air.
Amendments for Chemical Tankers:
- In the case of chemical tankers, it is accepted that inert gas need only be applied before commencing the unloading. This is in order to reduce cargo handling time. However, nitrogen will be the only accepted inert gas medium.
- The exemption for existing chemical tankers – having cargo tank volumes not exceeding 3000 m3 and having tank cleaning machine throughput not exceeding 17.5 m3/h per nozzle and total throughput not exceeding 110 m3/h per tank, does not apply to new chemical tankers constructed on or after 1st January 2016.
- However, the previous exemption for chemical tankers related to inert gas capacity still exists. When carrying flammable chemicals it is, therefore, acceptable that the unloading rate is reduced to 80% of the inert gas system capacity.
- A new paragraph is inserted in Chapter 8 of IBC code under “Cargo tank purging” which reads as “… before gas-freeing, the cargo tanks shall be purged with inert gas through outlet pipes with a cross-sectional area such that an exit velocity of at least 20 m/s can be maintained when any three tanks are being simultaneously supplied with inert gas. The outlets shall extend not less than 2 m above the deck level. Purging shall continue until the concentration of hydrocarbon or other flammable vapours in the cargo tanks has been reduced to less than 2% by volume. This is now in line with the oil tanker requirements.
- In case the chemical tanker is carrying oxygen dependent inhibitor and the tank is required to be inerted, then the inert gas should not be used before loading or during the voyage. Instead, the inert gas should be introduced in the tanks before commencing unloading. The minimum level of oxygen required is mentioned in the cargo manufacturer certificate and should be taken into account.