Characteristics Hazards

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INERT GAS SYSTEM&FLUE GASSYSTEM

Presented by H.V Sithuruwan fernando (Engineering Cadet/TPTM)1IntroductionThe combustion & explosion of petroleum gasses, the corrosion of steel occur only when oxygen is present

The object of the Inert Gas System is to reduce the oxygen content in the cargo tanks to below 8%, well below the Explosion Limit of 11.5%

This will prevent cargo tank explosion, reduces corrosion and increases Pumping capacity

What is inert gas Generator system ??Atmospheric airis burned using diesel in a combustion chamber and the exhaust collected, the resulting exhaustcontains less than 5%oxygen, thereby creating"inert gas".with a typical make-up of:

Oxygen: 2 4%

Carbon Dioxide: 12 14%

Nitrogen: 80%

Flammable Envelope

What is inert gas Generator ??A visual demonstrationInert gas generator (IGG)refers tomachineryon board marine producttankers

Inert gas generators consist distinctively of a gas producer and a scrubbing system

Atmospheric airis burned using diesel in a combustion chamber and the exhaust collected, the resulting exhaust contains less than 5%oxygen, thereby creating "inert gas".

The hot, dirty gas is then passed through a scrubbing tower which cleans and cools it using seawater.

This gas is then delivered to cargo tanks to prevent explosion offlammable cargo.

This generator is sometimes confused withflue gas systems, which draw inert gas from theboiler systemsof the ship.

Flue gas systems do not have a burner but only "clean" and measure the air before delivering it to the cargo hold.

The Inert Gas generator System

Alternatively an inert gas generator may be used when:

The ship does not have sufficient boiler capacity for the flue gas system Topping up units are fitted in addition to the flue gas system A high quality inert gas is required such as on gas and chemical tankers Typically a range between 0.2% to 2.0% oxygen content with No soot is available to suit client requirements.

Inert Gas Generator

A typical Inert Gas Generator system as shown here includessix major component parts, each having special features.

Inert Gas Generator Combustion Air Blower Deck Water Seal Pressure/ Vacuum Breaker Valves Control System

Inert Gas Generator Unique 'Sea Guardian' third generation design, incorporating innovative integrated burner, swirl vane separator and patented quench scrubber elements, eliminating the need for demister pads and spray nozzles.

Combustion Air Blower A Roots type blower of compact design. Selected for low noise and low maintenance characteristics.

In-built safety relief valve.

The picture represents a standard Inert Gas Generator (FU Type) complete with air fans, fuel pumps, deck water seal and P/V breaker

The Inert Gas SystemRegulations require the oxygen content in cargo tanks to be maintained at not more than 8% by volume

14Gas Pressure Regulation Valves

Deck Water SealThis is the principle barrier in the systemA water trap is fitted which permits inert gas to be delivered to the deck main but prevents any back flow of cargo gasEven when the IG plan is shut down

Deck Water Seal

Gas flow towards cargo tanksBack pressure in cargo tanks

Liquid Filled Pressure Vacuum Breakers

Inert Gas ProceduresGeneral Policy of Tank Atmosphere ControlTanks should be inerted prior to loading and maintained inert during loading, on the loaded passage, during discharge and during tank washingTanks should be kept in an inert condition whenever they contain cargo residues or ballast

Inert Gas ProceduresGeneral Policy of Tank Atmosphere ControlAfter cleaning and gas freeing it is not necessary to inert the tankThe transition from an inert condition to a gas free condition MUST be made without the tank atmosphere passing through the flammable range/envelope.This means purging the tank with inert gas until it is below the Critical Dilution Line

Inert Gas ProceduresPrimary InertingFrom a gas free condition the tanks should be inerted prior to the loading of cargo until the tanks have an oxygen content of less than 8% by volumeOn completion of inerting all the tanks are to be pressurised to a minimum of 100mm water gauge and also kept common with the gas mainA positive pressure being maintained on all tanks by topping up with IG as necessary

Inert Gas ProceduresLoading or BallastingThe IG plant is shut down and the deck isolating v/v closed, unless loading is taking place at the same time as deballasting (unusual???)Tank vents are opened and IG is thus vented from the tank as cargo or ballast is loaded into the tankDuring loading closed ullaging is used

Inert Gas ProceduresLoaded / Ballast PassageTanks are re-pressurised for the loaded or ballast passageA positive IG pressure should be maintained to prevent possible ingress of airLoss of pressure can normally be associated with leakage & falling air and/or sea temperaturesIf tank pressure falls below the low level it will necessitate starting the IG plant to restore the correct operating pressureInert Gas ProceduresCargo or Ballast DischargeOwners of the cargo, receivers and/or customs authorities may require manual ullaging, water dips and samples prior to discharge (regulations permitting)

This is acceptable provided a minimum number of tank openings are open at any one time

On completion of ullaging, and before commencing discharge the tanks are to be re-pressurised

All tanks are made common with the gas main

Tank openings including vents are closedThe IG plant is operated and the deck isolating v/v is opened to allow inert gas to replace the liquid being discharged

After discharge a hydrocarbon content of up to 4% may exist in an inerted tank however, it is preferable to maintain the content at 2% which is below the CDL

This condition should be monitored & maintained during the ballast passageInert Gas ProceduresTank Washing including COWTanks should be in the inert condition during tank washing

Inert Gas ProceduresGas FreeingTo gas free a tank it should first be purged with IG to reduce the Hydrocarbon content to 2% or less, below the CDL

The intended tank should always be isolated from other tanks prior to starting gas freeing

If gas freeing is to be carried out by means of portable/fixed fans connected to the cargo pipeline system then the IG inlet should be isolated

If the method of using the IG fan to draw fresh air is used then both the line back to the IG source and the IG inlet to other tanks being kept inert should also be isolatedA tank is said to be gas free when the entire tank is found to have an oxygen content of 21% by volume when tested with an oxygen analyser

Also a reading o less than 1% on a flammable gas indicator

If toxic gasses such as benzene or hydrogen sulphide are suspected to be present then gas freeing should continue until such time as tests with an appropriate device indicates that the content is below the Threshold Limit Value (TLV)Venting Inert Gas into the AtmosphereIn some regions of the world this is not allowedTo overcome this the piping system can be so arranged such that as cargo tanks are being loaded/gas freed the displaced IG can be transferred ashoreUnfortunately this may limit the speed of loading to within the speed at which the IG can be transferred ashore

Inert Gas FailureCrude Tankers:-Because of the dangers of ignition from pyrophoric deposits, inerted tanks should not be allowed to reach a flammable stateIn the event of failure of the IG plant before or during discharge of cargo or ballast then operations should cease and close the deck isolating valve until the IG plant is restored to full operation

Inert Gas FailureProduct Tankers:-

Discharge should not begin/resume without the permission of all interested parties

FLUE GAS SYSTEMS (FGS)

Generally used on crude oil tank ships where a higher quality inert gas is not required for the unrefined cargo product.

The gas in this case is available as a by-product of boiler combustion and is taken from the ships boiler uptake.

Typically a range between 3% to 5% O2content with some soot content dependent upon flue gas quality.

A typical Flue Gas System as shown here includes six major component parts, each having special features.

Scrubber Tower Gas Fans Deck Water Seal Pressure/Vacuum Breaker Valves Control SystemScrubber Tower

Combining a high energy venture scrubber with a packed tower scrubber enables high efficiency particulate removal, cool quenching, cleaning and final demisting of boiler flue gases.

Scrubber Tower36Inert Gas Scrubber System

Inert Gas System Schematic

Inert Gas System Schematic

39Re-circulating Arrangementto regulate the flow of inert gas to the IG deck main

Gas Fans

Single stage centrifugal inert gas fans are used.

The fan impellers are dynamically balanced and manufactured in corrosion resistant material.

The casings are internally glass flake coated to reduce maintenance.

Gas FansDeck Water Seal'Wet Type' design with an external seal pipe ensuring added safety Water is displaced during normal gas flow forming a semi wet seal

Entrained water is expelled by swirling the gas in the vessel and passing it through an integral demister.

Deck Water SealPressure/Vacuum Breaker Providing final protection by releasing excessive pressure or vacuum from the cargo tanks ensures that the tank design conditions cannot be exceeded.

The design is totally fail-safe.

Pressure/Vacuum BreakerValves Gas delivery and control valves are normally of the butterfly type with bonded rubber linings and corrosion resistant discs and shafts Where remotely controlled, these valves use double acting actuators. The boiler uptake valves are of the butterfly wafer pattern type suitable for high temperature with metal seating. These are normally pneumatically operated and fitted with limit switches forremote control and indications.

ValvesControl System Utilizes a programmable logic controller to give the required functions of control along with designated selective acceptance of alarms.

The instrumentation and control panels are generally located in the engine room, cargo room and bridge.

Control Panel

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Vent Pipe

Deck Master Valve

Pressure Vacuum Valve

Pressure Vacuum

Breaker

Riser Main

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Vapour

Line

Vent Pipe

Deck Master Valve

Pressure Vacuum Valve

Pressure Vacuum

Breaker

Riser Main

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Vent Pipe

Deck Master Valve

Pressure Vacuum Valve

Pressure Vacuum

Breaker

Riser Main

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Vent Pipe

Deck Master Valve

Pressure Vacuum Valve

Pressure Vacuum

Breaker

Riser Main

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Vent Pipe

Deck Master Valve

Pressure Vacuum Valve

Pressure Vacuum

Breaker

Riser Main

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Vent Pipe

Deck Master Valve

Pressure Vacuum Valve

Pressure Vacuum

Breaker

Riser Main