Marine Inert Gas Systems on Tankers

Marine inert gas systems (IGS) are the single most consequential fire-prevention measure on tankers. By displacing oxygen from cargo-tank vapour spaces with a non-combustible gas, an IGS removes one side of the fire triangle before cargo vapours or ignition sources can combine lethally. SOLAS Chapter II-2, as amended by Resolution MSC.365(93) adopted at MSC 93 in May 2014, requires every new oil tanker and new chemical tanker of 8,000 DWT and above constructed on or after 1 January 2016 to carry an IGS. Companion tools for the main operating calculations, including tank oxygen checks and IG supply rates, are available at Inert Gas O2 Check and Tanker IGS Supply.

Why the 8% oxygen threshold exists

Combustion requires fuel, oxygen, and an ignition source. In cargo tanks, fuel in the form of hydrocarbon vapour is unavoidable; ignition sources such as static electricity generated by high-velocity cargo washing, electrostatic charge from tank cleaning machines, and residual electrical fault currents cannot be reliably eliminated in normal operations. The practical control is oxygen reduction.

The lower explosive limit (LEL) and upper explosive limit (UEL) bracket the flammable range of a given hydrocarbon-air mixture. Crude oil vapour in air becomes ignitable at roughly 1% by volume (LEL) and ceases to be ignitable above approximately 10% by volume (UEL). Within that range, a spark of a few millijoules is enough to start a deflagration. Below the LEL, there is insufficient fuel; below the limiting oxygen concentration (LOC), there is insufficient oxygen regardless of fuel concentration.

For most petroleum cargo vapours, the LOC sits at 11 to 12% oxygen by volume. SOLAS Chapter II-2 Regulation 4.5.5 and FSS Code Chapter 15 set the operational ceiling at 8% oxygen, giving a 3 to 4 percentage-point margin below the LOC. The IG supply line to the deck main must contain no more than 5% oxygen by volume; the stricter supply limit exists because mixing of residual air in piping and tank headspaces can raise the local concentration before the gas disperses uniformly.

The consequence of breaching 8% is not immediate ignition but a loss of the inherent safety margin. At 9% oxygen in a tank containing crude vapour near the UEL, the addition of even a small air pocket from, say, a leaking hatch seal can produce a locally flammable mixture. The 8% ceiling keeps the tank firmly outside the flammable envelope under realistic mixing conditions.

SOLAS regulatory history and the MSC.365(93) threshold change

SOLAS first mandated IGS for oil tankers above 100,000 DWT in 1974, responding directly to a series of VLCC explosions in the late 1960s and early 1970s. The threshold dropped to 20,000 DWT under subsequent SOLAS amendments as the casualty record demonstrated that product tankers and smaller crude carriers carried the same ignition risk.

Resolution MSC.365(93), adopted by the Maritime Safety Committee at its 93rd session in May 2014 and entering force 1 January 2016, made two substantive changes. First, it extended the mandatory IGS requirement to new oil tankers of 8,000 DWT and above, down from 20,000 DWT. Second, it extended the same 8,000 DWT threshold to new chemical tankers, recognising that many products carried under the IBC Code are as flammable as crude oil. Ships built before 1 January 2016 above 20,000 DWT remained subject to existing requirements; vessels between 8,000 and 20,000 DWT built before that date are not retroactively required to install an IGS under SOLAS, though flag-state and class rules may differ.

The FSS Code Chapter 15 provides the detailed engineering standards for IGS design, adopted by Resolution MSC.98(73). MSC.1/Circ.1577, issued in 2016 as a revision of the earlier MSC.1/Circ.1389, provides the current operational guidelines and equipment performance criteria that administrations and flag states use when approving IGS designs.

Principle of operation: dilution and pressure balance

An IGS controls the cargo-tank atmosphere through two simultaneous actions: dilution and positive pressure.

Dilution replaces oxygen-containing atmosphere with inert gas. The rate at which oxygen concentration falls in a tank of volume $V$ when inert gas with oxygen fraction $y_{IG}$ displaces tank atmosphere with initial oxygen fraction $y_0$ is described, assuming ideal mixing, by:

where $Q$ is the volumetric flow rate of inert gas and $t$ is elapsed time. In practice, cargo tanks are far from ideally mixed; stratification, baffles, and trunk configurations mean the theoretical purge time must be multiplied by a safety factor of 1.5 to 2.0, and the actual oxygen concentration must be measured at multiple levels before operations proceed.

The Tanker Inerting IG Plant Start calculator uses the exponential dilution model with a configurable safety factor to estimate the blower run-time needed to reduce oxygen to the operational limit.

Positive pressure prevents air ingress. An IGS keeps the cargo-tank vapour space at a pressure slightly above atmospheric, typically 100 to 1,000 Pa above ambient depending on the PV valve settings. Air cannot flow into a tank that is already at positive pressure. If the IGS stops supplying gas and the pressure drops toward atmospheric, the PV vacuum breaker opens to admit inert gas (on systems with a pressurised reserve) or, as a last resort, ambient air, which is why continuous IGS availability during cargo discharge is operationally critical.

Inert gas sources: flue gas, dedicated generators, and nitrogen

Three source technologies serve tankers. The choice reflects the ship type, cargo compatibility, and IG purity requirements.

Flue gas from ship’s boilers. On steam-driven VLCCs and large crude tankers, main boiler exhaust provides a continuous, large-volume inert gas source at negligible additional fuel cost. Flue gas at the boiler uptake contains roughly 12 to 14% carbon dioxide, 74 to 78% nitrogen, 3 to 5% oxygen (depending on excess-air ratio), and variable quantities of sulfur dioxide and particulates that depend on fuel sulfur content. The scrubber and cooling section strips the sulfur compounds and particulates before the gas reaches the deck main. Oxygen content at the scrubber outlet is governed by the boiler’s combustion management; well-tuned steam boilers deliver IG at 2 to 3% oxygen.

Diesel-engine ships lack a boiler exhaust suitable for IG duty at tanker flow rates. Those ships use either a dedicated inert gas generator or a nitrogen plant.

Dedicated inert gas generators (IGG). An IGG burns marine diesel oil or fuel oil in a controlled-combustion chamber specifically designed for sub-stoichiometric combustion. Combustion air is metered to keep the excess-air ratio below 1.0, producing a flue gas with oxygen in the 1 to 3% range. The gas then passes through a scrubber and cooler identical in principle to the boiler-exhaust scrubber. IGGs are the standard solution on diesel-powered product tankers and VLCC newbuildings no longer fitted with oil-fired boilers. Fuel consumption is typically 0.8 to 1.5 kg of fuel per 100 cubic metres of inert gas produced.

Nitrogen generators: membrane and PSA. Chemical tankers and gas carriers frequently carry cargoes that react with carbon dioxide, sulfur dioxide, or condensed moisture in flue gas; nitrogen at 99%+ purity is required. Two technologies dominate.

Membrane nitrogen generators pass compressed air through hollow-fibre polymer membranes. Oxygen, carbon dioxide, and water vapour permeate through the membrane wall faster than nitrogen, so the retentate stream is nitrogen-enriched. A single-stage membrane unit delivers 95 to 97% nitrogen purity; double-stage units reach 99%+. Energy consumption runs 0.5 to 0.8 kWh per cubic metre of nitrogen at 97% purity.

Pressure swing adsorption (PSA) generators cycle between two molecular-sieve beds. One bed adsorbs oxygen from compressed air while the other regenerates under vacuum or low pressure. The output stream is 99 to 99.9% nitrogen. PSA units produce higher purity at lower flow rates and are common on chemical tankers where tank volumes are smaller but purity requirements are strict. The Nitrogen Generator Membrane PSA calculator covers both membrane and PSA sizing.

The table below compares the three source types across the criteria that determine selection:

System components: from source to tank

A tanker’s inert gas system can be traced from the gas source downstream through eight functional stages.

Scrubber and cooling tower

Flue gas enters the scrubber at 250 to 400 °C. Sea water is sprayed countercurrently to cool the gas to below 45 °C (the FSS Code Chapter 15 upper limit for gas delivered to the blowers), to absorb SO2, and to wet-scrub soot and carbon particulates. The scrubber also acts as a first-stage demister. Scrubber efficiency for SO2 removal depends on sea water pH and flow rate; FSS Code Chapter 15 requires the SO2 content at the scrubber outlet to not exceed 2 parts per million by volume under normal operating conditions on ships where cargo is sensitive to SO2. The Inert Gas Plant Flue Gas Scrubber calculator covers scrubber water flow and heat rejection sizing.

Blowers

Two or more blowers draw gas from the scrubber and deliver it to the deck main at the pressure needed to overcome the tank PV valve settings and pipe losses. FSS Code Chapter 15 requires the capacity to be sized so that the combined output of the running blowers can replace the maximum rate of cargo discharge without allowing tank pressure to fall below atmospheric. On a VLCC discharging at 20,000 cubic metres per hour, the blower array must match that flow rate at a delivery pressure of at least the PV valve cracking pressure, typically 1.4 kPa gauge.

Blowers on tankers are typically centrifugal or positive-displacement types. FSS Code Chapter 15 requires the blowers to be capable of supplying gas to the cargo tanks at not less than 125% of the maximum cargo discharge rate when both (where two are fitted) are running; the capacity of any single blower must cover 100% of the discharge rate in single-blower mode.

Deck water seal

The deck water seal is the primary non-return barrier between the IG plant and the cargo tank IG main. It consists of a water-filled chamber through which the IG main passes as an internal standpipe; gas can flow from the blower side to the deck side only by bubbling up through the water column, but cargo-tank vapour cannot flow backward past the water barrier. FSS Code Chapter 15 requires the deck water seal to be capable of preventing return flow of gas and vapour under all conditions of list and trim, including when the ship is at 30° list and 10° trim.

The water seal operates continuously; its water level is maintained automatically and alarmed on low level. If the water seal dries out, the non-return function is lost.

Mechanical non-return valve (NRV)

In addition to the deck water seal, FSS Code Chapter 15 requires a mechanical non-return valve, typically a flap or swing check valve, installed on the deck side of the water seal. The two non-return devices together provide defence-in-depth against backflow. The mechanical NRV is tested at every annual survey.

Deck isolating valve

A power-operated isolating valve on the IG main at the deck can shut off supply to the whole cargo system or to individual sections. The valve’s position must be remotely visible from the cargo control room. FSS Code Chapter 15 requires the isolating valve to default to the closed position on loss of actuator power.

Pressure-vacuum (PV) valves and the mast riser

Each cargo tank has a PV valve assembly. The pressure relief element opens at about 1.4 kPa above atmospheric to vent overpressure; the vacuum relief element opens at about 0.35 kPa below atmospheric to admit inert gas (from a header) or, if inert gas is unavailable, air through a flame-arresting screen. PV valves must incorporate an IMO-type-approved flame arrester element meeting the test requirements of MSC/Circ.677.

The mast riser, or high-velocity vent, provides a separate high-capacity pressure-relief path for the IG main header, venting tank over-pressure to high elevation and high velocity (typically above 30 m/s exit velocity) so that vapour is carried clear of the deck. MSC.1/Circ.1577 gives detailed guidance on mast riser sizing.

Oxygen analyser and recording system

The oxygen analyser is the primary instrument that verifies compliance with the 5% and 8% O2 limits. FSS Code Chapter 15 requires a fixed oxygen analyser to sample the gas at the outlet of the inert gas plant (verifying supply quality) and at the IG main header (verifying delivery quality). A second fixed analyser or portable instrument must be available to sample cargo-tank headspaces individually.

FSS Code Chapter 15 specifies that the analyser response time must not exceed 30 seconds. It must be calibrated against a known-concentration reference gas at intervals not exceeding one month, with the calibration logged.

Alarms required by FSS Code Chapter 15 and SOLAS Chapter II-2 Regulation 4.5.5 include:

• High oxygen alarm at the plant outlet, set at 5% O2 (trips an audible and visual alarm at the cargo control room and the navigation bridge, and must trigger automatic shutdown of cargo discharge pumps on some configurations)
• Low gas-pressure alarm at the IG main, set below a level that indicates inability to maintain positive tank pressure
• High gas-pressure alarm at the IG main
• Low water level alarm on the deck water seal
• High gas temperature alarm at the plant outlet, set at 45 °C

IG main and branch lines

The IG main runs from the deck water seal along the cargo deck to manifold connections at each cargo tank. Branch valves, remotely operable from the cargo control room, isolate individual tanks or tank groups. On modern VLCCs the IG main is typically 600 to 900 mm in diameter in the centre section, tapering toward the bow and stern. The pipe material is carbon steel; internal surfaces are often coated or lined to resist SO2-laden condensate corrosion.

Alarms, interlocks, and automatic safety actions

SOLAS Chapter II-2 Regulation 4.5.5 and FSS Code Chapter 15 together require a layered set of automatic responses, not just alarms.

If the oxygen content at the plant outlet rises above 5%, an alarm must sound. If the rise continues and the system cannot correct it (boiler load change, combustion fault on an IGG), FSS Code Chapter 15 requires automatic closure of the supply valve to the cargo deck and an alarm on the bridge. On installations where the IG system interlocks with the cargo pump control system, high oxygen at the plant outlet triggers automatic stopping of all cargo discharge pumps. The philosophy is that cargo discharge must not continue without a confirmed inert atmosphere.

If gas pressure in the IG main falls below the set minimum (indicating that the blowers can no longer maintain positive pressure in the tanks), an alarm must sound and, on ships with automatic cargo pump interlocks, the discharge pumps must stop.

The deck water seal low-level alarm, if not corrected within a defined time, must also trigger closure of the IG deck supply valve. A dry water seal in service is a direct backflow risk.

Cargo operations: inerting, purging, and gas-freeing

Four distinct atmospheric states govern the evolution of a cargo tank through the cargo cycle, and three controlled-gas-flow procedures move the tank between them.

Tank atmospheric states per ISGOTT 6th edition:

• Inert: oxygen below 8% by volume. The tank cannot support combustion regardless of hydrocarbon concentration. All normal cargo-handling operations, including crude oil washing, require this state.
• Gas free: hydrocarbon vapour concentration below 1% of LEL and oxygen at or near 20.9%. Personnel may enter. Welding and hot work are permitted.
• Flammable: hydrocarbon concentration between LEL and UEL, or oxygen above 8% with substantial hydrocarbon vapour present. No operations permitted; entry strictly forbidden.
• Inert but not gas free: the transitional state between the inert and gas-free states. Oxygen is below 8%, but hydrocarbons are above 1% LEL. This state is safe from fire but not safe for entry.

Inerting is the procedure for reducing a tank from air-filled or flammable to the inert state. The IG main delivers gas to the tank through the drop line or at the deck; displaced air vents through the PV valves or mast riser. The procedure continues until every measurement point in the tank reads below 8% O2. On a VLCC tank of 30,000 cubic metres with an initial oxygen concentration of 20.9%, reducing to below 8% requires displacing approximately 2.7 tank volumes of inert gas in ideal plug-flow terms. Actual volumes required, accounting for incomplete mixing, are typically 3.5 to 5 tank volumes. The Tanker Inerting Positive Pressure and Tanker Inerting Negative Pressure Check calculators address the pressure-management side of this procedure.

Purging moves the tank from the inert state toward the gas-free state by diluting hydrocarbon vapour to below 2% LEL before air is introduced. The reason for this sequence is the passage through the flammable range: if air were introduced directly into an inert, hydrocarbon-rich tank, the mixture would pass through the LEL to UEL range as oxygen climbed. Introducing more inert gas first reduces hydrocarbons below LEL, so that when air is subsequently admitted, the mixture passes from “inert with hydrocarbons below LEL” directly to “gas free” without entering the flammable envelope. The purge calculation uses the same dilution model as inerting:

where $y_{HC,0}$ is the initial hydrocarbon concentration as a fraction of LEL, $Q$ is the inert gas volumetric flow, $V$ is the tank volume, and $\eta$ is a mixing efficiency factor (0.5 to 0.7 for realistic tank geometries). The Tanker Inerting Topping Up calculator handles topping-up after partial loss of inert atmosphere.

Gas-freeing follows successful purging. Fresh air is introduced by fans or eductors; the tank atmosphere passes from “inert, hydrocarbons below LEL” to “gas free” without entering the flammable range because hydrocarbons are already below LEL. Gas freeing continues until both oxygen reads 20.9% (within 0.5%) and hydrocarbon reads below 1% LEL at all measurement levels. On a VLCC cargo tank, gas-freeing to entry standard can take 12 to 24 hours depending on fan capacity and tank geometry.

Personnel entry into any tank is forbidden unless the tank has been independently verified as gas free and oxygen-sufficient for breathing. This connects directly to Marine Confined Space Entry and Tank Inspection, which covers the permit-to-work system, atmosphere testing, and rescue arrangements required before entry.

Crude oil washing and the IGS precondition

Crude oil washing (COW) uses the ship’s own cargo as a solvent to clean tank residues during discharge, replacing sea-water washing. COW is permitted under MARPOL Annex I for crude oil tankers as an alternative to the clean-ballast-tank requirement, but SOLAS Chapter II-2 and Regulation 33 of MARPOL Annex I make an operating IGS a precondition for any COW operation. Before the first COW machine is started, the IGS must be running, delivering inert gas to the tank at the required pressure, and the tank oxygen content must be verified below 8%.

The reason is mechanical: COW machines rotate at high speed, throwing crude oil droplets against tank surfaces in a pattern that can generate static charge. If the atmosphere were not inert, even the small electrostatic discharge from a droplet-spray impact could ignite vapour. The operational procedures in ISGOTT 6th edition chapter 11 and the related Marine Tank Cleaning and Crude Oil Washing article cover the step-by-step sequence.

Chemical tankers: IBC Code requirements

The IBC Code (International Code for the Construction and Equipment of Ships Carrying Dangerous Chemicals in Bulk), adopted under SOLAS and MARPOL, classifies chemical cargoes by their fire and reactivity hazard. Chapters 3 and 9 of the IBC Code govern the cargo containment and atmosphere requirements.

For flammable cargoes in IBC Code ship type 2 and type 3 tanks, the code requires a fixed IGS when the cargo’s flashpoint is at or below 60 °C unless the ship is fitted with a vapour control system covering all tanks. The code additionally specifies that for cargoes that react with CO2 or SO2 (a category that includes several acids, oxidisers, and amine-group chemicals), only nitrogen is an acceptable inert gas. Flue-gas-derived IG, even well scrubbed, carries residual CO2 at 12 to 14% and traces of SO2; it is incompatible with such cargoes.

Chemical tanker nitrogen systems are sized differently from oil-tanker systems because tank volumes are smaller (typically 500 to 3,000 cubic metres per tank on a 20,000 DWT vessel) but purity requirements are higher. The IBC Code requires the delivered nitrogen purity to be at least 98% by volume for most cargoes, with specific cargoes demanding 99.9%+. Shore nitrogen supply via manifold connection is a common alternative on vessels without an onboard generator, but the ship is then dependent on port availability.

Gas carriers: IGC Code requirements

The IGC Code (International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk, 2016 edition) applies to ships carrying LPG, LNG, ethylene, ammonia, and other liquefied gases in bulk. Chapter 9 of the IGC Code covers cargo containment atmosphere requirements.

LNG carriers operate under different thermal and pressure conditions from oil tankers. The cargo containment system (membrane, Moss sphere, or independent tank) operates at or near the liquefied gas temperature of minus 163 °C for LNG. The vapour space above the cargo is an essentially pure methane atmosphere; inerting with nitrogen is the standard procedure for commissioning and decommissioning, not a continuous operation at sea. During normal cargo operations, the vapour space above LNG is already methane at close to the boiling point; there is no oxygen present, so the conventional IGS concept does not apply in the same form. Inerting becomes relevant during initial gas-up (introducing cargo into a dry, air-filled containment system) and cool-down phases, where the containment system is first blanketed with nitrogen and then progressively introduced to methane vapour before liquid cargo is admitted.

LPG carriers carrying propane, butane, or vinyl chloride monomer use nitrogen in the vapour space during operations. Because LPG tanks operate at ambient temperature but elevated pressure (propane vapour pressure at 40 °C is approximately 1.37 MPa), the nitrogen blanketing pressure must be managed carefully against the cargo vapour pressure. IGC Code chapter 9 provides the specific requirements for each cargo type and tank configuration.

Limitations of inert gas systems

IGS, properly maintained and operated, dramatically reduces the risk of cargo-tank fires and explosions, but it does not eliminate all risk and it introduces risks of its own.

Asphyxiation hazard. Inert gas with 3 to 5% oxygen is immediately incapacitating. A person entering a tank with inert atmosphere will lose consciousness within 15 to 30 seconds and will not self-rescue. Multiple fatalities across the tanker industry have resulted from tank entry under inert atmosphere without proper gas-free certification. The same confined-space entry protocols that protect crew from hydrogen sulfide and hydrocarbon vapour must be applied with equal rigour to inert-atmosphere tanks.

System availability dependency. Cargo discharge cannot safely continue without an operating IGS. A blower failure, water seal loss, or oxygen analyser failure during discharge requires immediate pump shutdown and atmosphere verification before repairs are made. Tankers operating on a single-blower basis (one blower failed, the second running) have a very limited margin; a second blower failure mandates discharge suspension.

SO2 and moisture in flue gas. Even with scrubbing, flue-gas-derived IG carries moisture and trace SO2. Over time this condenses in tank structures and contributes to corrosion of tank coatings and structural steel. Product tankers carrying white oils (jet fuel, kerosene, diesel) and chemical carriers with strict cargo purity requirements must use nitrogen to avoid this contamination.

Boiler-load dependency of flue gas quality. At low main boiler loads (ship at slow speed, manoeuvring, or at anchor), the flue gas oxygen content can rise above the 5% supply limit because excess air is higher at part load. FSS Code Chapter 15 requires an automatic high-O2 shutdown that prevents off-spec gas from reaching the deck main, but this means IG supply ceases when the ship most needs it, namely during slow discharge in port. IGGs eliminate this dependency at the cost of additional fuel burn.

Oxygen analyser single-point failure. If the fixed oxygen analyser fails mid-operation, the ship has no certified means of verifying the 5% supply limit. FSS Code Chapter 15 requires a second analyser or a certified portable instrument capable of covering this measurement, but the emergency is real: a failed analyser does not tell the operator whether the IG is in spec or out. The only safe response is to treat the atmosphere as unknown and act accordingly.

Thermal stratification in tanks. Large cargo tanks are not well-mixed. Inert gas introduced at the top of a Moss sphere or at the top of a conventional cargo tank settles as a density-gradient layer above the cargo vapour. Oxygen measurements taken only at the top of the tank can mislead; measurements at multiple levels, including near the cargo surface and in the corners below the heating coils, are required by ISGOTT 6th edition to certify the whole tank volume as inert.

Pressure-vacuum valve condition. PV valves are the last line of defence against either overpressure (from excess IG supply during loading) or air ingress (if IG supply drops). Fouled or corroded PV valves that stick in the closed position can allow dangerous overpressure to build; valves stuck open allow constant air ingress during sea passage. Annual PV valve testing and flame arrester inspection per FSS Code Chapter 15 is mandatory, but the testing interval means a valve that corrodes six months after its last annual survey may fail silently for up to 18 months.

Survey and maintenance obligations

FSS Code Chapter 15 and class society rules (applied by DNV, Lloyd’s Register, ABS, Bureau Veritas, ClassNK, and others) impose specific inspection obligations.

The oxygen analyser must be calibrated against certified reference gas at intervals not exceeding one month; calibration records must be retained on board for flag-state inspection. At annual class survey, the analyser is subject to verification of calibration, sample-line integrity, and alarm-function testing.

The deck water seal level and operation must be tested at each port arrival and departure when the IGS has been in service. The mechanical non-return valve must be inspected and pressure-tested at annual survey.

PV valves must be pressure-tested and flame arresters inspected at annual survey; in-service testing of PV valve settings is required every three to five months depending on class rules. Flame arrester elements that show distortion, corrosion, or contamination are replaced, not cleaned, because cleaning does not restore the original tested geometry.

Blower performance must be verified against rated flow and pressure at each annual survey. Bearing condition, shaft seal integrity, and impeller blade erosion are checked.

At drydock (typically every 2.5 or 5 years depending on flag state and class), the IG main piping is inspected internally for corrosion, the scrubber internals are cleaned and inspected, and all valves are overhauled to as-new condition.

Cross-system interactions

An inert gas system does not operate in isolation; it interacts with several other safety systems on board.

Fire detection and fixed fire-fighting. The Marine Fire Detection and Fixed Fire Fighting Systems article covers CO2 and halon-replacement systems for pump rooms and machinery spaces. The IGS protects cargo tanks; the fixed CO2 system protects the pump room. The two systems share the same organisational principle, elimination of oxygen, but are entirely separate pipe networks with separate detection and actuation logic. A CO2 flood of the pump room with the IG main open would not inert the cargo tanks.

SOLAS Chapter II-2 fire protection architecture. The IGS is one element of the SOLAS Chapter II-2 fire protection framework for tankers, alongside structural fire protection, means of escape, fire detection systems, and fire-fighting appliances. The interconnection of those elements is described in SOLAS Chapter II-2: Fire Protection, Fire Detection and Fire Extinction.

Cargo pump system. On tankers with IGS interlocks, the cargo pump control system receives a signal from the IGS oxygen analyser. A high-O2 alarm at the plant outlet triggers automatic pump shutdown. This interlock is required under FSS Code Chapter 15 for ships where the loss of IG supply creates an immediate fire risk, typically very large crude carriers where the tank volumes are so large that any air ingress rapidly creates a flammable zone.

Tank cleaning operations. Tank cleaning procedures, covered in detail at Marine Tank Cleaning and Crude Oil Washing, are tightly coupled to IGS status. No tank cleaning operation may begin unless the IGS is verified in service and the tank atmosphere is inert. Tank cleaning also generates heat, water vapour, and cargo-vapour aerosols that affect the accuracy of in-tank oxygen measurements; ISGOTT 6th edition section 11.6 requires measurements to be taken before the cleaning machine is started and at defined intervals throughout.

Enclosed-space entry. Once a tank has been inerted it is, by definition, an oxygen-deficient enclosed space, and no person may enter it until it has been purged of inert gas, ventilated with fresh air, and tested to confirm a safe atmosphere. The IGS status therefore governs the entry decision directly: a tank reading 8 percent oxygen is correctly inerted for fire safety but immediately fatal to an unprotected person. The full entry regime is set out in Marine Confined Space Entry and Tank Inspection, and the two requirements, keep the tank inert for fire safety yet make it breathable before entry, are managed as a single planned transition rather than as independent operations.

See also

• Tanker IGS Supply Calculator
• Inert Gas O2 Check
• Tanker Inerting IG Plant Start
• Tanker Inerting Positive Pressure
• Tanker Inerting Negative Pressure Check
• Tanker Inerting Topping Up
• Inert Gas Plant Flue Gas Scrubber
• Nitrogen Generator Membrane PSA
• Class LR IGS Calculator
• IGC Nitrogen Liquefied
• Marine Tank Cleaning and Crude Oil Washing
• Marine Confined Space Entry and Tank Inspection
• SOLAS Chapter II-2: Fire Protection, Fire Detection and Fire Extinction
• Marine Fire Detection and Fixed Fire Fighting Systems
• Oil Tanker
• Chemical Tanker
• LNG Cargo Containment Systems