On a laden tanker, the inert gas system is the single barrier standing between a cargo tank full of hydrocarbon vapour and an explosive atmosphere. It works by a simple principle — remove one leg of the fire triangle by keeping the oxygen in the tank too low to support combustion — and the whole of SOLAS builds on that principle with hard numbers: the system must deliver inert gas with an oxygen content of not more than five per cent by volume, and keep the cargo tanks under positive pressure with an atmosphere below eight per cent oxygen. Those thresholds are not aspirational; they are the line between a safe cargo operation and a catastrophe, and they hold only for as long as every component in the chain is maintained. A scrubber that no longer cools the flue gas, a deck seal running dry, an oxygen analyser reading two per cent low, a corroded non-return valve — any one of them quietly erodes the barrier while the panel still shows green. This guide is a practical maintenance reference for the people who keep that barrier intact: engineers, chief officers and superintendents responsible for the IG plant, the scrubber and deck seal, the pressure and vacuum protection devices, the oxygen analysers, the safety interlocks, and — on chemical and product tankers — the nitrogen generator. It works through each subsystem in turn, explains what maintenance each one needs and why, sets out the alarm and shutdown logic that must never be defeated, and closes with a maintenance-interval reference. To plan, schedule and record IG system maintenance with full evidence and certificate tracking across your fleet, start a free trial or book a demo.
INDUSTRY GUIDE · TANKER SYSTEMS
Inert Gas System Maintenance: IG Plant and Nitrogen Generator Service Guide
A technical maintenance reference for the barrier that keeps cargo tanks below the flammable limit — flue gas plant and scrubber, deck seal, pressure and vacuum protection, oxygen analysers, safety interlocks, and nitrogen generators — grounded in the SOLAS thresholds that define safe operation.
≤ 5%
O2 in the IG main by volume
≤ 8%
O2 in cargo tank atmosphere
125%
of max cargo discharge rate
What this guide covers
1 How the system works and why it matters
2 Scrubber and flue gas side
3 Deck seal and non-return devices
4 Pressure and vacuum protection
5 Oxygen analysers and calibration
6 Nitrogen generator service
7 Safety interlocks and alarms
8 Maintenance interval reference
1. How the System Works — and Why Maintenance Is Safety-Critical
Fire needs three things: heat, fuel and oxygen. Remove any one and combustion stops. The inert gas system removes the oxygen, flooding the cargo tank ullage space with gas low enough in oxygen that the hydrocarbon vapour cannot ignite regardless of any ignition source. Understanding the flow of the gas is the foundation for understanding what each maintenance task protects.
There are two ways to produce the gas, and they define two families of system. In a flue gas plant, the source is the exhaust from the ship's boiler — a high-temperature gas mixture drawn from the boiler uptake, which is already low in oxygen when the boiler is burning efficiently. In an inert gas generator, a dedicated combustion unit burns fuel independently to produce the gas. A third approach, the nitrogen generator, separates nitrogen from compressed air and is favoured on chemical tankers because it produces no soot or water carryover to contaminate sensitive cargoes. Whichever the source, the gas must be cleaned, cooled, delivered under positive pressure, and continuously verified for oxygen content.
In a flue gas system the path runs like this: flue gas is drawn from the boiler uptake through the flue gas isolating valves into the scrubber tower, where it is cooled and washed to remove soot, solids and sulphur; a demister strips entrained water; the inert gas blowers draw the cleaned gas and push it forward; an oxygen analyser after the blower verifies the gas is fit to deliver; the gas passes through the deck water seal and a mechanical non-return valve — the two devices that stop cargo vapour flowing back toward the machinery space — and out through the deck isolating valve into the deck main and the individual cargo tanks. Pressure and vacuum protection devices guard the tanks and the main against over- and under-pressure throughout.
Why every task on this list is safety-critical
The IG system is a protective barrier, and a degraded barrier can still appear to function. A scrubber losing cooling efficiency, a deck seal with a failing water supply, or an oxygen analyser that has drifted out of calibration will not necessarily announce itself — the plant runs, the deck main shows pressure, the panel looks normal — while the actual protection quietly falls below the standard. Maintenance is not about keeping the plant running; it is about keeping the barrier real. This is why the thresholds are verified continuously and why the interlocks that shut the plant down exist. Never treat an IG maintenance item as routine housekeeping deferrable to a convenient moment.
2. The Scrubber and Flue Gas Side
On a flue gas plant, the scrubber is where raw, dirty, hot exhaust becomes gas fit to enter a cargo tank. Its job is to purify the exhaust gas, remove solid substances and sulphur, and cool it — and it does that by intimate contact between the gas and large volumes of seawater. Everything about maintaining the scrubber follows from the fact that it is a wet, hot, corrosive, fouling environment.
Scrubber tower internals and nozzles
The spray nozzles, packing and trays that create gas-water contact foul with soot and scale and corrode in the acidic, salt-laden environment. Nozzles that clog reduce cooling and cleaning efficiency, so the gas arrives hotter and dirtier than the design intends. Internal inspection, nozzle cleaning and checking for corrosion are core periodic tasks, and the cooling water flow and distribution must be confirmed as effective, not merely present.
Cooling and scrubbing water supply
The scrubber depends on an adequate, well-distributed seawater supply. Water pumps, supply lines, strainers and the level and flow sensors that protect the tower all need attention — a high scrubber water level triggers an alarm and shutdown, and a loss of effective cooling lets hot gas pass forward. Confirming the water side is one of the most important checks on the flue gas plant.
Demister
After scrubbing, the demister separates entrained water droplets and remaining solids so the blower and downstream piping do not carry water forward. A fouled or damaged demister passes moisture into the system, promoting corrosion and carryover, so it is inspected and cleaned as part of the scrubber service.
Blowers and flue gas isolating valves
The blowers draw scrubbed gas and deliver it under pressure; regulations require the capacity to supply gas at a rate at least 125 per cent of the ship's maximum cargo discharge rate, typically through two blowers. Bearings, impellers, seals and drive need routine maintenance, and the flue gas isolating valves — which must be gas-tight when the boiler side is isolated for safe maintenance — need their seats and actuators checked, since a leaking isolating valve is a direct hazard to anyone working on the plant.
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The gas quality depends on the boiler, not just the plant
On a flue gas system the oxygen content of the produced gas depends on how efficiently the boiler is burning. At low boiler load the excess air rises and so does the oxygen in the exhaust, which is why boilers are typically held to a minimum load — often around twenty per cent — to keep the exhaust oxygen down near three per cent. This matters for maintenance because a plant that struggles to make good gas may be pointing at a boiler combustion or load issue rather than a fault in the IG plant itself. Reading the oxygen trend alongside boiler load is part of diagnosing the system correctly.
3. The Deck Seal and Non-Return Devices
Between the machinery space and the cargo tanks sit the devices whose entire purpose is to stop hydrocarbon gas from the tanks flowing back toward the engine room and its ignition sources. Regulations require at least two non-return barriers, one of which is a water seal, and maintaining them is among the most important work on the whole system because their failure defeats the primary safety function.
01
The deck water seal
A water-filled chamber that lets inert gas flow forward to the tanks while blocking any reverse flow of cargo vapour. Its water supply must run continuously — even when the system is shut down — which is why the seal is fed by pumps capable of maintaining it and why a low water level triggers an alarm and shutdown of gas delivery to deck. Deck seals come in wet, semi-dry and dry types, and maintenance covers the water supply and its pumps, the level sensors and float arrangements, the internal condition of the chamber, and the drains and heating that keep the seal from freezing or losing water. A seal that runs low is a barrier that has failed silently.
02
The mechanical non-return valve
Fitted in series with the water seal as the second barrier, this valve closes against reverse flow. Its seat, disc and hinge corrode and foul in the gas stream, so it is opened up, inspected and overhauled periodically to confirm it seats cleanly and closes reliably. Because it is the backstop if the water seal is compromised, its condition is checked with the same seriousness as the seal itself.
03
The deck isolating valve and relief
Downstream of the seal, the deck isolating valve separates the production side from the distribution side, and a relief arrangement after the deck seal balances built-up water-seal pressure when the system is shut down. In the event that both the deck seal and the non-return valve fail, this relief vents cargo-side gas to atmosphere rather than letting it track back — a last line of defence that must itself be kept functional.
Schedule every IG task and prove it was done
Inert gas maintenance is unforgiving of the item that slipped — the deck seal check deferred, the analyser calibration overdue, the non-return valve overhaul lost between crews. Marine Inspection turns the maintenance interval reference into scheduled, assigned jobs with photo evidence, tracks the survey and certificate deadlines tied to the system, and keeps a complete audit trail a class or port state surveyor will accept. See how it keeps a safety-critical system provably maintained across the fleet.
4. Pressure and Vacuum Protection
A cargo tank is a large, relatively fragile steel box that must be held under a slight positive pressure of inert gas and protected from both over-pressure, which can rupture it, and vacuum, which can collapse it. A set of devices manages this, and each has defined settings that maintenance must preserve.
Pressure/vacuum (PV) valves
Typically set around 200 mbar overpressure
Fitted to individual cargo tanks, PV valves relieve minor over- and under-pressure during normal breathing as cargo, temperature and operations change the tank volume. Their pallets, seats and springs stick, corrode and lose their set point over time, so they are opened, cleaned, freed and their settings verified. A PV valve stuck shut cannot protect the tank; one stuck open leaks the positive pressure that keeps air out.
PV breaker (mast riser breaker)
Typically set around 230 mbar overpressure
A liquid-filled last-resort device protecting the whole inert gas main against gross over- or under-pressure beyond what the PV valves handle, set slightly higher so it acts only when the primary devices are overwhelmed. Its liquid level and condition must be maintained and its flame arrestor kept clear, since it vents during loading and discharge and must not become an ignition path.
Mast riser
Manual, kept open during loading
Used to relieve pressure and maintain a controlled positive pressure during cargo loading, kept open during loading to avoid over-pressurising the tanks as they fill. Its valve and any flame screen are inspected and kept operable, since it is a routinely used relief path.
Flame arrestors and screens
On all vent paths to atmosphere
Every path where inert gas or cargo vapour can vent to atmosphere carries a flame arrestor or screen to stop an external flame propagating back into the tank. These corrode and clog, and a blocked arrestor also restricts the relief it protects, so cleaning and inspection are essential and a damaged element is renewed rather than patched.
5. Oxygen Analysers and Calibration
The oxygen analyser is the instrument that tells the system whether it is doing its job. Fitted after the blower, it separates the production and distribution parts of the plant, analysing the oxygen content of the gas and shutting the plant down if it exceeds the limit. Because the entire safety case rests on the oxygen number being correct, analyser maintenance and calibration are non-negotiable.
The fixed analyser continuously monitors the oxygen content of the delivered gas, and if it rises above the alarm limit — commonly eight per cent at the delivery point — it alarms and shuts down delivery to the deck. This only protects the ship if the reading is accurate, and analyser sensors drift with age and exposure. Regular calibration against known reference gases — a zero and a span check — is therefore a scheduled requirement, not an occasional nicety, and the analyser is verified before critical operations. A sensor reading low is the dangerous failure mode, because it under-reports the true oxygen and lets the plant deliver gas that is actually out of specification while the panel shows compliance.
Fixed analyser calibration
Calibrate the fixed IG main analyser on its scheduled interval using the correct span gas, checking both zero and span, and record the result. Confirm the reading against a portable instrument periodically as a cross-check, and never rely on a single uncalibrated source for a safety-critical measurement.
Portable oxygen meters
The portable meters used to verify tank atmospheres before entry or hot work are themselves calibrated and bump-tested before use. They are the independent check on the fixed system and the instrument crews stake their lives on during tank work, so their maintenance is treated with the same rigour.
Sensor life and renewal
Oxygen sensors have a finite life and lose accuracy toward its end even with calibration. Track sensor age and renew on the manufacturer's schedule rather than waiting for a calibration to fail, since a sensor that will not hold calibration during a cargo operation is a costly and dangerous time to discover the problem.
Sample lines and filters
The analyser only sees what the sample system delivers. Sample lines, filters, pumps and water traps must be kept clear and leak-free, because a blocked or leaking sample line gives a false reading regardless of how well the sensor is calibrated.
6. Nitrogen Generator Service
On chemical and product tankers, the nitrogen generator has largely replaced flue gas for inerting and blanketing, because it delivers clean, dry, high-purity nitrogen with no soot or water carryover to contaminate sensitive cargoes. It is a different machine from a flue gas plant, and its maintenance centres on one principle: the quality of the nitrogen depends entirely on the quality and cleanliness of the compressed feed air.
Two separation technologies are used. A membrane generator passes compressed air through hollow-fibre membranes that let oxygen and water permeate away faster than nitrogen, leaving a nitrogen-rich stream. A pressure swing adsorption, or PSA, generator passes compressed air through vessels of carbon molecular sieve that adsorb oxygen and carbon dioxide, letting nitrogen pass; the beds cycle between adsorption and regeneration. Both are typically designed around ninety-five per cent nitrogen purity delivered to a deck tank, and both live or die by their feed-air treatment.
STAGE 1
Feed-air compressor
The compressor supplying the generator is the front of the chain, and its maintenance is described by manufacturers as especially important. An oil-injected screw compressor that carries oil over into the air stream will foul membranes or poison molecular sieve, so compressor servicing, oil condition and separation are critical to protecting the expensive separation stage downstream.
STAGE 2
Aftercooler and refrigerated dryer
Compressed air is cooled and dried to strip out water that would otherwise condense and damage the separation stage. The aftercooler, the refrigerated dryer and their automatic condensate drains need routine service, since wet feed air is one of the fastest ways to degrade both membrane and PSA performance.
STAGE 3
Multi-stage pre-filtration
A train of filters — particulate, coalescing and activated-carbon — removes bulk liquids, oil aerosol and fine particulate before the air reaches the membranes or sieve beds. These filter elements are consumables with defined change intervals, and letting them go beyond life is exactly how oil and moisture reach and ruin the separation stage. Filter maintenance is the single most protective routine on the machine.
STAGE 4
Separation stage — membrane or PSA
The membranes or adsorber vessels are the heart of the generator and, if the feed air is kept clean and dry, have few moving parts and need little direct intervention. On PSA units the switching solenoid and check valves that cycle the beds are inspected and renewed as needed; on both types the separation stage is protected primarily by everything upstream of it rather than by servicing the modules themselves.
STAGE 5
Product oxygen analyser
The generator's own oxygen analyser verifies that the product nitrogen meets purity, diverting off-spec gas to waste until purity is reached. Like the IG main analyser it is calibrated on schedule and its sensor is a wear item — often renewed around every two years or per the manufacturer — since it is the gatekeeper on what actually reaches the tanks.
STAGE 6
Buffer tank and distribution
Nitrogen collects in a buffer or deck tank before distribution, buffering demand — essential on PSA systems with their cyclic output. The tank, its relief protection and the distribution valves are maintained as part of the system, closing the chain from air intake to tank delivery.
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The maintenance lesson for nitrogen generators
A nitrogen generator has very few moving parts in its separation stage, so its reliability is decided almost entirely by feed-air quality — compressor condition, drying and filtration. Operators who treat the compressor and filter maintenance as the priority get years of reliable service; those who let oil or moisture through find themselves replacing membranes or molecular sieve, which is expensive and slow. The counterintuitive point is that maintaining a nitrogen generator well means focusing on the unglamorous air-treatment components upstream rather than the separation technology everyone thinks of as the machine.
7. Safety Interlocks and Alarms — Never to Be Defeated
The inert gas system is protected by a set of automatic interlocks that alarm and shut the plant down when a condition threatens gas quality or the integrity of the barrier. These are the system's reflexes, and the single most important maintenance principle around them is that they are tested and kept functional, and never bypassed to keep the plant running through a fault.
Oxygen content high
Alarm and shutdown of gas delivery to deck when O2 exceeds the limit — commonly eight per cent — because gas above the limit no longer protects the tank.
Deck seal low water level
Alarm and shutdown of gas delivery to deck, because a seal that has lost its water can no longer prevent reverse flow of cargo vapour.
Scrubber high water level
Alarm and shutdown of the blower and scrubber, because high water risks carryover into the blower and downstream system.
Low IG deck pressure
Alarm on loss of positive pressure in the deck main, since a tank that loses positive pressure can draw air in and rise toward the flammable range.
Blower failure
Alarm and shutdown, because without the blower there is no gas delivery and no maintained pressure.
Power or control failure
Alarm and shutdown of the blower and scrubber, failing the plant to a safe stopped state rather than an uncontrolled one.
Emergency stop
Manual alarm and shutdown of the blower and scrubber from any emergency stop, giving the crew a hard override at all times.
Testing interlocks is maintenance; defeating them is not
Every one of these trips must be function-tested on a schedule, because an interlock that has quietly failed provides no protection while giving the appearance of it. Testing verifies that the sensor, the logic and the shutdown action all still work together. The opposite — bypassing or inhibiting an interlock to keep the plant running through a nuisance alarm or a known fault — removes the protection the whole system exists to provide, and has featured in serious tanker casualties. If a trip is nuisance-tripping, the fault is diagnosed and fixed, not bridged out. A plant that cannot run without its interlocks defeated is a plant that must not run.
8. Maintenance Interval Reference
The following reference organises the recurring tasks by rough interval. Actual intervals follow the manufacturer's instructions, the ship's planned maintenance system and class survey requirements, and this is a working structure to align them rather than a substitute for the equipment manuals. The principle throughout is that verification tasks — the ones that confirm the barrier is real — are done frequently, and intrusive overhauls on their own longer cycles.
Daily / each use in operation
Verify IG main oxygen content within limits and deck main positive pressure maintained
Confirm deck seal water supply and level, and scrubber cooling water flow
Check blower operation, temperatures and pressures against normal values
Confirm alarms and readings on the control panel are live and normal
Weekly / monthly
Calibrate the fixed oxygen analyser against reference gas; cross-check with portable
Function-test key alarms and shutdown interlocks
Service nitrogen generator feed-air filters and drain condensate traps
Inspect deck seal, non-return valve area and PV devices for obvious defects
Quarterly / periodic
Open, clean and free PV valves and PV breaker; verify set points and liquid level
Clean and inspect flame arrestors and screens on all vent paths
Inspect scrubber internals and nozzles; clean demister; check water side
Service nitrogen generator compressor, aftercooler and refrigerated dryer
Annual / overhaul cycle
Overhaul mechanical non-return valve and deck water seal internals
Overhaul blowers, flue gas isolating valves and deck isolating valve
Renew oxygen sensors per manufacturer life, typically around every two years
Full interlock and shutdown test, and survey items with class attendance as required
Maintaining an inert gas system well comes down to holding two ideas at once: it is an industrial plant with pumps, valves, filters and sensors that wear like any other, and it is a safety barrier whose degradation is often invisible until it is tested by an emergency. The maintenance that keeps it honest is therefore weighted toward verification — calibrating the analyser, testing the interlocks, confirming the deck seal water, checking that the gas leaving the plant is genuinely below the threshold — as much as toward the intrusive overhauls of the scrubber, the seal and the valves. The SOLAS numbers do not move; the five and eight per cent oxygen limits and the positive-pressure requirement are the standard the system exists to meet, and every task in this guide serves the goal of meeting them not just today but on the worst day, when the barrier is all that stands between the cargo and an explosion. The vessels that manage this best are the ones where IG maintenance is planned, scheduled, recorded and evidenced rather than remembered — where the deck seal check and the analyser calibration and the interlock test are never the thing that slipped. To plan and record inert gas system maintenance with full evidence, sensor-life tracking and survey deadlines across your fleet, start a free trial or book a demo.
Frequently Asked Questions
What oxygen level must an inert gas system maintain?
SOLAS requires the system to deliver inert gas with an oxygen content of not more than five per cent by volume in the IG main, and to maintain the cargo tanks under positive pressure with an atmosphere of not more than eight per cent oxygen. The fixed analyser after the blower alarms and shuts down delivery if the oxygen rises above the delivery limit, commonly eight per cent. These thresholds are the core of the safety case, which is why the oxygen analyser and its calibration are treated as non-negotiable maintenance items.
What is the difference between a flue gas plant and a nitrogen generator?
A flue gas plant takes exhaust from the ship's boiler, scrubs and cools it, and delivers it as inert gas — cheap to run but producing gas with soot and moisture that must be cleaned. A nitrogen generator separates nitrogen from compressed air using membranes or pressure swing adsorption, producing clean, dry, high-purity gas with no soot or water carryover, which is why chemical and product tankers favour it for sensitive cargoes. Their maintenance differs accordingly: the flue gas plant centres on the scrubber and water side, the nitrogen generator on feed-air quality.
Why does the deck seal need continuous water?
The deck water seal is a water-filled barrier that lets inert gas flow forward to the cargo tanks while preventing any reverse flow of hydrocarbon vapour back toward the machinery space and its ignition sources. If the water level falls, that barrier is lost, so the seal must be supplied with water continuously even when the system is shut down, and a low water level triggers an alarm and automatic shutdown of gas delivery to deck. Maintaining the water supply, its pumps, the level sensors and the chamber internals is among the most safety-critical work on the system.
How often should the oxygen analyser be calibrated?
The fixed oxygen analyser is calibrated on a regular scheduled interval against reference gases, checking both zero and span, and verified before critical cargo operations, with the exact frequency following the manufacturer's instructions and the ship's procedures. Portable oxygen meters are calibrated and bump-tested before each use. Sensors have a finite life and are renewed on the manufacturer's schedule — often around every two years — because a sensor reading low under-reports the true oxygen and lets the plant appear compliant while delivering out-of-specification gas.
What maintenance does a nitrogen generator need?
The reliability of a nitrogen generator depends almost entirely on the quality of the compressed feed air, so maintenance focuses upstream of the separation stage: servicing the feed-air compressor and controlling oil carryover, maintaining the aftercooler and refrigerated dryer and their condensate drains, and changing the multi-stage pre-filters on schedule. The membranes or PSA adsorber beds have few moving parts and need little direct service if the feed air is kept clean and dry. The product oxygen analyser is calibrated and its sensor renewed periodically, and PSA switching valves are inspected as needed.
What are the PV valve and PV breaker settings?
Pressure/vacuum valves on individual cargo tanks are typically set around 200 mbar overpressure and handle normal tank breathing, while the liquid-filled PV breaker protecting the whole inert gas main is set slightly higher, around 230 mbar overpressure, so it acts only as a last resort when the PV valves are overwhelmed. Maintenance frees and cleans the PV valve pallets and verifies their settings, and maintains the PV breaker's liquid level and its flame arrestor. Exact settings follow the vessel's specific equipment, so the equipment documentation governs.
Can an inert gas interlock be bypassed to keep operating?
No. The interlocks that shut the plant down on high oxygen, low deck seal level, high scrubber level, blower failure, power failure or emergency stop exist to protect the barrier, and bypassing one removes exactly the protection the system is there to provide. Defeating interlocks has featured in serious tanker casualties. If a trip is nuisance-tripping, the correct response is to diagnose and fix the underlying fault, not to bridge the interlock out. The interlocks are function-tested on schedule to confirm they still work, and a plant that cannot run without them defeated must not run.
Which tankers are required to have a fixed inert gas system?
Following amendments to SOLAS, the Fire Safety Systems Code and the International Bulk Chemical Code, a fixed inert gas system is required for tankers of 8,000 tonnes deadweight and over, constructed on or after 1 January 2016, broadening the earlier requirements that applied mainly to larger crude and product carriers. The system must be capable of inerting, purging and gas-freeing tanks and maintaining the required tank atmosphere, and of supplying gas at a rate of at least 125 per cent of the ship's maximum cargo discharge rate. The specific applicability to a given vessel follows its size, type and build date.
Keep the Barrier Real, and Prove It
Marine Inspection turns inert gas maintenance into scheduled, assigned jobs with photo evidence, tracks oxygen-sensor life and the survey and certificate deadlines tied to the system, and keeps a complete audit trail that class and port state surveyors accept. Make sure the deck seal check, the analyser calibration and the interlock test are never the item that slipped — and that a safety-critical system stays provably maintained across every tanker in your fleet.
