What to check on a Inert Gas Generator (N₂).
Crude and product tankers commonly inert their tanks with flue gas drawn from the boiler or a combustion-type inert gas generator burning marine diesel oil, because the cargo tolerates traces of SOx, CO2 and soot. Chemical tankers carrying IBC Code products cannot take that route: many cargoes react with combustion by-products or simply cannot carry the contamination risk. Instead they generate nitrogen on board by separating it from compressed air, either across hollow-fibre membranes or through pressure swing adsorption (PSA) over carbon molecular sieve beds. The gas produced is inert…
What sets this system apart
Crude and product tankers commonly inert their tanks with flue gas drawn from the boiler or a combustion-type inert gas generator burning marine diesel oil, because the cargo tolerates traces of SOx, CO2 and soot. Chemical tankers carrying IBC Code products cannot take that route: many cargoes react with combustion by-products or simply cannot carry the contamination risk. Instead they generate nitrogen on board by separating it from compressed air, either across hollow-fibre membranes or through pressure swing adsorption (PSA) over carbon molecular sieve beds. The gas produced is inert by being oxygen-depleted, not by combustion chemistry, so it leaves no soot, no acid gas and no moisture load beyond what the air dryer removes upstream.
Main components
Air supply train
A dedicated compressor feeds an air receiver, followed by coalescing pre-filters that strip oil aerosol and particulate before the air reaches the separation stage. Oil carryover is the single biggest threat to membrane life and PSA bed performance, so filter condition is checked far more often than most crews expect.
Separation stage
Membrane units pass dried, filtered air along bundles of hollow fibres; oxygen and water vapour permeate through the fibre wall faster than nitrogen, leaving a nitrogen-enriched stream at the bundle outlet. PSA units instead adsorb oxygen onto carbon molecular sieve in one vessel while a second vessel desorbs under vacuum or reduced pressure, switching beds roughly every one to two minutes.
Control and delivery
An oxygen analyzer on the product stream feeds a purity control valve that vents off-spec gas overboard until the set point is met, and a buffer receiver smooths flow to the tank mains distribution valve.
Selection criteria
| Parameter | Why it matters |
|---|---|
| Flow rate (Nm3/h) | Must match the fastest planned discharge rate plus tank breathing during loading |
| Purity (% O2) | Most inerting duties accept below 8% O2; some reactive cargoes need below 2%, which usually forces a PSA choice over membrane |
| Turndown | Port stays and slow steaming call for low continuous flow without losing purity |
| Footprint and weight | Retrofit projects on existing tonnage are often limited by space more than by required capacity |
Regulations and class
The IBC Code sets out which product groups require tank atmosphere control and to what oxygen level, and the ship's Certificate of Fitness lists the inerting requirement by cargo. Class treats the generator as essential cargo machinery: the oxygen analyzer is proven against a calibration gas at set intervals, and pressure vessels and safety valves in the air and product lines fall under periodic survey.
Typical faults
| Fault | Consequence |
|---|---|
| Oil carryover past a saturated coalescing filter | Membrane fibres foul and purity falls gradually, often unnoticed until the analyzer alarms |
| PSA valve actuator sticking | Bed switching timing drifts, one bed over-adsorbs and product purity swings |
| Oxygen analyzer left uncalibrated | Tank atmosphere is logged as compliant while the actual oxygen content is out of specification |
| Undersized or saturated air dryer | Moisture reaches the membrane or sieve bed and degrades separation efficiency over time |
What to look for in a supplier
- Purity held across the full turndown range, not only at rated flow
- Filter element consumption rate and local availability of spares
- Documented performance on the specific cargo range the vessel trades, since reactive cargoes may need tighter purity than the generator's default setting
- Analyzer type and its calibration gas requirement, since some sensors need a supply the vessel cannot easily source in every port
A membrane system that has sat idle for days often needs close to an hour of purge running before the purity reading stabilises; starting cargo operations straight off a cold start is a common way to load an under-inerted tank without the alarm ever tripping.