Nitrogen Generator
A membrane or pressure swing adsorption plant that strips oxygen from compressed air to produce inert nitrogen on demand, used where flue-gas inert gas is unsuitable, too wet, or too low in purity for the cargo carried.
Read more — Nitrogen Generator explained ▾
What makes this type distinct
A nitrogen generator produces inert gas by separating oxygen from compressed air rather than by scrubbing and cooling boiler or diesel engine flue gas. Two technologies are used aboard ship: hollow-fibre membrane separation, where air is forced through fibres that let oxygen and water vapour pass faster than nitrogen, and pressure swing adsorption (PSA), where compressed air is passed through vessels packed with carbon molecular sieve that adsorbs oxygen under pressure and releases it on depressurisation. Both give a dry, sulphur-free, oil-free gas stream, which is why nitrogen generators are fitted on LNG carriers, chemical tankers and product carriers where flue gas inert gas would introduce moisture, SOx or particulates the cargo or tank coating cannot tolerate.
Main components
Feed air compressor
Oil-free or oil-injected screw compressor supplying air at the pressure the membrane or PSA bed needs, typically 6-10 bar for membrane units and slightly higher for PSA.
Air pre-treatment train
Coalescing filters, an air dryer (refrigerant or desiccant) and a particulate filter protect the membrane or carbon molecular sieve from oil carryover, liquid water and dust, all of which shorten separator life.
Separation module
Membrane bundles or a pair of PSA vessels working in an alternating adsorb/regenerate cycle. PSA needs two vessels minimum so one regenerates while the other produces.
Buffer receiver and analyser
A receiver smooths the pulsed PSA output; an oxygen analyser on the outlet trips a vent valve if purity falls outside the set band before the gas reaches the deck main.
Selection and sizing
Sizing turns on required purity and flow, not on tank volume alone:
- Purity: 95-97% N2 covers general tank blanketing; LNG and ethylene carriers commonly need 99.5% or higher, which favours PSA over membrane.
- Flow rate: sized for the fastest planned discharge or purging rate, in m3/h at the working pressure, plus margin for topping-up losses through tank vents.
- Turndown: membrane units throttle down more smoothly for the low, continuous blanketing demand between cargo operations.
- Redundancy: two smaller trains rather than one large train let maintenance happen without losing inerting capability.
Regulations and class
SOLAS Chapter II-2 Regulation 4.5.5 permits nitrogen generator plants as an inert gas source where the arrangement gives protection equivalent to a flue gas system, subject to class approval of the specific installation. On LNG carriers the IGC Code expects inert gas dry enough not to form hydrates or ice in cargo piping, which is why membrane or PSA plants are the normal choice there rather than flue gas. Class societies survey the plant on the same cycle as other inert gas systems, with functional testing of the oxygen analyser and low-purity trip during annual surveys.
Typical faults
| Fault | Consequence |
|---|---|
| Oil carryover from the feed compressor fouling the membrane or carbon bed | Falling nitrogen purity, increased air consumption, eventual replacement of the affected module |
| Blocked or worn pre-filters | Reduced air flow to the separator, purity drops under load exactly when demand is highest |
| PSA switching valve failure | Loss of the regeneration cycle on one vessel, purity swings, possible automatic shutdown |
| Oxygen analyser drift or fouling | False low-purity trips or, worse, a failure to trip when purity is actually out of spec |
What to look for in a supplier
- Published purity-versus-flow curves at the actual feed pressure the ship's compressor delivers, not best-case laboratory figures.
- Membrane or carbon bed replacement intervals and whether they can be exchanged without a class attendance.
- Compatibility of the analyser and control system with the ship's existing inert gas alarm and monitoring panel.
- Track record on the specific cargo trade, since LNG, chemical and product tanker duty cycles stress the plant differently.
Purity drifts down quietly long before an alarm trips, so log the oxygen reading at the same point in the cargo cycle every voyage and watch the trend, not just the pass/fail.
2 manufacturers · 2 models
Alfa Laval
1
- Membrane fiber degradation from oil/water
- Pre-filter failure
- Flow control valve malfunction
- Compact footprint and lower capital cost than PSA systems
- Oil‑free operation when supplied with clean compressed air
- Continuous on‑line nitrogen production without storage tanks
- Simple control system and relatively low maintenance
- Maximum nitrogen purity limited to ~95–98%, lower than PSA units
- Membrane fibers are sensitive to oil or water contamination, requiring strict pre‑filtering
- Flow capacity is generally lower than PSA generators for the same size unit
- Membrane lifespan can be reduced by frequent exposure to contaminants
Parker (Domnick Hunter)
1- Carbon molecular sieve degradation
- Switching valve failure
- O2 analyzer drift
- Air compressor fault
- Eliminates the need for bulk liquid nitrogen storage and handling on board
- Provides continuous nitrogen supply with real‑time O₂ purity monitoring
- Compact footprint and integration with existing inert gas system controls
- CMS adsorbent cartridges last 10–15 years, reducing long‑term replacement cost
- Reduces deck space and safety hazards associated with cryogenic liquids
- Higher upfront capital cost compared with a simple liquid nitrogen feed system
- Performance depends on reliable power supply and compressor operation
- CMS degradation or switching‑valve failure can cause O₂ purity drift and downtime
- Requires regular pre‑filter changes and periodic O₂ analyzer calibration
- May be undersized for very large tankers with extremely high nitrogen demand