What to check on a Flue Gas IG System.
Some tankers carry a separate combustion unit whose only purpose is producing inert gas. A flue gas system instead taps off exhaust gas from the ship's main or auxiliary boiler, which already burns fuel continuously for steam generation, and diverts a portion of that exhaust through the scrubber and deck seal to the cargo tanks. This avoids running a second combustion process purely for inerting, but it means the inert gas supply depends on the boiler being fired at a rate that produces enough flue gas volume and quality —…
What sets a flue gas system apart from a dedicated IG generator
Some tankers carry a separate combustion unit whose only purpose is producing inert gas. A flue gas system instead taps off exhaust gas from the ship's main or auxiliary boiler, which already burns fuel continuously for steam generation, and diverts a portion of that exhaust through the scrubber and deck seal to the cargo tanks. This avoids running a second combustion process purely for inerting, but it means the inert gas supply depends on the boiler being fired at a rate that produces enough flue gas volume and quality — a boiler running light on load may not deliver the gas flow needed during a fast discharge.
Main components
Flue gas isolating and control valves
A dedicated take-off point on the boiler uptake, with isolating valves that route exhaust either up the funnel as normal or through the IG scrubber, depending on whether inerting is in progress. These valves and their actuators are checked for correct sealing, since any leakage path lets flue gas bypass the scrubber or lets tank atmosphere back-leak toward the boiler.
Scrubber and cooling section
Functions as in any IG system, cooling and washing the flue gas with sea water before it goes to the deck seal, but here the gas composition varies more with boiler load and fuel than it would from a dedicated generator burning at a steady, controlled rate.
Non-return / deck seal arrangement
The deck seal and non-return valves prevent any possibility of cargo vapour finding a path back toward the boiler flue system, a scenario the design must positively prevent rather than merely make unlikely.
Selection / operating considerations
- Boiler load profile during cargo operations, since flue gas volume and oxygen content both depend on firing rate and cannot simply be commanded up on demand the way a dedicated generator can.
- Fuel sulphur content, which directly affects the SO2 load the scrubber has to remove and, downstream, the corrosiveness of the gas delivered to the tanks.
- Redundancy — whether the vessel also carries a dedicated inert gas generator for periods when the boiler cannot supply adequate flue gas, such as in port with boilers on minimum fire.
Regulations and class
SOLAS Chapter II-2 requirements for inert gas systems apply equally to flue gas and dedicated generator types, with the same oxygen content limit (generally 5% by volume or less) required at the point of delivery to the tanks. IACS unified requirements address the oxygen and pressure monitoring, alarms and interlocks that must be in place regardless of gas source, and class surveys verify that the flue gas take-off, isolating valves and non-return arrangements function correctly and cannot allow tank atmosphere to migrate back toward the boiler.
Typical faults
| Fault | Consequence |
|---|---|
| Boiler running on minimum fire during slow discharge | Insufficient flue gas volume to maintain tank pressure, risking a vacuum condition |
| Isolating valve seat worn or damaged | Flue gas leaks past the valve into the normal exhaust path, or leaks the wrong way when switching modes |
| High sulphur fuel used without adjusting scrubber water rate | Scrubber cannot fully wash out SO2, delivering more corrosive gas to the tanks |
| Oxygen analyser fouled or uncalibrated | False readings mask an actual oxygen content excursion above the safe limit |
What to look for in a supplier
- Isolating valve and actuator design proven for the specific boiler uptake temperatures and flue gas chemistry involved.
- Oxygen and pressure monitoring instrumentation compatible with the vessel's existing IG control and alarm system.
- Spare parts support for scrubber internals, since these wear at a rate driven by fuel sulphur content and boiler duty cycle, which varies considerably between vessels.
Do not treat the flue gas system as available on demand the way a dedicated generator would be — check actual boiler load against the tank pressure and flow needed for the planned discharge rate before cargo operations start, not after tank pressure begins to fall.