Flue Gas Inert Gas Generator

What defines this type

Where a dedicated inert gas generator burns fuel in its own combustion chamber purely to make inert gas, a flue gas system takes a slipstream from the ship's existing auxiliary or exhaust gas boiler uptake and conditions it instead. This only works while a boiler is actually firing, which is normally the case during cargo operations on a tanker running its boiler for heating and steam services anyway. The gas arrives hot, wet and carrying soot and sulphur compounds from combustion, so the scrubbing and cooling stage does more work in this design than in a dedicated generator burning cleaner fuel under controlled conditions.

Main components

• Uptake takeoff and isolating valve – draws flue gas from the boiler uptake ahead of the funnel, with a valve to shut off the takeoff when inert gas is not needed.
• Scrubber tower – seawater spray strips soot and sulphur dioxide from the gas and cools it from boiler uptake temperature down to near seawater temperature.
• Demister – removes entrained water droplets before the gas reaches the blower.
• Gas blower – raises the cooled, scrubbed gas to the pressure needed to reach the cargo tanks against the resistance of the deck main and deck seal.
• Deck water seal – a water-filled non-return seal preventing any flammable cargo vapour from finding its way back to the machinery space through the inert gas main.
• Pressure/vacuum breaker – protects tanks from over- or under-pressure if the supply is interrupted.
• Oxygen analyser – continuously monitors and records oxygen content in the delivered gas.

Selection and sizing

Sizing is driven by the tank main's demand during the fastest planned cargo operation, not by the boiler's own steam output:

• Gas delivery rate matched to the vessel's maximum planned cargo discharge rate, since tanks must be replenished with inert gas as fast as cargo leaves them.
• Achievable oxygen content at the design flow rate, which depends on scrubber efficiency, not just blower capacity.
• Blower discharge pressure margin over deck main resistance and the deck seal's own back-pressure.
• Compatibility with the specific boiler's flue gas composition and turndown range, since a boiler running at low fire produces different flue gas conditions than one at full load.

Regulations

Tankers required to carry an inert gas system must maintain the delivered gas at no more than roughly 5% oxygen by volume under SOLAS requirements for tanker inert gas systems, with continuous monitoring and recording of oxygen content and pressure. The deck water seal and non-return arrangements are there specifically to prevent flashback into the machinery space, a requirement class societies check closely at survey given the consequence of failure.