Inert Gas System (Flue Gas)

What Sets the Flue-Gas Type Apart

Two ways exist to fill a tanker's cargo tanks with an oxygen-starved atmosphere: burn fuel in a dedicated inert gas generator (IGG), or tap exhaust that the propulsion or auxiliary boilers already produce. The flue-gas arrangement is the older and still the dominant choice on crude carriers and large product tankers, because the gas source is free running whenever the boiler is lit. The trade-off is that flue gas quality follows boiler load: light-off, slow-steaming or a badly tuned burner can push oxygen content above the safe limit, so the scrubbing and monitoring train has to work harder than on a purpose-built generator.

Main Components

Scrubber Tower

Raw flue gas enters near the bottom and rises through counter-flow seawater sprays that cool it from several hundred degrees to near ambient and wash out soot and sulphur oxides. A demister pad at the top strips entrained water before the gas leaves for the blowers.

Blowers

Two blowers, each rated for full system capacity, push scrubbed gas through the rest of the line. Running one at a time with the other as standby is normal; both are interlocked to trip on low scrubber water flow or high gas temperature.

Deck Water Seal

A water-filled, semi-dry or dry seal sits on deck and is the last physical barrier stopping hydrocarbon vapour from finding its way back to the engine room through the same piping that supplies inert gas. Low water level in a wet seal is one of the most common causes of a failed gas-free survey follow-up.

Non-Return / Deck Isolating Valve and PV Breaker

A mechanical non-return valve backs up the water seal, and a pressure/vacuum breaker on the mast riser vents the system to atmosphere if tank pressure ever runs outside the design envelope, independent of the individual tank PV valves.

Distribution Main and Branch Valves

From the mast the gas runs aft along the deck main with a branch and isolating valve to each tank, allowing individual tanks to be inerted, purged or gas-freed without disturbing the others.

Selection and Sizing

Capacity is set by the fastest discharge rate the ship can achieve, since inert gas has to replace cargo volume at least as quickly as it leaves the tanks, usually with margin for the fastest pump combination fitted. Typical delivery pressure at the deck main runs around 100-200 mmWG, enough to overcome distribution losses and still leave headroom below the tank PV valve setting. Scrubber sizing depends on boiler exhaust volume and sulphur content of the fuel burned, since higher sulphur fuel demands more seawater flow to hold pH and SO2 carryover within limits.

Regulations and Class

• SOLAS Chapter II-2, Regulation 16 requires a fixed inert gas system on crude oil tankers of 20,000 dwt and above and on product carriers of 8,000 dwt and above.
• Oxygen content in the gas supplied to the tanks must be continuously monitored and recorded, with alarms set to warn before the 8% by volume threshold in the tank atmosphere is approached.
• Class surveys check the deck seal, non-return valve and PV breaker at each renewal survey, and flag testing of the oxygen analyser and blower trips is part of the annual inert gas system verification.

Typical Faults

What to Look for in a Supplier

• Documented scrubber sizing calculation matched to the ship's actual boiler exhaust flow and fuel sulphur range, not a generic package.
• Materials specification for the scrubber shell and internals suited to continuous seawater and acid exposure, since premature corrosion here is a frequent warranty claim.
• Spare parts commonality with the blower and analyser models already carried on sister ships, to avoid a second parts inventory.
• Reference installations on a similar tanker size and trade, since scrubber performance data from a small coaster does not transfer cleanly to a VLCC.

Before opening any tank for inspection, do not trust the deck log alone: walk to the deck seal and confirm the water level and heater status yourself, because a seal that quietly ran dry overnight is the difference between a routine gas-free check and an incident report.