Onboard crude oil carriers (over 20,000 tonnes), inert gas is generated using either a flue gas system or by burning kerosene in a dedicated inert gas generator. The inert gas system is used to keep the atmosphere inside cargo tanks or bunkers from becoming explosive.
IG keeps the tank atmosphere’s oxygen content below 8% (on crude carriers, less on product carriers and gas tankers), making any air/hydrocarbon gas mixture in the tank too lean to ignite. IG is most important during discharging and ballast voyages, when there is more hydrocarbon vapour in the tank atmosphere. Inert gas can also be used to purge the tank of the volatile atmosphere prior to gas freeing, which involves replacing the atmosphere with breathable air, or vice versa.
Given the flue gas system draws its energy from boiler exhaust, it is critical that the fuel/air ratio in the boiler burners be properly regulated to produce high-quality inert gas. Too much air would result in an oxygen content higher than 5%, while too much fuel oil would result in dangerous hydrocarbon gas carryover. The scrubber tower filters and cools the flue gas. Various safety devices prevent overpressure, hydrocarbon gas return to the engine room, and the supply of IG with an excessively high oxygen content.
Since gas tankers and product carriers cannot use flue gas systems (because they require IG with an O2 content of 1% or less), inert gas generators are used instead. The inert gas generator is made up of a combustion chamber, a scrubber unit powered by fans, and a refrigeration unit to keep the gas cool.
A drier connected to the system removes moisture from the gas before it reaches the deck. Gas carrier cargo tanks are not inert, but the hold space around them is. This configuration allows the tanks to be kept cool by using a small heel of cargo while the vessel is in ballast while maintaining the explosion protection provided by the inert gas.
Kangrim Repair and maintenance services are also carried out in case issues arise in the IG system.
Working of Inert Gas Plant
The flue gas generated by the ship’s boiler serves as the foundation for inert gas production in the IG plant. To ensure the safety of the tank structure and atmosphere, the high temperature gas mixture from the boiler uptake is treated in an inert gas plant, which cleans, cools, and supplies the inert gas to the individual tanks via PV valves and breakers.
The system is divided into two major categories:
- A manufacturing plant that produces inert gas and delivers it under pressure to cargo tanks via blower(s).
- A distribution system that regulates the flow of inert gas into the appropriate cargo tanks at the appropriate time.
Brief working procedure:
- The flue gas isolating valve(s) to the scrubber unit draw boiler uptake gases to the scrubber unit.
- Before being supplied to the tanks, the gas is cooled, cleaned, and dried in the scrubber unit.
- Motorised inert gas blowers transport the treated gas from the scrubber tower to the tanks. They are supported by rubber vibration absorbers and are separated from the piping by rubber expansion bellows.
- The gas control valves regulate the amount of gas delivered to the deck, while the pressure controller manages the deck pressure. If the deck pressure is lower than the set point, the output signal is raised to open the valve more, and vice versa if the deck pressure is higher. These valves will then cooperate to maintain both the deck pressure and the blower pressure at their respective set points without starving or overfeeding the circuit.
- The gas passes through the deck water seal before entering the deck line, which also acts as a non-return valve, preventing the back-flow of explosive gases from the cargo tanks.
- When the system is turned off, an inert gas relief is installed after the deck seal to balance the built-up deck water seal pressure. The relief valve will vent the gases flowing from the cargo tank into the atmosphere if both the deck seal and the non-return valve fail.
- The oxygen analyser, which is installed after the blower, separates the plant’s “production” and “distribution” components and analyzes the oxygen content of the gas; if it is greater than 8%, the plant is alerted and shuts down.