On a ship, an inert gas system (IGS) is a system used to prevent an explosion in a tanker’s cargo tanks. While pumping out the flammable liquid cargo, an inert gas (sometimes from ship boiler flue gas) is also pumped in.

The job of an oil tanker is to transport various types of oil of varying quality, and these oils tend to emit vapours while being loaded for transportation. Even when there is no cargo on board, these vapours can be present. When these vapours come into contact with oxygen-rich air, they can cause severe and widespread devastation.

The resulting explosion can endanger not only human life and property, but it can also destroy marine life because it occurs in the middle of the sea or ocean. The smoke produced by the combustion of such vapours pollutes the atmosphere. Marine Repair and maintenance services for IGS can help mitigate this problem.

This is where the Inert Gas System comes into play, as it protects us from such an explosion. This can be accomplished with the help of an inert gas plant. The ship boiler’s flue gas can also be used for inerting.

The flue gas system washes and cools the boiler flue gas before delivering it to the cargo tanks during unloading and tank washing. This is sufficient because the flue gas already contains less than 5% oxygen.

How Inert Gas System Works?

The primary function of the Inert Gas System is to prevent the combustion of hydrocarbon gases. Inert gases are used for this purpose because they contain less than 8% oxygen, which is significantly less than what is required for combustion.

The Inert Gas System’s job is to spread the inert gas over the flammable one, the cargo oil, increasing the LEL (Lower Explosive Limit) and decreasing the UEL (Ultra Explosive Limit) (Upper Explosive Limit).

LEL represents the lower concentration at which the vapours can be ignited, and UEL represents the concentration at which the vapour explodes. All this is done to pull the concentration down to 10% which creates an atmosphere where the hydrocarbons inside the tank can no longer burn. The safety limit of inert gas is 5%.

All tanker ships use the Inert Gas System as a safety measure against explosion.

The Inert Gas System and its Components

The following components are used in the IG system of oil tankers:

1) Exhaust Gases Source

As the inert gas source, flue gas from the boiler or main engine exhaust is used.

2) Inert Gas Isolation Valve

When not in use, it acts as a supply valve from uptake to the rest of the system, isolating both systems.

3) Scrubbing Tower

Water sprays and baffle plates cool, clean, and moisten the flue gas as it enters from the bottom. This method reduces SO2 levels by up to 90% while also removing soot.

4) Demister

Demister absorbs moisture from treated flue gas. Polypropylene is the most common material.

5) Gas Blower

There are two kinds of them in general. One is for IG operations, while the other is for topping up. A steam-driven turbine blower is used for the former, while an electricity-powered blower is used for the latter.

6) I.G Pressure Regulating Valve

The pressure within the tank varies depending on the type of oil, its properties, and the atmospheric pressure.

To keep this change under control and to keep the blower fan from overheating, a pressure valve is attached next to the blower discharge and directs excess air back to the scrubbing tower.

7) Deck Seal

The deck seal keeps the blower gases from returning to the cargo tanks. Wet deck seals are typically used, and a demister is used to remove moisture from these gases.

8) Mechanical Non-Return Valve

It is a non-return mechanical device that is installed alongside the deck seal.

9) Deck Isolating Valve

This valve isolates the engine room completely from the deck.

10) Pressure Vacuum (Pv) Breaker

The PV breaker aids in preventing unwelcome changes in cargo pressurisation. It consists of a vent equipped with a flame trap that prevents ignition during port loading and discharging operations.

11) Cargo Tank Isolating Valves

Many cargo holds on a ship are equipped with isolating valves, the operation of which is controlled by the most responsible officers onboard. The valves regulate the flow of inert gas into the hold.

12) Mast Riser

The mast riser’s job is to keep the inert gas pressure positive while the cargo is being loaded. The mast riser is kept open to avoid pressurising the cargo tank during loading.

13) Safety And Alarm System

It goes without saying that a system of this complexity will require safety features to protect itself and its own machinery. This is accomplished through the use of the following alarm and automatic system shutdown mechanisms:

• When a certain level is reached in the scrubber, an alarm is triggered, and the blower and scrubber tower are turned off.

• When the scrubber tower experiences low pressure in the seawater supply, the blower shuts down and the alarm goes off (0.7 bar approximately).

• When the deck seal detects low pressure from the seawater supply (approximately 1.5 bar), the alarm is activated and the blower shuts down.

• If the inert gas temperature exceeds 70°C, an alarm is triggered, and the blower is turned off.

• Low pressure in the line after the blower (approximately 250mmWG) sets off the alarm and causes the blower to shut down.

• If the oxygen content exceeds 8%, an alarm is triggered, and gas delivery to the deck is suspended.

• The alarm is triggered when the level of the deck seal falls below a certain threshold. The blower is turned off, as is the scrubber tower.

• An emergency stop also raises the alarm, and both the blower and the scrubber tower are turned off.

In short, the alarms used in the system are:

• Scrubber low level
• High O2 Content (5%)
• Low O2 Content (1%)
• Deck seal High level
• Low lube oil pressure alarm

Marine Repair and IGS/IGG services are available to address any problems that may arise.

How the Inert Gas System Plant Works

The inert gas produced in the IG plant is derived from the flue gas produced in the ship’s boiler. The hot gas from the boiler is treated inside the inert gas plant, where it is cleaned and cooled. They are then fed into the tanks of individuals via PV valves and breakers. This ensures the tank structure and atmosphere are both safe.

According to their function, the system is divided into two parts:

• A manufacturing facility in charge of producing and delivering inert gas to cargo tanks via blowers.

• A distribution system that regulates the passage of the inert gas ensures that the gas is delivered to the correct cargo tank at the appropriate time.

In Brief: The Working Procedure of an Inert Gas System

1. The boiler gas is drawn into the scrubber via the flue gas isolating valves.
2. The gas is cooled, cleaned, and dried in the scrubber unit before being supplied to the tanks.
3. The scrubber’s treated gas is then blown into the tanks by motor-driven blowers. They are supported by rubber vibration absorbers and are separated from the piping by rubber expansion bellows.
4. The gas control valves control the amount of gas delivered, while the pressure controller controls the deck pressure. If the deck pressure falls below a certain level, the output signal is raised to open the valve slightly more, and if the deck pressure rises, the valve is closed to maintain the correct pressure. With the help of the valves, the system ensures that the circuit is not overfed or underfed during the process.
5. The gas passes through the deck water seal before entering the deck line, which also serves as a non-return valve, preventing the explosive gas from escaping from the cargo tanks.
6. After passing through the deck seal, the inert gas is mounted in order to achieve equilibrium with the deck water seal pressure. The system is shut down here. If both the deck seal and the non-return valve fail, the relief valve allows the tank’s gases to escape into the atmosphere.
7. If the oxygen content of the gas rises above 8% during the separation of the ‘production’ and ‘distribution’ components, the oxygen analyzer detects it, generates an alarm, and the plant is shut down.