The job of an oil tanker is to transport various types of oil of varying quality, and these oils tend to produce vapours while being loaded for transportation. These vapours can be present even when there is no cargo on board and can cause severe and widespread destruction if they come into contact with oxygen-rich air.
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 pollution of the environment is caused by the smoke produced by the combustion of such vapours. This is where the Inert Gas System comes in to save us from an explosion. With an inert gas plant, this is possible. The ship boiler’s flue gas can also be used for inerting.
The flue gas system cleans 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?
An Inert Gas System (IGS) on a ship is a system used to prevent an explosion in a tanker’s cargo tanks. While pumping out the flammable liquid cargo, an inert gas (often flue gas from ship boilers) is also pumped into the system.
The IGS’s primary function is to prevent the combustion of hydrocarbon gases. Inert gases are used for this 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 denotes the lowest concentration at which the vapours can be ignited, while UEL denotes the highest concentration at which the vapour explodes. All of this is done to reduce the concentration to 10%, which creates an atmosphere in which the hydrocarbons inside the tank can no longer burn. The inert gas safety limit is 5%. As an anti-explosion measure, all tanker ships use the Inert Gas System.
The Inert Gas System And Its Components
The following components are used in the IG system of oil tankers:
Exhaust Gases Source
The inert gas source is the flue gas from the boiler or the main engine exhaust.
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.
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.
Demister
Moisture is absorbed from treated flue gas. Polypropylene is the most common material.
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.
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.
Deck Seal
The deck seal keeps the blower gases from returning to the cargo tanks. Wet deck seals are typically used, as well as a demister, to remove moisture from these gases.
Mechanical Non-Return Valve
It’s a non-return mechanical device that’s installed alongside the deck seal.
Deck Isolating Valve
This valve isolates the engine room completely from the deck.
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.
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.
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.
Safety And Alarm System
It goes without saying that a system of this complexity will require its own safety features to protect itself and its own machinery.
Inert Gas System Maintenance
To ensure proper maintenance and operation of the inert gas system, the deck and engine departments should work closely together.
It is especially important to ensure that non-return barriers, particularly the deck water seal or block and bleed valves, function properly so that there is no possibility of petroleum gas or liquid petroleum passing back into the machinery spaces.
A record of inspection of the inert gas plant, including defects and their rectification, should be kept on board to demonstrate that the inert gas plant is fully operational and in good working order.
Degradation of Inert gas Quality
Tanker crews should be aware of the possibility of inert gas quality degradation within tanks as a result of air being drawn into the tanks as a result of inert gas or cargo system failure. Consider the following:
- If the pressure in the system drops due to temperature changes at night, the inert gas is not promptly replenished.
- Tank apertures are kept open for longer periods of time to allow for tank gauging, sampling, and dipping.
When water is drained from a non-inerted tank, air enters the drainings and may eventually enter inerted tank atmospheres.
If an eductor is used on recirculation to the slop tank, the volume of air entrained in this manner can be especially large. As a result, when liquid is drained to the slop tank, the inert gas quality in all tanks must be closely monitored.
