Inert Gas (IG) System on Ships: Importance and working system

Inert Gas System: Oil tankers transport many grades and qualities of oil, some of which tend to release combustible gasses and vapours during loading.

The hold may contain dangerous flammable gases even when there is no cargo on board.

An explosion that causes property damage, marine pollution, and fatalities can occur when the vapor from an oil cargo is combined with a specific amount of oxygen-dominated air.

Onboard, an inert gas system protects against these kinds of explosions. It can be used as flue gas from a ship’s boiler or as a stand-alone inert gas plant.

Inert Gas System: what are they?

Inert Gas System
(Credit: Knowledge of sea)

The most crucial integrated system for the safe operation of oil tankers is the inert gas system.

Gases with insufficient oxygen (often less than 8%) to prevent the combustion of combustible hydrocarbon gases are known as inert gases.

By dispersing the inert gas over the hydrocarbon mixture that makes up the oil cargo, the inert gas system lowers the higher explosion limit (HEL) and raises the lower explosion limit (LEL) (lowest concentration at which the vapours can catch fire). Hydrocarbon vapors cannot burn in the atmosphere formed inside the tank until the concentration exceeds about 10%. As a safety precaution, the inert gas content is maintained at or below 5%.

What are Barges? Essential Facts About These Floating Freight Transporters

Parts and explanation of Inert Gas System

An oil tanker’s standard inert gas system consists of the following parts:

  • Source of exhaust gases: since it contains flue gases, inert gas is obtained from the exhaust uptakes of the primary engine or boiler.
  • The inert gas isolating valve isolates both systems while not in use by acting as a supply valve from uptake to the remainder of the system.
  • Scrubbing tower: To cool, clean, and moisten the gases, flue gas enters the scrub tower from the bottom and travels through a number of baffle plates and water spray. Up to 90% of the SO2 level is removed, and the gas clears of soot.
  • Demister: Usually constructed of polypropylene, it draws water and moisture from the flue gas that has been treated.
  • Gas Blower: Typically, two kinds of fan blowers are utilized: an electrically powered blower for topping up and a steam-driven turbine blower for I.G. operation.
  • I.G. pressure regulating valve: The oil’s characteristics and the surrounding air quality affect the pressure inside the tanks. A pressure regulator valve is connected after the blower discharge to manage this variance and prevent the blower fan from overheating. This valve recirculates the excess gas back to the scrubbing tower.
  • Deck seal: The deck seal’s function is to prevent gasses from the blower from returning to the cargo tanks. Deck seals of the wet type are typically employed. To collect the moisture that the gases carry away, a demister is installed.
  • Mechanical non-return valve: This is an extra mechanical non-return device that runs parallel to the deck seal.
  • Deck isolating valve: This valve allows the engine room system to be completely isolated from the deck system.
  • ┬áPressure Vacuum (PV) breaker: This device assists in managing cargo tanks’ excessive or insufficient pressurization. In order to prevent fires from starting during loading or unloading operations while in port, the PV breaker vent is equipped with a flame trap.
  • Isolating valves for cargo tanks: A ship contains a number of cargo holds, and each hold has an isolating valve. Only a responsible officer within the vessel may operate the valve, which regulates the flow of inert gas to hold.
  • Mast riser: During cargo loading, the mast riser is left open to prevent pressurization of the cargo tank and is utilized to maintain a positive pressure of inert gas.
  • Safety and alarm system: To protect the tank and its own equipment, the inert gas plant is outfitted with a number of safety measures.

These are a few of the alerts (with shutdown) built into the ship’s inert gas plant:

  • Alarm and shutdown of the blower and scrubber tower are caused by a high level in the scrubber.
  • When seawater is supplied to the scrubber tower at low pressure (around 0.7 bar), the alert sounds and the blower is turned off.
  • Low pressure seawater supply to the deck seal (around 1.5 bar) triggers an alarm, which causes the blower to shut off.
  • Around 70 degrees Celsius of inert gas temperature causes an alert to sound and the blower to stop off.
  • Low pressure in the pipe following the blower (about 250 mm Wg) causes the blower to shut down and sound an alert.
  • High oxygen level (8%), which triggers an alert and stops the gas supply to the deck
  • Low deck seal level triggers an alarm and stops the deck’s gas supply.
  • An alarm and the blower and scrubber tower shutting down result from a power outage.
  • Alarm and blower and scrubber tower shutdown result from an emergency halt.

The inert gas plant is equipped with the following alarms:

  • Low level scrubber Deck seal elevated
  • Low Percentage of O2
  • Elevated O2 Concentration (5%)
  • Alarm for low lube oil pressure

Working of Inert Gas Plant

Working of Inert Gas Plant
(Credit: Marine insight)

The foundation for the IG plant’s inert gas manufacturing process is the flue gas that the ship’s boiler produces. In order to preserve the environment and tank structure, the high-temperature gas mixture from the boiler uptake is cleaned, cooled, and supplied to each tank via PV valves and breakers in an inert gas plant.

What is Emergency Position Indicating Radio Beacon (EPIRB)?

Two primary groupings can be identified within the system:

  1. A production facility to create inert gas and transport it to the cargo tanks under pressure using one or more blowers.
  2. A distribution system that regulates the flow of inert gas into the proper cargo tanks at the necessary intervals.

A brief operational process

  • Through one or more flue gas isolating valves, boiler uptake gases are drawn into the scrubber unit.
  • Before being fed to the tanks, the gas is cleaned, dried, and cooled in the scrubber unit.
  • The treated gas is sent from the scrubber tower to the tanks via motor-driven inert gas blowers. Rubber expansion bellows isolate them from the piping, and they are installed on rubber vibration absorbers.
  • The pressure controller controls the deck pressure, while the gas control valves regulate the amount of gas fed to the deck. The output signal will be increased to open the valve more if the deck pressure is lower than the set point, and vice versa if the deck pressure is higher than the set point. Then, these valves will cooperate to maintain the blower pressure and deck pressure at their respective setpoints without overfeeding or starving the circuit.
  • The gas travels through the deck water seal before entering the deck line. This seal also serves as an automatic non-return valve, stopping the backflow of explosive gases from the cargo tanks.
  • The inert gas relief is installed after the deck seal to balance the pressure that has built up for the deck water seal when the system is turned off. Should the deck seal and non-return valve both fail, the cargo tank’s gasses will be released into the atmosphere via the relief valve.
  • After the blower divides the plant’s “production” and “distribution” components, an oxygen analyser is installed. It measures the oxygen content of the gas and, if it is higher than 8%, triggers an alert and shuts down the plant.

Leave a Comment

Your email address will not be published. Required fields are marked *

Scroll to Top