Inert Gas or IG System on Ships

Oil tankers carry oil of different grades and quality, having the property to produce flammable vapours and gases when loaded for transportation.

Even with no cargo on board, there can be harmful flammable gases present in the hold.

When the vapour produced by an oil cargo is mixed with a certain concentration of air primarily containing oxygen, it can result in an explosion which results in damages to the property, marine pollution and loss of life

For safety against such explosions, an Inert gas system is used onboard. It can be through as a separate inert gas plant or flue gas produced by a ship’s boiler.

What is Inert gas and Inert gas system?

Inert gas system is the most important integrated system for oil tankers for safe operation of the ship.

Inert gas is the gas that contains insufficient oxygen (normally less than 8 %) to suppress the combustion of flammable hydrocarbon gases.

The inert gas system spreads the inert gas over the oil cargo hydrocarbon mixture which increases the lower explosion limit LEL (lower concentration at which the vapours can be ignited), simultaneously decreasing the Higher explosion limit HEL (Higher concentration at which vapour explodes). When the concentration reaches around 10 %, an atmosphere is created inside the tank in which hydrocarbon vapours cannot burn. The concentration of inert gas is kept around 5% as a safety limit.

Components and description of IG system:

The following components are used in a typical inert gas system in oil tankers:

1. Exhaust gases source: inert gas source is taken from exhaust uptakes of boiler or main engine as contains flue gases in it.

2. Inert gas isolating valve: It serves as the supply valve from uptake to the rest of the system isolating both the systems when not in use.

3. Scrubbing tower: Flue gas enters the scrub tower from the bottom and passes through a series of water spray and baffle plates to cool, clean and moist the gases. The SO2 level decreases up to 90% and gas becomes clear of soot.

4. Demister: Normally made of polypropylene, it is used to absorb moisture and water from the treated flue gas.

5. Gas Blower: Normally two types of fan blowers are used, a steam-driven turbine blower for I.G operation and an electrically driven blower for topping up purposes.

6. I.G pressure regulating valve: The pressure within the tanks varies with the property of the oil and atmospheric condition. To control this variation and to avoid overheating of the blower fan, a pressure regulator valve is attached after blower discharge which re-circulates the excess gas back to the scrubbing tower.

7. Deck seal: The purpose of the deck seal is to stop the gases to return back which are coming from the blower to cargo tanks. Normally wet type deck seals are used. A demister is fitted to absorb the moisture carried away by the gases.

8. Mechanical non-return valve: It is an additional non-return mechanical device in line with the deck seal.

9. Deck isolating valve: The engine room system can be isolated fully with the deck system with the help of this valve.

10. Pressure Vacuum (PV) breaker: The PV breaker helps in controlling the over or under pressurization of cargo tanks. The PV breaker vent is fitted with a flame trap to avoid the fire igniting when loading or discharging operation is going on when in port.

11. Cargo tank isolating valves: A vessel has number of cargo holds and each hold is provided with an isolating valve. The valve controls the flow of inert gas to hold and is operated only by a responsible officer in the vessel.

12. Mast riser: Mast riser is used to maintain a positive pressure of inert gas at the time of loading of cargo and during the loading time it is kept open to avoid pressurization of the cargo tank.

13. Safety and alarm system: The Inert gas plant is provided with various safety features to safeguard the tank and its own machinery.

Following are various alarms (with Shutdown) incorporated in the Inert Gas plant on board the ship:

High Level in scrubber leads to alarm and shutdown of blower and scrubber tower
Low-pressure seawater supply (approx. 0.7 bar) to scrubber tower leads to alarm and shutdown of blower
Low pressure seawater supply (approx. 1.5 bar) to deck seal leads to alarm and shutdown of blower
High inert gas temperature (approx. 70 deg C) leads to alarm and shutdown of blower
Low pressure in line after blower (approx. 250mm wg) leads to alarm and shutdown of blower
Oxygen content high (8%) leads to alarm and shutdown of gas delivery to deck
Low level in deck seal leads to alarm and shutdown of gas delivery to deck
Power failure leads to alarm and shutdown of blower and scrubber tower
Emergency stop leads to alarm and shutdown of blower and scrubber tower

Following are various alarms incorporated in the Inert Gas plant:

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

Working of Inert Gas Plant

The basis of inert gas production in the IG plant is the flue gas generated from the ship’s boiler. 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 to ensure the safety of the tank structure and atmosphere.

The system can be divided into two basic groups:

a) A production plant to produce inert gas and deliver it under pressure, by means of blower(s), to the cargo tanks.

b) A distribution system to control the passage of inert gas into the appropriate cargo tanks at the required time.

Brief working procedure

Boiler uptake gases are drawn to the scrubber unit via flue gas isolating valve(s) to the scrubber unit.
In the scrubber unit the gas is cooled, cleaned and dried before being supplied to the tanks.
Motor-driven inert gas blowers supply the treated gas from the scrubber tower to the tanks. They are mounted on rubber vibration absorbers and isolated from the piping by rubber expansion bellows.
Regulation of gas quantity delivered to the deck is taken care of by the gas control valves and the deck pressure is managed by the pressure controller. If the deck pressure is lower than the set point the output signal will be raised to open the valve more, and vice versa if the deck pressure is lower than the set-point. These valves will then work in cooperation to keep both the deck pressure/blower pressure at their respective setpoint without starving or overfeeding the circuit.
Before entering the deck line, the gas passes through the deck water seal which also acts as a non-return valve automatically preventing the back-flow of explosive gases from the cargo tanks.
After the deck seal, the inert gas relief is mounted to balance built-up deck water seal pressure when the system is shut down. In case of a failure of both the deck seal and the non-return valve, the relief valve will vent the gases flowing from the cargo tank into the atmosphere
The oxygen analyser which is fitted after the blower separates the “production” and “distribution” components of the plant and analyzes the oxygen content of the gas and if it is more than 8%, it alarms and shutdowns the plant

10 Points To Consider When Cleaning Cargo Tanks With IG System

It is very important to clean cargo tanks at regular intervals of time to avoid formation of sludge at the bottom, which reduces the cargo space, or in case of product tanker or OBO carrier- to avoid mixing of different cargoes.

The cargo tank cleaning is an important job carried out by ships crew and supervised by experienced on board officers (Master and Chief officer).

Safety of ship’s crew is of prime importance and cleaning a cargo tank requires step-by-step safety measure to avoid any accident and casualty.

Following steps to be taken when cleaning a tank which is supplied by inert gas via I.G generator or by I. G plant connected to ship’s boiler:

1. One Tank At A Time: Ensure only one tank at a time is chosen for tank washing. All crew involved in the operation are briefed about the hazard and safety aspects in a tool box meeting prior carrying out the operation.

The tank under cleaning must be isolated from main inert gas system, all the other tanks, and from any other common venting system

2. Provide Maximum Ventilation: Maximum ventilation output to be concentrated on tank well ahead of cleaning operation.

The ventilation to be arranged such that it provides good air circulation (free flow of air) from one end of the tank to the other. Officer must ensure enclosed space checklist is duly filled and followed. Rescue from the tank in case of any mishap must be planned ahead of the operation with all the equipment ready at the entry point.

3. Allow Atmosphere Test To Check Acceptable Limits: Washing operation should not commence until atmosphere tests are at acceptable limits at various levels of the cargo tank and the vapour content in any part of the tank is below 10% of the lower flammable limit.

4. Flush Tank Bottom With Water: The tank bottom should be flushed with water and later stripped. The piping lines, cargo pump equipment, cross overs and discharge lines also be flushed with water.

Clean, cold seawater can be used for washing. Ensure recirculating system are not used for tank washing

5. Test Tank Atmosphere Continuously: Testing the tank atmosphere to be carried out during washing at regular intervals. If the vapour level rises to 50% of the lower flammable limit, the washing operation must be immediately stopped until the vapour level has fallen to 20% of the lower flammable limit.

6. Check Washing Machine Capacity: If washing machines for tank cleaning are used with capacity exceeding 60m3 / hour, only one machine shall be used at any one time on ship.

7. Use Portable Machines With Utmost Care: If portable machines are used for washing, ensure all hose connection are properly drawn and bonding cables are tested for continuity before introducing them inside the tank. The should not be broken till the machines are out of the tank.

8. Drain Washing Water Properly: The draining of water from the tank should be continued during the washing operation. In the event of build up of wash water, the operation must be stopped and not to be resumed till all the water is stripped from the tank.

9. Avoid Chemical Additives: Avoid the usage of chemical additives

10. Close Deck Openings That Are Not Required: During washing operation all the deck openings, except those necessary for the operation, must be closed during the procedure

10 Safety Precautions To Take While Handling Inert Gas System On Ships

In November 2012, 5 officers on board an LPG tanker died when they “disassembled” a spectacle blind (also called spectacle flange or figure-8 blind used to prevent or start gas flow in a pipe system) without shutting off the inert gas supply, leading to their asphyxiation.

The investigation report said that six officers on board the LPG carrier fainted in the compressor room because of failure to shut off the inert gas supply and to ensure there was no inert gas in the pipe while removing the spectacle blind, leading to the release of inert gas into the compressor room causing asphyxiation.

Inert gas (IG) system is an integral part of cargo operations on tanker ships which must be handled with utmost care considering the hazardous effects of the inert gas on humans.

Officers handling the IG system must consider certain precautions to ensure their safety onboard ships.

1. Ensure Proper Maintenance of Inert Gas Safety Devices is Carried Out

Safety devices on IG systems are used to prevent the backflow of cargo gases to the machinery spaces. It is important that along with the non-return valve, a water seal and a vent is also fitted on the deck main for additional safety.

Sometimes an additional water seal is fitted at the bottom of the scrubber. It is important that these devices are properly maintained at all times.

2. Ensure Adequate Oxygen Level

Oxygen deficiency is extremely hazardous to the human body. It can not only damage the brain but can also lead to death easily.

In case of oxygen deficiency, the mind is likely to become apathetic and complacent and if escape is attempted at this stage, physical exertion will aggravate the weakness of the mind and body.

For this reason, it is necessary to ventilate the cargo tanks thoroughly to ensure that no pockets of oxygen deficiency remain and a steady reading of 21% is obtained at all times.

3. Ensure There are no Combustible Gases

An important point to note is that the inert gas does not affect the toxicity of hydrocarbon gases and thus the latter can be extremely dangerous (as it is flammable).

Gas freeing of tanks must be properly carried out to eliminate possible gas pockets. Any particular compartment must show a reading of Zero or 1% of the lower flammable limit (LFL) with a reliable combustible gas indicator.

4. Remove Toxic Components of Flue Gases

An approved combustible gas indicator should be used to measure the presence of flue gases in the tank. Flue gases contain sulphur dioxide, carbon monoxide and nitrogen which need to be properly measured during the gas freeing process.

After ventilation, the flue gas reading of the tank should be 1% or lower than the LFL along with an oxygen reading of 21%. Ventilation should be continued until a steady reading of 21% oxygen is obtained before entering.

5. Check Tank Pressure

Check the tanker pressure before opening any tank lids, ullage plugs or tank washing openings. Inerted cargo tank pressure must be adequately reduced before opening any tank.

6. Prevent Air From Entering the System

In the event of an inert gas system failing to deliver the required quality and quantity of inert gas, or is not able to maintain a positive pressure in the cargo tanks, action must be taken immediately to prevent air from being drawn into the tanks.

All cargo and ballast discharge from inerted tanks must be stopped, the inert gas deck isolating valve closed, the vent valve between it and the gas pressure regulating valve (if provided) opened, and other immediate actions must be taken to repair the inert gas system.

7. Take Measures to Prevent Electrostatic Ignition

The presence of hydrocarbons in the tanks can be dangerous. If the tank atmosphere contains flue gas, which has small particulate matter containing a small electrostatic charge, there is a possibility of an electrostatic ignition when the oxygen content of the tank rises due to the ingress of air.

Prevent any kind of ingress of air in the tanks.

8. Don’t Start Repair Work Without Gas Freeing

As Inert gas is asphyxiating, a person can quickly become unconscious even if the leakage of the gas has taken in the open air.

Extra precaution is thus required while doing any maintenance/repair work on the IG plant.

It is recommended that the I.G plant is completely gas freed before any work is started. Internal examination of any unit in the I.G. system should be done only after standard procedures for entry into enclosed spaces have been carried out.

9. Beware of Hydrogen Sulphide

When the oxygen content is reduced during the operation of the I.G. system, pyrophoric deposits are formed in the tankers, especially in those carrying sour crude oil.

These deposits along with crude form hydrogen sulphide, which is highly toxic in nature. Pyrophors and hydrogen sulphide formed during a loaded passage can persist even during subsequent ballast passages if they are not properly removed.

10. Ensure Proper Functioning of Blowers

Generally on oil tankers, blowers are used for gas freeing and hence an air inlet (suction from the atmosphere) at the suction side of the blower with blanking arrangement must be provided.

At normal operation, blanking arrangement is to be secured. During gas freeing, it is to be opened and the air is to be supplied by the blower to the tanks.

Construction and Working of an Explosimeter Used on Ships

Explosimeter is device that is used to determine the content of hydrocarbon in the atmosphere of pump room or tank spaces on ships. The scale used in the explosimeter is marked in terms of lower explosive or flammable limit and as a percentage of the lower limit (LEL). The scale may also be marked in parts per million (p.p.m).

Explisometer works on the principle of Wheatstone bridge. The Wheatstone bridge is supplied with a battery and there is no flow of current through the meter when the bridge resistance is balanced. One of the four resistances in the Wheatstone bridge is a hot filament. This resistance is enclosed in a chamber wherein a sample is drawn with the help of flexible tube and aspirator.

The combustion of the atmospheric sample takes place in the chamber in the presence of hot filament. The combustion of the gas drawn from the atmosphere causes an increase in the temperature, which further causes changes in resistance and imbalance in the Wheatstone bridge. Due to this imbalance, current flows through the meter and the reading is calibrated to indicate in percentage of LEL or P.P.M.

A false reading may be obtained if the percentage of oxygen in the sample is low or due to the presence of inert gas. This instrument is designed for detecting vapors in range up to lower flammable limit. False reading may be obtained if the percentage of gas is too high.

The instrument must be tested before use and should be purged enough so that there is no left over atmosphere in the chamber of last test. The sample should be taken from as many place as possible particularly from tank bottom to provide an accurate result.

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