Inert gas

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 system

Inert gas is the gas which contains insufficient oxygen (normally less then 8 %) to suppress combustion of flammable hydrocarbon gases.
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 vapors can be ignited), simultaneously decreasing the Higher explosion limit HEL (Higher concentration at which vapor explodes). When the concentration reaches around 10 %, an atmosphere is created inside tank in which hydrocarbon vapors 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 serve 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 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 purpose.
6)      I.G pressure regulating valve: The pressure within the tanks varies with the property of oil and atmospheric condition. To control this variation and to avoid overheating of blower fan, a pressure regulator valve is attached after blower discharge which re-circulates the excess gas back to scrubbing tower.
7)      Deck seal: 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 inline with 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 flame trap to avoid fire to ignite when loading or discharging operation is going on when in port.
11)  Cargo tank isolating valves: A vessel has numbers 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 cargo tank.

safety

Add caption

Definitions:

Inert gas- This is a gas which contains insufficient oxygen to support combustion of hydrocarbons

Inert conditions- This is where a space has had its oxygen content reduced to 8% or less

Inert gas plant- This is a system specially designed to supply cool, clean, pressurised, monitored and controlled inert gas.

Gas freeing- Opposite to inerting and is the replacement of an inert atmosphere with one of fresh air.

The choice of whether an inert gas system is fitted to a vessel is based on the initial cost of installation and maintenance, the planned cargos to be carried and the possibility of the being tainted by the inert gas and the possibility that the inert gas system in itself will introduce a risk. For an example of the latter procedures would have to ensure that the space is well ventillated of the inert gas before a person could enter.
Sources of inert gas
The use of the term Inert Gas is a misnomer in so far as the true inert gasses such as Helium and argon are prohibitively expensive to use.

Similarly the use of semi-inert gasses such as Nitrogen and Carbon-dioxide are too expensive to use on bulk, nitrogen is often seen in use on gas carriers in barrier spaces or for the clearing and inerting of pipelines and pumps.

The gas most commonly used is the exhaust product of combustion.

    This may have three sources
        Exhaust from an internal combustion engine such as a diesel engine or gas turbine. The relatively high Oxygen content in the output must be reduced to make it suitable and this is generally achieved by the use of an afterburner. This is an uncommon system and will not be dealt with currently
        Boilers
        Gas oil or heavy oil powered inert gas generator. This generally takens the form of an insulated combustion space similar in layout to a tank boiler with the insulation taking the place of the water,with rotary cup burner

    Purging- This is the introduction of inert gas into an inerted space to;
        further reduce O2 content
        reduce hydrocarbon level in the inert gas so that air may be introduced without the mix entering the flammable range.

Limits of flammability
Graph showing lower and upper explosive limits and area of combustion for hydrocarbons

Oxygen control
Theoretically any mix with less than 11.5% oxygen will not support combustion, However, for safety the level is reduced to 8% vol. This allows for calibration errors in monitoring equipment as well as any lack of homogeneity in the tanks.

The tank is kept at positive pressure to ensure no ingress of air.( say 100mmwg at the deck ).
Hydrocarbon control
The principle means of ensuring safe operation is the reduction in oxygen, high levels of HC should not effect the safe operation and may in fact aid by producing an over rich atmosphere.

If it is required to gas free then the level of HC must be reduced to prevent the mix entering the flammable range, then the HC level is reduced by purging.
Gas replacement
There are two principle means of gas replacement, these are;

Dilution-The important factors for these is that the vent is situated at the top of the tank and the inlet gas stream must have sufficient velocity to reach the bottom of the tank

Displacement- This requires a stable interface between the heavier and lighter gas, if the replacement gas is heavier it enters at the top with low velocity , the lighter gas is vented up a purge pipe reaching the base of the tank.
General policy of cargo tank atmosphere control.
It is the masters responsibility for keeping a non-explosive atmosphere within the tank, and to ensure all personnel concerned with the operation are well versed.

    To ensure the I.G. system is fulfilling its requirements it is the Chief Officers responsibility to
        Maintain Oxygen content at less than 8% especially when tank cleaning
        Maintain gas pressure at 100 mmWg
        Ensure correct level in Pv breaker
        Blanks and v/v's to be checked before operation
        Ensure no tank is overfilled when loading, ballasting.

The efficacy of the I.G. plant to produce inert gas at less than 8% Oxygen is the chief engineers responsibility.

    The correct operation of the I.G system should allow the following benefits;
        Closed loading procedures
        Reduced cargo discharge times
        Simplification of the tk cleaning procedures
        Reduction of oxygen dependent corrosion
        Limited repair work on hull without the need to gas free

Pyrophoric ignition
In an oxygen deficient atmosphere where there is Hydrogen sulphide present the iron oxide can be reduced to iron sulphide, with the reintroduction of air the iron reconverts to iron oxide with considerable heat and possible incandescence.

Hence, when gas freeing it is important to maintain the mix outside the flammable range.