Inert Gas Systems
Use of Inert Gas
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.
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
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 ).
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.
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
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.
Inert Gas plant
Components of plant
Boiler uptake valve- Provides a take off point for the flue gas, A cleaning arrangement is fitted to prevent soot build up.
Scrubber- Flue gas passes to the scrubber via the uptake valve, here it enters at the bottom via a waterseal and passes up through a series of sea water sprays and baffle plates being cooled and cleaned before exiting via a demister
The water is supplied via the scrubber pump, the sprays reduce the temperature to within 2oC of the sea water temperature, the sulphur dioxide content is reduced 90%, and the gas is clear of soot.
The tower is rubber lined and other parts are made of inconel or glassfibre to protect against the SO2 .The water seal at the bottom is provided by the weir arrangement fitted to the drain system.
The following alarms are fitted;
Fans-Two types of fans are fitted, a steam turbine driven one of sufficient capacity to supply I.G requirements during cargo ops, and an electrically driven unit with sufficient capacity for topping up only
Shut down of the fans occurs due to;
A blank on a stub pipe may be removed to allow the fans to blow fresh air up the I.G. main when gas freeing
Recirculating and regulating valves- The pressure within the tanks is controlled by automatic or manual operation of the regulating v/v, if the demand is low and the regulating v/v nearly closed then the recirculating v/v opens thereby reducing the possibility of the fan overheating by passing the gas back to the scrubber tower.
Oxygen monitor- Fitted just upstream of the regulating v/v and initiates an alarm if oxygen content above 8%.
Deck water seal- The inert gas leaving the engine room to deck passes through the deck water seal whose purpose is to prevent gases from passing back to the engine room from the cargo tanks. A demister is fitted on the outlet side.
The seal is internally rubber lined and a heating coil fitted to allow use in low temperatures. The weir controlled water level has a low level alarm fitted.
As the seal is a primary safety feature it is supplied from the general salt water system when the scrubber pump is not in use. Checks are made on the pipe from the seal to the regulating v/v to ensure no HC prescience which would indicate corrosion damage.
Non-return v/v- Purpose similar to the deck seal
Deck isolating valve-Allows the isolation of the deck system from the engine room system
Pressure/vacuum breaker ( PV)- This is the safety valve for the system and prevents both under and over pressurisation of the tanks. Flame screens are fitted to the PV breaker vent. The PV breaker may be of the mechanical or liquid type. The liquid type is filled with a liquid of the correct specific gravity, such as monoethylene Glycol and water.
Deck distribution system
After the PV breaker the inert gas is led down a main and valveless distributors to all the tanks, excepting the slops tanks whose lines contain isolating valves. On the main there are three PV hi velocity safety vents, the slops tanks are fitted with individual safety vents of same design.
On the for'd end of the main is the for'd pressure release which allows regulation of the inert gas pressure during loading
During loading the ballast is discharged at the same time as the cargo is loaded thereby limiting vapour release to deck, the I.G. plant is kept running in a state of readiness so in the event of sudden cargo stoppage pressure is not lost via the for'd pressure release; this may have to be throttled in to prevent overloading the fans.
This describes the common I.G. main connected to all the tanks, this has the following advantages
- The tanks are always kept at a positive pressure
- The volume of gas acts as a buffer for variations in loading/discharging rates
A disadvantage is that HC's can reenter a purged tank
Gas at inlet
Gas at outlet
5oC above sea water
less than 5%
SO2 & SO3
0.2 - 0.3%
less than 0.02%
12 - 14%
8 mg/m 3
Due to the corrosive nature of the atmosphere in the scrubber and the inside of the scrubber is specially considered and is normally coated with a glass fibre product or similar The scrubber is supplied by two sea water pumps. A small capacity seal water pump which is run on a continuous bases and ensures that the bottom of the scrubber is continuously full to the weir level thereby ensuring that there is a gas seal. A larger capacity spray water pump which is run when the system is in use supplying inert gas. The drains from the scrubber is very acidic and special consideration is given to the pipe run which may be internally coated
There are three types of deck seals in common use;
The seal is keep full using a continuously running seal water pump which may be backed up with a crossover from a secondary system as required. Should the pressure on the downstream side exced the upstream side the water is pushed up the inlet pipe. The height of this pipe ensures that the head pressure generated is greater than either the pressure release valve or any water seals
GAS VENTING SYSTEM
When loading cargo tanks at high rates, steps must be taken to avoid pressure build up which can cause structual damage. This requires the release of the vapours from the tank. This could be by pipes led up the mast with a release point via a flame trap. An alternative as shown provides a high velocity release which carries the gas well clear of the ship. Depending on the pressure, the area between the fixed cone and the orifice plate will change allowing the velocity to be maintained. At high pressures the orifice is forced upwards against the bellows force, thus increasing the flow area. At low pressure the counterweight pulls the orifce downwards thereby reducing the flow area and maintaining the gas velocity