Inert Gas System at ship (generally for Tankers) is of very high importance. At normal conditions, inert gas doesn’t react with other substances. It has very less reactivity.
It may be in elemental or compound form. Also, gases having less or very less oxygen content (insufficient for fire), called as inert gas.
Generally, (oxygen < 8 %) in gases or mixture of gases, assumed as inert gas.
Nitrogen and Carbon di-oxide gas are also considered as inert gas because of very less reactivity.
Purpose of inert gas on Ship
Single most important purpose is to prevent fire or explosion. Tankers (Oil/Chemical/Gas) are highly inclined towards the fire risk due to flammable vapours.
To prevent and reduce the risk of fire, inert gas is spread in various tanks. Actually inert gas increases the Lower flammable limit (LFL) and decreases the Upper flammable limit (UFL). If LFL and UFL, then range of flammability will be automatically reduced and tanks will be safer.
Now, for the generation of inert gas, there are several methods available. But considering different tankers and their working conditions, some systems are more commonly used as:
Systems for Inert Gas
- Flue-gas Inert Gas System – (Oil/Gas Tankers)
- PSA Nitrogen Generator – (Chemical/Gas Tankers)
- Membrane Separation Nitrogen Generator – (Chemical/Gas Tankers)
- Oil-fired Inert Gas Generator – (Oil/Chemical Tankers)
- Flue-gas Inert Gas System
Gas source: Exhaust gas of engine or Boiler uptake (flue gas), Flue gas ( < 5% oxygen) already.
Components and its function
- Inert gas isolating valve – just like supplying unit for IG system, valve open means IG system is on and valve close means IG system is off
- Scrubber Unit – It separates sulphur content
- Demister – Absorbs moisture and removes water content
- Inert gas blower (IG Blower) – Boosting speed
- Pressure regulating valve – Gas having high temperature and pressure is recirculated to scrubber unit
- Deck water seal – Prevents back flow of gas
- Mechanical non-return valve – For extra safety, It also prevents back flow of gas
- Deck isolating valve – For isolating deck and engine room supply
- P-V Breaker (Pressure Vacuum Breaker) – It controls the pressure (Control over under-pressurization and over-pressurization)
- Cargo tank isolating valve – For each cargo hold supply
- Mast Riser – It maintains (+) pressure during loading.
As we know, flue gas already has very less oxygen content, now we have to just clean, cool and maintain it uniformly. Flue gas is supplied via isolating valve to the lower part of scrubber. Scrubber unit removes sulphur content and gas passes through demister. Demister removes water content and gas passes through IG Blowers.
Blower boosts speed and pressure of gas and hot gas is taken back to the scrubber unit. Inert gas passes through deck water seal and mechanical non return valve, these prevent back flow of gas. PV breaker controls the pressure of gas and finally gases having less than 5% oxygen, is supplied to the cargo hold.
2. Pressure Swing Adsorption (PSA) Nitrogen Generator
- It’s a technology to separate some undesired gases from the mixture of gases
- Adsorption phenomenon is used (adsorption is the adhesion of molecules to a solid surface)
- For good adsorption process, good adsorbents are used (here we use carbon molecular sieve)
- Adsorption increases at high pressure and decreases at high temperature
Why we separate Nitrogen from air?
It’s simple, Nitrogen is 78 % of air and other are only 22 %. Also, molecular size of nitrogen is large. That’s why we can easily separate nitrogen from air.
PSA Technology: (Operation)
- It uses carbon molecular sieve material inside a vessel.
- Pressurized air is supplied to the vessel containing carbon molecular sieve.
- Oxygen, CO₂ and other molecules from air are captured (adsorbed) in molecular sieve.
- Nitrogen molecules are drawn off into a nitrogen receiving tank.
- Oxygen, CO₂, other molecules are flushed from the molecular sieve by de-pressuring the sieve bed and vessel is ready for new air and process continues.
3. Membrane Separation Nitrogen Generator
- It’s a least expensive method
- High purity & Reliability
- Lower operating and delivering pressure
- Due to difference in partial pressures on internal and external membrane surface, separation occurs
Membrane Technology: (Operation)
- When compressed air is forced into a vessel containing a hollow tube in the middle surrounded by a polymeric hollow fiber.
- These fibers selectively permeate (pass through) oxygen, water vapour and other molecules out of its side walls.
- Nitrogen is allowed to flow through its center as a final product.
- Pressure, flow rate and membrane size affect the nitrogen separation.
4. Oil Fired Inert Gas Generator
- Basic principle of this unit is to convert oxygen content into CO2 and nitrogen content is unaltered.
- Oxygen is converted into CO2 by simply combustion of oil.
- Oil is drawn from storage tank and pumped to main burner and pilot burner by gear pump.
- Air is supplied via root blower and air-fuel are mixed in a correct proportions in an atomizing burner fitted on top of the combustion chamber.
- Oil is burnt in a combustion chamber and combustion gas is passed through scrubber to cool and remove sulphur content.
- Cooling of gas is done by sea water.
- Gas is dried by passing it through either cooler or alumina dryer.
- Non return valves are fitted in series to act same as deck water seal.
- Quality of inert gas depends on quality of fuel and scrubbing process.
Safety Devices in IG System
There must be sufficient safeties for this (IG System) because of its critical nature and importance. Here is the following list of alarms & trips for the safe operation of the system.
- For High Oxygen content
- For Very low oxygen content
- For Low lube oil pressure
- For High temperature of gas
- For scrubber high level and low level
- In PSA system and membrane system, non return valves are there.
- For very low Lube oil pressure
- For very low or no flow scrubber water
- For no flow deck seal water
- For very high/low boiler pressure
Difference between PV-Valve and PV-Breaker
- PV-Valve is fitted on cargo tank and PV-Breaker is fitted in IG system.
- PV-Valve is fitted on each cargo tank but PV-Breaker is only one in IG system.
- PV-Valve doesn’t contain anti-freeze mixture but PV-Breaker contains antifreeze mixture.
- Both are used to prevent over and under pressurization.