Inert Gas Systems play a critical role in various industrial applications, ensuring safety and efficiency in processes involving volatile materials. These systems generate inert gases to displace oxygen and minimize the risk of combustion. Comprised of advanced components such as combustion systems, cooling units, and pressure control mechanisms, they maintain optimal conditions for operations.
Additionally, the use of measurement and control technologies for oxygen levels and dew points enhances the reliability of these systems. Overall, Inert Gas Systems are essential for protecting facilities and personnel while improving operational integrity.
General
The composition of Inert Gas Generatorinert gas depends on its method of production. On gas carriers inert gas is normally produced by combustion of a fuel in a purpose built inert gas generator but very rarely from the ships main boilers.
The inert gas produced is washed and filtered to remove soluble acid gases and to remove solid particles. It is then cooled and dried and delivered under pressure to the cargo tanks. Some vessels may be equipped to produce high purity Nitrogen by fractional distillation of air or by pressure swing absorption. Chemical gases will require the use of nitrogen gas for inerting purpose.
Note: Inert gas must not be used with Ammonia cargo due to the chemical reaction between it and the CO contained in the inert gas.
In general use:
- Nitrogen for VCM, butadiene, propylene oxide, PO – ethylene oxide mixes.
- Inert gas for hydrocarbon gases.
Inert Gas Composition
Inert gas is principally used to control General Overview of LNG Cargo Tanks (Typical Operations)cargo tank atmospheres and so prevent the formation of flammable mixtures. The primary requirement for an inert gas is low oxygen content. Its composition can, however, be extremely variable as can be seen from the table below which gives an approximate indication of inert gas components as a percentage by volume.
Regarding inerting levels, prior to gassing up a tank should have oxygen content of less than 5 % but sometimes loading terminals requires a lower figure. Prior to aeration, the inerting process should have achieved a hydrocarbon content of less than 2 %.
In addition to oxygen, another essential element regarding inert gas quality is its dryness. Any moisture contained within the gas can condense at the cold cargo temperatures encountered. Therefore, in order to prevent hydrate formation in the products loaded and to prevent serious condensation and corrosion in tanks and hold spaces, inert gas is thoroughly dried as it leaves the generator.
Inert Gas Plant Description
Inert gas generation plant consists following main components.
1 Combustion system.
2 Cooling and scrubber section.
3 Freon cooling unit.
4 Dryer unit.
5 Delivery pressure control.
6 Measurement and control of Oxygen and Dew point.
Combustion system
Inert gas is produced by stoichiometric combustion of fuel oil with air. Fuel oil is hydrocarbon and air contains 79 % nitrogen and 21 % oxygen. The combustion products are mainly 85 % nitrogen, 15 % carbon dioxide, water vapour and traces of oxygen, carbon monoxide and hydrogen.
Combustion air is supplied by Blower (e. g. Roots type blower). The quantity is controlled manually by regulating valve. This valve is to be adjusted so that oxygen content of inert gas is within limits. The automatic ignition sequence starts once the ignition start switch is turned ON. The sequence is as follows:
- 0 sec. – Blower and fuel oil pump starts.
- 60 sec. – Fuel oil supply solenoid valve to ignition pilot burner opens and flame is detected by UV flame detector.
- 85 sec. – Fuel supply solenoid valve to ignition main burner opens and pilot burner ignites main burner.
- 90 sec. – Pilot burner extinguishes. Fuel oil is shut to pilot burner and UV flame detector detects flame of main burner.
- 295 sec. – Inert gas ready for delivery indication will come provided that,
- A) Oxygen content is within limits.
- B) Dew point is below limits.
Cooling and scrubber section
Hot combustion gases are first cooled indirectly in the combustion chamber by seawater flowing in the jacket. The hot gases then pass through scrubber section where the seawater from sprayers ensures intense contact between gas and seawater, reducing gas temperature and also sulphur oxides are washed out.
Demister fitted at the outlet of scrubber prevents water carryover with inert gas. Water seal mounted under the cooling section avoids escape of gas through seawater.
Freon cooling unit
This unit is capable of cooling Inert gas from +35 degrees, 100 % relative humidity to +5 degrees, 100 % relative humidity. The capacity is controlled automatically from 0 % to 100 % to prevent condensing water from freezing. The dew point is brought to +5 degrees at outlet of cooler. The condensate formed is drained via condensate drain system with sight glass and water seal. The demister unit is mounted to remove last water droplets from cooled gas. Freon unit consists of four main parts Evaporator, Pilot receiver, compressor, Liquid separator. Compressor provides the driving force to freon, evaporator takes heat from inert gas, condenser passes heat to seawater.
Dryer unit
Dryer unit consists of heat generated, two vessel, and desiccant dryer. Here Inert gas or air is dehumidified further to require final dew point Starting condition is gas with dew point +5 degrees. The dryer is affected by a desiccant, adsorbing water from inert gas. Two vessels are provided, so that, when adsorption capacity of desiccant is reduced the vessel in use can be changed over to the other one and the vessel not in use can be regenerated by blowing hot air at about 150 degrees. The total time taken for regeneration is about 6 hrs. The regeneration takes place in following stages.
1 Parallel running for few minutes.
2 Hot air flushing.
3 Cooling desiccant to ambient temperature.
4 Purging with cold inert gas.
5 Decompression of vessel that will be regenerated.
Pressure control
Inert gas generator gas discharge pressure is controlled at 300 mbar. The parameters to be controlled for steady gas outlet pressure are combustion air pressure and fuel oil pressure. The generator outlet pressure is measured and amplified by pressure transmitter, whose output signal is fed to pressure controller, where it is compared with set value, and an output signal is fed to actuator for pressure regulating valve.
Read also: Gas Handling Equipment for Efficient Gas Processing
Measurement and control of oxygen and dew point
The regulation requires a continuous check (indication, registration & alarm) of the oxygen content. An analyzer to similar effect is provided. Dew point is continuously indicated, measured and monitored by an analyzer. If oxygen content or dew point goes beyond its set limits a visible and audible alarm is given and the gas produced is vented to the atmosphere. The oxygen analyzer should be calibrated regularly.
| A Component | B Inert Gas by Stoichiometric combustion | C Flue Gas frommain Boilers | D Nitrogen byfractional distillationor by pressure swingabsorption. |
|---|---|---|---|
| Nitrogen (N2) | 85 % | 83 % | 99,9 % |
| Carbon dioxide (CO2) | 14 % | 13 % | 1 ppm |
| Carbon Monoxide (CO) | 0,2 % | Present | 1 ppm |
| Oxygen (O2) | 0,3 % | 4 % | 4 ppm |
| Sulphur dioxide (SO2) | <0,1 % | 300 ppm | – |
| Oxides of nitrogen (NO) | 3 ppm | Present | – |
| Water vapor (H2O) | Present | Present | 5 ppm |
| Ash and soot (C) | Present | Present | – |
| Dew point | – 50 °C | May be high if not Dried | <-70 °C |
| Density (Air = 1,00) | 1,035 | 1,044 | 0,9672 |
Shipboard Nitrogen Generators
Gas carriers may be equipped with plant to strip the CO from the Inert Gas produced by the vessels Inert Gas Plant.
This equipment will normally produce nitrogen to the following composition:
| NITROGEN | Up to 99,8 % |
| OXYGEN | Less than 0,2 |
| CO2 | Less than 1 000 ppm |
| CO | Less than 1 000 ppm |
| Dew Point | Down to -50 °C |
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