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Inert gas, dry air & Nitrogen generator for LNG vessels

Inert gas generator: Inert gas means a gas (e.g. nitrogen) or mixture of gases, containing insufficient oxygen to support combustion. Many LNG vessels are equipped with an inert gas generator which may also be used to produce dry air. The inert gas and/or dry air is used for the inerting and gas freeing of cargo tanks, cargo pipes and void spaces when required prior to and after a refit or inspection period.



The operating principle is based on the combustion of a low sulphur content fuel and the cleaning and drying of the exhaust gases. The inert gas plant includes an inert gas generator, a scrubbing tower unit, two centrifugal fans, an effluent water seal, a fuel injection unit, an intermediate dryer unit (refrigeration type), a final dryer unit (adsorption type) together with an instrumentation / control system.

Inert gas is produced by the combustion of gas oil supplied by the fuel oil pump with air provided by blowers, in the combustion chamber of the inert gas generator. Good combustion is essential for the production of a good quality, soot free, low oxygen inert gas.

The products of the combustion are mainly carbon dioxide, water and small quantities of oxygen, carbon monoxide, sulphur oxides and hydrogen. The nitrogen content is generally unchanged during the combustion process and the inert gas produced consists mainly of 86% nitrogen and 14% carbon dioxide. Initially, the hot combustion gases produced are cooled indirectly in the combustion chamber by a sea water jacket. Thereafter, cooling of the gases mainly occurs in the scrubber section of the generator where the sulphur oxides are washed out. The sea water for the inert gas generator is supplied by one of the ballast pumps.

Before delivery out of the generator, water droplets and trapped moisture are separated from the inert gases by a demister. Further removal of water occurs in the intermediate dryer stage, where the refrigeration unit cools the gas to a temperature of about 5 degree C. The bulk of the water in the gas condenses and is drained away with the gas leaving this stage via a demister. In the final stage, the water is removed by absorption process in a desiccant dryer.

The oxygen level controls the ratio of the air/fuel mixture supplied to the burner. The oxygen content must be below 1% by volume and the inert gas must have an acceptable dewpoint and minimal levels of soot. These minimum levels can be found in the ship specific cargo information book.

The inert gas generator can produce dry-air instead of inert gas with the same capacity. For the production of dry-air:
  1. There is no combustion in the generator.
  2. There is no measure of oxygen content.
  3. The oxygen signal is overridden when the mode selector is on dry-air production
After the processes of cooling and drying and, subject to satisfactory dew point, the dry air is supplied to the cargo system.

The testing of the IG system should be included within the vesselís planned maintenance system along with maintenance routines recommended by the plant manufacturers. Any defects to the IG Plant must be reported to the management office.

The following precautions should be observed:-
  1. The whole system should be visually checked before starting up, in particular the deck non return valves.
  2. The piping system to the vent outlet should be opened to release any pressure and prevent back-flow. Temporary connections / spool pieces to the cargo system should be connected.
  3. The scrubber water supply should be started before beginning combustion.
  4. The gas produced should be vented to atmosphere until it is of sufficiently good quality for use. It is important that the gas is clean. Reducing the oxygen level to very low levels can result in excessive soot being delivered with the IG
  5. The gas quality should be continually monitored while the plant is in use.
  6. After use, the temporary connections / spool pieces to the cargo system must be disconnected and the flanges blanked securely.


Nitrogen generator

Most LNG vessels are equipped with nitrogen generators, installed in the engine room, which produce gaseous nitrogen for:
The operating principle is based on the hollow fibre membranes through which compressed air flows and is separated into oxygen and nitrogen. The oxygen is vented to the atmosphere and the nitrogen is stored in a buffer tank.

The nitrogen generators are equipped with an oxygen analyzer, which continually monitors the oxygen content in the nitrogen output. If the level of oxygen rises above 1% of the design value, then an alarm is activated. If the level of oxygen rises further, the high high alarm operates, redirecting the flow to atmosphere and closing the discharge line to the buffer tank.




It is important to appreciate that the exhaust from the nitrogen plant will be oxygen-rich compared to the surrounding atmosphere.

The testing of the nitrogen system should be included within the vesselís planned maintenance system along with maintenance routines recommended by the plant manufacturers. Any defects to the N2 Plant must be reported to the management office.

The following precautions should be observed:-
  1. The whole system should be visually checked before starting up.
  2. Temporary connections / spool pieces to the cargo system should be connected.
  3. The gas produced should be vented to atmosphere until itís of sufficiently good quality for use. The gas quality should be continually monitored while the plant is in use.
  4. After use, the temporary connections / spool pieces to the cargo system must be disconnected and the flanges blanked securely.

Nitrogen from shore

When vessels fitted with only an inert gas plant are to load oxygen-critical cargoes, supply of pure nitrogen should be taken from shore as the quality of ship-generated inert gas is inadequate. Supply of nitrogen is normally by road tanker or barge in liquid form and therefore a nitrogen vaporiser is needed to inert the cargo tanks prior to loading these cargoes.

Inert Gas Systems

Inert gas has an important role in maintaining safety aboard a gas carrier and the inert gas system should be kept in good working order. Regardless of frequency of use it should be tested regularly to prevent deterioration and enable any faults to be detected and rectified.

The following precautions should be observed:

Below is more guideline for safe cargo operation on board gas carriers

Procedure for safe Cargo handling in Liquefied Gas Carriers

Procedure for commissioning the cargo system

Preparation for Cargo Transfer

Procedure for discussion prior cargo transfer

Procedure for loading Liquefied Gas Cargoes

Procedure for Cargo Conditioning in Liquefied Gas Carriers

Cargo Transfer between Vessels (STS Operation)

Procedure for segregation of Liquefied Gas Cargoes

Procedure for Stripping Liquefied Gas Cargoes

Procedure for Changing Liquefied Gas Cargoes

Displacing Atmosphere with Inert Gas (Inerting)

Displacing with Vapour of the Next Cargo (Purging)

Procedure for Water washing after Ammonia Cargoes






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