| GTM-8.4 |
Gas Tanker Manual GTM Inert Gas and Nitrogen Systems |
Form No.: GTM-8.4
Revision: 01 Date: 15 Oct 2024 Issued by: DPA Approved by: MD |
4. INERT GAS & NITROGEN SYSTEMS
4.1 GENERAL
Gas carriers use various forms of inert gas and these are:-
- Inert gas from combustion-type generators
- Nitrogen from shipboard production systems, and
- Pure nitrogen taken from shore.
Chemical / Oxygen-critical gases will require the use of Nitrogen. In general use:-
- Nitrogen for VCM, Butadiene, PO-EO mixes
- Inert Gas for Hydrocarbons.
Reference should always be made to the Cargo Data sheet provided by the Shipper and Appendix A of the ICS publication Tanker Safety Guide – Liquefied Gas.
Shipboard-generated inert gas must not be used for ammonia cargoes.
4.2 INERT GAS GENERATORS
The composition of inert 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 ship's 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.
The testing of the IG system should be included within the vessel’s 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:-
- The whole system should be visually checked before starting up, in particular the deck non-return valves. The start-up of the IG Plant should be subject to a Critical Operations checklist SAF 16.
- 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.
- The scrubber water supply should be started before beginning combustion.
- The gas produced should be vented to the 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.
- The gas quality should be continually monitored while the plant is in use.
- After use, the temporary connections / spool pieces to the cargo system must be disconnected and the flanges blanked securely.
Inert gas activities should be recorded on Form TNK 09 Inert Gas Log.
4.3 INERT GAS COMPOSITIONS
Inert gas is principally used to control 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.
| A Component |
B Inert Gas by Stoichiometric Combustion (by calculation) |
C Flue Gas from Main Boilers |
D Nitrogen by Nitrogen Generator |
|---|---|---|---|
| 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 vapour (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 |
Note: Inert Gas must not be used with Ammonia due to the chemical reaction between it and the CO2 contained in the inert gas.
4.4 SHIPBOARD NITROGEN GENERATORS
The most common system utilised for the production of nitrogen on ships is an air separation process. This system works by separating air into its component gases by passing compressed air over hollow fibre membranes. The membranes divide the air into two streams – one stream essentially nitrogen and the other contains oxygen, CO2 plus some other trace gases.
This equipment will normally produce nitrogen to the following composition:-
- NITROGEN: up to 99.8%
- OXYGEN: less than 0.2%
- CO2: less than 1000 ppm
- CO: less than 1000 ppm
- Dew Point: down to -60°C
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 vessel’s 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:-
- The whole system should be visually checked before starting up. The start-up of the IG Plant should be subject to a Critical Operations checklist SAF 16.
- Temporary connections / spool pieces to the cargo system should be connected.
- The gas produced should be vented to the atmosphere until it’s of sufficiently good quality for use. The gas quality should be continually monitored while the plant is in use.
- After use, the temporary connections / spool pieces to the cargo system must be disconnected and the flanges blanked securely.
Inert gas activities should be recorded on Form TNK 09 Inert Gas Log.
4.5 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.
The process of taking nitrogen from shore and inerting the tanks should be subject to a risk assessment and critical operations checklist.