Inerting and Gassing-Up Procedures on Gas Carriers
For any marine engineer or cargo officer working on LNG (Liquefied Natural Gas) or LPG (Liquefied Petroleum Gas) carriers, mastering cargo conditioning procedures is paramount. Among the most critical of these operations are inerting and gassing-up. These processes prevent the formation of explosive mixtures within the cargo tanks and associated piping, ensuring the vessel’s safety during loading, discharging, and dry-docking. This technical guide delves into the physicochemical principles and practical execution of these procedures.
The Principles of Inerting
Inerting is the process of replacing the atmosphere in a cargo tank (usually air or flammable vapor) with an inert gas. The primary goal is to reduce the oxygen concentration below the Limiting Oxygen Concentration (LOC) or the flammable gas concentration below its Lower Explosive Limit (LEL), thereby eliminating the fire triangle.
On modern gas carriers, this is typically achieved using an on-board Inert Gas Generator (IGG) or a Nitrogen Generator. The IGG burns low-sulfur marine gas oil (LSMGO) in a carefully controlled environment to produce a gas mixture primarily composed of nitrogen and carbon dioxide, with an oxygen content typically below 1%.
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Controlling Dew Point and Moisture
A critical aspect of inerting on gas carriers, unlike crude oil tankers, is moisture control. Cryogenic cargoes like LNG (-163°C) and LPG (down to -48°C) will instantly freeze any residual moisture in the tanks, potentially blocking valves, damaging pumps, and compromising structural integrity. Therefore, the inert gas must be thoroughly dried. The IGG system includes a refrigeration unit and desiccant dryers to achieve a dew point of at least -40°C (and often lower for LNG).
The Gassing-Up Procedure
Gassing-up is the process of replacing the inert gas in the cargo tanks with the vapor of the cargo to be loaded. This is a mandatory step before loading liquid cargo because loading liquid directly into an inerted tank would cause the inert gas to freeze (as CO2 freezes at -78°C, which is warmer than LNG) or cause significant contamination of the cargo.
Execution and Safety Controls
The gassing-up process is usually conducted using vapor supplied from the shore terminal or generated on board using a cargo vaporizer. The heavier cargo vapor is introduced at the bottom of the tank, slowly displacing the lighter inert gas out through the top (or vice versa, depending on the relative densities of the gases).
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During this phase, strict monitoring is required:
- Gas Concentration: Continuous sampling using calibrated gas analyzers (e.g., infrared or catalytic sensors) to ensure the inert gas is fully expelled and the cargo vapor concentration is sufficient for cooling down.
- Pressure Control: The tank pressure must be carefully managed to prevent lifting safety relief valves or causing structural damage. The displaced inert gas is safely vented to the atmosphere via the mast riser.
Failure to strictly adhere to inerting and gassing-up protocols can lead to catastrophic consequences. It requires a deep understanding of thermodynamics, gas laws, and the specific operational manual of the vessel’s cargo plant.