What is Gassing Up on gas carriers?
23 Sep '21, 17:23
The terms “gassing up” and “purging” mean the same in the gas shipping industry. “Purging” is normally used to describe inerting operations on chemical tankers – so “gassing up” will be used here. When inerting is completed, cargo vapour can be introduced safely into the cargo system. Gas up the associated piping systems systematically to ensure distribution of gas through the relevant pipe work.
The most common inert gases, nitrogen and carbon dioxide, cannot be condensed by the ships reliquefaction plant. Both gases are above their respective “critical temperatures” at cargo temperatures.
For this reason, remove inert gas from the cargo tanks and system before loading. Gas up using vapour of the cargo to be loaded and vent the incondensibles. Ensure cargo vapour concentration of at least 90-95% so the reliquefaction plant can operate efficiently. Charterer may request different gas concentrations for specific cargoes.
If high cargo condensing pressures are anticipated, continue gassing up to reach cargo concentrations of 98-99%. Small volumes of nitrogen in the cargo vapour will cause abnormally high cargo condenser pressures and cargo compressor discharge temperatures. This is a particular problem with ethylene cargoes.
During gassing up maintain a slight over-pressure to ensure that air is not drawn into the system, creating a potentially flammable mixture.
Minimise turbulence by controlling the inlet gas pressure and velocity to reduce mixing between the incoming gas and existing tank atmosphere. However, if the operation is prolonged then diffusion will occur between the gas layers. As the interface passes each sample point, a rapid change in gas concentration will be observed.
To avoid condensation cargo tank atmospheres, structures and associated pipe-work must be warmer than the dew-point of the incoming gas. Likewise, condensation can occur if the system pressure rises above the saturation pressure of the incoming vapour.
23 Sep '21, 17:25