Monitoring of the inertization of cargo tanks of LNG class vessels

The results of monitoring the inertization of cargo tanks of vessels intended for the transportation of liquefied natural gas are given. It is determined that the mandatory stage of cargo operations in the port of unloading of liquefied natural gas is the inertization of tanks. It is noted that on gas-carrying vessels, the following sources of inert gases can be: flue gases of vessel's auxiliary boilers; gases generated in the inert gas generator during the burning of liquid fuel in them; directly chemically pure inert gas (usually nitrogen). It is also stated that the inertization of cargo tanks is carried out by one of two methods: either dilution of the gas atmosphere (which is the process of mixing two environments), or replacement of the gas atmosphere (in which the gases supplied to the tank form a dividing layer and gradually displace the residual vapors cargo). The stability and integrity of the dividing layer determines the inertization quality of cargo tanks. Control of the state of the dividing layer in the cargo tank is impossible with optical or visual means of control, which is caused by the opaque environment inside the tank. In this regard, it is proposed to determine the integrity of the separating layer, as well as the level at which it is located in the cargo tank, by measuring the concentration of inert gas in the volume of the tank. Research was carried out on a gas carrier with a cargo capacity of 42,563 m3. Inertization of the vessel's cargo tanks was ensured with the help of nitrogen, which was generated by an inert gas generator using the Pressure Swing Adsorption technology. Nitrogen concentration monitoring in the volume of the cargo tank was performed at levels corresponding to 5 %, 20 %, 50 %, 80 % and 95 % of the tank depth. Research on determining the effect of nitrogen pressure entering the cargo tanks for their inertization on the stability and integrity of the dividing layer was carried out in the range of 0.95–1.05 MPa. The duration of the experiment was 210 minutes, fixation of nitrogen concentration values was performed every 30 minutes. As a result, optimal pressure values were established, according to which the inertization is ensured in the minimum time. The critical pressure values at which the separation layer breaks down were also determined.