Analysis of Vapor Insulation System for Cargo Tank of LNG and LH2 Carrier

Abstract

For storage and transport of LNG or LH 2 (liquefied hydrogen gas) it is of great importance to control the loss of boil off gas, and the thermal insulation and the effective utilization of the boil off gas are here playing an important roll. Regarding the reduction of the evaporation quantity of the cargo, increasing the insulation thickness is probably the most common solutions chosen. But due to the fact that there might be limited space available in hold space, other method could be preferable. The authors, however, have directed their focus of engineering on a recovery of cold energy imposed on the evaporated vapor and tried to take advantage of the boil off gas which is a result of heat passing through the protective thermal insulation from the surrounding atmosphere. This theory of Vapor insulation system was applied to the spherical tanks of LNG carrier and LH 2 carrier. Cold nitrogen gas which was heat-exchanged with the boiled off LNG or LH 2 vapor was introduced into the annular space around the tank, in which the gas flows upward along the tank surface bringing the heat absorbed from the hold space out of the cargo containment system before reaching the valuable liquid cargo. In order to optimize the Vapor insulation system, the effect of several important parameters such as gas flow quantity, gas temperature and location of gas passage and others were analyzed carefully and realistic examples of design were proposed.