Transport and storage tanks for cryogenic LNG are double wall vacuum insulated vessels to minimize the heat influx from a source in the surroundings – ambient air, solar radiation, fire exposure – into the liquid. A temperature increase of the liquid will result in a pressure increase in the tank. Pressures above the design temperature should be prevented through activation of a safety valve. Furthermore, the strength of the pressurized shell will start to decrease at temperatures above 300°C. Research is focused to study the accident situation, since fire engulfment of the tank is a worst case, low frequency but nevertheless credible scenario. Progressive weakening of the shell strength could lead to a tank rupture.
These phenomena have been studied in detail in this work in order to prove that earlier conservative assumptions about potential external risks associated with storage and transport of LNG were correct. The following 3 research questions were addressed:
- Has a double-walled pressure tank – at least – equivalent heat load resistance to those of single-walled pressure tank fitted with heat resistant coating?
- Can a double-walled pressure tank endure an exposure to radiation intensities of 35 kW/m2 or more without any catastrophic rupture?
- What heat load and duration of exposure are necessary to catastrophically rupture a full size double-walled pressure tank (i.e. time to failure)?
A large scale bon fire test was conducted by TNO in cooperation with the German BAM Institute in Berlin where a double wall vacuum insulated tank (3m3) was exposed for about 2 hours to a heat stress experiment with heat loads of 75 kW/m2. It was observed that the vacuum was lost and the outer tank showed restricted buckling but the perlite insulation maintained its performance and the inner tank filled with 2 m3 liquid nitrogen never at any point showed temperatures above 50°C, well below the temperature of 300°C were the strength of the inner tank steel would start to deteriorate.
It was concluded that the experiment generated a positive answer to the first two research questions. As for the third question, for a full size LNG storage tank (45 m3) with a much larger content, it takes many hours (6-8) before the pressure relief valve would start venting. If this would lead to further catastrophic conditions, very much will depend on local conditions and the actual design of the storage vessel and the design and performance of the safety valve.