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Number of Tanks
Once the total storage capacity is established the number of tanks should be decided. In larger terminals, where a single tank is not sufficient, the minimum number of tanks can be determined based on the total storage volume and the maximum capacity of a single tank. The latter number, however, is not fixed, and will depend on the type of containment, type of construction and applicable codes. This is a typical instance where the LNG storage configuration depends on several inter-related factors.
Most of the LNG tanks in service have capacities of 100,000 cubic meters or less. Aboveground tanks with an inner metal wall have been built for capacity as high as 140,000 cubic meters. With today's technology it should be possible to build these up to 200,000 cubic meters capacity. However, the maximum capacity may be limited depending on the applicable codes and hydrostatic test requirements. Below-ground tanks using the membrane type design with reinforcing concrete have been built for capacities as high as 200,000 cubic meters. Above-ground tanks with concrete inner and outer walls have been proposed for 250,000 cubic meters capacity.
A typical grassroots terminal with 2.5 million tpa capacity might require 250,000 to 300,000 cubic meters of storage. This can be accomplished with two to three LNG tanks.
For smaller terminals, with a small overall storage requirement, a single-tank may be an option to consider. Despite the excellent record of reliability with LNG tanks many owners might prefer two smaller sized tanks instead of a single large tank. The baseload nature of the facility and the implications of a long-term take or pay contract often favor multiple tanks. For example, the Fukuoka terminal in Japan, commissioned in 1993, is designed for annual sendout of 0.15 to 0.36 million tpa. This small terminal has two LNG tanks, each with a capacity of 35,000 cubic meters.
In specifying storage tank capacity it is important to remember that the "usable" volume in the tank is less than the built up volume. The minimum level to which the LNG in the tank can be lowered to will be limited by the LNG pumps' ability. Similarly, to avoid tank overfill it will be necessary to limit the maximum fill level to less than the full height of liquid container. The ratio of usable volume to built-up volume will depend on the tank height, the pumpout arrangement, the LNG pump characteristics, and the instrumentation/control philosophy. Typically, only about 95% of the volume is usable.