The selection of the type of LNG storage tank for your project should always include the consideration of the following:
- site specifics
- local regulatory requirements
- risk of impact on outer tank walls
- economics of number of tanks and tank size
- tank foundation heating requirements
- prior operating experience
LNG storage includes several design types, such as spherical metal tanks, buried concrete tanks, and in-ground frozen storage. The double-wall type is probably the most commonly used design variation today. The double-wall design is utilized for a wide variety of free standing, cylindrical storage vessels, commonly referred to as double containment vessels.
Industry experience with the double-wall LNG tank design can be summarized with the following economic and technical advantages:
- No geographic limitation for location of storage unit
- Controlled heat influx, allowing low heat influx by design.
- Long life and low maintenance
- Proved by hydrostatic and pneumatic testing to overload conditions.
- Proven technology through years of operating experience.
- Complete accessibility for inspection or for removal from service for modification.
- No contamination of stored product.
- Rapid and inexpensive cool-down (or warmup).
- Fixed costs with all costs known before selection and installation; predictable construction schedule.
- Minimum technical and economic risk due to well established material properties and quality assurance procedures.
Double-wall LNG tanks are generally flat-bottomed tanks, cylindrical in shape and have a domed roof. The outer tank consists of a shell made of carbon steel (or concrete) and acts as a vapor/moisture barrier and container of insulation. The inner tank is manufactured from a material suitable for cryogenic temperatures, typically aluminum or nickel steel. An insulation deck is suspended from the dome of the outer tank over the top of the inner tank. An insulation layer of expanded inorganic material separates the two tanks. The inner tank is separated from the conventional, pile supported, concrete foundation by a rigid insulating material, such as foamglass.
Careful consideration must be given to the LNG tank internal components, including the facilities to allow adequate liquid loading (top or bottom), purge, and cool-down devices. Also, there are advantages to having the pump located internally, submerged in the LNG, as opposed to locating it out-of-tank. Tanks are designed with the nozzle connections located on the top of the tank where possible in order to avoid penetrating the tank below the liquid level.