Common LNG storage tank structures include: vertical LNG tanks, horizontal LNG tanks, vertical mother-daughter tanks, and atmospheric pressure tanks. For large LNG receiving terminals and peak-shaving stations, the mainstream LNG storage tanks are full-containment tanks and membrane tanks. Full-containment tanks (such as 9% Ni steel tanks) employ a double-layer structure: the inner tank is made of cryogenic 9% Ni steel, the outer tank is reinforced concrete, and the space between them is filled with insulation materials such as perlite. Membrane tanks separate the structural, insulation, and airtightness functions; the inner tank is a stainless steel membrane, offering advantages such as steel savings, simple construction, and large effective volume.
Small vertical LNG tanks: Common volumes are 50 cubic meters, 100 cubic meters, 150 cubic meters, and 200 cubic meters. Small horizontal LNG tanks: Common volumes are 60 cubic meters and 100 cubic meters. Vertical mother-daughter tanks refer to an inner tank composed of multiple daughter tanks connected in parallel to meet large-capacity storage needs. Multiple daughter tanks are assembled side-by-side within a large outer tank. The number of daughter tanks is 3-7, generally not exceeding 12. The volume of a single daughter tank should not be too large, typically between 100-150 cubic meters, with a maximum of 250 cubic meters. Common volumes are 1000 cubic meters, 1750 cubic meters, and 2000 cubic meters. Atmospheric pressure storage tanks include large and medium-sized atmospheric pressure LNG storage tanks and extra-large atmospheric pressure LNG storage tanks. Currently, China has built several 220,000 cubic meter LNG storage tanks and is constructing the world's largest 270,000 cubic meter LNG storage tank.
The foundation types for large LNG storage tanks are mainly ground-mounted and elevated. Ground-mounted tanks require bottom heating devices, making maintenance difficult and prone to soil structure damage due to low temperatures, leading to ice crystal formation and soil expansion, posing potential risks to tank operation. Elevated storage tanks, using pile foundations to lift them off the ground, effectively avoid the aforementioned problems.
The storage tank's process system includes a system for handling boil-off gas (BOG). Under normal operating conditions, BOG primarily flows in two ways: firstly, directly into the external pipeline; secondly, via a low-pressure pump into a recondenser, where it is reliquefied with LNG before being exported. In case of abnormal conditions such as tank overpressure, the BOG is sent to the combustion flare.