The API 650 gives the range as 0.8 x tank diameter up to a maximum of 1.2 x tank diameter.
This involves the use of spherically-pressed plates, which are expensive to produce. This type of roof is usually confined to small, high pressure tanks, or for tanks where
internal linings and an internal corrosion allowance or stainless steel materials are required.
☐ Radial Rafter Support
This structure consists of a series of curved radial steel beam sections connected to the shell at their outer end and to a center crown ring at the center of the tank. A series of circumferential rings provide
lateral support for the beams and cross bracing in the plane of the roof is provided in some bays to give the structure torsional stability. This type of roof can be used in all sizes of tank and has an advantage over the truss
type of structure when dealing with tanks over say 50 meters in diameter where the truss type structure becomes quite massive.
There is a further advantage because, unlike the truss type structure, the domed structure is completely clear of the stored product. Also, if an internal floating cover is to be installed in the tank, there is no loss of tank capacity.
One disadvantage is that this type of roof is not frangible and therefore if frangibility is a desirable feature then it can not be used.
☐ Externally-Framed Support
This again consists of a supporting structure composed of a series of curved radial rafters. In this case the roof sheeting is attached to the underside of the supporting rafters. This type of arrangement is ideal for internally-lined or stainless steel tanks, which can have a carbon steel external structure.
☐ Geodesic Dome Roofs
This type of roof is a fully triangulated, spherical, space frame structure, generally designed to be self-supporting from it's periphery with an integral peripheral tension ring to take the horizontal forces. They are usually constructed in reinforeced plastic or aluminium.
They are particularly suited to water and wastewater applications where their corrosion resistant properties are a distinct advantage, also these relatively lightweight structures lend themselves to being retrofitted to existing tanks for the containment of vapor, gasses and odours, as they can be erected along side a tank
and lifted into position in one piece.
They are also used in the petrochemical industry, again for the containment of vapors or as weatherproof covers for floating roof tanks containing moisture sensitive products.