Advantages of internal floating roofs
There was a time, 50 years ago, that most hydrocarbon storage tanks had internal floating roofs (IFRs), not because they were an environmental requirement but because they were good for business. In the 1920s the petroleum industry began putting floating roofs on crude oil and petroleum products to conserve product and to prevent fires. For many decades the cost of a floating roof was justified on the basis of lost product from evaporation and risk of fire. The IFR reduces the risk of fire because the vapour in the space between the IFR and the tank roof is normally well below the explosive limit.
Times changed; IFRs are now required by environmental regulations to control hydrocarbon emissions. In most circumstances an IFR is the most cost effective method of controlling vapour emissions. The typical IFR used in the petroleum and ethanol industry is constructed from an aluminum skin and beams, and floats on pontoons. Steel pan type IFRs and steel pontoon IFRs are also used but have a higher initial cost. Aluminum is suitable for ethanol produced in a distillation process such as that which is normally used for fuel grade ethanol. The residual water serves as an inhibitor. Methanol is produced in a chemical process and contains no water and therefore no inhibitor.
The storage of methanol
typically requires either a
steel IFR or a hybrid. A
hybrid uses stainless steel for
the parts in direct contact
with the product. These parts
include the pontoons, rim
plates, legs, and all other
parts that dip into the
product. The deck skin, clamp
beams and parts above the
product can be
manufactured from
aluminum. A hybrid IFR is
more expensive than an
aluminum one but is
typically less expensive than
a steel one.
The IFR, whether steel,
aluminum or a hybrid, does
not prevent loss of product by
holding the contents under
pressure. There is essentially
no pressure differential
between the under side and
top side of the floating roof. An
IFR works by preventing the
circulation of air at the surface
of the product.
