Calculated Vapour Savings

A.P.I. Publication 2517 (Evaporation Loss from External Floating Roof Tanks, second edition, Feb. 1980) provides a sample calculation for a 100 foot diameter floating roof tank, containing gasoline of 10 psi RVP and fitted with a mechanical shoe primary seal. The calculated total loss from the seal area was 132 bbl / year.

Using similar criteria and applying the site characteristics provided for a particular U.K. terminal:-

Tank diameter = 120 feet
Average windspeed = 12 mph
Vapour pressure = 6 psia
Metal shoe primary seal

Calc. evaporation loss = 233 barrels / year

The same tank fitted with an M.N.P. secondary seal would result in losses of about 11 barrels / year.

Savings = 222 barrels / year

If the primary seal fitted is a vapour mounted foam seal then the loss is about 1,703 barrels / year.
An M.N.P. secondary seal would reduce this to about 598 barrels / year.

Savings = 1,105 barrels / year.