not be described as a secondary feature of secondary seals, but
secondary seals do certainly reduce the likelihood of ignition in
the rim area and limit the consequences of any accidental fire.
Rim space fires account for the majority of incendive incidents
involving floating roof tanks.
of all types, by reducing the availability of hydrocarbons in the
seal zone, contribute to safety in the floating roof. Category ii)
secondary seals have advantages over category i) secondary seals
in this, as in other respects; however those seals having metal
compression plates present the best safety features of all. We believe
that the best protection is afforded by the seal type where the
metal compression plates are joined together and only the narrow
edge of the sealing gasket could be exposed to fire. Those seals
using polyurethane fabric vapour barriers have also performed well
in fire tests; however it is conceivable that in a fire the vapour
barrier could be consumed.
Testing of secondary
seals in fire simulated conditions has been conducted mainly in
the U.S.A. In Appendix A is a report on the Fig.8 type seal (from
Motherwell Nayler Petroseals). Generally, following ignition of
a gasoline sample beneath the seal, the seal would burp upwards
with the increase of pressure then draw back onto the shell, starving
the flames of oxygen and putting out the fire.
In some cases
clients, after considering the benefits of the secondary seal in
fire prevention, have dispensed with foam pourers. If foam pourers
and foam dams are used however it should be noted that the top of
the foam dam should be higher than the top of the secondary seal
for best protection.