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Safety Considerations

Safety should 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.

Secondary seals 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.

Fig. 8