Atmospheric Storage Tanks Excessive Vapor Generation
Excessive Vapor Generation. Excessive vapor generation is the result of a deviation of temperature or routing of products more volatile than the design fluid. For tanks provided with internal heaters, adequate level should be maintained above the surface of the heater so as not to overheat the tank contents and cause vapor generation or reach the autoignition temperature. Adequate venting capacity should be provided for excess vapor generation or coil rupture.
The polymerization of materials in a tank can yield sudden high overpressure combined with elevated temperatures in the tank. In this situation standard pressure relief valves may not be enough, both because very large two-phase flows may be involved, and because solid, polymerized materials may plug the relief valve. In these cases rupture discs with ducting leading to the atmosphere may be used, with the relief effluent being directed to a safe area of the plant. If the polymerization of the tank can yield potentially hazardous materials, the safe area may include an isolation or containment tank or sump for the hazardous material. Additionally, the discharge piping should be anchored, and the pipe elbows braced to counteract the thrust placed on them by a discharge of this type.
Another example of excessive vapor generation occurs in pressure sphere storage tanks where rollover has occurred. This phenomenon is due to stratification of the tank contents causing a dense upper layer which ultimately rolls to the bottom releasing a massive vapor load from the lower (warmer) tank contents. This phenomenon is avoided by carefully keeping the contents mixed and by not loading warm material into the bottom of the sphere using a dip leg. A recent accident attributed to a process similar to rollover is described by Kletz. Warm (+1O0C) ammonia was loaded into the bottom of a tank of refrigerated (-330C) ammonia. Kletz suggests that the warm ammonia rose to the surface, evaporated, and the overpressure overwhelmed the relief valves, resulting in the tank failing. The tank split from top to bottom, releasing 7000 tonnes of liquid ammonia, forming a pool about 2 feet (0.7 m) deep which caught fire; the fire subsequently spread and resulted in loss of life and many injuries.