The potential for Rollover in an Ammonia Storage Tank became a concern after the Lithuania Incident in 1989. Confusing statements have been published and reported in the literature. To help re- duce the confusion, we can define two classes of events. The first class (“thermal overload”) occurs when warm liquid is introduced into a tank containing cold liquid. This will generate large amounts of vapor as the system comes to thermal equilibrium. The second class (“Rollover”) has been reported in LNG tanks where saturated liquid is introduced to the tank, where it can stratify due to density differences associated with composition differences. Normal heat leak occurs, but the associated boil-off does not take place due to the stratification and the effect of the liquid head pressure on the boiling point. After a certain time, the accumulation of heat in the lower layer results in a decrease in density and then the lower layer becomes lighter and suddenly moves upward causing a large movement of the liquid (i.e. a Rollover). The boil-off that had been suppressed for hours or days comes off the liquid in a short period of time and may overwhelm the relief system. Stabilization of these stratified systems (a prerequisite for Rollover) is possible only when compositional differences exist that allow a denser layer to exist. Rollover is not possible in pure ammonia storage tanks.
Small amounts of water are added into pure ammonia to prevent stress corrosion cracking in storage & handling equipment. It is speculated that if the concentration of water is maintained at less than 0.5% (5000 ppm) in the ammonia, the compositional variation is expected to be too small to result in density stratification. Without density stratification, rollover in storage tanks containing anhydrous ammonia with small amounts of water is impossible. This paper will provide a discussion of the rollover and thermal overload phenomena, different causes, and their prevention in design & operation.

This paper was presented at the AICHE Ammonia Safety Conference 2023 in Munich