Action If Stratification Is Suspected

In addition to management of effective mixing, the real problem arises in deciding what to do, if it is suspected that there is still some stratification present in one or more tanks, after filling is completed. If nothing is done, then a rollover will undoubtedly occur, if the suspicion is correct. 

On a tank farm with several storage tanks, one effective, and popular, procedure is to transfer liquid intermittently or continuously from tank to tank via bottom emptying and top spray filling, 

The tank vapour pressure is determined by the rate of production of this additional vapour and can therefore be controlled, in principle, by the rate of liquid transfer. Should the vapour space pressure rise too high, the liquid pumps should be stopped to try and reduce the rate of dissipation of thermal overfill energy, and allow the vapour pressure to fall back. 

On a multi-tank farm, the safest procedure is to carry out tank-to-tank transfers continuously, when the overall rate of vapour generation should become constant. Initially, inter-tank mixing will produce a high rate of evaporation as any thermal overfill introduced during a fill with fresh, dense liquid is dissipated. 

With a single tank in service, the only effective option is to circulate liquid out from the bottom and in through the top fill sprays. Again, the tank vapour pressure rise is proportional to the additional evaporation produced by heat in-leak during the liquid transfer and the rate of dissipation of thermal overfill. It can again, in principle, be controlled by the rate of hquid transfer, and the pumps stopped if the pressure rises out of control. However, the circulation must be restarted, perhaps slowly at first, in order to totally remove the thermal overfill energy that has built up. There is no other option  

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