Effect of Sub-cooling Difference Between Layers

This problem has been widely experienced, when any stratified liquid is taken from a pressurised VI tank. One quick answer is to raise the tank pressure by feeding back from the pump exit into the vapour space. 

3 Beware of Pressure Cycling Non-condensing Purge Gas to Empty a Tank 

The pressure cycling of a non-condensing purge gas, such as nitrogen or helium, to completely empty a tank of residual LNG for maintenance purposes, should be approached with caution. 

The non-condensing gas will tend to reduce the partial-pressure of residual natural gas vapour below the saturation pressure of any residual LNG liquid left in the tank. In addition, the purge gas may dissolve in the surface layers of the residual liquid, creating an impedance and restriction to the required surface evaporation. 

The liquid is then superheated, with respect to its partial pressure and temperature, and can boil explosively as a QHN event. Such an event may cause mechanical damage to the tank and fittings. 

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