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Pressure Build-up by Evaporation

Bild eines Geysires der gerade Wasser ausstößt
Cryogen liquids do expand by a factor of 500 to 1500 when evaporated and warmed up to room temperature. Locking in of cryogen liquids or elevated heat input can therefore lead to significant pressure build-up and may in consequence lead to the burst of the cryogen container. Possible reasons for an elevated heat input are:
 
  • fast cooldown of components or cryogenic installations,
  • large heat production within the object to be cooled e.g. during a quench (>>),
  • loss of the insulation vacuum,
  • thermoacoustic oscillations (>>),


 

Further mechanisms which can lead to a pressure increase:
  • boiling retardation
    In very clean vessels boiling may not start but at temperatures above the boiling point. In this case the boiling can be from violent to explosion like. Boiling retardation can be prevented by introducing porous material, e.g. ceramics, as boiling nuclei.
  • stratification
    If the cryogen in a large tank is not disturbed for some time, a temperature stratification may occur. The stratification causes a larger pressure rise than expected due to the elevated temperature in the liquid surface layer. [12].
  • roll-over in LNG (liquefied natural gas) tanks
    In large LNG tanks independent horizontal convection cells can develop. The heat introduced via the tank walls leads in the lower layers to an expansion of the liquid and in the topmost layer to the evaporation of lighter fractions. Thereby the density of the topmost layer increases and the density of all other layers decreases. This effect can lead to a sudden roll-over and mixing of the layers. Due to the fact that the lower layers are overheated, the roll-over goes along with violent boiling [13].
  • the release of cryopumped gas
    Gas leaking into the insulation vacuum of a cryostat can be condensed by the cold surfaces and accumulate. Upon eventual warmup of the cryostat the desorbing gas can create a pressure in the interspace which can both damage the internal and the external vessels.