Abstract
The dimensioning of pressure relief devices (PRD) for helium cryostats requires detailed knowledge of the pressure increase during incidents. In case of the loss of insulating vacuum (LIV), this is induced by heat input due to deposition of atmospheric air on the surface of the helium vessel. Instead of considering the process dynamics, the dimensioning of PRD according to established standards is based on constant heat flux values, yielding potential oversizing. However, the heat flux depends on the rate of air reaching the cold surface, the thermal capacitance of and the heat transfer resistance inside the helium vessel, which are temperature-/time-dependent.In order to improve the theoretical basis for the dimensioning of cryogenic PRD, this work presents a one-dimensional heat transfer model to calculate the heat flux dynamics based on dominating physical mechanisms. The results are compared with experimental data measured in the cryogenic safety test facility PICARD.
Highlights
The sizing of cryogenic pressure relief devices (PRD) according to established standards is often based on constant heat flux values
In case of the loss of insulating vacuum (LIV), this is induced by heat input due to deposition of atmospheric air on the surface of the helium vessel
Instead of considering the process dynamics, the dimensioning of PRD according to established standards is based on constant heat flux values, yielding potential oversizing
Summary
The sizing of cryogenic PRD according to established standards is often based on constant heat flux values. The resulting deposition heat is transferred through the wall to the helium inside the cryogenic vessel, where it causes an isochoric temperature and pressure increase until the relieving pressure of the PRD is reached. The PRD opens and releases the discharge mass flow rate at constant pressure while the temperature evolution depends on the relieving pressure and the filling level It increases either at supercritical pressure or at subcritical pressure if only gaseous helium is stored in the cryostat. In order to combine the air deposition in the vacuum space, the heat conduction in the cryogenic vessel and the convection in the helium, the time-dependent wall temperature profile on the outer surface of the cryogenic vessel TW is calculated by the one-dimensional transient heat equation dTW dt. A discrete proportional integrate (PI) step-size controller is used as described in [16] in order to overcome the problem of oscillating step-size sequences that typically
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