The flash calculation problem for cryogenic vessels
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Hello everyone! I work on the design of cryogenic vessels, and in my job I encounter flash calculation problems related to the following two operating conditions; I hope to get some help from everyone. 1. Exhaust steam loss: The tank contains a highly saturated liquid; by opening the vapor phase valve, the vapor pressure is reduced to a certain level, and the mass of steam that needs to be exhausted is calculated. 2. Pressure drop during discharge: The storage tank contains a highly saturated liquid; when the liquid phase valve is opened without any self-pressurization to discharge a certain amount of liquid, the corresponding pressure drop is calculated. The difference between these two operating conditions and the flashing that occurs in a flash tank is that the temperature of the steam or liquid discharged during the process decreases gradually; as a result, both the gas phase pressure in the storage tank and the specific enthalpy of the medium also decrease gradually, which makes it impossible to calculate directly based on the constancy of enthalpy. My own approach is to divide the emission process into multiple stages, and carry out approximate calculations by assuming that the enthalpy remains constant in each stage; the specific enthalpy of the emitted medium is taken as the average value at the beginning and end of the corresponding stage. I’m not sure if this method is valid.
It is closely related to speed and the degree of adiabaticity. Take a liquid nitrogen tank as an example: to reduce the pressure of gas from 1 Mpa to 0.5 Mpa, it may be necessary to release all the gas in the tank, or it might only be necessary to release a portion of it. By releasing some liquid nitrogen, the gas-phase pressure may remain unchanged
Ignore the external heat input during the discharge process. What you’re describing is actually related to the temperature of the medium. If the medium remains saturated, then during pressure relief the liquid will flash and cool down, requiring the removal of a large amount of gas in order to reduce the pressure ； If the liquid is subcooled and the pressure in the gas phase is generated by the vaporizer heating the liquid, then when pressure is released only the excess gas is discharged; the liquid does not flash, and as a result the amount of fluid released is very small.