The invention relates to a method and a device for monitoring the filling of a tank with a liquid gas.
Liquid-gas tanks, or cryotanks, have an insulated storage container in which liquefied, deep-frozen gas can be stored in a pressurized state. Examples of liquid compressed gases are liquid hydrogen (LH.sub.2), liquefied natural gas (LNG) and liquid nitrogen (LN.sub.2). The low temperature of the liquefied gases is maintained over prolonged periods by means of a vacuum insulation of the storage container. Cryotanks are used, for example, as fuel tanks in automotive engineering.
There are various standards which prescribe a filling-level limitation for liquid-gas storage. For example in ISO/WD 13985 of Dec. 15, 1994 for liquid hydrogen tanks, it is stipulated that the tank must be fitted with a filling-level measuring device. This is intended to ensure that the tank is not overfilled. The aim is to end the filling operation at 98% of the maximum permissible filling level. In accordance with the standard TRG 103 for liquid, deep-frozen compressed gases, only so much liquid gas may be charged into a tank that, at the activation pressure of the pressure valves provided on the tank, the latter is filled to a maximum of 95%.
Since the pressure and the volume of the liquid gas increase as the temperature rises, when filling the storage container care must be taken in any case to ensure that the maximum filling level is not exceeded either when the maximum permissible internal storage pressure is reached. The maximum volume of liquid gas which may be charged into a tank thus depends on the pressure and temperature of the liquid gas during filling and on the theoretically possible increase in volume until the maximum tank pressure is reached during operation.
In the prior art, a system with an overflow has previously been used as a filling-level limitation, in which case the height of the overflow took the maximum increase in volume into account. With this type of registering or limiting of the filling level, because the phase boundary is only determined independently of the density, the maximum permissible filling level must be set to allow for the maximum possible increase in volume of the liquid gas. If, for example, supercooled liquid hydrogen at the boiling temperature at 1 bar is charged into the tank at a pressure of 15 bar, the tank may only be filled to a maximum of 65%. When the liquid hydrogen gradually assumes its state of equilibrium and heats up to the boiling temperature at 15 bar, its volume expands under ambient pressure to approximately 95% of the tank volume.
Since the conventional filling-level limitation with an overflow always takes into account the theoretically maximum possible increase in volume of the liquid gas, accordingly less mass can be stored than if the increase in volume actually possible were to be taken into account based on the actually prevailing pressure and temperature condition of the liquid gas introduced.
For instance, the maximum permissible filling quantity of liquid hydrogen in a 140-liter tank at a permissible pressure of 6 bar is calculated as 7.8 kg, corresponding to 110 liters at a boiling-point pressure of 1 bar. Since only the volume in the storage tank is measured and the theoretically maximum possible increase in volume for the increase in pressure from the ambient pressure level to the maximum permissible pressure has to be taken into account, at a boiling-point pressure of 3 bar, for example, only 7.2 kg and, at a boiling-point pressure of 6 bar, only 6.5 kg can be stored.