Method and device for monitoring the fill level of a liquid in a liquid container
A method for monitoring the fill level of a liquid in a liquid container, in particular for monitoring the coolant fill level in a reactor pressure vessel of a pressurized water reactor system, wherein a determination is made about the drop in the liquid level below the installed height of a heated thermal element by way of the temperature difference measured between said heated thermal element and an unheated thermal element associated therewith, both of said elements being disposed in the liquid container, said method comprising the especially reliable triggering of an alarm with a short response time while maintaining a low level of physical and controls engineering complexity when the liquid level to be monitored falls below a critical value. To this end, according to the invention the temporal progression of the temperature difference is continuously monitored for any significant increase, in particular sudden increase, within a time interval of pre-determined duration prior to the respective evaluation time, wherein an alarm signal is issued as soon as the change in the temperature difference within said time interval reaches or exceeds a pre-determined limit value.
1. A method of monitoring a fill level of a liquid in a liquid container, which comprises:
providing a liquid container with a heated thermocouple mounted in the container at a given installation height level and an associated unheated thermocouple;
inferring that the liquid level has sunk below the given installation height level of the heated thermocouple from a measured temperature difference between the heated thermocouple and the unheated thermocouple;
continuously monitoring a temporal profile of the temperature difference for a significant increase within a time interval, which occurs before a respective evaluation time point, of a duration that is previously fixed;
outputting an alarm signal as soon as a change in the temperature difference within the time interval reaches or exceeds a previously fixed limit value;
thereby ascertaining a sequence of temperature differences over a large number of periodically sequential measurement time points, wherein the differences between the last ascertained sequence element and all of its predecessors, the measurement time points of which fall within a time interval of pre-specified length, are formed at an evaluation time point, and the alarm signal is output as soon as at least one of the differences reaches or exceeds a previously fixed limit value.
2. The method according to claim 1 , which comprises monitoring the temporal profile of the temperature difference for a sudden increase within the time interval before the respective evaluation time point.
3. The method according to claim 1 , which comprises monitoring a coolant fill level in a reactor pressure vessel of a pressurized-water reactor plant.
4. The method according to claim 1 , which comprises repeating the evaluation process iteratively at each measurement time point.
5. The method according to claim 1 , which comprises predetermining a temporal gap between two immediately successive measurement time points to lie in a range between 50 ms and 1000 ms.
6. The method according to claim 5 , which comprises setting the temporal time gap in a range between 100 ms and 350 ms.
7. The method according to claim 1 , which comprises pre-specifying the length of the time interval to lie in a range between 30 s and 100 s.
8. The method according to claim 7 , which comprises pre-specifying the length of the time interval to be approximately 50 s.
9. The method according to claim 1 , which comprises temporarily storing the measured temperature differences in a FIFO memory.
10. The method according to claim 1 , which comprises pre-specifying a plurality of different limit values, and triggering respectively different alerts when the limit values are reached or exceeded.
11. The method according to claim 1 , which comprises deactivating an alarm, once triggered, if, after the alarm has been triggered, the temperature difference has dropped by a pre-specified amount within a pre-specified time interval.
12. The method according to claim 1 , which comprises deactivating an alarm, once triggered, if, after the alarm has been triggered, the temperature difference has dropped to, or below, a threshold value that has been pre-specified in a temperature-independent fashion.
13. The method according to claim 1 , which comprises measuring one or both of the temperature and the pressure of a medium, which is to be monitored with respect to its fill level height, in the liquid container and wherein an alarm, once triggered, is deactivated if, after the alarm has been triggered, the temperature difference has dropped to, or below, a threshold value which is pre-specified in dependence on one of both of an instantaneous temperature and an instantaneous pressure.
14. An electronic evaluation and control unit for a device for monitoring a fill level of a liquid in a liquid container, comprising:
a signal input for measurement signals of an associated heated thermocouple and a signal input for an unheated thermocouple associated with the heated thermocouple;
a subtraction unit configured to form from the measurement signals a temperature difference signal characteristic for a temperature difference with respect to the heated and unheated thermocouples;
an evaluation unit with means for continuously monitoring a temporal profile of the temperature difference for a significant increase within a time interval of predetermined duration, which time interval occurs before a respective evaluation time point;
an alarm output for outputting an alarm signal as soon as an increase in the temperature difference within the time interval reaches or exceeds a predetermined limit value;
said evaluation unit being configured to ascertain a sequence of temperature differences over a large number of periodically sequential measurement time points, wherein the differences between a last ascertained sequence element and all predecessors thereof, the measurement time points of which fall within a time interval of pre-specified length, are formed at an evaluation time point, and to output an alarm signal as soon as at least one of the differences reaches or exceeds a previously fixed limit value.
15. The device according to claim 14 , wherein said evaluation unit is configured to monitor the temperature difference for a sudden increase.
16. A device for monitoring a fill level of a liquid in a liquid container, comprising:
a heated thermocouple mounted in the liquid container and an associated unheated thermocouple in the liquid container; and
an electronic evaluation and control unit according to claim 14 connected to receive the output signals from the heated theremocouple and the unheated thermocouple.
17. An electronic evaluation and control unit for a device for monitoring a fill level of a liquid in a liquid container, comprising:
a signal input for a temperature difference signal generated by a circuit with a heated thermocouple and an unheated thermocouple;
an evaluation unit with means for continuously monitoring a temporal profile of the temperature difference for a significant increase within a time interval of predetermined duration, which time interval occurs before a respective evaluation time point;
an alarm output for outputting an alarm signal as soon as an increase in the temperature difference within the time interval reaches or exceeds a predetermined limit value;
said evaluation unit being configured to ascertain a sequence of temperature differences over a large number of periodically sequential measurement time points, wherein the differences between a last ascertained sequence element and all predecessors thereof, the measurement time points of which fall within a time interval of pre-specified length, are formed at an evaluation time point, and to output an alarm signal as soon as at least one of the differences reaches or exceeds a previously fixed limit value.
18. The device according to claim 17 , wherein said evaluation unit is configured to monitor the temperature difference for a sudden increase.
19. A device for monitoring a fill level of a liquid in a liquid container, comprising:
a heated thermocouple mounted in the liquid container and an associated unheated thermocouple in the liquid container; and
an electronic evaluation and control unit according to claim 17 connected to receive the output signals from the heated theremocouple and the unheated thermocouple.