How should \"three-out-of-two\" and \"two-out-of-two\" in the interlock system for tank level gauges be understood? Does three and two refer to the liquid level signal or...
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I would like to ask the experienced colleagues about the interlock issues related to level gauges. How should I understand \"three-out-of-two\" and \"two-out-of-two\"? Do the numbers 3 and 2 refer to level signals or the number of level gauges? And what are the applicable conditions for two-out-of-three and two-out-of-two? Under what operating conditions should a tank be equipped with a two-out-of-three system?
Two out of three: the “three” here refers to the three values obtained for the same parameter from three transmitters, while “two” means that two of these three values meet the interlock conditions. As for when to use three-out-of-two, two-out-of-two, or one-out-of-one, it is determined through analysis and calculation based on the specific operating conditions. Fully consider reliability, fault tolerance, and security
Can it be understood that the three-out-of-two principle means that three gauges measure the high liquid level simultaneously, and if the readings from two of these gauges meet the specified conditions, the valve will be closed?
The number of signals for three and two fingers; a “standalone signaling device” is required. That is, three level gauges (level switches) ； The choice between three options and two options is actually driven by considerations of system security and availability. Availability refers to the ability of a system, under a redundant configuration, to ensure that the production process continues without interruption, even when one of the systems fails, by relying on the redundant systems to maintain the necessary safety functions. Safety refers to the ability of the safety instrumented system to keep the process in a safe state when any potential hazard occurs. A fail-safe system refers to a system or its associated components that can bring the controlled device back to a safe state in the event of a failure. For automation engineering, achieving fault safety means including failures within the control system itself under the same monitoring scope as abnormalities in the process equipment and processes. Choose two out of three: in the event of a fault, there is a 2/3 probability that the signal will be correct (with a 1/3 probability that it will be incorrect) ； Choose one: when a failure occurs, there is a 1/2 chance that the signal will be correct (in reality, the signal is unavailable) ； Choose two out of three: the ability to ensure uninterrupted production is higher than that of choosing two out of two (higher availability). Choose two out of three: when a fault occurs, there is a 1/3 probability that the signal is incorrect ； It’s a choice of two: when a failure occurs, it’s 100% certain that there is indeed a fault present ； With a choice of two, when the signals are inconsistent, measures can be taken immediately to ensure that the process remains in a safe state; thus, the level of safety is higher than that in the case of a choice of three (higher safety level).
Regarding the interlock issues of level gauges, the 2-out-of-3 and 2-out-of-2 configurations are used to determine the safety level for such interlocks; a higher safety level can be achieved by using either 2-out-of-3 and 2-out-of-2 or 2-out-of-3 and 2-out-of-1 configurations.