Issues related to safety instrumented systems
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I’ve been reading GB/T50770-2013, the Standard for Design of Safety Instrumented Systems in Petrochemical Industries. There are some points in it that I don’t quite understand. It states that when a system requires high safety levels, an AND logic structure should be used; when high availability is required, an OR logic structure should be employed. When a system needs to balance both high safety and high availability, a two-out-of-three logic structure is recommended. What do these AND, OR, and two-out-of-three logic structures mean? What do “three” and “two” refer to here? Thank you everyone for helping me understand this
It seems that three devices need to be installed at one location for measurement, with at least two values needing to be normal
Location? Measurement points? Three solenoid valves? Doesn’t it waste resources to need three devices for one point, just for safety’s sake?
It’s usually the case with regular measuring devices – things like level and pressure sensors. They’re installed at key and important points. I’ve seen systems that use two sensors for measuring level; I’m not sure about other devices
Please study some books on process safety... fault trees, accident trees...
If we follow what’s mentioned above, the instrument’s operation logic is 1, which means that the fault signal is 1 (usually, safety interlocks are set to 0). With an OR logic, the operation will take place as long as there is a 1 output; if it’s 0, then AND logic must be used, which of course enhances safety. The two-out-of-three principle means that three instruments are installed at one point; interlock action is triggered only when two of them send out fault signals. This is done to prevent errors and reduce the likelihood of false signals