Sensors and actuators in the SIS loop
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Question 1: In a SIS loop, sensors are arranged in a 1oo2 (or 2oo3) configuration, but the sensors are not instruments with an SIL rating; that is, if ordinary instruments are used, can they still meet the SIL2 requirements for the loop? Question 2: For actuators, such as cut-off valves, if only one cut-off valve is used but two solenoids are employed for actuation (1oo2), can this configure a 1oo2 mode for the cut-off valve? (Doesn’t it need to take into account failures in the valve’s mechanical mechanism?) (None of the above takes into account the reliability of the system; only its safety is considered.)
For the first question, yes; if it is 2003, SIL3 can be achieved. Regarding the second question, redundant solenoids can only improve the safety integrity level of the solenoid assembly to a limited extent, and they cannot enhance the safety integrity level of the control valves.
1. The manufacturer needs to provide some data to calculate PFDavg
Meeting the SIL level requires consideration of both hardware fault margin and the probability of hardware failure in the SIS circuit. The claim that field instruments of types 1oo2 and 2oo3 can meet SIL2 or SIL3 standards only reflects the requirement related to hardware fault margin; in fact, the hardware fault safety margin is also related to the instrument’s own safety failure score, so it’s not an absolute rule. In addition to hardware fault margin, it is also necessary to evaluate the probability of hardware failure for the entire circuit, which is what we commonly refer to as SIS circuit calculation.