It is very important to understand the types of failures in the Safety Instrumented Systems (SIS) and methods to improve the reliability.
There are two types of failures that are important to be considered in the design of SIS and to improve safety instrument & system performance:
This is a type of failure occurring at a random time, which results from one or more degradation mechanisms (viz. Ageing of sensors, Damaged diaphragm, O rings leak, etc).
Random failures can be effectively predicted with statistics and are the basis for the probability of failure on demand-based calculations requirements for safety integrity level.
A failure that happens in a deterministic (not in a random way) predictable fashion from a certain cause, can only be eliminated by a modification of the design or of the manufacturing process, operational procedures, documentation, or other relevant factors.
Note: Systematic failures are not included in failure rate calculations
Discrepancies between expected behavior and the actual behavior of the SIS should be analyzed and, where essential and necessary modifications to be made such that the designed safety is maintained.
This shall include monitoring the following:
NOTE: It is recommended to analyze the discrepancies between expected behavior and actual behavior using Root Cause Analysis.
Reliability ensures that an SIS element is able to perform a required function over a stated period of time.
Reliability is defined on a yearly basis in % and is inversely related to false trip probability.
Reliability analysis is used to study the frequency that it will falsely trip the plant.
Evaluate & optimize the occurrence of false trips.
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