Safety instrumented systems common-mode failure

The DAS is included wilhin the instrumentation and control architecture in order to support the risk goals in the APIOOO PRA for analyzed events. The DAS reduces the probability of a severe accident resulting from Ihe imlikely coincidence of a Postulated Initiating Event (PIE) and postulated common-mode failures in Ihe protection and control systems. Common-mode failure between the Protection and Safety Monitoring System and Ihe DAS is unlikely because each runs on a different operating system from the other, and there are no sensors shared between the two systems (see Section 8.4.4.4 of this chapter for a further enhancement to the DAS to reduce common-cause failure even more). 

An important issue to be considered at an early stage is whether there are any common oause failures between redundant parts within each layer (for example, between 2 pressure relief valves on the same vessel), between safety layers or between safety layers and the BPCS. An example of this could be where failure of a basic process control system measurement could oause a demand on the safety instrumented system and a device with the same characteristics is used within the safety instrumented system. In such cases it will be necessary to establish if there are oredible failure modes that could cause failure of both devices at the same time. Where a common cause of failure is identified then the following actions can be taken. 

Degradation of instrument air systems can lead to failures in safety related systems, including possible common-mode failures. Failures have occurred in several systems, including auxiliary feedwater, residual heat removal, main steam isolation, BWR scram, service water, emergency diesel generators, containment isolation, and fuel pool seals. 

Malfunctions in the control room cabinets can produce a common mode failure in vital instrumentation required to control the safety systems both in the automatic and in the manual mode, and could simultaneously threaten several safety functions. 

Safety instrumented systems (SIS) in the oil and gas industry generally function in the low demand mode, which means that regular inspection and testing are necessary to reveal their failures. Past experiences indicate that the occurrences of common cause failures are a serious threat to SIS... 

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