BPCS function and initiating cause independence illustration

For example, consider the case where a flow control loop is the initiating cause. This initiating cause includes a flow transmitter, a controller, and a control valve. In order to allocate risk reduction to a pressure control loop in the BPCS, the pressure transmitter should be wired to an independent controller, modulating an independent final element (for example, vent valve to flare system). 

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5 Requirements for preventing common cause, common mode and dependent faiiures 

1 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. 

It should be noted that any sensors or actuators which are shared by the BPCS and SIS are very likely to introduce common cause failures and that the approach to such sharing of devices should be as discussed in this subclause. 

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Figure 2 - BPCS function and initiating cause independence illustration...

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