Requirement on the basic process control system as a layer of protection

4 Requirement on the basic process control system as a layer of protection 

1 The basic process control system may be identified as a protection layer subject to certain conditions. If functions are implemented in the BPCS for the purpose of reducing the process risk, the BPCS can be allocated a risk reduction for the identified risks it is intended to reduce. 

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 

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