Low-demand examples

The hazardous event is initiated by the failure of either of the BPCS control valves when the reactor is at high pressure ( 5 Barg). The scenario will result in a loss of containment due to overpressure of equipment in the gas processing unit. Due to the scenario risk, an SIL 1 SIF is proposed to detect high pressure and isolate the vent line. 

The design team considered various options for implementing the proposed SIF and evaluated each proposal s mode of operation. 

Case 1 When high pressure in the reactor is detected, the BPCS control valves are not enabled. 

The SIF could be implemented using a dedicated pressure transmitter on the reactor and dedicated solenoid-operated valve on the each control valve. The SIF would not allow air to control valve positioners unless the pressure was 5 Barg. This SIF design was determined to not be sufficiently independent of the initiating cause since it relied on the control valve for isolation. 

Mode of operation The scenario is caused by failure of the BPCS control valves. Since the SIF also uses these valves, the SIF would also fail if the control valves failed to seat. The SIF operates in demand mode, but it is not independent of the process control system. 

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