By Florin Omota, Fellow on Process Control and Functional Safety - Fluor

The automation of industrial processes normally relies on two systems, a Basic Process Control System (BPCS) accessible to the operator and an independent Safety Instrumented System (SIS).
The Basic Process Control System (BPCS) is a system which handles process control and monitoring for a facility or piece of equipment. It takes inputs from process instrumentations or sensors to provide outputs based on design control strategy. The Basic Process Control System is responsible for maintaining the process parameters at optimum operating conditions within the required boundaries, therefore being also the first layer of protection against hazards.
The Safety Instrumented System (SIS) is designed according to IEC 61511:2016 standard to implement very specific Safety Instrumented Functions (SIF’s). A SIF is composed of one or more sensors, a logic solver and one or more final elements (e.g. pumps to stop or valves to close).
Sharing a sensor signal in BPCS and SIS is often seen as unacceptable in risk analysis studies, like Hazard and Operability (HAZOP) and Layer of Protection Analysis (LOPA). An innovative approach is proposed to quantify the level of protection provided by BPCS in conjunction to the classical SIL verification method. Any extra BPCS protection layer can offer risk reduction for the SIS. Without considering the safety contribution of the BPCS, the SIS system would be overdesigned resulting in extra cost.
As a case study, sharing three sensors between SIS and BPCS will be explained in more detail. SIS offers the possibility of using the same three sensors in 2 out of 3 (2oo3) voting configuration. BPCS is using the middle out of three (Moo3) value for more reliable process control and additional protection.
This study demonstrates that sharing BPCS and SIS instrumentation can improve both safety and controllability, increase the overall availability of the plant and reduce both CAPEX and OPEX. 

Presentation file here