Hazop analyzes SIL1 levels, BPCS (DCS) and SIS?

Basic concepts『BPCS』

Basic process control system: Responds to input signals from process, system-related equipment, other programmable systems, and/or an operator, and produces a system that makes the process and system-related equipment operate as required, but it does not perform any instrumentation safety functions with the declared SIL≥1. (Excerpt: GB/T 21109.1-2007 (IEC 61511-1:2003, IDT) Functional safety of safety instrumented systems in the process industry – Part 1: Framework, definitions, system, hardware and software requirements 3.3.2)

Basic Process Control System: Responds to input signals from process measurements and other related equipment, other instruments, control systems, or operators. According to the process control law, algorithm and method, the output signal is generated to realize the operation of process control and its related equipment. In petrochemical plants or plants, the basic process control system usually uses a distributed control system (DCS). Basic process control systems should not perform safety instrumented functions for SIL1, SIL2, SIL3. (Excerpt: GB/T 50770-2013 Code for design of petrochemical safety instrumented systems 2.1.19 )

『SIS』

Safety instrumented system: An instrumented system used to implement one or several instrument safety functions. SIS can consist of any combination of sensor, logic solver, and final element.

Instrument safety function; SIF has a specific SIL to achieve functional safety safety safety functions, which can be both an instrument safety protection function and an instrument safety control function.

Safety integrity level; SIL is used to specify discrete levels (one of 4 levels) for the safety integrity requirements of instrumentation safety functions assigned to safety instrumented systems. SIL4 is the highest level of safety integrity and SIL1 is the lowest.
(Excerpt: GB/T 21109.1-2007 (IEC 61511-1:2003, IDT) Functional safety of safety instrumented systems for the process industry Part 1: Framework, definitions, system, hardware and software requirements 3.2.72/3.2.71/3.2.74)

Safety instrumented system: An instrumented system that implements one or more safety instrumented functions. (Excerpt: GB/T 50770-2013 Code for design of petrochemical safety instrumented systems 2.1.1);

The difference between BPCS and SIS 

Safety instrumented system (SIS) independent of the process control system BPCS (such as distributed control system DCS, etc.), production is normally dormant or static, once the production device or facility may lead to safety accidents, can be instantaneously accurate action, so that the production process safely stop running or automatically import a predetermined safety state, must have high reliability (that is, functional safety) and standardized maintenance management, if the safety instrumented system fails, often lead to serious safety accidents. (Excerpt: General Administration of Safety Supervision No. 3 (2014) No. 116, Guiding Opinions of the State Administration of Safety Supervision on Strengthening the Management of Chemical Safety Instrumentation Systems)

The meaning of SIS independence from BPCS: If the normal operation of the BPCS control loop meets the following requirements, it can be used as an independent protective layer, the BPCS control loop should be physically separated from the safety instrumented system (SIS) functional safety loop SIF, including the sensor, controller and final element.

Difference between BPCS and SIS:

Different purpose functions: production function / safety function;

Different operating states: real-time control / over-limit time interlock;

Different reliability requirements: SIS requires higher reliability;

Different control methods: continuous control as the main / logic control as the main control;

Different methods of use and maintenance: SIS is more stringent;

BPCS and SIS linkage

Whether BPCS and SIS can share components can be considered and determined from the following three aspects:

Requirements and provisions of standard specifications, safety requirements, IPL methodology, SIL assessment;

Economic evaluation (provided that basic safety requirements are met), e.g., ALARP (as low as reasonably practicable) analysis;

Managers or engineers are determined based on experience and subjective will.

Either way, the minimum requirement to meet the requirements of regulations and standards is required.