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The SIS System is engineered to provide exceptional security and reliability in hazardous environments. It ensures the safe operation of industrial processes by detecting unsafe conditions and automatically taking corrective actions, thereby minimizing risks. Key features include real-time monitoring, fail-safe designs, and redundancy to enhance system integrity. The SIS is compliant with industry safety standards, ensuring robust performance under various operational scenarios. Its applications span across industries such as oil and gas, chemical manufacturing, and power generation, where safety is paramount. By integrating with existing control systems, the SIS not only enhances safety but also improves overall operational efficiency, making it an essential component for safeguarding personnel and equipment in high-risk environments.
1.Concept
SIS is a Safety instrumented system, named after the definition of the safety system control system by the American Instrument Association (ISA). Also called emergency shutdown system (ESD) safety interlocking system (SIS) or instrument protection system (IPS), Safety instrumented system refers to a system that can realize one or more safety functions. Used to monitor the operation of production devices or independent units. If the production process exceeds the safe operating range, it can be put into a safe state to ensure that the device or independent unit has a certain degree of safety. The safety system is different from the process interlocking of batch control, sequence control, and process control. When the process variables (temperature, pressure, flow, liquid level, etc.) exceed the limit, mechanical equipment failure, system failure or energy interruption, the Safety instrumented system automatically (manually if necessary) completes the preset action, so that the operators and process devices are in a safe state. The SIS system can be used to implement a control system for one or multiple safety instrument functions. Mainly for the alarm and interlocking parts of the factory control system, implementing alarm actions or adjusting or stopping control on the detection results in the control system is an important component of automatic control in factories and enterprises.
2.System composition
The Safety instrumented system mainly consists of measurement unit, logic control unit and execution unit, together with corresponding software. Usually, there are communication requirements with the basic process control system (such as DCS system), which together form the process instrument control system of the production device.
2.2Safety Integrity Level
Safety integrity level is an international "standard language", which aims to divide the safety level requirements of each safety instrument loop in the process unit in a simple way. It is an important safety indicator that must be followed in the design of SIS systems.
According to the IEC standard, it is divided into SIL1-SIL4 from low to high, SIL1-SIL3 from ISA 84.01, and AK1-AI8 from low to high according to DIN V VDE0804. The corresponding relationship between them is as follows.
Safety integrity level
2.3Safety Lifecycle
The safety life cycle of the Safety instrumented system is also a very important concept. To ensure the safe production and operation of the process unit, not only the appropriate control system should be selected, but also there are strict requirements for the risk assessment of the process, the classification of the safety loop, and the maintenance and management of the control system. The entire safety lifecycle of the SIS system can be divided into three stages: analysis, engineering implementation, and operation and maintenance. In the analysis stage, potential hazards of the process should be identified, and their consequences and possibilities should be analyzed to determine process risks and necessary risk reduction requirements. During the implementation phase of the project, the main tasks are to complete the engineering design, instrument selection, hardware configuration, software configuration, and system integration of the SIS, as well as training for operation and maintenance personnel, installation and debugging of the SIS, and safety verification of the SIS. The operation and maintenance phase has the longest time interval throughout the entire safety lifecycle, including operation and maintenance, modification, and SIS deactivation.
After the design and selection of SIS system, the dangerous failure probability or dangerous failure frequency of safety instrument function shall be calculated according to the reliability data and operation mode to assess whether it meets the Functional safety requirements of the target safety instrument. This is an important link to ensure necessary risk reduction and Functional safety and Functional safety of instruments. At the same time, after SIS operation, routine maintenance, modification management, periodic inspection and testing, Functional safety audit, etc. are also the core work of Functional safety.
