The difference between SIS system and DCS system

The State Administration of Work Safety Supervision and Administration Division III Safety Supervision Administration No. 3 [2014] No. 116 document requires chemical plants to be equipped with chemical safety instrumented systems (SIS), and many companies are also using DCS systems. The following introduces five aspects of DCS systems and SIS safety instrumented systems. a difference. The concept of DCS and SIS SIS system is the abbreviation of Safety Interlocking System (Safety Instrument Syetem) safety interlocking system/safety instrumented system. SIS includes three parts: field instrument, logic solver, and actuator. These three parts must be designed safely. The conventional ESD system is only the logic solver part of the SIS, and of course it must be designed safely. The feature is still PID control. With its powerful communication function, it is a system that integrates all automation equipment in the factory. SIS is more stringent than DCS in terms of reliability and availability. In IEC61508, IEC61511 and other standards, SIS and DCS hardware are required to be set independently. This type of system is often used in high-risk industries such as petrochemicals. Once the action is taken, the entire site will be stopped, which is a defense line to ensure safety at the expense of production efficiency. SIS certification, the petrochemical industry usually has a grade of 3. DCS system is the abbreviation of DistributedControlSystem distributed control system. The DCS system is a process control system, which is dynamic and requires frequent manual intervention, which may cause human error; while the SIS (ESD) is static and does not require human intervention. This setting can avoid human error. PLC is usually used as a control scheme for complete sets of equipment, such as vacuum, waste water, container, compressor, etc. forming a self-contained system. Their characteristics are often that they do not require a lot of analog control, and are extremely effective, convenient, reliable and economical for digital control. It is usually used as a sub-site of DCS for easy monitoring. From the perspective of safety, the production device can be divided into three layers: the first layer is the production process layer, the second layer is the process control layer, and the third layer is the safety instrumented system parking protection layer. SIS and DCS play different roles in petroleum and petrochemical production. The difference between DCS system and SIS safety instrumented system 1. SIS is a 'static' system. It is used to monitor the operation status of the production equipment, quickly deal with abnormal conditions, minimize the possibility of failure, and keep people and equipment in a safe state; under normal conditions, it always monitors the operation of the equipment, and the system The output remains unchanged and does not affect the production process; under abnormal conditions, it will perform logical operations according to the pre-designed strategy to make the production device stop safely; DCS is a 'dynamic' system. It is used to continuously detect, calculate and control process variables (continuous, sequential, intermittent, etc.), dynamically control the production process, ensure the smooth operation of the production equipment, and ensure product quality and product quality. 2. SIS must test potential faults; DCS can automatically display faults. 3. The maintenance time of SIS is very critical. If it is not done well, the entire line of the device will be shut down; DCS does not have strict requirements on the length of maintenance time. 4. SIS is never allowed to run offline, otherwise the production device will lose its safety protection barrier; DCS can switch between automatic and manual. 5. The reliability and availability requirements of SIS are stricter than those of DCS. IEC61508 ISA S84.01 strongly recommends independent setting of SIS and DCS hardware. The DCS system only controls general interlocking, pump start and stop, and sequence control, and the safety level requirements are not as high as SIS.