The difference between SIS and ESD, DCS, PLC

SIS is a safety instrument system, ESD is an emergency stop system, and ESD is a part of SIS.

SIS includes three parts: on-site instruments, logic solvers, and actuators, all of which are designed for safety. Conventional ESD systems are only the logic solvers of SIS, and of course, they are also designed for safety. The characteristic is still PID control, which integrates all automation equipment in the factory with its powerful communication function. SIS has stricter requirements for reliability and availability than DCS. In standards such as IEC61508 and IEC61511, SIS and DCS hardware are required to be set independently. This type of system is often used in high-risk industries such as petrochemicals, and once activated, it is a defense line that sacrifices production efficiency and ensures safety. SIS certification, the petrochemical industry usually has a level of 3.

The DCS system is a process control system that is dynamic and requires frequent manual intervention, which may cause human error; SIS (ESD) is static and does not require human intervention, which can prevent human error. From the figure below, we can see that SIS includes ESD, ITCC, etc., while DCS includes PLC, FCS, etc.
PLC: Usually used as a control scheme for complete equipment, such as vacuum, wastewater, containerization, compressors, etc., forming a self-contained system. Their characteristic is often that they do not require a lot of analog control, and are extremely effective, convenient, reliable, and economical for digital control. Usually used as a sub site of DCS for easy monitoring.