What is the difference between a safety instrumented system and a DCS system + field instruments?
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Since its functions can be handled by both DCS and instruments, why was such a concept introduced? Is it mainly for interlocking and emergency shutdown? I need some advice. Our factory doesn’t have this SIS; we are in the process of learning it.
In my opinion, they’re more or less the same; it’s called SIS when used for safety control, and DCS when used for controlling process parameters... This is just my personal view for reference
The two systems have different starting points; the SIS system is generally used for interlocking unit control, based on a fail-safe configuration. DCS is not fail-safe. Simply put, the two system designs have different starting points: one focuses on security, while the other focuses on practicality
You might want to check out the relevant posts in the control section; they cover this topic!
The instrument safety system focuses more on the speed of response to fault conditions, whereas DCS and instruments focus on process handling.
SIS is a safety instrumented system primarily used for the multiple monitoring of ESD and key parameters； DCS is mainly used for chemical production control, and its safety level is generally lower than that of SIS. Safety requirements are quite stringent these days; it is recommended that you pay attention to SIS and use it if possible.
There are quite a few entries on this topic in this section; the original poster should do some searching themselves.
1. Different SIL levels； 2. Different functions ； 3. The most crucial point: The hardware foundation of the SIS system is relays (or soft relays). Action determination is based on the actual flow of current in the circuit (or digital algorithms in soft relays). Each action represents a complete current flow process; there is no intersection between relays, and no software reference relationships exist. Such a circuit structure enables the calculation of the failure probability of the entire system using the accident chain algorithm, and allows the accident probability to be determined based on this ； DCS often executes software algorithms based on CPU interrupt technology. Due to the cross-referencing relationships of software interrupt commands, inevitable software common-cause failures exist among multiple functional actions. Therefore, the failure probability and accident probability cannot be accurately determined.
1. SIS is completely different from DCS; SIS focuses on production safety and remains in a static state under normal conditions, taking action only in the event of major incidents to ensure the safe shutdown of equipment and large-scale installations. DCS is dynamic in nature and is used for real-time control of production processes. 2. In terms of structure and hardware, SIS gives much greater consideration to safety than DCS; generally, redundancy is sufficient for a DCS structure. However, SIS takes triple or quadruple redundancy and fault tolerance into account; the requirements vary from the controller to the field instruments, but the configurations differ depending on the safety level specified in the design. 3. The processing speed of SIS is also several orders of magnitude higher than that of DCS (to detect faults promptly and take action in a timely manner). ) In short, whether SIS is necessary depends on a comprehensive assessment of the safety level of your equipment, with appropriate configuration. It can’t be determined just by thinking.
On professional forums like HaiChuan, it’s common to see many questions that are not of this nature; it’s therefore easy to understand why explosions in domestic chemical plants occur so frequently.
It seems that SIS also aims to be innovative; if DCS isn’t safe, who would use it?