Understanding of chains and understanding of SIS
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Recently, the workshop organized a series of training sessions on interlocks, including DCS interlocks and SIS interlocks. It’s really difficult to find effective ways to help those with less experience understand these concepts; therefore, I hope everyone will discuss more and share their own understanding. In my opinion, interlock logic refers to a mechanism by which the control system can take automatic action based on preset parameters (trigger values) when process parameters deviate from the specified limits, potentially leading to quality issues with the product, damage to equipment, or safety hazards, and when no manual intervention is carried out in a timely manner. DCS systems allow for some level of control, whereas SIS systems are generally not controllable – aside from a few systems that have manual buttons, they can only be activated or deactivated, and they react passively based on preset values.
Subtract one to get an explanation: what is SIS, and what is DCS.
DCS controls product quality, while SIS prevents accidents from occurring
Take out the interlock flowchart and understand the location of each interlock, its function, the interlock values, and the procedures to be followed after an interlock occurs. Small amounts add up over time, and one instance can lead to understanding many similar cases. It’s not enough to just know how to operate; understanding the instruments and equipment as well will naturally help one learn faster. Being responsible, inquisitive, and willing to ask questions – there is nothing that cannot be learned with such an attitude.
DCS and SIS are two separate systems; they are completely independent in terms of field detection, actuators, and so on. DCS is control-oriented, making real-time adjustments to certain parameters for the operability of the process. SIS, on the other hand, are measures taken compulsorily when certain parameters deviate significantly from their controlled values; they are not controlled by DCS operations and represent forced enforcement methods. For example, in terms of pressure control in the reactor, when the pressure is too high, the DCS operator can use the internal interlocks of the DCS to adjust the opening degree of the pressure control valve in order to release pressure; this falls under the scope of DCS interlocks. SIS, on the other hand, triggers an immediate shutdown via interlock when the pressure exceeds a certain value.
Strictly speaking, DCS cannot be used as an interlock system. SIS stands for Safety Instrumented System, and it has no control functions. Interlocks include process interlocks, equipment interlocks, etc. Interlock design requires analysis using methods such as HAZOP in order to determine whether interlocks are necessary and their level of importance. The logic in DCS systems, which are part of control systems, is generally positive logic, that is, a \"1\" logic, with TRUE serving as the trigger. SIS is exactly the opposite of “0” logic. In general, process interlocks take into account whether exceeding the allowable values for a process could lead to unsafe consequences for subsequent production ； Equipment interlocking is primarily concerned with preventing damage to the equipment or other unsafe consequences such as explosions. And interlocking is meant to reduce these safety risks.
What is the main difference between DCS and SIS? Composition of the system: A DCS generally consists of a human-machine interface operation station, a communication bus, and field control stations ； The SIS system consists of sensors, logical processors, and final components. That is, DCS does not contain the detection and execution section. Functions achieved: DCS is used for continuous process measurement, as well as for operational control in various modes such as continuous, sequential, and intermittent; it ensures that the production process operates under optimal conditions under normal circumstances ； And SIS is to go beyond ultimate safety by bringing processes and equipment into a safe state. Operating status: DCS is active and dynamic; it continuously monitors, processes, and controls process variables to ensure dynamic control of the production process, thereby guaranteeing product quality and output. The SIS system is passive and dormant. Security level: The DCS has a low security level and does not require security certification ； The SIS system has a high level of security and requires security certification. Response to failure modes: Most failures in DCS systems are obvious; they become apparent during the dynamic course of production, with few cases of hidden failures ； When the SIS fails, the effect is not so apparent; therefore, the only way to determine whether such a dormant system can still function properly is to conduct periodic diagnostics or tests on it. Therefore, safety instrument systems require periodic manual offline or online inspection and testing, while some safety systems have built-in self-diagnosis capabilities.