Is it in line with ** and industry standards to have both DCS interlocks and SIS interlocks based on the same causal logic?

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For valves, it is acceptable under certain conditions. For moving equipment, this is unreasonable and non-compliant.

For valve actuation interlocking, two solenoid valves are required, with signals being transmitted via separate cables

Since it is interlocking, a SIL rating assessment must be carried out. If the assessment indicates that a SIS is not necessary, then a DCS will suffice. However, if a certain SIL rating needs to be achieved, then a SIS is required. But if you have sufficient funds, after implementing interlocking in the SIS, it is still necessary to set up an identical system in the DCS as well. Such over-protection not only results in higher costs but also increases the likelihood of interlocking actions occurring.

Theoretically, the control logic of BPCS and SIS differs due to the differences in the controlled objects and their intended purposes; refer to IEC61508 and China’s HG standards.

I checked the national standards; it seems that SIL1 allows the use of both DCS and SIS, while SIL2 is not suitable for using both DCS and SIS. For SIL3, DCS and SIS should not be used simultaneously in interlock systems

““The fact that SIL3 should not have both DCS and SIS present for interlocking” means that the sensing and actuating components cannot be shared, but it is possible to implement the same interlocking function; it is best to set the interlocking values so that the DCS takes action before the SIS.

The SIS system and the DCS system operate at different safety levels within the production facility. The interlocks in the DCS system are of the type commonly used in production processes, such as switch interlocks or interlocks for starting and stopping equipment, and they are part of normal operations； Most of the interlocks in SIS are related to the safety interlocks of personnel and equipment (especially shutdown interlocks), and they represent fault and accident interlocks for which the SIL level is strictly defined. The nature and configuration of the two are different. 1. DCS system: ① DCS is used for continuous measurement of processes, conventional control (continuous, sequential, intermittent, etc.), and operation control management, thereby ensuring the stable operation of production facilities. ② DCS is a “dynamic” system that continuously monitors, processes, and controls process variables, enabling dynamic control of the production process to ensure product quality and output levels. ③DCS can automatically display faults ； DCS is not demanding in terms of the length of maintenance time or the requirements involved ； DCS can switch between automatic and manual mode. ④The DCS system is used only for general interlocks, starting and stopping of motors, sequencing, etc., and its safety requirements are not as high as those of a SIS. ⑤A DCS system generally consists of a human-machine interface operation station, a communication bus, and field control stations; it does not include detection and actuation components. ⑥ To achieve automation in the production process, operators often change certain output actions of the DCS system. 2. SIS system: ① The SIS is used to monitor the operating conditions of production facilities, to promptly address any abnormal situations, thereby minimizing the likelihood of failures and ensuring the safety of both people and equipment. ②SIS is a static system; under normal operating conditions, it continuously monitors the operation of the equipment, with no change in system output and thus no impact on the production process. In abnormal conditions, it performs logical operations according to pre-designed strategies to ensure the safe shutdown of the production equipment. ③SIS must test for potential faults ； The SIS maintenance time is extremely critical; severe cases can lead to the shutdown of the entire plant ； The SIS system must never operate offline, otherwise the production facility will lose its safety protection barrier. ④Compared to DCS, SIS requires stricter standards in terms of reliability and availability; IEC 61508-11 and ISA S84.01 strongly recommend that SIS and DCS hardware be installed separately. ⑤The SIS system consists of sensors, logic processors, and final units. ⑥The SIS system remains silent in normal operation and does not take any action; it only activates when interlocks are triggered.