Principles and bases for setting interlock circuits in chemical processing units

The principles and criteria for setting up interlock circuits in chemical processing plants mainly include the following: 1. Safety principle: The purpose of designing interlock circuits in chemical processing plants is to ensure that equipment and systems operate within safe parameters, thereby preventing accidents and incidents. Therefore, potential dangerous situations should be taken into account in the design, and corresponding interlock measures should be implemented. 2. Equipment protection principle: Interlock circuits can also be used to protect equipment from damage caused by overload, overheating, overvoltage, etc. For example, the feed valve is closed when the liquid level is too high to prevent overflow. 3. Process operation principles: Interlock circuits can provide the necessary sequential and logical control in process operations. For example, under certain process conditions, it is necessary to raise the temperature first and then add the material, and an interlock circuit can be used to ensure the correct sequence of operations. As for whether there are clear standards or specifications for the instrument interlock circuits in chemical engineering projects, generally, corresponding norms and standards are established based on different **regions and specific industries. For example, commonly used international standards include ANSI/ISA-84 from the International Automation Association and IEC 61511 from the International Electrotechnical Commission. Generally, the interlocks for parameters such as liquid level, temperature, and pressure need to be configured based on specific process conditions and operational requirements. For example, when the liquid level is too high, an interlock circuit can be set to automatically close the feed valve; or when the temperature exceeds a set value, the heater can be turned off. Regarding material mixing, whether it is necessary to install interlock circuits also depends on the specific circumstances. If mixing could lead to dangerous reactions or unstable process conditions, it is recommended to implement appropriate interlock measures. As for whether inter-opening control is required for the pair of storage tanks, this depends on the specific process design and operational requirements. Generally speaking, if the liquid levels in two tanks affect each other or if it is necessary to share inlet and outlet pipes, mutual control can be considered to ensure the continuity and safety of operations. .

Thank you for the guidance. Generally, the design is carried out in accordance with the requirements of the manufacturing process, but for this interlock system, no standardized examples of various equipment and sections have been found. It would be quite meaningful for automation professionals if **a relevant standard were established to regulate and guide industries such as oil refining, chemical processing, and coal chemical processing**. At the very least, it would enable them to understand the common process procedures, as well as the protection mechanisms, operational interlock circuits, and their purposes. I think there is always something problematic with using two on-off valves to control the filling and discharging of a storage tank, as well as using two such valves to perform pressing or pressure relief operations on a single tank. In all these cases, two-position operation of the final actuating elements is involved; in other words, two interlock circuits are working towards one output. It’s only possible to determine whether reset or setting should have priority based on the degree of importance, and the output state isn’t unique. Can this type of control not be classified as an interlock? In terms of mixing, as long as no dangerous reactions occur, is it sufficient to simply control the total amount of feed to prevent overflow, that is, to use a high-level interlock, without the need to set up separate interlocks for each feeding component? After all, there are various formulations for the feed of different products; can such formulation-based feed control be managed through manual operation or fixed-value settings, rather than via interlocks?

Also, are the analysis nodes considered in the HAZAOP analysis and LOPA risk assessment reports? For SIL1 and above, it goes into the SIS; so, for SILA, interlocks are implemented in the DCS?

1. It is not particularly necessary to understand the principles of interlock settings for instrument maintenance; this falls under the category of process engineering； 2. Interlock settings are generally specified by the process package; interlock levels and verification are part of the protection layers established as a result of HAZOP and LOPA analyses, with the process being the primary guiding factor in most cases ； 3. **Or it is impossible for the industry to establish specific regulations regarding interlock settings. 4. The pressure increase and relief control of storage tanks involves the risk of high-pressure fluid migrating into low-pressure areas; the SIL level is determined based on the pressure levels, material characteristics, and the consequences of such pressure migration ； If the risk is low, a simple sequential control can be used without W interlocking ； Otherwise, it won’t work.