[2026 Interlock Examples] DCS and SIS Interlock Examples
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Example scenario: Exothermic chemical reaction reactor. Process description: A reaction reactor is undergoing an exothermic chemical reaction. The reaction temperature needs to be maintained within a specific range (for example: 120°C – 130°C) to ensure product quality and reaction efficiency. If the temperature exceeds the upper limit (for example, 135°C), the reaction may get out of control, leading to a sharp increase in pressure and a risk of explosion. Control objective: Normal control: Maintain the reaction temperature at the set value (e.g., 125°C). Basic safety: Preventing uncontrolled reactions due to excessive temperatures (primary protection). Core safety: In cases of extremely high temperatures (e.g., 150°C) or failure of the primary protection system, an emergency shutdown is triggered to prevent explosions (as a last line of defense).
Solution: DCS interlock vs. SIS interlock 1. DCS interlock (part of the Basic Process Control System – BPCS) Functionality: A temperature sensor (TI-101) continuously measures the temperature of the reactor. Interlock logic (implemented in the DCS): If the temperature measurement (TI-101) is above 135°C and the rate of temperature increase is too high (optional condition), then the feed valve (FV-101) shall be closed, the cooling water valve (TV-101) shall be opened to 100%, and the standby mixing motor shall be started (optional). The DCS controller runs a PID control algorithm to regulate the opening degree of the cooling water control valve (TV-101), thereby maintaining the temperature at around 125°C.
Purpose: To intervene when significant deviations occur in the process, attempting to bring it back within the normal operating range and preventing the activation of higher-level safety measures. This is process optimization and the first line of defense against risks.
Features: High-frequency operation: The PID control valve operates frequently. Complexity: Control algorithms can be complex (such as cascade, feedforward, etc.). Availability orientation: The main goal is to maintain production continuity and stability. Failure mode: Failures in the DCS system (including sensors, logic controllers, and actuators) can lead to hazards (such as valves getting stuck in the open position and preventing cooling). Its design has a low tolerance for random hardware failures. Independence: It usually shares sensors, controllers, and end components (valves) with process control, or even if independent, its design standards and safety lifecycle management are inferior to those of SIS. Maintenance: It can be carried out online, with relatively little impact on production.
2. SIS Interlock (Safety Instrumented System – SIS) Function: Independent temperature sensors (TIS-101, which may be configured redundantly, such as 2oo3) are used to measure the temperature of the reactor for safety purposes. Independent safety logic controllers (such as safety PLCs). Independent final actuation elements (such as the shut-off valve XV-101 for emergency shutdown of the feed, and the emergency cooling valve XV-102 for full opening of the cooling water). Safety interlock logic (implemented within the SIS): If the independent safety temperature measurement (TIS-101) is above 150°C, the feed valve (XV-101) is shut off urgently, the cooling water valve (XV-102) is opened fully urgently, and the mixing motor (MS-101) is stopped urgently.
This action brings the reaction vessel into a safe shutdown state. Purpose: To independently and reliably execute predefined safety actions when process deviations reach a dangerous level (close to the safety limits) or when the BPCS (DCS interlocks) are unable to provide effective control, thereby bringing the process back to a safe state (usually by shutting it down) and preventing serious safety incidents such as explosions, fires, or toxic substance leaks. This is the final automated line of defense for risk reduction.
Features: Low-frequency operation (requirement): Ideally, it should never operate; it should only act in dangerous situations. Simplicity: The logic is usually simple and definite (for example, a high-high-high alarm triggers shutdown). Safety/reliability orientation: The core goal is to prevent the occurrence of specific hazardous events, achieving high reliability and high availability (safe availability). Failure mode: Designed to be fail-safe. The failure of the system (sensors, logic devices, actuators) should, as much as possible, lead to a safe state (such as valves closing due to power loss). Strict control requirements apply to random hardware failures and systematic failures. High independence: Physically and functionally independent from the BPCS (DCS) as much as possible, using separate sensors, controllers, and final actuators. SIL Level: The entire Safety Instrumented Function (SIF) must have its Safety Integrity Level (SIL 1-4) determined based on a risk analysis, and it must be designed, selected, installed, tested, and maintained in accordance with the requirements of that level. Maintenance: Maintenance (especially regular functional testing) requires strict standards, and it may be necessary to shut down the system in order to verify its functional integrity.
Detailed explanation of the standard specifications: IEC 61511 / ISA 84.00.01 (Functional safety standards for the process industry): Core standards specifically designed for safety instrumented systems in the process industry. Make a clear distinction between BPCS and SIS: The standard emphasizes from the outset that the primary purpose of BPCS (which usually includes DCS) is process control; although it can also provide some level of risk reduction, it cannot be relied upon as the main safeguard against hazardous events, especially for high-risk situations. SIS is specifically designed to provide the required risk reduction. A Process Hazard Analysis (PHA – such as HAZOP) is required: to identify potential hazardous scenarios. Protection Layer Analysis (LOPA) is required: to quantitatively assess the risk reduction capability of existing protection layers (including alarms, BPCS interlocks, pressure relief devices, etc.), determine the Risk Reduction Factor (RRF) that needs to be provided by the SIS, and thereby establish the Safety Integrity Level (SIL 1-4). Define the safety life cycle of SIS: It covers the entire management process from conceptual design, SIF definition, SIL selection, SIS design, installation, commissioning, operation, maintenance, modification to decommissioning, in order to ensure functional safety.
Mandatory periodic functional testing (PFT): Each SIF is subjected to a complete test at intervals determined based on the SIL level, in order to verify its functional integrity and keep its PFDavg (average failure probability under required conditions) within the specified range. Certification requirements: The key components of SIS (sensors, logic processors, actuators) usually require SIL certification by an independent third-party organization in accordance with IEC 61508.
IEC 61508 (Functional Safety of Electrical/Electronic/Programmable Electronic Safety-Related Systems – Fundamental Standard): This is the general fundamental standard for functional safety. IEC 61511 is a field-specific standard developed for the process industry based on IEC 61508. It defines basic concepts such as functional safety and Safety Integrity Level (SIL). It specifies general technical and management requirements that cover the entire safety lifecycle of E/E/PE safety-related systems. It provides manufacturers of SIS devices with a framework for designing and evaluating the safety performance of their products.
In this example: The DCS interlock (activated at 135°C) is an extension of the process control system; it is designed to intervene when significant deviations occur in the process, aiming to prevent shutdowns, maintain production continuity, and serve as an initial barrier against risks. Its failure itself may be the reason that necessitates an SIS action. It is not designed and certified according to SIL levels.