【2026 Basic Knowledge】Working Principle of Instrument Interlocks and Interlock Examples
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This post was last edited by The one on 2026-7-26 10:48. I. Definition A SIS (Safety Instrumented System) is a safety protection circuit that operates independently of the DCS control system; when process parameters exceed safe thresholds, it automatically triggers device actions to eliminate hazardous sources and prevent explosions/leaks/over-temperature or over-pressure conditions, and it constitutes a mandatory safety facility. Simple logic: Process measurement point → Instrument acquisition → Logical judgment → Actuator action.
II. Complete workflow 1. Detection unit (field instrument) – Responsible for real-time collection of process hazard parameters; it serves as the \"eyes\" of the interlock system. Pressure: pressure transmitters, pressure switches. Liquid level: liquid level transmitter, float switch, tuning fork level gauge. Temperature: thermocouples/thermistors, temperature switches. Flow rate: flow meters, flow switches. Equipment status: Motor operation feedback, valve responder.
Operation mode: The instrument continuously sends 4-20mA analog signals or digital contact signals to the SIS cabinet； Once the parameter exceeds the set value (interlock value), the instrument output signal changes.
2. Logical operation unit (SIS safety controller): The “brain” of interlocking; it is separate from ordinary DCS systems, features hardware redundancy, and offers a higher safety level (SIL1~SIL3). ①. Receive signals from on-site instruments and compare them with the preset interlock action values. Example: The normal pressure of the storage tank is ≤0.6 MPa, with an interlock value of 0.8 MPa.
②. Execute preset logic: AND/OR/NOT, XOR, XNOR, 2-of-2, 3-of-2 voting. Two-out-of-two: Interlock is triggered only when two pressure transmitters exceed the set pressure simultaneously (to prevent false activation due to a fault in a single instrument). Two-out-of-three: If 2 out of 3 measurement points exceed the limits, the process is considered hazardous; this method is commonly used for high-pressure vessels in the petrochemical industry.
③. Output the switch command and upload the alarm to the operation station, with pop-up and audio-visual alarms to alert the operator.
3. Execution unit: The “hands and feet” of interlocking, which acts immediately upon receiving a signal to prioritize disconnection. Common action taken against hazard sources: – Cut off feeding: Emergency shut-off valve (ESD valve, which closes automatically in the event of loss of air or power). -Pressure relief protection: vent valves, safety valves, and emergency pressure relief valves are activated. -Shut-down protection: The pumps, compressors, and reactor mixing motors stop operating. -Protection displacement: The nitrogen purge valve opens automatically to dilute combustible gases.
Key feature: Interlocked valves are mostly fail-safe (FC/FO) type; they automatically move to a safe position in the event of a loss of air supply or power, thereby preventing loss of protection in the event of a failure in the control system.
4. Alarm and Recording Unit Operation Station: Displays the interlock alarm screen, indicating the triggering point, time, and reason for the interlock. SOE event sequence recording: Precise millisecond-level recording of interlock activation, reset, and valve operation sequences for post-incident troubleshooting. On-site sound and light alarm: A built-in speaker and strobe light are used to alert maintenance personnel.
III. Examples: Complete operation of the overpressure interlock in a high-pressure reactor 1. Two pressure transmitters measure the reactor pressure in real time. 2. When the pressure rises to 0.8 MPa (interlock value), both transmitters output alarm signals simultaneously. 3. The SIS logic performs a \"two-out-of-two\" check and outputs the interlock command. 4. Execute action ① The feed cut-off valve closes automatically, stopping the supply of raw material to the reactor. ② The circulating compressor has stopped. ③ The top vent valve is opened to release pressure. 5. Pop-up alarms and audio-visual alarms in the control room; the system records SOE events. 6. Once the pressure drops to a safe level, manual reset of the interlock is required before the system can be restarted (to prevent secondary hazards caused by automatic restoration).
IV. Other key points 1. Interlock bypass: When maintaining instruments, it is necessary to temporarily disable the interlocks; this must be done by two persons, with proper documentation, and the interlocks must be reactivated within a specified time frame. The risk associated with the device increases during this bypass period, making it a topic that requires close supervision.