Modes of Operation of a SIF
The demand mode of a Safety Instrumented Function (SIF) refers to the operational mode in which the SIF is activated only when a specific demand occurs, such as a process deviation or an unsafe condition. The demand mode is classified based on the frequency of such demands.
Mode of Operation of a SIF is the way in which a SIF operates which may be either Low Demand Mode, High Demand Mode or Continuous Mode.
1. Low Demand Mode
It is the mode of operation where the SIF is only performed on demand, in order to transfer the process into a specified safe state, and where the frequency of demands is no greater than one per year.
Examples of Low Demand SIF
1.Emergency Shutdown Systems (ESD): Used to shut down processes in response to critical failures or unsafe conditions. Example: Stopping a chemical reactor during a temperature or pressure excursion to prevent a runaway reaction.
2. Fire and Gas Detection Systems: Designed to detect fires, gas leaks, or other hazardous conditions and activate mitigation measures like alarms, fire suppression systems, or area isolation. Example: Gas detection system shutting down process units when a combustible gas concentration is detected.
3. Boiler Emergency Shutdown Systems: Activated during unsafe boiler conditions such as low water levels, high steam pressure, or loss of flame. Example: Shutting down the fuel supply in a boiler during a flameout.
2. High Demand Mode SIF
High demand mode is the mode of operation where the SIF, is only performed on demand, in order to transfer the process into a specified safe state, and where the frequency of demands is greater than one per year.
In the event of a dangerous failure of the SIF, a hazardous event can only occur;
- if the failure is undetected and a demand occurs before the next proof test
- if the failure is detected by the diagnostic tests but the related process and its associated equipment has not been moved to a safe state before a demand occurs.
SIF operating in low demand mode or high demand mode is called the demand mode of SIF.
Examples of High Demand Mode SIF
1. Tank Overfill Prevention in Frequent Operations: Tanks that are filled and emptied multiple times a year but not continuously. Example: A system that activates a high-level shutdown valve to stop filling during periodic batch processes.
2. Furnace or Heater Trips Due to Operational Deviations: Heaters in facilities where operational upsets or deviations trigger emergency actions a few times annually. Example: A burner management system that shuts off fuel due to flame loss or pressure anomalies during seasonal production changes.
3. Continuous Mode SIF
Continuous mode is the mode of operation where the SIF retains the process in a safe state as part of normal operation. Burner or turbine speed control functions may be specified as continuous mode of operation. SIF operating in continuous mode is called, continuous mode SIF
- In the event of a dangerous failure of the SIF a hazardous event will occur without further failure unless action is taken to prevent it within the process safety time.
- Continuous mode covers those SIF which implement continuous control to maintain functional safety.
Examples of Continuous Mode SIF
1. Cooling Water Systems for Reactor Temperature Control: Ensures reactors stay within safe temperature limits by operating continuously to avoid overheating.
2. Burner Management Systems (BMS): Used in boilers, heaters, and furnaces to continuously monitor and control combustion processes. Ensures proper ignition, fuel shut-off, and flame monitoring.
