Alarm values and interlock values
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Production facilities are generally equipped with production automation control systems, including Basic Production Control Systems (BPCS) such as DCS, FCS, PLC, etc., Safety Instrumented Systems (SIS), and Flammable and Toxic Gas Detection and Alarm Systems (GDS). Production facilities install one or several of these systems as needed. SH/T 3007-2014 “Code for Design of Tank Farms in Petrochemical Storage and Transportation Systems” specifies how to set up high-level alarms and ultra-high-level alarms. It can be seen that there should be a height equivalent to the maximum liquid inflow volume of the tank over 10 min to 15 min between the high liquid level alarm and the very high liquid level alarm; this provides time for operators to respond to the BPCS alarm. If no effective response is given within a certain period of time (such as 10 min to 15 min), then the very high liquid level alarm/interlock is triggered, hence the BPCS alarm precedes the SIS interlock. GB/T 50770-2013 \"Code for Design of Safety Instrumented Systems in Petrochemical Industries\" – Clause 2.1.7 describes the typical multi-layer protection structure for petrochemical plants or units. It is clear from this that the sequential relationship between the BPCS alarm values and the SIS interlocks exists, that is, BPCS alarms occur before SIS interlocks. SH/T 3184-2017 \"Design Specifications for Automation Systems in Petrochemical Tank Farms\" specifies indirect requirements, and it also shows the interrelationships between them, namely that BPCS alarms precede SIS interlocks, along with more detailed requirements regarding instrument configuration. GB/T 41394-2022 \"Functional safety requirements for overflow prevention systems of storage tanks containing explosive hazardous chemicals\" specifies the relevant requirements based on the monitoring methods for storage tanks and the configuration of instruments. In CCPS’ “Guidelines for Safe and Reliable Instrumented Protective Systems”, Figure 4.4, “Process Condition Changes Over Time”, shows the sequential relationship between BPCS alarms and SIS interlocks. GB/T 41261-2022 \"Management of Alarm Systems in Process Industries\" also provides valuable reference information. API RECOMMENDED PRACTICE 556 《Instrumentation, Control, and Protective Systems for Gas Fired Heaters» also shows the sequential relationship between BPCS alarms and SIS interlocks. DEP 32.80.10.14-Gen-2017 《ALARM MANAGEMENT Figure 3.1 Example of Limit and Constraint Hierarchy» Table 3.1 Engineering Constraints – This also shows the hierarchical relationship between BPCS alarms and SIS interlocks. In summary, the relationship between BPCS alarms and SIS interlocks is that BPCS alarms occur prior to SIS interlocks.
For the automation control systems of production facilities, alarm values and interlock values are typically set to ensure the safety of the production process. Alarm values are primarily intended to alert operators that a certain parameter (such as temperature, pressure, or level) has exceeded the normal range, requiring prompt intervention or action. This is achieved through the Basic Production Process Control System (BPCS). For example, in the case of storage tanks, once the liquid level reaches the high-level alarm threshold, the operator must take action within 10 to 15 minutes; otherwise, the level will continue to rise. The interlock value represents a more stringent threshold; if the parameters exceed this value, the Safety Instrumented System (SIS) will immediately initiate a series of pre-defined automated actions, such as closing valves or starting emergency pumps, in order to prevent potential catastrophic consequences. In the aforementioned example of the liquid storage tank, if the liquid level exceeds the high-high level alarm/interlock value, the system will take action automatically, without the need for human intervention. Therefore, the main relationship between the alarm value and the interlock value is that when the operating parameters of the equipment exceed the alarm value but do not reach the interlock value, manual intervention is required ； When the parameters exceed the interlock value, the system will automatically implement preset protection measures to ensure production safety. .
The relationship between BPCS alarms and SIS interlocks is that BPCS alarms occur prior to SIS interlocks.