Every industrial plant and manufacturing facility has inherent risks due to factors like fire, explosion, or chemical exposure. Eliminating these risks by not building or operating such plants is impractical because they produce materials that are essential in our everyday lives. To mitigate these risks and create a safe operating environment, process control systems are installed, operated by trained personnel, and equipped with robust alarm detection and reporting systems.
However, even with these measures in place, accidents can still occur. To address this issue, organizations like OSHA and professional groups in the chemical industry developed standards around the concept of functional safety. This led to the creation of the Safety Instrumented System (SIS), which serves as an additional layer of protection to reduce the risk of accidents.
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What is a Safety Instrumented System?
A Safety Instrumented System (SIS) consists of sensors, logic solvers, and final control elements that work together to bring a process to a safe state when predetermined conditions are violated. Unlike the basic process control system, the SIS is a separate set of devices designed specifically for the purpose of risk reduction. It operates independently and is not interlinked with the basic process control system.
The logic solver, a specialized and hardened device similar to a PLC, executes the logic to determine the state of the SIS outputs for each Safety Instrumented Function (SIF). Each SIF is designed to address a specific risk and is associated with a process function in the plant. By implementing an SIS, an additional level of protection is provided, enhancing the overall safety of the process.
Understanding Risk Reduction and Safety Integrity Levels
Reducing risk is crucial in ensuring the safety of industrial processes. When designing a Safety Instrumented System, a detailed risk analysis is conducted to identify potential risks and determine which of them require a Safety Instrumented Function. The level of risk that is tolerable must be established by each individual company, taking into account industry benchmarks.
To assess the reliability and performance of the Safety Instrumented System, a metric called the Probability of Failure on Demand (PFD) is used. The PFD measures the probability that a device within the SIS will fail to respond when called upon. Safety Integrity Levels (SIL) are assigned to SIS based on the desired PFD. A higher SIL indicates a more reliable and robust safety instrument function.
Redundancy and Design Principles
Redundancy, or the use of duplicate components or systems, is another strategy to enhance the reliability of a Safety Instrumented System. Redundancy adds cost to the system but can significantly reduce risk. For example, a 2 out of 3 fault-tolerant system provides a higher level of safety response compared to a 1 out of 2 system, but it may come at a higher installation cost.
When designing a Safety Instrumented System, following design principles and best practices is crucial. These include preventing online changes to the logic solver, implementing testing procedures for SIFs, and establishing a Management of Change process for making any modifications to the system. Adhering to these principles ensures the integrity and effectiveness of the SIS.
FAQs
Q: What is the purpose of a Safety Instrumented System?
A: The purpose of a Safety Instrumented System is to reduce the risk of accidents or injuries in industrial plants and manufacturing facilities. It provides an additional layer of protection beyond the basic process control system.
Q: How does a Safety Instrumented System differ from the basic process control system?
A: A Safety Instrumented System is separate from the basic process control system and consists of specialized devices such as sensors, logic solvers, and final control elements. It operates independently and is designed specifically for risk reduction.
Q: How is the reliability of a Safety Instrumented System assessed?
A: The reliability of a Safety Instrumented System is assessed using the Probability of Failure on Demand (PFD). The PFD measures the probability that a device within the SIS will fail to respond when required. A higher Safety Integrity Level (SIL) indicates a more reliable system.
Conclusion
In summary, a Safety Instrumented System is a crucial component of industrial plants and manufacturing facilities. By providing an additional layer of protection, the SIS reduces the risk of accidents or injuries. With sensors, logic solvers, and final control elements working together, it ensures the process can be brought to a safe state when abnormal conditions occur. Adhering to design principles and industry standards is essential in developing an effective and reliable Safety Instrumented System.
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