What is a Safety Instrumented System?

Summary

This video script delves into the critical role of Safety Instrumented Systems (SIS) in safeguarding chemical and manufacturing plants. It underscores the necessity of SIS as an additional layer of protection, beyond basic process control, to mitigate risks that could lead to accidents. The script explains the structure of SIS, including sensors, logic solvers, and final control elements, and their purpose of driving the process to a safe state. It also touches on the importance of risk analysis, Safety Integrity Levels, and redundancy in enhancing system reliability. The video aims to educate on the measures that contribute to a safer operating environment in high-risk industries.

Takeaways

• 🔐 A Safety Instrumented System (SIS) is a critical component in industrial plants, designed to ensure safety by taking the process to a safe state when predetermined conditions are violated.
• 🏭 The SIS is an additional layer of protection that complements the basic process control system, alarms, and operator intervention, aiming to reduce the risk of injury, fire, or explosion.
• 📉 The SIS should provide at least a 10-fold decrease in the risk of operation, known as a risk reduction factor.
• ⚠️ Historical accidents, such as those in Flixborough, Bhopal, and Texas City, highlight the necessity for SIS to mitigate risks that basic process controls might not address.
• 🛠️ The SIS consists of sensors, logic solvers, and final control elements, operating independently from the basic process control system to ensure integrity.
• 🔍 A detailed risk analysis is essential for designing an SIS, identifying potential risks and determining which require a Safety Instrumented Function (SIF).
• 📊 The Probability of Failure on Demand (PFD) is a key metric used to assess the reliability of SIFs, with lower PFD indicating higher reliability.
• 🔢 Safety Integrity Levels (SIL) are used to categorize the required reliability of a SIF, with SIL 4 representing the highest level of reliability, though it may not always be practical.
• 💡 Redundancy in SIS design can increase reliability and reduce risk, but it also adds to the cost of the system.
• 👨‍🔧 Standards like ISA-84/IEC-61511 provide a framework for the development and documentation of SIS, emphasizing principles like no online logic solver changes and strict testing requirements.

Q & A

What is a Safety Instrumented System (SIS)?

A Safety Instrumented System (SIS) is a separate set of devices from the basic process control system, designed to take the process to a safe state when pre-determined conditions are violated. It includes sensors, logic solvers, and final control elements.

Why are SISs important in industrial plants?

SISs are crucial in industrial plants because they provide an additional layer of protection to reduce the risk of injury, fire, explosion, or other hazards to a tolerable level, ensuring the safety of the process, equipment, personnel, and the community.

How does an SIS differ from a basic process control system?

An SIS is separate and independent from the basic process control system. It is designed to provide a risk reduction factor of greater than 10X and is not interlinked with the basic process control system to avoid its shortcomings.

What is a Safety Instrumented Function (SIF)?

A Safety Instrumented Function (SIF) is an individual function within a plant that is designed to perform a specific safety task, such as 'reactor overpressure protection,' using the components of the SIS.

What is the role of a logic solver in an SIS?

The logic solver in an SIS is a specialized, hardened PLC-like device that processes inputs from sensors and determines the appropriate state of the SIS outputs to maintain safety in response to abnormal conditions.

What is the significance of the Probability of Failure on Demand (PFD) in SIS design?

The Probability of Failure on Demand (PFD) is a measure of the likelihood that a device within the SIS will fail to respond when called upon. It is used to determine the Safety Integrity Level (SIL) required for each SIF to ensure the system meets the necessary reliability standards.

How is redundancy used to enhance the reliability of an SIS?

Redundancy in an SIS involves having multiple layers or components that perform the same function, which increases the system's reliability by providing backup in case one component fails. Examples include 1 out of 2 or 2 out of 3 fault-tolerant systems.

What standards guide the development and documentation of an SIS?

The development and documentation of an SIS are guided by standards such as ISA-84/IEC-61511, which prescribe methodologies for designing, testing, and managing changes to the system to ensure its effectiveness and safety.

Why is a detailed risk analysis important in designing an SIS?

A detailed risk analysis is essential in designing an SIS because it identifies all potential risks and determines which risks require a Safety Instrumented Function to be defined. This analysis helps in deciding the tolerable level of risk and the necessary safety measures.

How does the Fatal Accident Rate (FAR) compare between the chemical industry and driving a car?

The Fatal Accident Rate (FAR) in the chemical industry is 4, which is significantly lower than the FAR of driving a car, which is 40. This comparison highlights the effectiveness of safety measures, including SISs, in reducing risks in the chemical industry.