Safety Instrumented Systems (SIS) in Critical Infrastructure: Definition, Uses & Examples

Instructor: Erik Rodriguez

Erik has experience working in Cybersecurity and has a Master's of Science in Information Systems.

In this lesson, we will discuss what safety instrumented systems are and the elements they are composed of. We will also review examples of how SIS are used in the real world.

Safety First!

Much like how your car has various safety features such as airbags and predictive collision alerts that are designed to protect you on your daily commute, so do industrial plants. Due to safety regulations, most, if not all, industries must employ the use of safety instrumented systems to create a safe work environment for its employees. Without these systems, accidents could be more common and the likelihood for human endangerment would increase exponentially. However, what exactly are safety instrumented systems and what do they consist of?

SIS Defined

As the name suggests, Safety Instrumented Systems, or SIS, are those systems tasked with the operational safety of the production process. SIS are employed to ensure that emergency processes are enacted in the event of fires, equipment malfunction, accidents resulting from human error, and other emergency events. SIS can and should be able to trigger alarms, automatic shutdowns for equipment, sprinkler systems, and even send out alerts to personnel informing them of the triggering event. Ultimately, SIS are meant to protect human lives, minimize financial loss, and lessen the damage and impact on the environment and general population in the event of a workplace accident.

Elements of an SIS

Although different SIS have different uses, they are all comprised of the same basic elements. These include sensors, a logic system, and the final control element. These three elements work together to detect when a safety issue occurs and trigger any emergency procedures in order to stop or contain the event.

Sensors

Sensors form the first component in an SIS. Sensors are used to measure production parameters such as temperature, weight, water flow, air pressure, and much more. Personnel dictate what are acceptable levels for these parameters. Throughout the production process, sensors provide continuous monitoring and feed that information to the logic system.

Logic System

Perhaps the most important element in an SIS, the logic system is tasked with analyzing the information it receives from sensors in order to determine what action, if any, needs to be taken. As sensors feed information to it, the logic system processes the data nearly instantly. During its analysis, the logic system will determine whether all operational processes are running smoothly or if emergency procedures need to be deployed. Should the logic solver detect a safety issue, it will forward that information to the final control element.

Final Control Element

As the name suggests, the final control element is the last component in an SIS. The final control element is responsible for deploying any actions deemed necessary by the logic system. This can include shutting down equipment and machinery, triggering alarms, calling emergency services, or any other action predetermined by personnel. As such, the final control element is directly responsible for preventing accidents and other catastrophes.

Examples and Uses for SIS

Wastewater Treatment

One common use of SIS can be seen in wastewater treatment plants. As wastewater goes through the treatment process, it passes along a series of pipes into holding tanks. These holding tanks contain sensors which detect when the water level has reached its limit. This information is then analyzed by the logic system and transferred to the final control element. The final control element then shuts off the valves that feed the water into the holding tanks to prevent an overflow or other accidents from occurring.

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