What Are Safety Instrumented Systems (SIS)?
A safety instrumented system (SIS) is a system that contains control elements, sensors, and logic solvers to monitor parameters and values of processes within a plant and ensure that they remain within defined limits. In case those limits are violated, safety instrumented systems deploy a series of actions to take the process to a safe state.
Safety conditions in a plant must be clearly defined and must be followed closely by the employees. SIS ensures that in case the safety of certain processes is compromised, appropriate action is triggered to mitigate risk and make those processes safe again.
The Main Components of Safety Instrumented Systems
Safety instrumented systems are independent from the process control system, and include three main components: sensors, logic solver, and the final element.
The sensors are mounted on machines, cables, and valves, and are used to detect any abnormal deviations. Sensors are often the first point in safety instrumented systems, and can categorize deviations as low-level or high-level.
The Logic Solver
The logic solver or a programmable logic controller (PLC) is a component that takes data from the sensor, evaluates it, and then issues a command. For instance, if the sensor detects high voltage in one of the cables, higher than the predetermined levels, the logic controller will immediately issue a command to close the element.
Logic solvers are specialized devices that are often installed in rough conditions, so they usually have a hard casing. Multiple processing units are installed to execute the logic while ensuring the integrity of the system.
The final element is the last guard within a safety instrumented system, and plays a critical role in maintaining the overall safety of the entire system. It acts upon the command issued by the logic solver by terminating connections, opening or closing valves, or taking relevant steps to take the process to a safe state.
Understanding Safety Instrumented Functions
Safety instrumented systems take into account discrete functions throughout the facility. These are known as Safety Instrumented Functions (SIF).
Data from the sensors for each SIF is entered into the system, and the logic solver then determines acceptable SIS outputs for each SIF. These acceptable outputs must meet Safety Integrity Level (SIL) targets.
IEC 61508 — The Basis of Safety Instrumented Systems
For many years, safety systems followed key design principles proposed by German standards DIN V 19250 and DIN V VDE 0801. A global standard, IEC 61508, was then adopted, which is used in almost all operational safety systems, including electrical, electronic, and programmable machinery.
Safety systems must provide steps to mitigate three important failures:
- Failures due to common causes
- System failures
- Failures detected in random hardware
As digital instrumentation and equipment becomes more and more mainstream, it is important that those in charge of health and safety have proper qualifications and have the required knowledge needed to evaluate the level of safe performance. This helps determine safe performance limits which are used for calibrating the safety instrumented systems.
How Do Safety Instrumented Systems Work?
A distributed control system is generally responsible for monitoring inputs, such as temperature, process values, pressure, liquid flow, voltage, or other variables. The SIS is capable of manipulating the final element, which includes components like actuators or valves.
The SIS issues an alarm when the values increase beyond safe levels, which prompts the operator to take remedial steps. But, if the operator is unable to contain the situation, the SIS takes steps to reduce the possibility of a major incident.
Safety instrumented systems are commonly used in process facilities, including nuclear or chemical plants, and refineries. They provide protection against accidents caused due to high fuel gas pressure or rising reactor temperatures, for instance.
Example of a Safety Instrumented System at Work
A common example of safety instrumented systems at work is in wastewater treatment plants. Wastewater that undergoes different treatments is often moved from one holding tank to another using pipes with a varying diameter.
Sensors are installed within the tanks to determine when the maximum limit of a holding tank is reached. The logic solver monitors the levels and shuts off the main valves as soon as process variables exceed the limit. This prevents overflow risk and ensures safe performance.
In this scenario, the SIF would be to stop the pumps from filling the tank when a safe level is reached.
This prevents the tank from overfilling. Safety instrumented functions are implemented by the SIS to move processes to a “safe state.”
Similarly, if high temperature is a major hazard in any process, the SIS should have built-in functionality to activate a relay to cut power once the temperature reaches its maximum safe levels.
The Safety Life Cycle
The Safety Life Cycle has been mentioned in two voluntary standards, namely:
The Safety Life Cycle serves as a framework that organizations can use to identify process hazards, and determine which ones require the implementation of safety instrumented systems to reduce risks.
This is a cyclical process, so in case any changes are made to the operating conditions or the design of the process, the framework must be repeated altogether to ensure new changes are properly integrated.
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It allows you to aggregate data from multiple safety instrumented systems within the organization, evaluate their efficacy, and allow for a more comprehensive analysis of how your safety processes are performing. There’s a built-in notification system, so managers are always in the know without having to access the system!