What is a safety instrumented system?

Organizations that handle hazardous materials as part of their core business processes rely on safety instrumented systems to keep workers safe and reduce the likelihood of catastrophic incidents like fires, explosions, toxic chemical releases and nuclear meltdowns.

Safety instrumented systems are closely related to another type of business process system known as a basic process control system (BPCS). However, while a BCPS focuses on maintaining normal business operations, an SIS focuses exclusively on safety. When an SIS detects hazardous conditions, it initiates a predefined response like releasing pressure from a valve or activating an emergency shutdown system.

Safety instrumented systems are primarily used in process industries, which are industries that manufacture products by transforming raw materials through chemical, physical, biological or thermal processes. These industries include oil and gas, chemical manufacturing, power generation, pharmaceuticals and water treatment. To manage risk, businesses operating in these industries deploy safety-related systems to detect dangerous conditions and restore systems to a safe state.

The primary goal of an SIS is to restore a system or process to a safe state, which is the condition in which a facility or individual piece of equipment is safe again following a dangerous event.

At the core of every safety instrumented system are one or more safety instrumented functions (SIFs), actions the SIS takes when it detects hazardous conditions. For example, if an SIS detects dangerous levels in a chemical reactor, its emergency shutdown valves and pump controls can automatically prevent any more material from flowing into the reactor.

All safety instrumented systems are closely regulated through standards established by The International Electrotechnical Commission (IEC). Standards IEC 61508 and IEC 61511 provide detailed frameworks for organizations to follow when building SIS architectures.

A safety instrumented system (SIS) functions by monitoring systems and processes and taking predefined actions when it detects dangerous conditions. System designs vary by application and industry, but most SIS architectures consist of three primary components:

• Sensors
• Logic solvers
• Final control elements

The first layer of an SIS consists of sensors and transmitters that constantly measure subtle changes in pressure, temperature, level and flow inside a system or process. These small devices provide real-time data about operating conditions and detect deviations that can indicate that hazardous conditions are present.

Most modern SIS tools rely on multiple sensors to provide redundancy and reduce the likelihood of false alarms. Redundant architectures lower the likelihood of a single device failure eluding detection and help make software-instrumented systems more reliable.

The logic solver is the decision-making component of an SIS, responsible for receiving input from sensors and transmitters and evaluating whether the data meets predefined safety conditions. If the data the logic solver evaluates indicates a hazardous condition exists, it can automatically execute a safety response.

In most modern industrial automation environments, logic solvers rely on specialized controllers known as safety programmable logic controllers (PLCs) to receive and evaluate signals and initiate safety functions. Unlike conventional PLCs, which focus on optimizing production, safety PLCs only control functional safety applications inside an SIS.

Final control elements are the actual, physical components (operational technology) inside the SIS that execute changes in a process and restore it to a safe state. Some common final control elements are valves, isolation valves, motor starters, relays, dampers and actuators. When the logic solver detects hazardous conditions, the final control elements take a predefined protective action.

For example, when a sensor indicates to the logic control that the levels of hazardous material in a pipe exceed preset limits, the logic solver deploys actuators to physically close the valves and stop the flow of material. If performed correctly, this process restores the system to a safe state.

Safety lifecycle management is a tightly structured process that organizations rely on to ensure a safety instrumented system (SIS) and its instrumented functions perform correctly.

Three key components are critical to effectively managing the safety lifecycle and ensuring an SIS is effective:

• Safety integrity levels: Safety integrity levels (SILs) are the level of risk reduction required by a specific safety instrumented function in an SIS. SILs range from SIL 1 to SIL 4, with higher levels indicating increased reliability. For most organizations, SIL 2 and SIL 3 are acceptable because they provide a significant risk reduction while still being practical and cost-effective.
• Safety requirement specifications: A safety requirement specification (SRS) defines the reliability and operational requirements for a specific safety function in an SIS. While setting up an SIS, organizations typically conduct different types of hazard and risk assessments. The most common are hazard and operability studies (HAZOPs) and layers of protection analysis (LOPA). These widely accepted methodologies help machine operators and technicians determine the right safeguards to have in place around potentially hazardous processes.
• Proof testing: Finally, organizations that design an SIS and put it into operation must constantly test to verify its safety functions work properly. Proof testing involves periodic inspections of safety instrumented systems and carefully coordinated tests to verify the SIS will operate correctly when a hazardous condition arises. In addition to proof testing, organizations also track key metrics around equipment involved in hazardous processes like failure rate and probability of failure on demand (PFoD).

Modern industrial facilities are increasingly reliant on complex automation technologies like artificial intelligence (AI) and the Internet of Things (IoT). While these technologies improve efficiency and increase productivity, they also introduce new risks to environment, health and safety (EHS) and business continuity. For example, a single equipment malfunction in a modern facility can escalate into a hazardous event if adequate safeguards are not in place.

Implementing a safety instrumented system (SIS) helps organizations increase the reliability of their processes, improve worker safety and better protect their most important assets. According to a recent report, demand is rising for SIS tools, with its current market of USD 5.26 billion projected to reach USD 7.32 billion over the next 5 years.

Here’s a look at some common benefits of deploying an SIS at the enterprise level.

An SIS enhances process safety management by continuously monitoring processes and systems that can result in hazardous conditions. According to one report, an SIS can reduce workplace incidents by up to 80% through its detection and response capabilities. By automatically tracking variables and initiating protective actions, safety instrumented systems help reduce the likelihood of accidents and catastrophic failures.

Workers in industrial facilities, for example, often handle hazardous and combustible chemicals as part of their daily job. Safety shutdown systems—initiated by an SIS—can alert them to potential exposure and take corrective action. When conditions exceed certain tolerance thresholds, an SIS can respond faster and more consistently than a human.

Many of the processes and systems that an SIS monitors are core to business operations management, the design and optimization of business processes to facilitate efficiency and resilience. In addition to presenting safety issues, asset failures can result in significant financial loss.

By preventing damage to reactors, pipelines, compressors, turbines and other critical assets, an SIS can reduce repair costs and unplanned downtime. For example, large centrifugal compressors are some of the most critical and expensive assets in the natural gas industry. Technicians and machine operators program safety instrumented systems to monitor flow rate, pressure and vibration levels to prevent a condition called surge, where gas flowing through the compressor becomes unstable and can damage the asset.

Designing and operating an SIS helps organizations improve their compliance monitoring capabilities in several ways. First, process industries are often subject to strict safety regulations and standards because of the materials that are core to their business processes. Complying with IEC 61508 and IEC 61511 demonstrates an organization’s commitment to worker safety and shows that they have properly implemented best practices for functional safety management.

Second, regulators and insurance providers often require that facilities maintain documented safety programs that include their SIS design, proof testing documentation and recorded maintenance activities.

A reliable SIS helps an organization reduce the likelihood of a false shutdown by stopping equipment only when conditions are truly dangerous. Without an SIS, machine operators must rely on manual shutdown, increasing the likelihood of false shutdowns because manual processes aren’t as reliable as automated ones.

Also, operating equipment under unsafe conditions increases the likelihood of failure. A properly designed SIS prevents catastrophic equipment damage and helps reduce the number of emergency repairs, collateral failures and unplanned outages.

Safety instrumented systems help improve risk management initiatives by providing an independent layer of protection (ILP) against hazardous events. While proper procedures and operator intervention help reduce risk during training, an SIS can actively monitor conditions and intervene when they become unsafe.

Here are some common examples of hazardous events that safety instrumented systems help prevent:

• Failure of control valves to regulate toxic chemical levels
• Failure of process control systems to maintain safe conditions
• Failure of operators to recognize hazardous conditions when they exist
• Failure of bearings, pipes, motors and other hardware components

Safety instrumented systems are primarily used in process industries to protect the processes that transform raw materials like oil, water and chemicals into products. Here are five of its most common instances:

• Oil and gas: The oil and gas sector relies on safety instrumented systems to monitor refineries, offshore drilling platforms, pipelines and processing plants. Safety instrumented systems designed specifically for oil and gas regularly perform high-pressure shutdowns, detect dangerous chemicals and perform automated isolation of equipment.
• Chemical manufacturing: Chemical manufacturers rely on safety instrumented systems to monitor reactive, toxic or flammable liquids and gases. In a chemical manufacturing plant, an SIS monitors reactor temperatures, liquid and gas pressures and chemical concentrations to keep worker’s safe and prevent catastrophic failures.
• Power plants: Power plant operators use safety instrumented systems to protect valuable assets like turbines, boilers and auxiliary systems. Safety functions for an SIS in a power plant typically include turbine overspeed protection, boiler shutdown and monitoring of cooling systems.
• Pharmaceutical manufacturing: In the pharmaceuticals industry, safety instrumented systems help ensure safe operation of production equipment and critical systems. Many pharmaceutical plants use flammable solvents, toxic chemicals and high-pressure systems. An SIS designed for a pharmaceutical plant workload helps regulate synthesis reactors, solvent storage systems, distillation units and high-pressure reactors.
• Wastewater treatment facilities: Water treatment facilities use SIS technology to carefully manage chemical dosing systems, storage tanks and pumping stations. Safety functions for wastewater treatment help prevent a wide range of potentially dangerous incidents like conveyor system malfunctions, gas and toxic chemical leakage, fires and explosions.