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.

Outlines

00:00

🔐 Introduction to Safety Instrumented Systems

The video introduces the concept of Safety Instrumented Systems (SIS), emphasizing their critical role in ensuring the safe operation of chemical, refining, and manufacturing plants. It acknowledges the inherent dangers in these industries, such as fire, explosion, and chemical exposure, and the impracticality of eliminating these risks by not operating such plants. The video sets the stage for discussing how SIS, along with process control systems and trained personnel, forms the first line of defense against these risks. Historical incidents like the Flixborough, Bhopal, and Texas City disasters are mentioned to highlight the importance of SIS in reducing the risk to a tolerable level.

05:06

🛠 The Role of Safety Instrumented Systems in Risk Mitigation

This section delves into the specifics of what a Safety Instrumented System is and its function in reducing operational risks. It explains that an SIS is a separate system from the basic process control, designed to bring the process to a safe state when predetermined conditions are violated. The paragraph introduces the concept of Safety Instrumented Functions (SIFs) and how they are identified and designed through a detailed risk analysis. The video also discusses the concept of tolerable risk levels and how they are determined by each company, often with industry benchmarks. It further explains the components of an SIS, including sensors, logic solvers, and final control elements, using a reactor overpressure protection example to illustrate how these components work together to provide an additional layer of safety.

10:09

📊 Safety Integrity Levels and Redundancy in SIS Design

The paragraph explores the importance of Safety Integrity Levels (SIL) in the design of SIS, which are determined by the Probability of Failure on Demand (PFD) for each SIF. It explains how PFD values are derived from vendor data or industry databases and how they influence the design of SIS to meet specific safety integrity levels. The video also discusses the concept of redundancy in SIS design, explaining how adding redundant systems can increase reliability and safety but also add to the cost. It outlines different redundancy configurations, such as 1 out of 2 and 2 out of 3 systems, and their impact on safety and cost. The paragraph concludes with a mention of the ISA-84/IEC-61511 standards, which provide a methodology for developing and documenting SIS, including design principles and management of change processes.

15:10

🏢 Conclusion and Call to Action for Further Learning

The final paragraph summarizes the importance of Safety Instrumented Systems in reducing the risk of accidents and injuries in industrial processes. It reiterates that SIS is one of many protective layers used by plants to safeguard processes, equipment, personnel, and communities. The video concludes with a call to action, encouraging viewers to visit realpars.com for more training material on PLC programming and to subscribe to their training series for further learning in the field of automation and controls engineering.

Keywords

💡Safety Instrument System (SIS)

A Safety Instrument System (SIS) is a separate and critical component in industrial plants designed to ensure safety by taking the process to a safe state when pre-determined conditions are violated. It is a layer of protection that operates independently of the basic process control system. The SIS is crucial for risk reduction in hazardous environments, as it includes sensors, logic solvers, and final control elements. In the video, the SIS is highlighted as an essential tool for preventing accidents in chemical and refining plants, with examples given of how it can stop a process to prevent over-pressurization.

💡Functional Safety

Functional safety refers to the concept of ensuring that systems perform their intended safety functions correctly and reliably. It is a key principle behind the development of standards like ISA 84 and IEC 61508, which were later harmonized into ISA-84/IEC-61511. The video emphasizes functional safety as a holistic approach to managing risks in industrial processes, moving beyond isolated risk assessments to consider the entire processing function.

💡Risk Reduction Factor

The Risk Reduction Factor (RRF) is a measure of how much a safety measure, such as an SIS, can reduce the likelihood of an accident. The video explains that the SIS should provide a risk reduction factor of at least 10, meaning it should decrease the risk of an operation by an order of magnitude. This is a critical metric in determining the effectiveness of safety measures in an industrial setting.

💡Basic Process Control System

The Basic Process Control System is the primary system that manages the day-to-day operations of a plant, including monitoring and controlling variables like pressure, temperature, and flowrate. It is the first line of defense in maintaining safe operations. The video contrasts this system with the SIS, emphasizing that while the basic system is essential, it may not be sufficient to reduce risks to a tolerable level, hence the need for an additional layer like the SIS.

💡Alarm Detection and Reporting System

An Alarm Detection and Reporting System is a component of the process control system that alerts operators to abnormal conditions, enabling them to take corrective actions before a risk becomes an accident. The video mentions this system as part of the initial layers of protection in a plant, highlighting its role in safety but also indicating that it may not be enough on its own to prevent serious incidents.

💡Safety Integrity Level (SIL)

Safety Integrity Level (SIL) is a classification used to specify the required reliability of a safety-related control system. The video discusses how the SIL is determined based on the Probability of Failure on Demand (PFD), with higher SILs indicating a lower probability of failure. This is crucial for designing SISs to ensure they provide the necessary level of safety.

💡Probability of Failure on Demand (PFD)

The Probability of Failure on Demand (PFD) is a measure of the likelihood that a safety device will fail to perform its required function when needed. The video explains how PFD values are used to determine the Safety Integrity Level required for a Safety Instrumented Function, with lower PFDs corresponding to higher SILs and more reliable safety functions.

💡Tolerable Risk Level

The Tolerable Risk Level is the acceptable threshold of risk that a company sets for its operations. The video discusses how each company must determine its own tolerable risk level, which is then used to design safety measures like the SIS. This concept is central to the video's discussion of risk management and the implementation of safety systems.

💡Safety Instrumented Function (SIF)

A Safety Instrumented Function (SIF) is a specific safety function within an SIS that is designed to perform a particular safety task, such as preventing overpressure in a reactor. The video uses the example of 'reactor overpressure protection' to illustrate how a SIF is implemented with a pressure sensor, logic solver, and control valve to provide an independent layer of protection.

💡Redundancy

Redundancy in the context of safety systems refers to the inclusion of backup or duplicate components to increase the reliability of the system. The video discusses how adding redundancy, such as a 1 out of 2 or 2 out of 3 system, can improve safety by providing alternative pathways to achieve a safe state if the primary components fail. However, it also notes that redundancy adds cost and may not always be practical or economically feasible.

Highlights

A Safety Instrument System (SIS) is crucial for maintaining the safety of chemical, refining, and manufacturing plants.

Process control systems and alarm detection systems are installed to maintain plant safety, but they might not be sufficient to reduce risks to a tolerable level.

Historical accidents like Flixborough, Bhopal, and Texas City highlight the need for more robust safety measures beyond basic process controls.

OSHA and industry groups developed standards like ISA 84 and IEC 61508 to address functional safety and risk mitigation.

The SIS is an additional layer of protection that should provide at least a 10-fold decrease in operational risk.

Chemical industry's Fatal Accident Rate (FAR) is lower than that of driving a car, indicating the effectiveness of safety measures.

Safety Instrumented Functions (SIFs) are designed to take the process to a safe state when predetermined conditions are violated.

The SIS consists of sensors, logic solvers, and final control elements, operating independently from the basic process control system.

A detailed risk analysis is necessary to identify potential risks and decide which require a SIF.

The Probability of Failure on Demand (PFD) is a key metric used to determine the reliability of SIFs.

Safety Integrity Levels (SIL) are used to categorize the required reliability of a SIF, with SIL 4 being the highest.

Redundancy in SIS design can increase reliability but also adds cost.

ISA-84/IEC-61511 standards provide a methodology for developing and documenting SIS.

The SIS is one of many layers of protection used in plants to safeguard processes, equipment, personnel, and the community.

RealPars offers training materials for PLC programming and automation controls engineering.

Transcripts

In this video, you will learn what a Safety Instrument System is,

how it is constructed, and how it plays an important role in keeping our chemical,

refining, and other manufacturing plants running safely

and as productive community partners and employers.

Before we get into today’s video, if you love our videos,

be sure to click the like button below,

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Chemical, petrochemical, mining, gas compression,

and many other types of plants and manufacturing facilities

can be very dangerous places to work due to the presence of risk:

risk due to fire, explosion, tank overflow, gas release, or chemical exposure.

The only way to eliminate these risks is to not build or operate these types of plants.

But that is not practical.

These plants produce materials that are useful,

necessary, and important in our everyday lives.

Even a product like dry powdered laundry detergent is made

via a process that includes pumping liquids at high pressure,

spraying droplets into very hot air,

and collecting the product below which may be dusty and pose an inhalation hazard.

In order to minimize these risks,

process control systems are installed to maintain a safe operation of the plant,

assisted by a robust alarm detection and reporting system,

and operated by trained, qualified personnel.

But often, these measures alone cannot reduce the risk of injury,

fire, explosion, or other risks to a tolerable level.

Regardless of the types of risks, the process design itself,

the basic process control system, alarms, and operator intervention,

provide the first layers of protection for the process.

Each of these layers provides approximately a 10-fold

or greater protection to the process plant than the layer below.

In the process design, care is taken to specify lines,

equipment, and valves with the right sizes,

materials of construction, and proper accessories.

The basic process control system is installed with the appropriate instruments,

controls, and monitoring logic to allow the plant to be operated

within the safest ranges for pressure, temperature and flowrate.

Alarms are configured to allow the operators to react to abnormal conditions

and take corrective actions before a risk becomes an accident.

Even with all of these layers of protection in place,

the risks may still be too great to prevent an accident from happening.

A couple of examples illustrate this.

In 1974, a nylon plant in Flixborough, England,

exploded, killing 28 and injuring more than 100.

In 1984, a gas leak in a fertilizer plant in Bhopal, India,

killed over 3000 and injured 200,000.

More recently, in 2005, an explosion at a Texas City refinery

killed 15 and injured more than 150.

All three of these plants had control systems, alarms, and trained operators.

But these first three layers of protection do not reduce a hazardous plant’s risk to a tolerable level.

The risks associated with production at Flixborough were not all well-defined,

and the proper controls were not in place to minimize those risks.

At Bhopal, systems were in place to prevent the resulting gas leak

but did not take into account the scenario that led to the accident.

In Texas City, several technical and operational shortcomings led to an explosion.

In order to mitigate risks like the ones above, OSHA,

The Occupational Safety and Health Administration,

and several companies in the chemical industry,

along with ISA and other professional groups,

embraced the idea of defining risks, not as isolated processing line or tank risks,

but as risks associated with processing functions as a whole.

Standards ISA 84 and IEC 61508 were developed around the concept of functional safety.

Later, these standards, ISA in the US and IEC in Europe,

were harmonized in a single standard, ISA-84/IEC-61511.

The way functional safety would be addressed in a plant

in order to reduce functional risks was to install a separate,

well-designed, Safety Instrumented System.

The Safety Instrumented System, or SIS,

represents an additional layer of protection above the first three layer discussed previously.

This layer should provide at least a 10-fold decrease in the risk of the operation.

This decrease can be called a risk reduction factor of equal to or greater than 10.

So as we have seen, many levels of protection are required

to reduce the risk of an operation to a tolerable risk level.

This level of tolerable risk must be determined by each individual company,

but there are benchmarks for many industries,

such as chemical, oil & gas, food & beverage, and others.

Overall, the chemical industry has a Fatal Accident Rate, or FAR, of 4.

Driving a car has an FAR of 40.

Fatal Accident Rate is just one way that overall risk can be measured.

And in addition to the layers discussed so far,

others can be added to reduce the overall risk even greater,

like physical protection devices, such as relief valves and dikes,

and plant and community response teams, like fire departments.

So, now let’s answer what a Safety Instrumented System is.

A Safety Instrumented System is comprised of sensors,

logic solvers, and final control elements

for the single purpose of taking the process to a safe state

when pre-determined conditions are violated.

This means that the Safety Instrumented System, or SIS,

is a separate set of devices from the basic process control system.

In order to provide a risk reduction factor of greater than 10X,

it cannot be interlinked with the basic process control system,

and any of shortcomings of that system.

The logic solver is a specialized, hardened PLC- like device

that may have multiple processors executing the logic in parallel

to insure integrity of the logic and resulting action.

The SIS is designed around individual functions in the plant,

called Safety Instrumented Functions, or SIF for short.

The logic solver takes the SIS inputs

and determines what the state of the SIS outputs should be for that SIF.

Consider this process for transferring a liquid from a tank to reactor.

Normally, the flow controller,

which resides in the basic process control system,

can easily make the transfer of liquid in a very controlled, repeatable manner.

When the reactor level reaches a high alarm point,

the flow is stopped by shutting the control valve in order to keep the closed tank from over-pressurizing.

Let’s define our Safety Instrumented Function as “reactor overpressure protection”.

Now, let’s add the pieces of the SIS that are required to implement the components required for this function.

As you can see, we keep the basic process flow control loop in place,

operating as it normally does.

But now, we add a pressure sensor, logic solver,

and a positive shutoff valve to stop the flow independent of the flow controller

and the basic process control logic.

We have provided an independent layer of protection against reactor overpressure.

This improves the overall safety of the process.

In designing a Safety Instrumented System,

the design team must do a detailed risk analysis,

identifying all of the potential risks and deciding which of the risks

require a Safety Instrumented Function to be defined.

A detailed risk matrix can be used to identify the level of risk that is tolerable,

and at what point a function require as a SIF to be defined.

This can be done qualitatively,

or quantitatively by assigning numerical values to the expected frequency and severity of the risk.

Even a Safety Instrumented System has a probability to fail.

What if the pressure sensor in the previous example does not detect the high pressure condition?

What if the isolation valve does not close when it is told to?

The probability that a device, whether input, output, or logic solver,

will fail causing the SIF to not respond when called upon,

is called the Probability of Failure on Demand, or PFD.

For instance, a pressure regulator has approximately a 1 in 10,

or 1 x 10-1 , probability of failure in a years’ time.

Failure of an isolation valve is about 1 in 100, or 1 x 10-2.

These values can be obtained from vendor data for specific devices,

or from industry databases of typical PFD’s for each type of device.

When we design an overall safety instrumented system for each safety instrumented function,

we need to determine the overall Probability of Failure on Demand

or PFD for each function that is required.

If we determine the PFD should be less than 0.01, or 1 x 10-2,

then our SIF needs to be designed to a Safety Integrity Level of 2.

Similarly, a PFD of less than 1 x 10-1 requires a safety integrity level of 1,

and a PFD of less than 1 x 10-3 requires a safety integrity level of 3.

We can look up the PFD values for each of the devices

and logic solver elements we would like to use,

but to determine the overall PFD for an individual SIF usually requires a computer program.

Suffice it to say, the higher the safety integrity level,

the more reliable the safety instrument function will be.

A Safety Integrity Level of 4 is possible,

or a PFD of 1 x 10-4, but is usually not practical or economically feasible.

Another way to reduce risk is to add redundancy.

Redundancy adds cost, but generally will increase the reliability of the system and reduce risk.

A 1 out of 2 system will provide a greater

level of safety response than a simplex system.

A 2 out of 3 fault-tolerant system

can provide a greater level of safety response than a 1 out of 2 system.

While the 2 out of 3 system may be more reliable,

it may be installed at a much higher cost than a 1 out of 2 system.

Likewise, a 1 out of 2 system will have a higher cost than a simplex system.

When designing a Safety Instrumented System,

the ISA-84/IEC-61511 standards prescribe a methodology

for developing and documenting the system.

Certain design principles should be followed,

such as not allowing on-line changes to a logic solver,

requirements for testing the SIF, and a Management of Change process

for making any changes to the system once the design has been approved.

To review, past accidents and fatalities have led to a new way of looking at risk in a processing plant.

We now look at Safety Instrumented Functions in order to mitigate risk

and provide a safer operating environment.

The goal of the Safety Instrument System is to reduce the risk of accident or injury.

The SIS is only one of many layers of protection

that a plant uses to safeguard the process,

equipment, personnel, and the community.

But when implemented correctly, it can provide a very large reduction in the overall risk profile.

Safety Instrumented Systems are comprised of sensors,

logic solvers, and final control elements which are separate from all basic process control system elements,

and the logic solver drives the final control elements to the state required

to provide a safe state if the inputs indicate an abnormal situation.

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