Sunday, March 14, 2021

SAFETY INSTRUMENTED SYSTEM IN INDUSTRIAL CONTROL PROCESS


INTRODUCTION

Chemical, petrochemical, gas plant, and many other manufacturing industries can be hazardous to work in because of hazards that could result in an explosion and death. A fire outbreak, liquid overflow, gas release, or undue exposure to lethal doses of radioactive radiation could all pose a threat. Manufacturing facilities generate items that are helpful, essential, and significant in our everyday lives, necessitating the continuation of production.

A process control system is installed to maintain a safe operation of the manufacturing facility, helped by robust detection and sensing systems controlled by experienced and qualified staff, in order to limit or possibly eliminate risk.In many circumstances, however, this technique is inadequate to reduce the risk of operational and safety dangers. A safety instrumented system, which is an involves the use of computational safety system, is required.

WHAT IS SAFETY INSTRUMENTED SYSTEM?

When the conditions appear to be hazardous and dangerous if action is not done, a safety instrumented system is a combination of complex automatic control systems that moves a manufacturing or industrial process to a safe state. They're utilized to keep track of the state of a plant's many metrics while keeping them within operational limitations. A safety instrumented system consists of both hardware and software technologies that are used to control a process' critical value. The unpredictability of essential process parameters has an impact on a plant's critical functioning. There may be temperature, cooling rate, spindle speed, pressure flow, overflow, as well as other essential features involved.


LAYERS OF PROTECTION PROCESS

Layers of protection process include the following:

 1.     Design process

2.     Basic process control system

3.     Alarm system

4.     Safety instrumented system

5.     Physical protection

6.     Plant emergency response

7.     Community emergency response

DESIGN PROCESS

The Design Process is the initial step in breaking down massive, complicated industrial processes into manageable state that ensure safety. Control engineers use the design process to describe the processes required to complete a project in the process industries safely. The design process includes explaining the challenge, collecting data and photos, data analysis with ideas, designing solutions, and getting feedback from others.

BASIC PROCESS CONTROL SYSTEM

A process control and monitoring system for a facility or piece of equipment is known as a basic process control system. The Basic Process Control System is in charge of ensuring that an industrial plant runs smoothly, and it is frequently employed to safeguard against dangerous situations.

ALARM SYSTEM

The goal of an alarm is to safeguard employees, equipment, or the process from potentially hazardous situations, or to notify the operator when conditions occur that could affect process quality.

SAFETY INSTRUMENTED SYSTEM

Safety Instumented System is designed to lower the frequency or severity of an identified emergency event in comparison to the top layers of protection. Employees, equipment, and the environment are all guaranteed to be safe. The job of the Safety Instrumented System is to monitor the process for potentially hazardous circumstances and act as needed to maintain it safe.. When specified conditions are breached, a Safety Instrumented System is made up of sensoring devices, logic solvers, and some control devices that controls the process to a comfortable state which guarantee better safe operation.

PLANT EMERGENCY RESPONSE

An emergency response is any planned action to a potentially dangerous or unforeseen event.. An emergency response procedure's purpose is to reduce the event's impact on people and the environment. People come first in emergency response strategies, followed by property. To contain, control, or end the issue should be the purpose of emergency responses. This includes evacuation buildings, extinguishing fires, disconnecting utilities, and other emergency responses.

COMMUNITY EMERGENCY RESPONSE

Volunteers are trained in basic disaster response skills such as fire safety, light search and rescue, team organization, and disaster medical operations through the Community Emergency Response program, which educates volunteers about disaster preparedness for risks that may affect their industry or community.

The first three layers of protection described above are provided by the process design, basic process control system, and alarm system. Each of these layers safeguards the process plant by ten times and more than the previous layers.The function of the last two layers of protection process can be inconsequential if a well integrated safety instrumented system is put in place. Even with the first three layers of safety in place, the danger of an accident occurring may be too significant to prevent. For example, in 2005, an explosion at a refinery in Texas City killed 15 people and injured more than 150 others. All of their plants were equipped with a control system, alarms, and well-trained workers. However, the protection systems in place were unable to reduce the risk to a bare minimum. Because comprehensive process controls systems were not in place to mitigate the risk, the risk remained. Despite the presence of control measures, frequent evaluations would have avoided the accident.

Occupational safety and health administration, as well as ISA and other professional groups like as IEC, support the concept of risk definition in order to manage risk. Standard ISA84 and IEC61508 were designed on the principle of functional safety to isolate risk associated with processing function. After some time, the ISA and IEC standards were merged. At order to limit functional risk, functional safety would be handled in a plant by installing a separate well-designed safety instrumented system.

Above and above the first three levels of protection, the Safety Instrumented System (SIS) provides an additional layer of safety. The level of protection procedure in a safety instrumented system should give at least a 10-fold reduction in the operation's risk. This decrease is referred to as a risk reduction factor of 10 or above (=>10).

Physical personal protection equipment (PPE) can also be employed to further limit danger. Planting a community response team, such as a fire department, can also aid in risk reduction.

 STRUCTURE OF SAFETY INSTRUMENTED SYSTEM

Sensors, logic solvers, and final control elements such as valves or actuators make up a safety instrumented system (SIS). Separate from the main control system, a safety instrumented system is a collection of specialized complex equipment. It can interlink with the core process system to provide a risk reduction factor of larger than 10 (>10) times.



A logic solver is a special PLC-like device with many processors that execute logic in parallel to verify the integrity of the logic and the action it produces. Because a safety instrumented system is built around the function of a safety instrumented system (SIF). The logic solver can take the SIS input and figure out what the state of the safety instrumented system output for that safety instrumented function should be (SIF).

Consider the movement of a liquid from a tank to a reactor. The flow controller, when used in conjunction with a standard control system, can swiftly and precisely transfer liquid. When the liquid level in the reactors reaches the high level mark, the flow is stopped by closing the control valve, preventing overpressurization of the reactor tank. The reactor overpressure protection is what defines a safety instrumented function (SIF).

RISK ASSESSMENT AND ANALYSIS

Although labor has numerous economic and other benefits, it also has a variety of industrial hazards that put people's health and safety at risk. Chemicals, biological agents, physical factors, unfavorable ergonomic conditions, allergens, a complex network of safety risks, and a wide range of psychosocial risk factors are just a few of them. Before I go any further, let me define hazard. A hazard is anything or any scenario that has the potential to cause injury to a worker. There are two types of dangers: safety hazards that result in physical injuries to workers and health hazards that result in the development of disease.. It's vital to remember that a "hazard" is just a possibility for harm. The toxicity of the health threat, the amount of exposure, and the duration of exposure will all influence whether it causes harm. Hazards can also be categorized based on the degree of harm they produce.

In the overall risk assessment and risk management process, hazard assessment is critical. Individual workplace hazards are identified, assessed, and controlled as near to the source as practicable. Hazard analysis shifts control closer to the source of the hazard as technology, resources, social expectations, and legal requirements evolve.

Prior to initiating an intervention, most modern industrial safety and health laws require a risk assessment and analysis. It's important to remember that risk management necessitates keeping risk to as low a level as is reasonably practicable. The likelihood or probability of the harm occurring and the severity of the repercussions are used to calculate risk. This can be described quantitatively as a rating or qualitatively as a description of the circumstances in which the harm may occur. When there is a substantial change in work procedures, the assessment should be recorded and reviewed on a regular basis.

When building a safety instrumented system, the design team must do a thorough risk analysis, identifying all potential risks and determining which ones necessitate the definition of a safety instrumented function (SIF). A thorough risk matrix can be used to determine the acceptable amount of risk. This is accomplished by providing a numerical value to the risk's expected frequency and instability. A risk assessment matrix is a tool that allows for a quick overview of potential risks on a single sheet, based on the risk's likelihood or probability and the severity of the consequences.


 SAFETY INTEGRITY AND RISK REDUCTION

A Safety Instrumented Level (SIL) is a metric for assessing the performance of a safety system in terms of the likelihood of failure on demand (PFD).

Even if devices in safety instrumented systems have a chance of failing, the probability that they will fail and cause the safety instrumented system to fail to respond when it is needed is known as Probability of Failure on Demand (PFD).The safety integrity level concept was adopted based on numbers: it's easier to communicate the chance of failure than it is to explain the probability of proper performance (e.g., 1 in 100,000 vs. 99,999 in 100,000). Safety Integrity Level is divided into four levels of integrity: Safety Integrity Level 1, Safety Integrity Level 2, Safety Integrity Level 3, and Safety Integrity Level 4. The greater the Safety Integrity Level, the higher the related safety level, and the less likely a system would fail to perform as intended. As the Safety Integrity Level rises, so do the system's installation and maintenance costs and complexity.

For example, a pressure regulator has a 1*10-1 PFD failure chance on demand (1 in 10). The probability of an isolation valve failing on demand is around 1 in 100 = 1*10-2 PFD. This value can be found on the device data sheet provided by the vendor.

For each SIF, we must compute the overall probability of failure on demand (PFD) for each function required in any safety instrumented system design.

 Safety Integrity Level 4 systems, in particular for the process industries, are so complex and expensive to develop that they are not cost-effective. Furthermore, if a process poses such a high level of risk that it necessitates the use of a Safety Integrity Level 4 system to get it to a safe condition, there is a fundamental flaw in the process design that must be addressed by a process change or other non-instrumented technique.

SAFETY INTEGRITY LEVEL

PROBABILITY OF FAILURE ON DEMAND (PFD)

4

0.0001

3

0.001

2

0.01

1

0.1

The higher a device's safety integrity level, the more trustworthy its safety integrity factor (SIF). Adding a redundancy control system is another method to reduce risk. It would improve system reliability while lowering risk. Two systems, obviously, would provide a higher level of safety response than a single one. A higher level of safety response would be provided by two out of three safety systems than by one out of two safety systems. The ISA-84/IEC61511 standards provide the approach for developing and documenting safety instrumented systems when they are being designed. We must, as a matter of course, adhere to the established requirements for testing safety instrumented functions (SIF). A Safety Instrumented Function (SIF) is a safety function that is implemented by the Safety Integrity System to reach or maintain a safe condition and has a specified Safety Integrity Level (SIL).



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