Basic Fundamentals of Safety Instrumented Systems SIS
The operation of many industrial processes involve inherent risks due to the presence of dangerous material like gases and chemicals. Safety Instrumented Systems SIS are specifically designed to protect personnel, equipment and the environment by reducing the likelihood (frequency) or the impact severity of an identified emergency event.
Explosions and fires account for millions of dollars of losses in the chemical or oil and gas industries each year. Since a great potential for loss exists, it is common to employ Safety Instrumented Systems SIS to provide safe isolation of flammable or potentially toxic material in the event of a fire or accidental release of fluids.
This online training tutorial will explain the basic concepts, definitions and commonly used terms in Safety Instrumented Systems SIS and provide a basic understanding of related concepts.
Basics of Safety and Layers of Protection
Safety is provided by layers of protection. These layers start with safe and effective process control, extend to manual and automatic prevention layers, and continue with layers to mitigate the consequences of an event.
The first layer is the Basic Process Control System BPCS. The control system itself provides significant safety through proper design of process control.
The next layer of protection is also provided by the control system and the system operators. Automated shutdown sequences in the process control system combined with operator intervention to shut down the process are the next layer of safety.
The third layer is the Safety Instrumented System SIS. It is a safety system independent of the process control system. It has separate sensors, valves and logic system. No process control is performed in this system, its only role is safety.
These layers are designed to prevent a safety related event. If a safety related event occurs there are additional layers designed to mitigate the impact of the event.
The fourth layer is an active protection layer. This layer may have valves or rupture disks designed to provide a relief point that prevents a rupture, large spill or other uncontrolled release that can cause an explosion or fire.
The fifth layer is a passive protection layer. It may consist of a dike or other passive barrier that serves to contain a fire or channel the energy of an explosion in a direction that minimizes the spread of damage.
The final layer is plant and emergency response. If a large safety event occurs this layer responds in a way that minimizes ongoing damage, injury or loss of life. It may include evacuation plans, fire fighting, etc.
Overall safety is determined by how these layers work together.
Basics of Safety Instrumented Systems SIS
Typically, Safety Instrumented Systems consist of three elements: A Sensor, a Logic Solver and a Final Control Element
Field sensors are used to collect information necessary to determine if an emergency situation exists. The purpose of these sensors is to measure process parameters (e.g. temperature, pressure, flow, etc.) used to determine if the equipment or process is in a safe state. Sensor types range from simple pneumatic or electrical switches to Smart transmitters with on-board diagnostics. These sensors are dedicated to the Safety Instrumented System SIS.
The purpose of this component of Safety Instrumented Systems SIS is to determine what action is to be taken based on the information gathered. Highly reliable logic solvers are used which provide both fail-safe and fault-tolerant operation. It is typically a controller that reads signals from the sensors and executes pre-programmed actions to prevent a hazard by providing output to final control elements.
Final Control Element:
It implements the action determined by the logic system. This final control element is typically a pneumatically actuated On-Off valve operated by solenoid valves.
It is imperative that all three elements of the SIS system function as designed in order to safely isolate the process plant in the event of an emergency.
Probability of Failure upon Demand PFD
By understanding how components of an Safety Instrumented System SIS can fail, it is possible to calculate a Probability of Failure on Demand PFD. There are two basic ways for SIS to fail. The first way is commonly called a spurious trip which usually results in an unplanned but safe process shutdown. While there is no danger associated with this type of SIS failure, the operational costs can be very high. The seconf type of failure does not cause a process shutdown or nuisance trip. Instead, the failure remains undetected, permitting continued process operation in an unsafe or dangerous manner. If an emergency demand occurred, the SIS would be unable to respond properly. These failures are known as covert or hidden failures and contribute to the probability PFD of the system failing in a dangerous manner on demand.
The PFD for the Safety Instrumented System SIS is the sum of PFD's for each element of the system. In order to determine the PFD of each element, the analyst needs documented, historic failure rate data for each element. This failure rate (dangerous) is used in conjunction with the Test Interval TI term to calculate the PFD. It is the test interval TI that accounts for the length of time before a covert fault is discovered through testing. Increases in the test interval directly impact the PFD value in a linear manner; e.g. if you double the interval between tests, you will double the Probability of Failure on Demand, and make it twice as difficult to meet the target Safety Integrity Level SIL.
The governing standards for Safety Instrumented Systems SIS state that plant operators must determine and document that equipment is designed, maintained, inspected, tested and operated in a safe manner. Thus, it is imperative that these components of Safety Instrumented Systems be tested frequently enough to reduce the PFD and meet the target SIL.