What is a Safety Instrumented System?

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 is productive community partners and employers [Music] before we get into today's video if you love our videos be sure to click the like button below and make sure to click Subscribe and the bell to receive notifications of new real Parrs videos this way you never miss another one 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's 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 sprang 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 tenfold 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 flow rate 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 flix borough 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 plants risk to a tolerable level the risks associated with production of flix burrow 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 embrace the idea of defining risks not as isolated processing line or tank risks but as risks associated with

processing functions as a whole standards is a84 and IEC 61508 were developed around the concept of functional safety later these standards is a in the u.s. and IEC in Europe were harmonized in a single standard is a 84 IEC six one five one one 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 layers discussed previously this layer should provide at least a tenfold 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 the level of tolerable risk must be determined by each individual company but there are benchmarks for many industries such as chemical oil and gas food and beverage and others overall the chemical industry has a fatal accident rate or F AR of for driving a car has an F AR 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 planting 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 predetermined 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 ten times it cannot be interlinked with the basic process control system and any of the 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 ensure 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 a 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 requires an 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's told to the probability that a device weather 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 times 10 to the negative power of one probability of failure in a year's time failure of an isolation valve is about 1 in 100 or 1 times 10 to the negative power of 2 these values can be obtained from vendor data for specific devices or from industry databases of typical PFDs 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 point zero one or one times ten to the negative power of two then our SIF needs to be designed to a safety integrity level of two similarly a PFD of less than one times 10 to the negative power of one requires a safety integrity level of 1 and a PFD of less than 1 times 10 to the negative power of 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 PFD of 1 times 10 to the negative power of 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 one out of two system will provide a greater level of safety response than a simplex system a two out of three fault tolerance system can provide a greater level of safety response than a one out of two system while the two-out-of-three system may be more reliable it may be installed at a much higher cost than a one out of two system likewise a one out of two system will have a higher cost than a simplex system when designing a safety instrumented

system the ISA 84 IEC six one five one one standards prescribed a methodology for developing and documenting the system certain design principles should be followed such as not allowing online 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 make sure that you head over to real parse comm to find even more training material for all of your plc programming needs we offer many videos to assist you in learning plc programming and landing that job in the high-paying highly sought after field of automation and controls engineering go to real farce comm and subscribe to our highly effective training series now [Music]