Safety Instrumented System HW 2 Name: Mustafa Essam ID:
5295
INTRODUCTION
Ayman Ismail 5837
Ensuring that a presses or a plant is safe is made up of many layers. Plant design is at the core of plant safety. The Basic Process Control System (BPCS) comes next, followed y operator inter!ention, alarm systems, and the Safety "nstrumented System (S"S). The S"S is an important layer of protection ecause it can pro!ide the highest le!el of protection when compared with other layers shown in the figure elow.
# Safety Instrumented System (SIS) consists of an engineered set of hardware and software controls which are especially used on critical process systems. # critical process system can e identified as one which, once running and an operational prolem occurs, may need to e put into a $Safe State$ to a!oid ad!erse Safety, %ealth and En!ironmental(S%&E) conse'uences. Examples of critical processes ha!e een common since the eginning of the "ndustrial #ge. ne of the more well*nown critical processes is the operation of a steam oiler. Critical parts of the process would include the lighting of the urners, controlling the le!el of water in the drum and controlling the steam pressure.
Basic Fundamentals of Safety Instrumented Systems SIS
The operation of many industrial processes in!ol!e inherent ris*s due to the presence of dangerous material li*e gases and chemicals. Safety "nstrumented Systems S"S are specifically designed to protect personnel, e'uipment and the en!ironment y reducing the li*elihood (fre'uency) or the impact se!erity of an identified e mergency e!ent. 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 "nstrumented Systems S"S to pro!ide safe isolation of flammale or potentially toxic material in the e!ent of a fire or accidental release of fluids.
Basics of Safety and Layers of rotection
Safety is pro!ided y layers of protection. These layers start with safe and effecti!e process control, extend to manual and automatic pre!ention layers, and continue with layers to mitigate the conse'uences of an e!ent. The first layer is the Basic Process Control System BPCS. The control system itself pro!ides significant safety through proper design of process control. The next layer of protection is also pro!ided y the control system and the system operators. #utomated shutdown se'uences in the process con trol system comined with operator inter!ention to shut down the process are the next layer of safety.
The third layer is the Safety "nstrumented System S"S. "t is a safety system independent of the process control system. "t has separate sensors, !al!es and logic system. +o process control is performed in this system, its only role is safety. These layers are designed to pre!ent a safety related e!ent. "f a safety related e!ent o ccurs there are additional layers designed to mitigate the impact of the e!ent. The fourth layer is an acti!e protection layer. This layer may ha!e !al!es or rupture dis*s designed to pro!ide a relief point that pre!ents a rupture, large spill or other uncontrolled release that can cause an explosion or fire. The fifth layer is a passi!e protection layer. "t may consist of a di*e or other passi!e arrier that ser!es to contain a fire or channel the energy of an explosion in a direction that minimies the spread of damage. The final layer is plant and emergency response. "f a large safety e!ent occurs this layer responds in a way that minimies ongoing damage, in-ury or loss of life. "t may include e!a cuation plans, firefighting, etc. !erall safety is determined y how these layers wor* togethe r.
Basics of Safety Instrumented Systems SIS Typically, Safety "nstrumented Systems consist of three elements # Sensor, a /ogic Sol!er and a 0inal Control Element
Sensors:
0ield 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 e'uipment or process is in a safe state. Sensor types range
from simple pneumatic or electrical switches to Smart transmitters with onoard diagnostics. These sensors are dedicated to the Safety "nstrumented System S"S.
Logic Solver:
The purpose of this component of Safety "nstrumented Systems S"S is to determine what action is to e ta*en ased on the information gathered. %ighly reliale logic sol!ers are used which pro!ide oth failsafe and faulttolerant operation. "t is typically a controller that reads signals from the sensors and executes preprogrammed actions to pre!en t a haard y pro!iding output to final control elements. Final Control Element:
"t implements the action determined y the logic system. This final control element is typically a pneumatically actuated nff !al!e operated y solenoid !al!es. "t is imperati!e that all three elements of the S"S system function as d esigned in order to safely isolate the process plant in the e!ent of an emergency.
ro!a!ility of Failure u"on Demand FD By understanding how components of a Safety "nstrumented System S"S can fail, it is possile to calculate a Proaility of 0ailure on 1emand P01. There are two asic ways for S"S to fail. The first way is commonly called a spurious trip which usually results in an unplanned ut safe process shutdown. 2hile there is no danger associated with this type of S"S failure, the operational costs can e !ery high. The second type of failure does not cause a process shutdown or nuisance trip. "nstead, the failure remains undetected, permitting continued process operation in an unsafe or dangerous manner. "f an emergency demand occurred, the S"S would e unale to respond properly. These failures are *nown as co!ert or hidden failures and contriute to the proaility P01 of the system failing in a dangerous manner on demand. The P01 for the Safety "nstrumented System S"S is the sum of P013s for each element of the system. "n order to determine the P01 of each element, the analyst needs documented, historic failure rate data for each element. This failure rate (dangerous) is used in con-unction with the Test "nter!al T" term to calculate the P01. "t is the test inter!al T" that accounts for the length of time efore a co!ert fault is disco!ered through testing. "ncreases in the test inter!al directly impact the P01 !alue in a linear manner4 e.g. if you doule the inter!al etween tests, you will doule the Proaility of 0ailure on 1emand, and ma*e it twice as difficult to meet the target Safety "ntegrity /e!el S"/.
The go!erning standards for Safety "nstrumented Systems S"S state that plant op erators must determine and document that e'uipment is designed, maintained, inspected, tested and operated in a safe manner. Thus, it is imperati!e that these components of Safety "nstrumented Systems e tested fre'uently enough to reduce the P01 and meet the target S"/.
#$am"le% Ty"ical features of safety instrumented systems # flammale li'uid is pumped into a uffer tan* from which it is consumed in a process. # typical le!el control loop is pro!ided in the process control system to maintain the tan* le!el at 567 full. # haard will occur if the le!el control fails for any reason and the tan* ecomes full. The reasons for le!el control failure include
8ammed open le!el !al!e (dirt or seiure) 0ailed le!el transmitter false low signal Control loop left on manual with !al!e open /ea*ing control !al!e
# simple S"S would re'uire the features shown in the 0igure ellow a separate le!el switch set to detect high le!el in the tan* causes an automatic shutoff !al!e to close off all li'uid feed to the tan*. The shutoff !al!e remains closed until the prolem has een rectified.
&uestions 9. Explain what safety integrity le!els (S"/) are: Safety integrity le!el (S"/) is defined as a relati!e le!el of ris*reduction pro!ided y a safety function, or to specify a target le!el of ris* reduction. "n simple terms, S"/ is a measurement of performance re'uired for a safety instrumented function (S"0). The re'uirements for a gi!en S"/ are not consistent among all of the functional safety standards. "n the European functional safety standards ased on the "EC ;956< standard four S"/s are defined, with S"/ = the most dependale and S"/ 9 the least. # S"/ is determined ased on a numer of 'uantitati!e factors in comination with 'ualitati!e factors such as de!elopment process and safety life cycle management. >. Explain why S"S uses different from other process control systems (1CS and P/C) instruments: S"S (Safety "nstrumented System) is implemented to Safety Critical 0unction to ring the process to Safe State when ?nsafe Condition is detected. nly operation on demand asis. Typically, Safety "nstrumented Systems consist of three elements # Sensor, a /ogic Sol!er and a 0inal Control Element. # distriuted control system (1CS) is a control system for a process or plant, wherein control elements are distriuted throughout the system. 1CS is implemented for process control with process critical interloc*, se'uencing, day to day operation and in continuously on demand. # 1CS typically uses custom designed processors as controllers and uses oth proprietary interconnections and standard communications protocol for communication. "nput and output modules form component parts of the 1CS. The processor recei!es information from input modules and sends information to o utput modules. The input modules recei!e information from input instruments in the process (or field) and the output modules transmit instructions to the output instruments in the field. The inputs and outputs can e either analog signal which are continuously changing or discrete signals which are > state either on or off. Computer uses or electrical uses connect the processor and modules through multiplexer or demultiplexers. Buses also connect the distriuted controllers with the central controller and finally to the %uman@machine interface (%A") or control consoles. The elements of a 1CS may connect directly to physical e'uipment such as switches, pumps and !al!es and to %uman Aachine "nterface (%A") !ia SC#1#. The differences etween a 1CS and SC#1# is often sutle, especially with ad!ances in technology allowing the functionality of each to o!erlap. . "f reliaility of a !al!e (proaility of nonfailure within a specified period of time) is 6., what is reliaility of (a) parallel connection of > identical !al!es4 () series connection of > !al!es4 (c) parallelDseries connection of = !al!es:
