# ACHIEVING ACCEPTABLE RISK Level of Protection Analysis

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ACHIEVING ACCEPTABLE RISK Level of Protection Analysis
HAZARD IDENTIFICATION 1. Check lists 2. Dow Relative Ranking 3. HAZOP - Hazard and Operability LAYER OF PROTECTION ANALYSIS 1. Express risk target quantitatively 2. Determine risk for system 3. Reduce risk to meet target HAZARD ASSESSMENT - Fault Tree - Event Tree - Consequence analysis - Human Error Analysis ACTIONS TO ELIMINATE OR MITIGATE - Apply all engineering sciences Semi-quantitative analysis to give order-of-magnitude estimate We will use our group skills and knowledge of safety layers in applications. More accurate

Safety Layer of Protection Analysis
1. Express risk target quantitatively FAR: Fatal Accident Rate - This is the number of fatalities occurring during 1000 working lifetimes (108 hours). This is used in the U.K. Fatality Rate = FAR * (hours worked) / 108 OSHA Incidence Rate - This is the number of illnesses and injuries for 100 work-years. This is used in the USA.

Safety Layer of Protection Analysis
1. Express risk target quantitatively FAR Data for typical Activities What is FAR for cigarette smoking? What is the fatality rate/year for the chemical industry?

Safety Layer of Protection Analysis
1. Express risk target quantitatively One standard used is to maintain the risk for involuntary activities less (much less?) than typical risks such as “staying home” - Results in rules, such as fatality rate < 10-6/year - See Wells (1996) Table 9.4 - Remember that many risks exist (total risk is sum) Are current risks accepted or merely tolerated? We must consider the inaccuracies of the estimates We must consider people outside of the manufacturing site.

Safety Layer of Protection Analysis
1. Express risk target quantitatively People usually distinguish between voluntary and involuntary risk. They often accept higher risk for voluntary activities (rock climbing). People consider the number of fatalities per accident Fatalities = (frequency) (fatalities/accident) .001 = (.001) (1) fatalities/time period .001 = ( )(100,000) fatalities/time period We need to consider frequency and consequence

Safety Layer of Protection Analysis
1. Express risk target quantitatively The decision can be presented in a F-N plot similar to the one below. (The coordinate values here are not “standard”; they must be selected by the professional.) 1.00E-07 “Acceptable risk” “Unacceptable risk” The design must be enhanced to reduce the likelihood of death (or serious damage) and/or to mitigate the effects. Probability or Frequency, F (events/year) 1.00E-08 1.00E-09 1 10 100 Deaths per event, N

Safety Layer of Protection Analysis 2. Determine the risk for system
In Level of Protection Analysis (LOPA), we assume that the probability of each element in the system functioning (or failing) is independent of all other elements. We consider the probability of the initiating event (root cause) occurring We consider the probability that every independent protection layer (IPL) will prevent the cause or satisfactorily mitigate the effect

Safety Layer of Protection Analysis 2. Determine the risk for system
X is the probability of the event Yi is the probability of failure on demand (PFD) for each IPL Unsafe, Yn unsafe I P L n   Unsafe, Y2 I P L 3 Unsafe, Y1 I P L 2 Initiating event, X I P L 1 Safe/ tolerable

Safety Layer of Protection Analysis 2. Determine the risk for system
1 Initiating event, X 2 3 Unsafe, Y1 Y2 n Safe/ tolerable unsafe Recall that the events are considered independent The probability that the unsafe consequence will occur is the product of the individual probabilities.

Safety Layer of Protection Analysis
2. Determine the risk for system How do we determine the initiating events? How do we determine the probability of the initiating event, X How do we determine the probability that each IPL will function successfully? How do we determine the target level for the system? HAZOP Company, industry experience Company, industry experience F-N plot, depends on consequence

Safety Layer of Protection Analysis 2. Determine the risk for system
Some typical protection layer Probability of Failure on Demand (PFD) BPCS control loop = 0.10 Operator response to alarm = 0.10 Relief safety valve = 0.001 Vessel failure at maximum design pressure = 10-4 or better (lower) Source: A. Frederickson, Layer of Protection Analysis, May 2006

Safety Layer of Protection Analysis 2. Determine the risk for system
Often, credit is taken for good design and maintenance procedures. Proper materials of construction (reduce corrosion) Proper equipment specification (pumps, etc.) Good maintenance (monitor for corrosion, test safety systems periodically, train personnel on proper responses, etc.) A typical value is PFD = 0.10

Safety Layer of Protection Analysis
3. Reduce the risk to achieve the target The general approach is to Set the target frequency for an event leading to an unsafe situation (based on F-N plot) Calculate the frequency for a proposed design If the frequency for the design is too high, reduce it - The first approach is often to introduce or enhance the safety interlock system (SIS) system Continue with improvements until the target frequency has been achieved

Safety Layer of Protection Analysis
Process examples The Layer of Protection Analysis (LOPA) is performed using a standard table for data entry. Likelihood = X Probability of failure on demand = Yi Mitigated likelihood = (X)(Y1)(Y 2)  (Yn)

Safety Layer of Protection Analysis
Process examples Class Exercise 1: Flash drum for “rough” component separation for this proposed design. Feed Methane Ethane (LK) Propane Butane Pentane Vapor product Liquid Process fluid Steam FC-1 F2 F3 T1 T2 T3 T5 TC-6 PC-1 LC-1 AC-1 L. Key Split range PAH LAL LAH cascade

Safety Layer of Protection Analysis
Process examples Class Exercise 1: Flash drum for “rough” component separation. Complete the table with your best estimates of values. Assume that the target mitigated likelihood = 10-5 event/year

Safety Layer of Protection Analysis
Process examples Class Exercise 1: Some observations about the design. The drum pressure controller uses only one sensor; when it fails, the pressure is not controlled. The same sensor is used for control and alarming. Therefore, the alarm provides no additional protection for this initiating cause. No safety valve is provided (which is a serious design flaw). No SIS is provided for the system. (No SIS would be provided for a typical design.)

Safety Layer of Protection Analysis
Safety Layer of Protection Analysis Process examples Class Exercise 1: Solution using initial design and typical published values. Much too high! We must make improvements to the design.

Safety Layer of Protection Analysis
Process examples Class Exercise 1: Solution using enhanced design and typical published values. Enhanced design includes separate P sensor for alarm and a pressure relief valve. Sketch on process drawing. The enhanced design achieves the target mitigated likelihood. Verify table entries.

Safety Layer of Protection Analysis Class Exercise 1: Solution.
Process examples Class Exercise 1: Solution. Feed Methane Ethane (LK) Propane Butane Pentane Vapor product Liquid Process fluid Steam FC-1 F2 F3 T1 T2 T3 T5 TC-6 PC-1 LC-1 AC-1 L. Key Split range LAL LAH cascade P-2 PAH

Safety Layer of Protection Analysis
Process examples Class Exercise 1: Each IPL must be independent. For the solution in the LOPA table and process sketch, describe some situations (equipment faults) in which the independent layers of protection are Independent Dependent For each situation in which the IPLs are dependent, suggest a design improvement that would remove the common cause fault, so that the LOPA analysis in the table would be correct. Hints: Consider faults such as power supply, signal transmission, computing, and actuation

Safety Layer of Protection Analysis Approaches to reducing risk
The most common are BPCS, Alarms and Pressure relief. They are typically provided in the base design. The next most common is SIS, which requires careful design and continuing maintenance The probability of failure on demand for an SIS depends on its design. Duplicated equipment (e.g., sensors, valves, transmission lines) can improve the performance A very reliable method is to design an “inherently safe” process, but these concepts should be applied in the base case

Safety Layer of Protection Analysis Approaches to reducing risk
The safety interlock system (SIS) must use independent sensor, calculation, and final element to be independent! We desire an SIS that functions when a fault has occurred and does not function when the fault has not occurred. SIS performance improves with the use of redundant elements; however, the systems become complex, requiring high capital cost and extensive ongoing maintenance. Use LOPA to determine the required PFD; then, design the SIS to achieve the required PFD.

Safety Layer of Protection Analysis
Approaches to reducing risk Performance for the four SIL’s levels for a safety interlock system (SIS) Safety Integrity Level (SIL) Probability of Failure on Demand SIL-1 0.10 to 0.001 SIL-2 0.01 to 0.001 SIL-3 0.001 to SIL-4 Less than

Safety Layer of Protection Analysis
Approaches to reducing risk Two common designs for a safety interlock system (SIS) Failure on demand False shutdown 1 out of 1 must indicate failure T100 s 5 x 10-3 5 x 10-3 Better performance, more expensive T100 T101 T102 Same variable, multiple sensors! 2 out of 3 must indicate failure s 2.5 x 10-6 2.5 x 10-6

Safety Layer of Protection Analysis
Process examples Class Exercise 2: Fired heater to increase stream’s temperature.

Safety Layer of Protection Analysis
Process examples Class Exercise 2: Fired heater to increase stream’s temperature.

Safety Layer of Protection Analysis
References Dowell, A. and D. Hendershoot, Simplified Risk Analysis - Layer of Protection Analysis, AIChE National Meeting, Indianapolis, Paper 281a, Nov. 3-8, 2002 Dowell, A. and T. Williams, Layer of Protection Analysis: Generating Scenarios Automatically from HAZOP Data, Process Safety Progress, 24, 1, (March 2005). Frederickson A., Layer of Protection Analysis, May 2006 Gulland, W., Methods of Determining Safety Integrity Level (SIL) Requirements - Pros and Cons, Haight, J. and V. Kecojevic, Automation vs. Human Intervantion: What is the Best Fit for the Best Performance?, Process Safety Progress, 24, 1, (March 2005) Melhem, G. and P. Stickles, How Much Safety is Enough, Hydrocarbon Processing, 1999 Wiegernick, J., Introduction to the Risk-Based Design of Safety Instrumented Systems for the Process Industries, Seventh International Conference on Control, Automation, Robotics and Vision, Singapore, Dec

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