• 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 Level of Protection Analysis Present By: Prakash Thapa ExxonMobil Ltd, Canada
• 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
Level of Protection Analysis
Present By: Prakash Thapa
ExxonMobil Ltd, Canada
• 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 Analysis1. Express risk target quantitatively
Activity FARChemical Industry 4Steel Industry 8Coal Mining 40Construction 67Uranium 70Asbestos (old data?) 620
Staying home 3Traveling by automobile 57Traveling by airplane 240Cigarette smoking ???
What is the fatality rate/year for the chemical industry?
What is FAR for cigarette smoking?
Safety Layer of Protection Analysis1. Express risk target quantitatively
FAR Data for typical Activities
• 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 Analysis1. 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 = (.0000001)(100,000) fatalities/time period
We need to consider frequency and consequence
Safety Layer of Protection Analysis1. Express risk target quantitatively
1.00E-09
1.00E-08
1.00E-07
1 10 100
Deaths per event, N
Pro
bab
ility
or
Fre
qu
ency
, F
(ev
ents
/yea
r)
“Acceptable risk”
“Unacceptable risk”
The design must be enhanced to reduce the likelihood of death (or serious damage) and/or to mitigate the effects.
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.)
Safety Layer of Protection Analysis1. Express risk target quantitatively
Safety Layer of Protection Analysis2. 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 Analysis2. Determine the risk for system
IPL1
Initiating event, X
IPL2
IPL3
Unsafe, Y1
Unsafe, Y2
IPLn
Unsafe, Yn
Safe/tolerable
unsafe
X is the probability of the event
Yi is the probability of failure on demand (PFD) for each IPL
Safety Layer of Protection Analysis2. Determine the risk for system
Recall that the events are considered independent
The probability that the unsafe consequence will occur is the product of the individual probabilities.
n
iieconsequenc YXP
1
)(
IPL1
Initiating event, X
IPL2
IPL3Unsafe,
Y1
Unsafe, Y2
IPLn
Safe/tolerable
unsafe
…
Safety Layer of Protection Analysis2. 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 Analysis2. 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, www.safetyusersgroup.com, May 2006
Safety Layer of Protection Analysis2. 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 Analysis3. 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 AnalysisProcess examples
The Layer of Protection Analysis (LOPA) is performed using a standard table for data entry.
Likelihood = X Probability of failure on demand = Yi
Connection (tap) for pressure sensor P1 becomes plugged
0.10 0.10 1.0 0.10 1.0 PRV 0.01
.00001 Pressure sensor does not measure the drum pressure The PRV must exhaust to a separation (knock-out) drum and fuel or flare system.
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 AnalysisProcess examples
Feed
MethaneEthane (LK)PropaneButanePentane
Vaporproduct
Liquidproduct
Processfluid
Steam
FC-1
F2 F3
T1 T2
T3
T5
TC-6 PC-1
LC-1
AC-1
L. Key
Split range
LALLAH
cascade
P-2PAH
Class Exercise 1: Solution.
Safety Layer of Protection AnalysisProcess examples
Safety Layer of Protection AnalysisProcess 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 AnalysisApproaches 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 AnalysisApproaches 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 AnalysisApproaches to reducing risk
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 0.0001
SIL-4 Less than 0.0001
Performance for the four SIL’s levels for a safety interlock system (SIS)
1 out of 1 must indicate
failure
T100s
2 out of 3 must indicate
failure
T100T101T102
Same variable,multiple sensors!
s
Falseshutdown
Failure on demand
5 x 10-35 x 10-3
2.5 x 10-6 2.5 x 10-6
Better performance,more expensive
Safety Layer of Protection AnalysisApproaches to reducing risk
Two common designs for a safety interlock system (SIS)
Safety Layer of Protection AnalysisProcess examples
Class Exercise 2: Fired heater to increase stream’s temperature.
![[PPT]Layer of Protection Analysis, Introduction - My …mycommittees.api.org/standards/cre/soics/Shared Documents/2012... · Web viewQuality assurance for data analysis. ... i.e.](https://static.documents.pub/doc/80x56/5af733317f8b9a92719149e2/pptlayer-of-protection-analysis-introduction-my-documents2012web-viewquality.jpg)