TOPIC : SAFETY ANALYSIS & PREVENTION

DATE : 22 FEBRUARY 2012, WEDNESDAY

LECTURE 5.2 : DETAILED HAZARD ANALYSIS, RISK ANALYSISHealth, Safety & Environment (CBB 2012)

Lecture Content Part 2Detailed Hazard AnalysisFailure mode and effect analysis (FMEA) Hazard and operability review (HAZOP)Technique of operation review (TOR)Human error analysis (HEA)Fault tree analysis (FTA)Risk analysis

Lecture OutcomeYou should be able to identify, define and differentiateDetailed Hazard Analysis (for complex system)Failure mode and effect analysis (FMEA) Hazard and operability review (HAZOP)Technique of operation review (TOR)Human error analysis (HEA)Fault tree analysis (FTA)Risk analysis

Proceeds as follows:Critically examine the system in question.Divide the system into its various components.Each component is studied to determine how it could fail. Rate each potential failure according to its consequences (0 to 4).FMEAWeaknesses: no human error factor. does not account for component interfaces.

Failure mode and effect analysis is a tool that examines potential product or process failures, evaluates risk priorities, and helps determine remedial actions to avoid identified problems. Proceeds as follows:Critically examine the system in question.Divide the system into its various components.Each component is studied to determine how it could fail. Rate each potential failure according to its consequences (0 to 4).Failure Mode and Effect Analysis (FMEA)Weaknesses: no human error factor. does not account for component interfaces.

FMEA Example

HAZOPOriginally for use with new processes in the chemical industry.Requires a team of experienced people with various backgrounds relating to the process/system.Guide words are used.Weakness: no human error factor.Basic idea - consider all possible ways that process and operational failures can occur.

Preparation for HAZOPDetailed information on the process must be availableProcess flow diagrams (PFD), process and instrumentation diagrams (P&Ids), detailed equipment specifications, materials of construction and mass & energy balances are very essential for the study.

Process Flow Diagram (PFD)

HAZOP Guide WordNoLessMorePart ofAs well asReverseOther thanNever noneQuantitative decreaseQuantitative increaseQualitative decreaseQualitative increaseOpposite of forwardComplete substitutionGuide words describe ways in which the component may deviate from its design.Guide wordsMeaning

ProcedureStages of HAZOP as applied to each component of a chemical processing system are described below:

Identify the design intent of the selected part of process.Consider each condition or action using the HAZOP guide word to suggest possible deviationsConsider causes and consequences of the deviationDefine and note the action required to address the problems.

Stages of HAZOP as applied to each component of a chemical processing system are described below:

1-Beginning2-Select a vessel3-Explain the original intention4-Select a line5-Explain the intention of the line6-Apply guide word7-Develop a meaningful deviation8-Examine possible causes9-Examine consequencesProcedure

10- Detect hazard or operating problems11- Make suitable records/improvement action12- Mark line as having been examined13- Repeat steps 6 12 for other guide words14- Exercise end

EXAMPLEThe phosphoric acid and ammonia are mixed, and a non-hazardous product, diammonium phosphate (DAP), results if the reaction of ammonia is complete. If too little phosphoric acid is added, the reaction is incomplete, and ammonia is produced. Too little ammonia available to the reactor results in a safe but undesirable product. The HazOp team is assigned to investigate "Personnel Hazards from the Reaction".

SOLUTION

HEA It is best to perform HEA together with either FMEA or HAZOP. This will enhance the effectiveness of all three processes.For predicting human error before accidents occur by:observing employees at work and noting hazards.actually performing job tasks to get a firsthand feel for hazards.

TORA method that brings supervisors and employees to work together to analyse workplace accidents, failures and incidents.Seeks to identify the factors leading to an accident by following a simple yes/no sequence written in easy-to-understand terms. Weakness - it is an after-the-fact process, triggered by an accident.Strength - its involvement of line personnel in the analysis.

FTAThe analysis process is displayed visually.The end effect (called the top event) sits at the top of the model, with the events leading to the top event successively at the bottom layers. Uses symbols derived from Boolean algebra.The resultant model looks like a logic diagram or a flow chart.

OR gateAND gateTop eventIntermediate eventBasic eventUndeveloped eventExternal or house eventTransfer IN-OUTFTA Logic and Event Symbols

The Fault Tree ConceptTop EventContributing EventsUndeveloped EventBasic Event

Fault TreeFault tree: top-down approach starting with the unwanted consequences as the top event & identifying all factors that could contribute to the top event.

Used to think through possible causes of a loss, to find most probable sequence of events leading to the loss & to quantify the probability of loss.

Steps: Fault TreeTo draw a fault tree take the following steps.Determine undesirable event, which is to be the Top Event.Determine the Basic Events, which could immediately cause the Top Event.Determine the relationship between the Basic Events and the Top Event in terms of AND and OR gates.Determine whether any of the Basic Events need further analysis, if so repeat steps 2 & 3.

Example: Fault TreeConsider a case of a motor overheated. The Basic Events could be the primary motor fails or excessive current load to the motor.

The current load might be excessive due to excess current flow in the circuit and failure of the fuse.

It could be either short circuiting or a power surge that contributed to the excess current flow.

Example: Fault TreeThe probability of the Top Event (the motor overheated) is obtained by combining the base events according to the logic rules:

For this case, probability = [(C+D) x B] + AFor an OR gate (ADD the probabilities)For an AND gate (MULTIPLY the value)

Example: Fault TreeThe probability of the Top Event (the motor overheated) is obtained by combining the base events according to the logic rules:For this case, probability = [(0.007+0.003) x 0.1] + 0.05 = 0.051

Event Tree AnalysisGraphical model that identifies possible outcomes from a given initiating event. Suited for complex processes involving several layers of safety systems.First: Identify an initiating event that could lead to failure of the system (e.g. human error, utility failure etc.)Second: Identify intermediate events/action which resulted from the initiating event (usually are safety features). Each intermediate event leads to two branches, one for a successful, and the other for an unsuccessful operation.

Event Tree AnalysisTo analyse risk of possible consequences of an initiating event or failureStarting with initiating event which might produce variety of consequences depending on subsequent eventCan be used to investigate the feasible outcome of an event and estimate relative probability of each outcome

An Event Tree Analysis for a Fluid Flow Problem

Steps: Event Tree1st step: Initiating event column.2nd step: Subsequent events (usually branches in to 2 paths (YES it will happen or NO it wont). These events might be more than one, hence need to list all.3rd step: Two columns drawn for overall outcome/consequence and to calculate its probability.

Example: Event TreeConsider the event of a steering wheel failure (initiating event).Suppose that there is a 50% chance that the driver is able to the failure and pull to safely. If he/she cannot correct the failure, there may be a collision with another car. There is also the possibility that driver himself might crash into the steering wheel upon impact.The probability of a collision with another car is 0.2 and the probability of the driver crashing into the steering wheel is 0.3.Construct an event tree to determine the probabilities of events that could occur should there be a steering wheel failure.

Example: Event Tree A B C Consequences Probability Correct Avoid Avoid Steering Collision InjuryYESA = 0.5NOB = 0.2NOB = 0.8YESYES C = 0.7NO C = 0.3 NO C = 0.3YES C = 0.7STEERINGFAILSCollision AvoidedCollision AvoidedInjury to SelfCollisionCollision + Injury0.500.280.120.070.031.00

Risk AnalysisDecision-making tool normally associated with insurance and investments.

It can also be used to analyse the workplace, identify hazards and develop strategies for overcoming hazards.

Focuses on TWO questions:How frequently does a given event occur?How severe are the consequences of a given event?

What is a risk?Risk may be considered as the potential for adverse effects resulting from an activity or eventAcceptable level of riskThis is generally determined by what is prepared to be lost balanced against possible gains

The Risk Management ProcessThe total procedure associated with identifying a hazard,

assessing the risk,

putting in place control measures,

and reviewing the outcomes.

Hazard IdentificationRemember Hazard:

An expression of possible loss in terms of SEVERITY and PROBABILITYRISK = (PROBABILITY) x (CONSEQUENCES)Decrease risk by decreasing the frequency and severity of hazard-related eventsRisk

Risk Assessment

Risk AssessmentRisk: The possibility of an unwanted event occurring

Likelihood: The chance of an event actually occurring.

LikelihoodVery Likely -- Could happen frequently Likely -- Could happen occasionally

Unlikely -- Could happen, but only rarely

Highly Unlikely -- Could happen but probably never will

LikelihoodWhen evaluating the likelihood of an accident, a factor that will modify the likelihood category, is exposure. Very Rare -- Once per year or less Rare -- A few times per year Unusual -- Once per month Occasional -- Once per week Frequent -- Daily Continuous -- Constant

ConsequencesFATALDeathMAJOR INJURIESNormally irreversible injury or damage to health requiring extended time off work to effect best recovery.MINOR INJURIESTypically a reversible injury or damage to health needing several days away from work to recover. Recovery would be full and permanent.NEGLIGIBLE INJURIES Would require first aid and may need the remainder of the work period or shift off before being able to return to work.

Risk Assessment Matrix

Risk Control

Hierarchy of ControlControl: the measures we take to eliminate or reduce the risk to an acceptable level.Hierarchy of Control: The order in which controls should be considered when selecting methods of controlling a risk.

Hierarchy of ControlElimination Substitution Isolation Engineering Controls Administrative Controls Provide Personal Protective Equipment .

Hazard Prevention/ DeterrenceInformation obtained from the various hazard analysis methods may be used to PREVENT accidents and illnesses.

Implementing Hazard Control MeasuresAdjust asnecessaryAssess effectivenessMonitor/ observeApply the method(s)Select a method(s)

Eliminate the source of hazards.Substitute a less hazardous equivalent.Reduce the hazards at the source.Remove the employee from the hazard.Isolate the hazard.

Dilute the hazard.Apply appropriate management strategies.Use appropriate PPE.Provide employee training.Practice good housekeeping.

Step 1: Select a Method

Step 2: Apply the MethodPut the selected method to practical use to test cases.

Purpose is to test and analyse the suitability of the method in a real life situation.

Step 3: Monitor and ObserveMonitor and observe for any:

complaints from workers. complaints from clients. change in worker performance. other side effects.

Step 4: Assess EffectivenessTabulate all complaints.

Tabulate workers performance (productivity).

Analyse the data.

Positive results that show no noticeable effect.

Step 5: Adjust as NecessaryE.g. Change of working schedule.

E.g. Use a different concentration.

Or if all fail, go back to Step 1.

Hazard identification, risk assessment, control and review is not a task that is completed and then forgotten about. Hazard identification should be properly documented even on the simplest of situations

Risk assessment should include a careful assessment of both likelihood and consequence. Control measures should conform to the recommendations of the hierarchy of control. The risk management process is an on going one.Conclusion

SummaryEnd of LectureThank you.

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