Certified PHA HAZOP Leader Course

Day 4
Course Timings:
• 09:00 - 10:30 : First Session

• 10:30 - 11:00 : Prayer and Break

• 11:00 - 12:30 : Second Session

• 12:30 - 01:00 : Second Break

• 01:00 – 02:30 : Last Session

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Contents:

Fault Tree Analysis

Event Tree Analysis

Failure Mode and Effect Analysis

Final Exam

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PART IV
FTA
Models, hardware failures, and external events, and how they cause an accident

The most serious outcome is the top event

constructed by relating the sequences of events, individual (OR), or in-

combination (AND)

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FTA Symbols

Top Event And gate &

Basic Event
Or gate OR

Undeveloped Event

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Construct a fault tree

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Fault Tree Analysis
vessel failure
due to over
pressured

AND

Pressure rises PSV does not

relieve

AND OR

Process Control Set point

pressure fails high Fouling inlet too high
rises or outlet
PSV too PSV stuck
small closed

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 6

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Event Trees
ETA

Models the possible consequences of an event that can produce

an accident

It begins with an initiating event

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Accidents do happen!

When an accident or deviation occurs, safety systems come into

play to prevent the accident.

These safety systems either fail or succeed.

The probability of success or failure is used to determine the overall

probability of each final outcome.

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  Identify an initiating event of interest.
 Identify the safety functions
 Construct the event tree
 Describe the resulting sequences.

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Failure Mode & Effects Analysis

To identify and prevent problems before they occur

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Steps to Conduct a FMEA

S O C Current design D R
Item Action Results
e c l controls e P Response
Potential
Potential Potential v c a t N Recommende & target
Causes/ S O D R
Failure Effects of u s e d complete
Mechanisms of Action E C E P
Mode Failure r Prevent Detect Actions date
failure s c taken V C T N
Function

Identify Determine and

Identify Prioritize
failure modes and assess
causes and controls
effects actions

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Item/Function
S C Current design D R
e O l e P Action Results
Item controls
v c a t N Response
Potential Potential Potential Causes/ c s e Recommende & target S O D R
Failure Effects of Mechanisms of u d complete
s c E C E P
Mode Failure failure r Actions date Action
Prevent Detect V C T N
Function taken

Describe what the system or component is designed to do

EXAMPLE: Car HVAC system

 defog windows

 heat or cool cabin to 70 degrees in conditions (-40 to 100 degrees)

 within 3 to 5 minutes

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Failure Mode
S O C Current design D R
Action Results
e c l controls e P Response
v Potential c a t N
Potential Potential Recommend & target
Causes/ u s S O D R
CE` Failure Effects of e ed complete
Mechanisms of r E C E P
Mode Failure s Detec c Actions date Action
failure Prevent
t taken V C T N

Failure to meet design intent

Examples:
 HVAC system
• does not heat vehicle or defog windows
• takes more than 5 minutes to heat vehicle
• does not heat cabin to 70 degrees in below zero temperatures

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Failure Mode Keywords
Rupture Spurious start
Crack Loss of function
Leak High pressure
Plugged Low pressure
Failure to open High temperature
Failure to close Low temperature
Failure to stop Overfilling
Failure to start Hose bypass
Failure to continue Instrument bypassed
Spurious stop

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Effect(s) of Failure

EXAMPLE:
 Cannot see out of front window
 Air conditioner makes cab too cold
 Takes too long to heat up

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Potential Effect(s) of Failure
Effects of the failure mode on the function (consequences)

Consider Potential failure modes Under different

 Operating Conditions

 Usage

 Incorrect service operations

Examples of Potential Effects

 loss of function

 no/low output

 Intermittent operations

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Severity

Severity of the effect of the failure.

For multiple effects, select the highest Rating

Scale: 1 = Not Severe, 10 = Very Severe

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Severity Rating
Effect Rank Criteria

None 1 No effect

Very Slight 2 Negligible effect on Performance. Some users may notice.

Slight 3 Slight effect on performance. Non vital faults will be noticed by many
users
Minor 4 Minor effect on performance. User is slightly dissatisfied.

Moderate 5 Reduced performance with gradual performance degradation. User

dissatisfied.
Severe 6 Degraded performance, but safe and usable. User dissatisfied.

High Severity 7 Very poor performance. Very dissatisfied user.

Very High Severity 8 Inoperable but safe.

Extreme Severity 9 Probable failure with hazardous effects. Compliance with regulation is
unlikely.
Maximum Severity 10 Unpredictable failure with hazardous effects almost certain. Non-
compliant with regulations.

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Severity

EXAMPLE:
 Cannot see out of front window – severity 9
 Air conditioner makes cab too cold – severity 5
 Takes too long to heat up – severity 3
 Doesn’t get warm enough – severity 3

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Cause(s) of Failure

Potential Cause: Is an indication of a design weakness, the consequence of which is the

failure mode
 Examples:
• Incorrect routing of vent hoses
• Inadequate coolant capacity for application

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Potential Cause
Examples of Potential Cause

 Vibration

 Corrosion

 Fatigue

 Wear

 Over-stressing

 Over-load

 Imbalance

 Inadequate maintenance instruction

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Occurrence

It is the likelihood that a specific cause will occur

Ratting scale: 1 = Not Likely, 10 = Very Likely
EXAMPLE:
 Incorrect location of vents – occurrence 3
 Incorrect routing of vent hoses – occurrence 6
 Inadequate coolant capacity for application – occurrence 2

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Occurrence Ranking
Occurrence Rank Criteria
Extremely Unlikely 1 Less than 0.01 per thousand
Remote Likelihood 2 0.1 per thousand rate of occurrence
Very Low 3 0.5 per thousand rate of occurrence
Likelihood
Low Likelihood 4 1 per thousand rate of occurrence
Moderately Low 5 2 per thousand rate of occurrence
Likelihood
Medium Likelihood 6 5 per thousand rate of occurrence
Moderately High 7 10 per thousand rate of occurrence
Likelihood
Very High Severity 8 20 per thousand rate of occurrence
Extreme Severity 9 50 per thousand rate of occurrence
Maximum Severity 10 100 per thousand rate of occurrence

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Classification

Classification critical or significant Examples:

must have recommended actions Cannot see out of front window
1. Critical (9 - 10 severity with ≥2 occurrence)  severity 9
2. Significant (4 - 8 severity with ≥4  incorrect vent location – occurrence 2
occurrence) Air conditioner makes cab too cold
 severity 5
 Incorrect routing of vent hoses (too close
to heat source) occurrence 6

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Current Design Controls
Design Controls
3 types of Controls
 Prevention
 Detect cause mechanism
 Detect the failure mode

Examples:
 Engineering specifications – preventive  Functional testing – detective control
control  1 = Likely to Detect, 10 = Not Likely to Detect

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Detection

Detection rates the likelihood that the problem will be detected.

Example:
 Functional testing – detection 3

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Detection Ranking
Detection Rank Criteria

Extremely Likely 1 Can be corrected prior to prototype/ Controls will almost certainly detect

Very High Likelihood 2 Can be corrected prior to design release/Very High probability of detection

High Likelihood 3 Likely to be corrected/High probability of detection

Moderately High Likelihood 4 Design controls are moderately effective

Medium Likelihood 5 Design controls have an even chance of working

Moderately Low Likelihood 6 Design controls may miss the problem

Low Likelihood 7 Design controls are likely to miss the problem

Very Low Likelihood 8 Design controls have a poor chance of detection

Remote Likelihood 9 Unproven, unreliable design/poor chance for detection

Extremely Unlikely 10 No design technique available/Controls will not detect

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RPN (Risk Priority Number)

Severity X Occurrence X Detection = RPN

RPN is used to prioritize concerns/actions

Example:
 Cannot see out of front window – severity 9,
 Incorrect vent location – occurrence 2,
 Functional testing – detection 3,
 RPN - 54

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Recommended Actions

All critical or significant class must have actions

Examples of Recommended actions
 Visual Inspection (D)
 Process instructions (P)
 Training (P)

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Responsibility & Target Completion Date

Action Results

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Exercise Design FMEA
Perform A FMEA on a pressure cooker

Pressure Cooker Safety Features

 Safety valve.

 Thermostat.

 Pressure gage.

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Pressure Cooker Block Diagram

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