PROCESS HAZARDS ANALYSIS. Process Hazards Analysis n WHAT ? –Fire, Explosions, Toxic Releases –Consequences, Mechanism, Improvement n WHY ? –Ensure Safety

PROCESS HAZARDSPROCESS HAZARDSANALYSISANALYSIS

Process Hazards AnalysisProcess Hazards Analysis

WHAT ?WHAT ?– Fire, Explosions, Toxic ReleasesFire, Explosions, Toxic Releases– Consequences , Mechanism, ImprovementConsequences , Mechanism, Improvement

WHY ?WHY ?– Ensure Safety to the Public and Employees Ensure Safety to the Public and Employees – Risk ManagementRisk Management

WHO ?WHO ?– Performed by process engineers and plant Performed by process engineers and plant

personnelpersonnel

Process Hazards AnalysisProcess Hazards Analysis Report Contents Report Contents

1. Hazards Identification1. Hazards Identification

2. Hazardous Events & 2. Hazardous Events & Consequences AnalysisConsequences Analysis

3. Lines of Defense3. Lines of Defense

4. Recommendations4. Recommendations

Process Hazards AnalysisProcess Hazards AnalysisPart 1 - Hazard Part 1 - Hazard IdentificationIdentification

Properties of Materials Properties of Materials – Reactive - Mix wrong proportions, Reactive - Mix wrong proportions,

abnormal chemicals, temperature or abnormal chemicals, temperature or pressure excursionspressure excursions

– FlammableFlammable– Explosive Explosive – Toxic - humans, ecologyToxic - humans, ecology

– Comparison to Other MaterialsComparison to Other Materials

Part 2 - Hazard Events & Part 2 - Hazard Events & Consequence AnalysisConsequence Analysis

Toxic ReleaseToxic Release– Toxic Concentrations - Indoor, DownwindToxic Concentrations - Indoor, Downwind

Fire (Radiation)Fire (Radiation) Explosion (Physical Explosion, Explosion (Physical Explosion,

Chemical Explosion)Chemical Explosion)– Pressure Wave, Fireball, MissilesPressure Wave, Fireball, Missiles

Consequence Analysis SpreadsheetConsequence Analysis Spreadsheet

Hazard Identification Hazard Identification - Hazardous Events- Hazardous Events

Loss Of ContainmentLoss Of Containment– ChecklistsChecklists– What-if (Brainstorming) SessionWhat-if (Brainstorming) Session

Open-ended Manual Valves, Valve Sheared OffOpen-ended Manual Valves, Valve Sheared Off Pump Seal FailuresPump Seal Failures Heat Exchanger Tube RuptureHeat Exchanger Tube Rupture

Operation at Abnormal ConditionsOperation at Abnormal Conditions– What-if SessionWhat-if Session– HAZOP methodHAZOP method– FMEA methodFMEA method

Hazard IdentificationHazard IdentificationConsequence AnalysisConsequence Analysis

Not all hazards require a numerical Not all hazards require a numerical quantification of the hazard. quantification of the hazard.

Hazards may be evaluated by Hazards may be evaluated by Qualitative means using engineering Qualitative means using engineering judgement.judgement.– A 1/8” dia hole in a water line has no off-A 1/8” dia hole in a water line has no off-

site consequencessite consequences– Deinventory of 3000 lb. of methyl Deinventory of 3000 lb. of methyl

isocyanate (chemical in Bhopal Incident)isocyanate (chemical in Bhopal Incident)


For Consequences that are not For Consequences that are not obvious or that are serious enough obvious or that are serious enough that more detail is warranted. Use that more detail is warranted. Use Quantitative techniques.Quantitative techniques.

Step 1. Determine the Release RateStep 1. Determine the Release Rate Step 2. Determine the EffectsStep 2. Determine the Effects


Determining The Release RateDetermining The Release Rate– Assume a scenarioAssume a scenario

Pick a ‘most likely’ scenario - corrosion Pick a ‘most likely’ scenario - corrosion causes a 1/8” diameter hole in pipecauses a 1/8” diameter hole in pipe

Pick a ‘worst case’ scenario - pipe is Pick a ‘worst case’ scenario - pipe is sheared off by forkliftsheared off by forklift

– Use standard engineering calculations Use standard engineering calculations to determine the release rate.to determine the release rate.


Standard Flow Equations (orifices)Standard Flow Equations (orifices)– Liquid Flow from a tank/pipe under pressLiquid Flow from a tank/pipe under press

PgCAQ comass 2

A - area of holeCo - Orifice Coefficient (usually 0.6 for sharp edge hole) - densitygc - gravitational constantP - Pressure differential


Standard Flow Equations (orifices)Standard Flow Equations (orifices)– Sonic Vapour Flow from a tank/pipe under pressSonic Vapour Flow from a tank/pipe under press

11

1

2

og

coomass TR

MgAPCQ

Q - mass flow (sonic exit velocity)

Co - Discharge Coef

A - Area of hole

Po - Inlet Pressure (abs)

- Cp/ Cv

gc - gravitational constant

M- molecular weigth

Rg - Gas Coef

To - temp (abs)


Flashing LiquidsFlashing Liquids– A liquid operated above it’s boiling point A liquid operated above it’s boiling point

will flash in a release. will flash in a release. Case 1. The fluid path is very short (through Case 1. The fluid path is very short (through

the wall of a vessel) and non-equilibrium the wall of a vessel) and non-equilibrium conditions exist. The liquid does not have time conditions exist. The liquid does not have time to flash within the hole. Use Liquid Eqt.to flash within the hole. Use Liquid Eqt.

Case 2. The fluid path is greater than about 10 Case 2. The fluid path is greater than about 10 cm then flashing occurs. Use a mixed vap/liq cm then flashing occurs. Use a mixed vap/liq density based on the flash, Pdensity based on the flash, Ptank tank - P- Psatsat for for P and P and the liquid Eqt.the liquid Eqt.

Hazard Identification Hazard Identification - Consequence Analysis- Consequence Analysis

Toxic ReleasesToxic Releases– Types:Types:

Ground Level, Elevated, Lighter than Air, Ground Level, Elevated, Lighter than Air, Heavier than Air, Neutral buoyant, Heavier than Air, Neutral buoyant, Continuous Release, Puff ReleaseContinuous Release, Puff Release

– Consequences:Consequences: Health, Environmental, On-site or Off-siteHealth, Environmental, On-site or Off-site

– Causes:Causes: (LOSS of CONTAINMENT) - Leakage (vessel (LOSS of CONTAINMENT) - Leakage (vessel

failure, pump or pipe failure, flange failure), failure, pump or pipe failure, flange failure), drain points, splashesdrain points, splashes

Consequence AnalysisConsequence Analysis

Toxic Releases - Ground LevelToxic Releases - Ground Level

Consequence AnalysisConsequence Analysis Toxic Releases - Heavier Than AirToxic Releases - Heavier Than Air


Modelling Toxic ReleasesModelling Toxic Releases

SAFER - Real Time Release Calculations


Gaussian Distribution ModelsGaussian Distribution Models– Assume Assume

distribution is ‘normal’distribution is ‘normal’ Wind SpeedWind Speed Surface RoughnessSurface Roughness Atmospheric StabilityAtmospheric Stability Sampling Period (Momentary Conc’s high Sampling Period (Momentary Conc’s high

for shorter periods of time)for shorter periods of time)

Consequence AnalysisConsequence Analysis- Toxic Releases- Toxic Releases

Gaussian ModelGaussian Model

Y

X

Z

GroundLevelConc.

ElevationConc.

Consequence Analysis Consequence Analysis - Toxic Releases- Toxic Releases

Gaussian ModelGaussian Model

22

5.0exp),,(zyzy

zy

u

QzyxConc

Q = Release Rate

u = Wind Velocity

x = downwind distance

y = cross wind distance

z = elevation

y = Standard Dev in y direction

z = Standard Dev in z direction


Typical Values for the Standard DeviationTypical Values for the Standard Deviation

22

5.0exp),,(zyzy

zy

u

QzyxConc

Distance Downwind y, m z, m

< 300 m 0.0873 x 0.92 0.0736 x 0.84

300- 4000 m 0.0873 x 0.92 0.01771 x 0.69

For E Atmospheric Stability, Complicated Terrain


Gaussian Model - SimplificationsGaussian Model - Simplifications– Conc is max at the centre of the plumeConc is max at the centre of the plume– Worst Case Wind Speed = 1.5 m/sWorst Case Wind Speed = 1.5 m/s– Substitute Substitute yyz z = 0.0224x= 0.0224x22 for x < 500 m and for x < 500 m and

yyz z = 0.394x= 0.394x1.541.54 x > 500 m (for night time x > 500 m (for night time

conditions in a urban release)conditions in a urban release)– Empirical correction factor for elevated Empirical correction factor for elevated

releaserelease

Chemical Engineering - Aug 1998

Hfx

QxConc

5.10224.0)(

2

Conc

Qfx H08.3


Maximum ConcentrationsMaximum Concentrations– EPRG 2 - Emergency Planning Response EPRG 2 - Emergency Planning Response

Guideline 2Guideline 2– LOC - Level of ConcernLOC - Level of Concern– LD 50 - Lethal Dose , 50% of samplesLD 50 - Lethal Dose , 50% of samples– LC 50 - Lethal Concentration , 50% of LC 50 - Lethal Concentration , 50% of

samplessamples– IDLH - Immediately Dangerous to Life and IDLH - Immediately Dangerous to Life and

Health LevelHealth Level– TLV - Threshold Limit ValueTLV - Threshold Limit Value


Maximum ConcentrationsMaximum Concentrations– EPRG 2 - The concentration below which EPRG 2 - The concentration below which

almost all people could be exposed for almost all people could be exposed for one hour without irreversible or other one hour without irreversible or other serious health effects or symptoms that serious health effects or symptoms that would impair their ability to take would impair their ability to take protective actionprotective action

MechanismMechanism– Inhalation, Skin Contact, SwallowingInhalation, Skin Contact, Swallowing


Lines of Defense (Mitigation)Lines of Defense (Mitigation)– Deinventory SystemsDeinventory Systems– Leak Detection (Air Monitors)Leak Detection (Air Monitors)– Isolation SystemsIsolation Systems– Water Sprays (Scrubber Systems, Water Sprays (Scrubber Systems,

Tank Sprays)Tank Sprays)– DikingDiking– Operating ProceduresOperating Procedures


BhopalBhopal– A Release involving Methyl IsocyanateA Release involving Methyl Isocyanate– Methyl Isocyanate - EPRG 2: 0.5 ppmMethyl Isocyanate - EPRG 2: 0.5 ppm– >50,000 lbs released over 2 hours>50,000 lbs released over 2 hours– 2500 deaths2500 deaths– Caused by a disgruntled employee who Caused by a disgruntled employee who

diverted water into a storage tank.diverted water into a storage tank.– Union Carbide president cited for criminal Union Carbide president cited for criminal

negligence charges in India.negligence charges in India.


FIRESFIRES– Types:Types:

Pool Fires, Vapour Cloud Fires (flash fire), Pool Fires, Vapour Cloud Fires (flash fire), Jet FireJet Fire

– Consequences:Consequences: Radiant Heat, Sympathetic IgnitionRadiant Heat, Sympathetic Ignition

– Causes:Causes: (LOSS of CONTAINMENT) - Leakage (LOSS of CONTAINMENT) - Leakage

(vessel failure, pump or pipe failure, (vessel failure, pump or pipe failure, flange failure), drain points, Insulation flange failure), drain points, Insulation fires, auto decompositonfires, auto decompositon

Fires - Pool FireFires - Pool Fire

Fires - Vapour Cloud FireFires - Vapour Cloud Fire

Fires - Jet FireFires - Jet Fire

FIREFIRE

Fire TriangleFire Triangle

Flammable RangeFlammable Range– LFL, UFLLFL, UFL– LEL, UELLEL, UEL

OxidizerOxidizer Ignition Source (they come for free)Ignition Source (they come for free)

Flam. Range0 % VOL 100 % VOL

Fire - Flammability LimitsFire - Flammability Limits

AcetoneAcetone AcetyleneAcetylene Carbon MonoxideCarbon Monoxide CyclohexaneCyclohexane EthyleneEthylene Methane (Nat Methane (Nat

Gas)Gas) PropanePropane

1313

100100

7474

7.87.8

36*36*

1515

9.59.5

2.62.6

2.52.5

12.512.5

1.31.3

2.72.7

55

2.12.1

LEL UEL (% vol)

* 100 % at pressures > 7 MPa (7,000 kPa = 1000 psig)

Fire - IgnitionFire - Ignition

Heat Heat – autoignition temperaturesautoignition temperatures– flash pointflash point

Electrical (spark, static, lightning…)Electrical (spark, static, lightning…) Open Flames (welding, fired heaters, Open Flames (welding, fired heaters,

flares)flares)

OpenCup

Fire - SurpressionFire - Surpression

02468

10121416

0 20 40 60

Added Inert Gas (% v / v)

Conc

of M

ethan

e (%

v /

v)

CO2N2

EFFECT OF INERT GASES ON FLAMMABILITY LIMITS

Fire - ConsequencesFire - Consequences

Financial LossFinancial Loss Personnel Personnel

LossLoss

FIRE - CONSEQUENCE FIRE - CONSEQUENCE ANALYSISANALYSIS

Vapour Cloud Fires - Fire Ball SizeVapour Cloud Fires - Fire Ball Size– Diameter (meters) = 5.8 Mass(kg)Diameter (meters) = 5.8 Mass(kg)1/31/3

Fire Ball DurationFire Ball Duration– Time(sec) = 0.45 Mass (kg)Time(sec) = 0.45 Mass (kg)1/31/3

Radiant Heat DamageRadiant Heat Damage– heat evolved and radiated, orheat evolved and radiated, or– surface emissive power, orsurface emissive power, or– flame temperature and emissivityflame temperature and emissivity

FIRE - CONSEQUENCESFIRE - CONSEQUENCES

Radiant Heat Damage (cont’d)Radiant Heat Damage (cont’d)– Heat Release MethodHeat Release Method

24 r

QFI r

API RP 521 Method; Fr = 0.16 to 0.38, use 0.3

r

x

Fire - ConsequencesFire - ConsequencesDose Duration Result

kJ / m2 sec

838 1.43 mortality of 99% of people580 10 mortality of 50% of people125 30 1st degree burns1.6 1 Continuous Exposure to People Okay

37.5 1 Damage caused to process equipment30 1 spontaneous ignition of wood19 1 cable insulation degrades15 1 Ignition of wood

Fire - ConsequencesFire - Consequences

FireFire

Prevention - Lines of DefensePrevention - Lines of Defense– Flame ArrestersFlame Arresters– ContainmentContainment– Dilution (below the LEL)Dilution (below the LEL)– Emergency IsolationEmergency Isolation– Water, Foam ...Water, Foam ...

ExplosionsExplosions– Types:Types:

Deflagration versus DetonationDeflagration versus Detonation Vapour Cloud Explosions, Physical Vapour Cloud Explosions, Physical

(vessel), BLEVE, Dust Explosions, Nuclear(vessel), BLEVE, Dust Explosions, Nuclear

– Consequences:Consequences: Overpressure, Blast WaveOverpressure, Blast Wave MissilesMissiles FireballFireball

– Causes:Causes: Fire -> ExplosionFire -> Explosion Vessel OverpressureVessel Overpressure Chemical ReactionChemical Reaction

Explosions - PhysicalExplosions - Physical

Typically a gas filled container Typically a gas filled container catastrophically failingcatastrophically failing– most likely to fail at 4 x the vessel most likely to fail at 4 x the vessel

design pressure (mechanical over design pressure (mechanical over design)design)

– higher temperatures (fire exposure, higher temperatures (fire exposure, process excursions) can weaken the process excursions) can weaken the steel resulting in lower than expected steel resulting in lower than expected burst pressureburst pressure

Explosions - Physical Explosions - Physical

Isentropic expansion of the gas Isentropic expansion of the gas equationequation

1

11 b

Sb

P

PVPE

E - Ideal Energy Release (Joules)

Pb - Burst Pressure (Pa)

Ps - Surroundings Pressure (Pa)

k = Cp/Cv

Energy Converted to Blast Wave is usually 40 to 80%

Source: Bodurtha

Explosions - Vapour CloudExplosions - Vapour Cloud

Difference between Fire and Difference between Fire and Explosion is the occurrence of Explosion is the occurrence of OverpressureOverpressure

Conditions RequiredConditions Required– Ignition SourceIgnition Source– Gas Concentration in Range for Gas Concentration in Range for

DetonationDetonation– Oxidizer ?Oxidizer ?0 % VOL 100 % VOL

Detonation

ExplosionsExplosions Detonation Ranges & FlammabilityDetonation Ranges & Flammability

CompoundDetonation Limits(confined) (unconfined)

FlammabilityLimits

Lower Upper Lower Upper Lower Upper

Ethylene (Pres > 7MPa)

3.3 100

Ethylene (Press < 7Mpa

3.3 14.7 2.7 36.

Propane 2.6 7.4 3 7 3 12.4

ExplosionsExplosions

Damage CalculationsDamage Calculations– Step 1. Calculate the TNT EquivalentStep 1. Calculate the TNT Equivalent– Step 2. Determine Overpressure at Step 2. Determine Overpressure at

different distances from the explosion different distances from the explosion centercenter

– Step 3. Determine damage from Step 3. Determine damage from missilesmissiles

– Step 4. Decide if off-site consequences Step 4. Decide if off-site consequences existexist


Vapour Cloud Explosion - TNT Vapour Cloud Explosion - TNT EquivalentEquivalent

Mass of Fuel x Heat Of Combustion

Heat of Combustion of TNTTNTEquivalent

= Explosionx Efficiency (2 %)


Overpressure at DistancesOverpressure at Distances– method of ‘scaled distance’method of ‘scaled distance’

Shock Wave Parameters

0.01

0.1

1

10

100

1 10 100 1000

Scaled Distance ft/lb^1/3

Ov

erp

res

su

re (

ps

i)

Source:

Bodurtha

ExplosionsExplosionsOverpressure Damagepsi0.03 Large glass windows which are already under strain are broken0.15 Typical pressure for glass failure0.3 95% probability of no serious damage0.1 large and small windows are l00% shattered0.7 Minor damage to house structures3 Non-reinforced concrete or cinder walls completely shattered3 Steel frame building distorted and pulled from foundations4 Rupture of oil storage tanks is complete10 Probable total destruction of buildings300 Limit of crater lip100 Lethality (low)200 Lethality (high)30 lung damage (low)37 lung damage (high)5 ear drum rupture

Explosion - ConsequencesExplosion - Consequences

Explosions - PreventionExplosions - Prevention

Avoidance of Flammable MixturesAvoidance of Flammable Mixtures– fuel rich, fuel lean, oxygen deficient, fuel rich, fuel lean, oxygen deficient,

inert gasesinert gases Elimination of Ignition Sources - Elimination of Ignition Sources -

impossible ?impossible ? Avoidance of Runaway ReactionsAvoidance of Runaway Reactions Avoidance of Excessive Fluid Avoidance of Excessive Fluid

PressuresPressures

Explosion - ProtectionExplosion - Protection

Explosion Relief (vessels, pipes, blgd)Explosion Relief (vessels, pipes, blgd)– minimizes the degree of overpressureminimizes the degree of overpressure

Flame Arresters - prevents passage of Flame Arresters - prevents passage of flameflame

Separation - plant layoutSeparation - plant layout Containment - blast walls, barricades …Containment - blast walls, barricades … Automatic IsolationAutomatic Isolation Automatic Explosion SuppressionAutomatic Explosion Suppression

Explosion - ProtectionExplosion - Protection

Part 3 - Lines Of DefensePart 3 - Lines Of Defense

Relief ValvesRelief Valves Control System (high temp interlock)Control System (high temp interlock) Deinventory SystemsDeinventory Systems Redundant systemsRedundant systems Operating ProceduresOperating Procedures

ARE THE LINES OF DEFENSE ARE THE LINES OF DEFENSE ADEQUATE ?ADEQUATE ?

Part 4 - RecommendationsPart 4 - Recommendations

For those consequences that are For those consequences that are very serious and are likely to occur very serious and are likely to occur make recommendationsmake recommendations

‘‘Likely’ - those things that could Likely’ - those things that could reasonably occur within the reasonably occur within the lifetime of a plantlifetime of a plant

‘‘Very Serious’ - All Off Site Very Serious’ - All Off Site ConsequencesConsequences

PHA - ExamplePHA - Example

Materials are - Ethylene, Steel, Materials are - Ethylene, Steel, Water Water

EthyleneWater

To Atm.


Material Properties - From MSDS Material Properties - From MSDS SheetsSheets– Ethylene Ethylene

Explosion limits: 2.7 - 36% Relative to most hydrocarbons high range of limits, at pressures > 7Mpa UEL = 100%

Toxicity - Considered an asphyxiantToxicity - Considered an asphyxiant

– Water & SteelWater & Steel No explosion limits or ToxicityNo explosion limits or Toxicity


Chemical Interaction MatrixChemical Interaction Matrix

Ethylene

Water

Steel

Ethylene Water Steel

X

none X

none yes X

Triple/Multiple combinations: None

PHA - ExamplePHA - Example Hazard Identification - What ifHazard Identification - What if

Fire

Rupture of shell and subsequent ignition

Hazard Mechanism

Shell ruptures from poor quality workmanship. Ignition highly likely.

Consequences

1. Jet fire likely causing localized property damage.

2. VCE possible (1000 kg material)

Risk

1. Low - Onsite and, probability of shell failing low.2. Off site!

Lines of Defense

Initial hydrostatic testing of equipment


Toxic

Release of Ethylene into water system

Hazard Mechanism Consequences Risk Lines of Defense


RecommendationsRecommendations– Ensure vessel construction has Ensure vessel construction has

appropriate quality control appropriate quality control (hydrotesting).(hydrotesting).

– Maintenance and Inspection of Maintenance and Inspection of ExchangerExchanger

Documents

PROCESS HAZARDS ANALYSIS. Process Hazards Analysis n WHAT ? –Fire, Explosions, Toxic Releases –Consequences, Mechanism, Improvement n WHY ? –Ensure Safety