Process Hazards Analysis n WHAT ? –Fire, Explosions, Toxic Releases –Consequences, Mechanism, Improvement n WHY ? –Ensure Safety to the Public and Employees –Risk Management n WHO ? –Performed by process engineers and plant personnel
Process Hazards Analysis Part 1 - Hazard Identification n Properties of Materials –Reactive - Mix wrong proportions, abnormal chemicals, temperature or pressure excursions –Flammable –Explosive –Toxic - humans, ecology –Comparison to Other Materials
Part 2 - Hazard Events & Consequence Analysis n Toxic Release –Toxic Concentrations - Indoor, Downwind n Fire (Radiation) n Explosion (Physical Explosion, Chemical Explosion) –Pressure Wave, Fireball, Missiles Consequence Analysis Spreadsheet
Hazard Identification - Hazardous Events n Loss Of Containment –Checklists –What-if (Brainstorming) Session n Open-ended Manual Valves, Valve Sheared Off n Pump Seal Failures n Heat Exchanger Tube Rupture n Operation at Abnormal Conditions –What-if Session –HAZOP method –FMEA method
Hazard Identification Consequence Analysis n Not all hazards require a numerical quantification of the hazard. n Hazards may be evaluated by Qualitative means using engineering judgement. –A 1/8” dia hole in a water line has no off- site consequences –Deinventory of 3000 lb. of methyl isocyanate (chemical in Bhopal Incident)
Hazard Identification Consequence Analysis n For Consequences that are not obvious or that are serious enough that more detail is warranted. Use Quantitative techniques. n Step 1. Determine the Release Rate n Step 2. Determine the Effects
Hazard Identification Consequence Analysis n Determining The Release Rate –Assume a scenario n Pick a ‘most likely’ scenario - corrosion causes a 1/8” diameter hole in pipe n Pick a ‘worst case’ scenario - pipe is sheared off by forklift –Use standard engineering calculations to determine the release rate.
Hazard Identification Consequence Analysis n Standard Flow Equations (orifices) –Liquid Flow from a tank/pipe under press A - area of hole Co - Orifice Coefficient (usually 0.6 for sharp edge hole) - density g c - gravitational constant P - Pressure differential
Hazard Identification Consequence Analysis n Standard Flow Equations (orifices) –Sonic Vapour Flow from a tank/pipe under press Q - mass flow (sonic exit velocity) Co - Discharge Coef A - Area of hole P o - Inlet Pressure (abs) - Cp/ Cv g c - gravitational constant M- molecular weigth R g - Gas Coef T o - temp (abs)
Hazard Identification Consequence Analysis n Flashing Liquids –A liquid operated above it’s boiling point will flash in a release. n Case 1. The fluid path is very short (through the wall of a vessel) and non-equilibrium conditions exist. The liquid does not have time to flash within the hole. Use Liquid Eqt. Case 2. The fluid path is greater than about 10 cm then flashing occurs. Use a mixed vap/liq density based on the flash, P tank - P sat for P and the liquid Eqt. Case 2. The fluid path is greater than about 10 cm then flashing occurs. Use a mixed vap/liq density based on the flash, P tank - P sat for P and the liquid Eqt.
Hazard Identification - Consequence Analysis n Toxic Releases –Types: n Ground Level, Elevated, Lighter than Air, Heavier than Air, Neutral buoyant, Continuous Release, Puff Release –Consequences: n Health, Environmental, On-site or Off-site –Causes: n (LOSS of CONTAINMENT) - Leakage (vessel failure, pump or pipe failure, flange failure), drain points, splashes
Consequence Analysis n Toxic Releases - Ground Level
Consequence Analysis n Toxic Releases - Heavier Than Air
Consequence Analysis n Modelling Toxic Releases SAFER - Real Time Release Calculations
Consequence Analysis n Gaussian Distribution Models –Assume n distribution is ‘normal’ n Wind Speed n Surface Roughness n Atmospheric Stability n Sampling Period (Momentary Conc’s high for shorter periods of time)
Consequence Analysis - Toxic Releases n Gaussian Model Y X Z Ground Level Conc. Elevation Conc.
Consequence Analysis - Toxic Releases n Gaussian Model 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
Consequence Analysis - Toxic Releases n Typical Values for the Standard Deviation Distance Downwind y, m z, m < 300 m0.0873 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
Consequence Analysis - Toxic Releases n Gaussian Model - Simplifications –Conc is max at the centre of the plume –Worst Case Wind Speed = 1.5 m/s –Substitute y z = 0.0224x 2 for x 500 m (for night time conditions in a urban release) –Empirical correction factor for elevated release Chemical Engineering - Aug 1998
Consequence Analysis - Toxic Releases n Maximum Concentrations –EPRG 2 - Emergency Planning Response Guideline 2 –LOC - Level of Concern –LD 50 - Lethal Dose, 50% of samples –LC 50 - Lethal Concentration, 50% of samples –IDLH - Immediately Dangerous to Life and Health Level –TLV - Threshold Limit Value
Consequence Analysis - Toxic Releases n Maximum Concentrations –EPRG 2 - The concentration below which almost all people could be exposed for one hour without irreversible or other serious health effects or symptoms that would impair their ability to take protective action n Mechanism –Inhalation, Skin Contact, Swallowing
Consequence Analysis - Toxic Releases n Lines of Defense (Mitigation) –Deinventory Systems –Leak Detection (Air Monitors) –Isolation Systems –Water Sprays (Scrubber Systems, Tank Sprays) –Diking –Operating Procedures
Consequence Analysis - Toxic Releases n Bhopal –A Release involving Methyl Isocyanate –Methyl Isocyanate - EPRG 2: 0.5 ppm –>50,000 lbs released over 2 hours –2500 deaths –Caused by a disgruntled employee who diverted water into a storage tank. –Union Carbide president cited for criminal negligence charges in India.
Consequence Analysis n FIRES –Types: n Pool Fires, Vapour Cloud Fires (flash fire), Jet Fire –Consequences: n Radiant Heat, Sympathetic Ignition –Causes: n (LOSS of CONTAINMENT) - Leakage (vessel failure, pump or pipe failure, flange failure), drain points, Insulation fires, auto decompositon
FIRE n Fire Triangle n Flammable Range –LFL, UFL –LEL, UEL n Oxidizer n Ignition Source (they come for free) Flam. Range 0 % VOL100 % VOL
Fire - Flammability Limits n Acetone n Acetylene n Carbon Monoxide n Cyclohexane n Ethylene n Methane (Nat Gas) n Propane 13 100 74 7.8 36* 15 220.127.116.1118.104.22.1682.1 LELUEL (% vol) * 100 % at pressures > 7 MPa (7,000 kPa = 1000 psig)
Fire - Ignition n Heat –autoignition temperatures –flash point n Electrical (spark, static, lightning…) n Open Flames (welding, fired heaters, flares) Open Cup
Fire - Surpression EFFECT OF INERT GASES ON FLAMMABILITY LIMITS
Fire - Consequences n Financial Loss n Personnel Loss
FIRE - CONSEQUENCE ANALYSIS n Vapour Cloud Fires - Fire Ball Size –Diameter (meters) = 5.8 Mass(kg) 1/3 n Fire Ball Duration –Time(sec) = 0.45 Mass (kg) 1/3 n Radiant Heat Damage –heat evolved and radiated, or –surface emissive power, or –flame temperature and emissivity
FIRE - CONSEQUENCES n Radiant Heat Damage (cont’d) –Heat Release Method API RP 521 Method; Fr = 0.16 to 0.38, use 0.3 r x
Fire - Consequences DoseDurationResult kJ / m2 sec 8381.43mortality of 99% of people 58010mortality of 50% of people 125301st degree burns 1.61Continuous Exposure to People Okay 37.51Damage caused to process equipment 301spontaneous ignition of wood 191cable insulation degrades 151Ignition of wood
Fire n Prevention - Lines of Defense –Flame Arresters –Containment –Dilution (below the LEL) –Emergency Isolation –Water, Foam...
Explosions –Types: n Deflagration versus Detonation n Vapour Cloud Explosions, Physical (vessel), BLEVE, Dust Explosions, Nuclear –Consequences: n Overpressure, Blast Wave n Missiles n Fireball –Causes: n Fire -> Explosion n Vessel Overpressure n Chemical Reaction
Explosions - Physical n Typically a gas filled container catastrophically failing –most likely to fail at 4 x the vessel design pressure (mechanical over design) –higher temperatures (fire exposure, process excursions) can weaken the steel resulting in lower than expected burst pressure
Explosions - Physical n Isentropic expansion of the gas equation 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 Cloud n Difference between Fire and Explosion is the occurrence of Overpressure n Conditions Required –Ignition Source –Gas Concentration in Range for Detonation –Oxidizer ? 0 % VOL100 % VOL Detonation
Explosions n Damage Calculations –Step 1. Calculate the TNT Equivalent –Step 2. Determine Overpressure at different distances from the explosion center –Step 3. Determine damage from missiles –Step 4. Decide if off-site consequences exist
Explosions n Vapour Cloud Explosion - TNT Equivalent Mass of Fuel x Heat Of Combustion Heat of Combustion of TNT TNT Equivalent = Explosion x Efficiency (2 %)
Explosions n Overpressure at Distances –method of ‘scaled distance’ Source: Bodurtha
Explosions OverpressureDamage psi 0.03Large glass windows which are already under strain are broken 0.15Typical pressure for glass failure 0.395% probability of no serious damage 0.1large and small windows are l00% shattered 0.7Minor damage to house structures 3Non-reinforced concrete or cinder walls completely shattered 3Steel frame building distorted and pulled from foundations 4Rupture of oil storage tanks is complete 10Probable total destruction of buildings 300Limit of crater lip 100Lethality (low) 200Lethality (high) 30lung damage (low) 37lung damage (high) 5ear drum rupture
Explosions - Prevention n Avoidance of Flammable Mixtures –fuel rich, fuel lean, oxygen deficient, inert gases n Elimination of Ignition Sources - impossible ? n Avoidance of Runaway Reactions n Avoidance of Excessive Fluid Pressures
Explosion - Protection n Explosion Relief (vessels, pipes, blgd) –minimizes the degree of overpressure n Flame Arresters - prevents passage of flame n Separation - plant layout n Containment - blast walls, barricades … n Automatic Isolation n Automatic Explosion Suppression
Part 3 - Lines Of Defense n Relief Valves n Control System (high temp interlock) n Deinventory Systems n Redundant systems n Operating Procedures ARE THE LINES OF DEFENSE ADEQUATE ?
Part 4 - Recommendations n For those consequences that are very serious and are likely to occur make recommendations n ‘Likely’ - those things that could reasonably occur within the lifetime of a plant n ‘Very Serious’ - All Off Site Consequences
PHA - Example n Materials are - Ethylene, Steel, Water Ethylene Water To Atm.
PHA - Example n Material Properties - From MSDS Sheets –Ethylene n n Explosion limits: 2.7 - 36% Relative to most hydrocarbons high range of limits, at pressures > 7Mpa UEL = 100% n Toxicity - Considered an asphyxiant –Water & Steel n No explosion limits or Toxicity
PHA - Example n Chemical Interaction Matrix Ethylene Water Steel EthyleneWaterSteel X none X none yes X Triple/Multiple combinations: None
PHA - Example n Hazard Identification - What if Fire Rupture of shell and subsequent ignition HazardMechanism 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 hydrost atic testing of equipm ent
PHA - Example Toxic Release of Ethylene into water system HazardMechanismConsequencesRisk Lines of Defense
PHA - Example n Recommendations –Ensure vessel construction has appropriate quality control (hydrotesting). –Maintenance and Inspection of Exchanger