Transcript 49(Lecture - Process Hazards Analysis)

PROCESS HAZARDS
ANALYSIS
Process Hazards Analysis

WHAT ?
– Fire, Explosions, Toxic Releases
– Consequences , Mechanism, Improvement

WHY ?
– Ensure Safety to the Public and Employees
– Risk Management

WHO ?
– Performed by process engineers and plant
personnel
Process Hazards Analysis
Report Contents
1. Hazards Identification
2. Hazardous Events & Consequences
Analysis
3. Lines of Defense
4. Recommendations
Process Hazards Analysis
Part 1 - Hazard Identification

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

Toxic Release
– Toxic Concentrations - Indoor, Downwind
Fire (Radiation)
 Explosion (Physical Explosion, Chemical
Explosion)

– Pressure Wave, Fireball, Missiles
Consequence Analysis Spreadsheet
Hazard Identification
- Hazardous Events

Loss Of Containment
– Checklists
– What-if (Brainstorming) Session




Open-ended Manual Valves, Valve Sheared Off
Pump Seal Failures
Heat Exchanger Tube Rupture
Operation at Abnormal Conditions
– What-if Session
– HAZOP method
– FMEA method
Hazard Identification
Consequence Analysis
Not all hazards require a numerical
quantification of the hazard.
 Hazards may be evaluated by
Qualitative means using engineering
judgement.

– A 1/8” dia hole in a water line has no offsite consequences
– Deinventory of 3000 lb. of methyl
isocyanate (chemical in Bhopal Incident)
Hazard Identification
Consequence Analysis

For Consequences that are not obvious
or that are serious enough that more
detail is warranted. Use Quantitative
techniques.
Step 1. Determine the Release Rate
 Step 2. Determine the Effects

Hazard Identification
Consequence Analysis

Determining The Release Rate
– Assume a scenario


Pick a ‘most likely’ scenario - corrosion causes a
1/8” diameter hole in pipe
Pick a ‘worst case’ scenario - pipe is sheared off
by forklift
– Use standard engineering calculations to
determine the release rate.
Hazard Identification
Consequence Analysis

Standard Flow Equations (orifices)
– Liquid Flow from a tank/pipe under press
Qmass  A Co 2  g c P
A - area of hole
Co - Orifice Coefficient (usually 0.6 for sharp edge hole)
 - density
gc - gravitational constant
P - Pressure differential
Hazard Identification
Consequence Analysis

Standard Flow Equations (orifices)
– Sonic Vapour Flow from a tank/pipe under
press
 1
 g c M  2   1


Qmass  Co APo
Rg To    1 
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)
Hazard Identification
Consequence Analysis

Flashing Liquids
– A liquid operated above it’s boiling point
will flash in a release.


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, Ptank - Psat for DP
and the liquid Eqt.
Hazard Identification
- Consequence Analysis

Toxic Releases
– Types:

Ground Level, Elevated, Lighter than Air, Heavier
than Air, Neutral buoyant, Continuous Release, Puff
Release
– Consequences:

Health, Environmental, On-site or Off-site
– Causes:

(LOSS of CONTAINMENT) - Leakage (vessel failure,
pump or pipe failure, flange failure), drain points,
splashes
Consequence Analysis

Toxic Releases - Ground Level
Consequence Analysis

Toxic Releases - Heavier Than Air
Consequence Analysis

Modelling Toxic Releases
SAFER - Real Time Release Calculations
Consequence Analysis

Gaussian Distribution Models
– Assume





distribution is ‘normal’
Wind Speed
Surface Roughness
Atmospheric Stability
Sampling Period (Momentary Conc’s high for
shorter periods of time)
Consequence Analysis
- Toxic Releases

Gaussian Model
Y
Ground
Level
Conc.
Z
Elevation
Conc.
X
Consequence Analysis
- Toxic Releases

Gaussian Model
2


 y   z
Q

Conc( x, y, z ) 
exp  0.5    
yz u
  y    z



Q = Release Rate



2


 

u = Wind Velocity
x = downwind distance
y = cross wind distance
y = Standard Dev in y direction
z = elevation
z = Standard Dev in z direction
Consequence Analysis
- Toxic Releases

Typical Values for the Standard
Deviation
2


 y   z
Q

Conc( x, y, z ) 
exp  0.5    
yz u
  y    z



Distance Downwind
y, m



2


 

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
Consequence Analysis
- Toxic Releases
Q
Conc( x) 
fH
2
 0.0224x 1.5

x  3.08 f H
Q
Conc
Gaussian Model - Simplifications
– Conc is max at the centre of the plume
– Worst Case Wind Speed = 1.5 m/s
– Substitute yz = 0.0224x2 for x < 500 m
and yz = 0.394x1.54 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

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

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

Mechanism
– Inhalation, Skin Contact, Swallowing
Consequence Analysis
- Toxic Releases

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

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

FIRES
– Types:

Pool Fires, Vapour Cloud Fires (flash fire), Jet
Fire
– Consequences:

Radiant Heat, Sympathetic Ignition
– Causes:

(LOSS of CONTAINMENT) - Leakage (vessel
failure, pump or pipe failure, flange failure),
drain points, Insulation fires, auto decompositon
Fires - Pool Fire
Fires - Vapour Cloud Fire
Fires - Jet Fire
FIRE

Fire Triangle

Flammable Range
Flam. Range
– LFL, UFL
0 % VOL
– LEL, UEL
100 % VOL
Oxidizer
 Ignition Source (they come for free)

Fire - Flammability Limits
LEL







Acetone
Acetylene
Carbon Monoxide
Cyclohexane
Ethylene
Methane (Nat Gas)
Propane
2.6
2.5
12.5
1.3
2.7
5
2.1
UEL (% vol)
13
100
74
7.8
36*
15
9.5
* 100 % at pressures > 7 MPa (7,000 kPa = 1000 psig)
Fire - Ignition

Heat
– autoignition temperatures
– flash point
Open
Cup
Electrical (spark, static, lightning…)
 Open Flames (welding, fired heaters,
flares)

Fire - Surpression
Conc of Methane (% v /v)
EFFECT OF INERT GASES ON FLAMMABILITY LIMITS
16
14
12
10
8
6
4
2
0
CO2
N2
0
20
40
Added Inert Gas (% v / v)
60
Fire - Consequences
Financial Loss
 Personnel Loss

FIRE - CONSEQUENCE
ANALYSIS

Vapour Cloud Fires - Fire Ball Size
– Diameter (meters) = 5.8 Mass(kg)1/3

Fire Ball Duration
– Time(sec) = 0.45 Mass (kg)1/3

Radiant Heat Damage
– heat evolved and radiated, or
– surface emissive power, or
– flame temperature and emissivity
FIRE - CONSEQUENCES

Radiant Heat Damage (cont’d)
– Heat Release Method
Fr Q
I
2
4 r
r
x
API RP 521 Method; Fr = 0.16 to 0.38, use 0.3
Fire - Consequences
Dose Duration
Result
kJ / m2 sec
838
580
125
1.6
1.43
10
30
1
mortality of 99% of people
mortality of 50% of people
1st degree burns
Continuous Exposure to People Okay
37.5
30
19
15
1
1
1
1
Damage caused to process equipment
spontaneous ignition of wood
cable insulation degrades
Ignition of wood
Fire - Consequences
Fire

Prevention - Lines of Defense
– Flame Arresters
– Containment
– Dilution (below the LEL)
– Emergency Isolation
– Water, Foam ...
Explosions
– Types:


Deflagration versus Detonation
Vapour Cloud Explosions, Physical (vessel),
BLEVE, Dust Explosions, Nuclear
– Consequences:



Overpressure, Blast Wave
Missiles
Fireball
– Causes:



Fire -> Explosion
Vessel Overpressure
Chemical Reaction
Explosions - Physical

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

Isentropic expansion of the gas
equation

Pb V   PS
E
1  
  1   Pb

E - Ideal Energy Release (Joules)
 1








Energy
Converted to
Blast Wave
is usually 40
to 80%
k = Cp/Cv
Pb - Burst Pressure (Pa)
Ps - Surroundings Pressure (Pa)
Source: Bodurtha
Explosions - Vapour Cloud
Difference between Fire and Explosion
is the occurrence of Overpressure
 Conditions Required

– Ignition Source
– Gas Concentration in Range for Detonation
– Oxidizer ?
Detonation
0 % VOL
100 % VOL
Explosions

Detonation Ranges & Flammability
Compound
Detonation Limits
(confined)
(unconfined)
Lower Upper
Ethylene (Pres > 7
MPa)
3.3
100
Ethylene (Press < 7
Mpa
3.3
14.7
Propane
2.6
7.4
Flammability
Limits
Lower Upper Lower Upper
3
7
2.7
36.
3
12.4
Explosions

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

Vapour Cloud Explosion - TNT
Equivalent
TNT
=
Equivalent
Mass of Fuel x
Heat Of Combustion
Heat of Combustion of TNT
Explosion
x Efficiency
(2 %)
Explosions
Overpressure at Distances
– method of ‘scaled distance’
Shock Wave Parameters
Overpressure (psi)

100
10
1
0.1
0.01
1
10
100
Scaled Distance ft/lb^1/3
1000
Source:
Bodurtha
Explosions
Overpressure Damage
psi
0.03
Large glass windows which are already under strain are broken
0.15
Typical pressure for glass failure
0.3
95% probability of no serious damage
0.1
large and small windows are l00% shattered
0.7
Minor damage to house structures
3
Non-reinforced concrete or cinder walls completely shattered
3
Steel frame building distorted and pulled from foundations
4
Rupture of oil storage tanks is complete
10
Probable total destruction of buildings
300
Limit of crater lip
100
Lethality (low)
200
Lethality (high)
30
lung damage (low)
37
lung damage (high)
5
ear drum rupture
Explosion - Consequences
Explosions - Prevention

Avoidance of Flammable Mixtures
– fuel rich, fuel lean, oxygen deficient, inert
gases
Elimination of Ignition Sources impossible ?
 Avoidance of Runaway Reactions
 Avoidance of Excessive Fluid Pressures

Explosion - Protection

Explosion Relief (vessels, pipes, blgd)
– minimizes the degree of overpressure
Flame Arresters - prevents passage of
flame
 Separation - plant layout
 Containment - blast walls, barricades …
 Automatic Isolation
 Automatic Explosion Suppression

Explosion - Protection
Part 3 - Lines Of Defense
Relief Valves
 Control System (high temp interlock)
 Deinventory Systems
 Redundant systems
 Operating Procedures

ARE THE LINES OF DEFENSE ADEQUATE ?
Part 4 - Recommendations
For those consequences that are very
serious and are likely to occur make
recommendations
 ‘Likely’ - those things that could
reasonably occur within the lifetime of a
plant
 ‘Very Serious’ - All Off Site
Consequences

PHA - Example
Water

Ethylene
To Atm.
Materials are - Ethylene, Steel, Water
PHA - Example

Material Properties - From MSDS Sheets
– Ethylene


Explosion limits: 2.7 - 36%
Relative to most hydrocarbons high range of
limits, at pressures > 7Mpa UEL = 100%
Toxicity - Considered an asphyxiant
– Water & Steel

No explosion limits or Toxicity
PHA - Example

Chemical Interaction Matrix
Ethylene
Water
Steel
Ethylene X
Water
none
X
Steel
none
yes
Triple/Multiple combinations: None
X
PHA - Example

Hazard Identification - What if
Hazard
Mechanism
Consequences Risk
Lines of
Defense
Shell ruptures
from poor
quality
workmanship.
Ignition highly
likely.
1. Jet fire
likely causing
localized
property
damage.
Initial
hydrost
atic
testing
of
equipm
ent
Fire
Rupture of
shell and
subsequent
ignition
1. Low Onsite
and,
probability
of shell
failing
2. VCE
low.
possible (1000
2. Off site!
kg material)
PHA - Example
Hazard
Toxic
Release of
Ethylene
into water
system
Mechanism
Consequences Risk
Lines of
Defense
PHA - Example

Recommendations
– Ensure vessel construction has appropriate
quality control (hydrotesting).
– Maintenance and Inspection of Exchanger