DCMH.

CH4051_Process Safety
Fire

Prepared by:

Dr. Mardhati Zainal Abidin

Universiti Teknologi PETRONAS

AP Ir. Dr. Risza Rusli

University of Doha for Science & Technology

For:
Ho Chi Minh City University of Technology

1
A quick recap on dispersion model..

2
 Objectives

At the end of this chapter, students should be able to

• Understand the flammability characteristics of liquid, vapor and gasses and the nature of fire
• Prevent fire by controlling ignition source, fuel concentration range and oxygen content
• Estimate the impact of fire hazard

3
 Chapter Outline

▪ Hazardous material release & fire

▪ Fire triangle
▪ Flammability characteristics of liquids and vapours
▪ Flammability diagram
▪ Fire consequence model

4
Consequences Analysis Procedure
Loss of containment
•Rupture or break in pipeline
Selection of a Release Incident •Hole in a tank or pipeline
•Runaway reaction
To describe release accident •Fire external to vessel
•Total quantity released
Selection of a Source Model
•Release duration
•Release rate Neutrally buoyant models
Results from the models
Selection of a Dispersion Model •Downwind concentration
•Area affected
•Duration

Models Flammable/Toxic
•TNT Equivalency
•Multi-Energy Explosion •Response vs dose
•Fireball •Probit model
Selection of Fire Selection of •Toxic response
Results
& Explosion Model Effect Model •No. of individuals affected
•Blast overpressure
•Radiant heat flux •Property damage
•Escape
•Emergency Response Mitigation Factors
•Containment dikes
•PPE

Consequence Model

5
 Hazardous Material Release

Hazardous materials are typically contained in storage or process vessels (as a

gas, liquid or solid).

Release of Hazardous Material

The release is significant if the solid is:
• An unstable material such as an explosive
• Flammable or combustible solid (petroleum coke)
• Toxic or carcinogenic (either in bulk or as dust)
• Soluble in water and spill occurs over water (dissolves into the water)
• Dust (which can cause clouds and impact respiration)

Release of Liquids or Gases from Containment

Release from containment will result in:
• Instantaneous release if there is a major failure/rupture
• Continuous release if a hole develops in a vessel

6
 Hazardous Material Release

Consequences of Release from a Pressurised Storage Tank

Flammable Gas Release

No Ignition = vapour cloud (could be toxic)
Immediate ignition = jet fire
Delayed ignition = vapour cloud explosion

Flammable Liquid Release

No ignition = toxic health issues
Immediate Ignition – pool fire
Pool fire under or near a pressure vessel can lead to a Boiling
Liquid Expanding Vapour Explosion (BLEVE)

7
 Consequence - Fire

To prevent these accidents, need to be familiar with:

(1) Fire, explosion and toxicity properties of materials
(2) Nature of fire, explosion and toxic release process
(3) Procedures to reduce fire, explosion and toxic release hazards

Fire is the rapid exothermic oxidation of an ignited fuel.

Fuel may be in solid, liquid or vapour but combustion is always in the

vapour phase. Solids and liquids vaporize before combustion.

8
 Consequence - Fire

Event Type Event Mechanism Hazard Concern

Fires
Gas/Vapour - Jet fire, flash fire, fireball Thermal radiation, flame
Liquid - Pool fire, tank fire, running fire, spray impingement, combustion
fire, fireball products, initiation of
Solids - Bulk fire, smouldering fire further fires
Explosions
Confined - Runaway reactions, combustion Blast pressure waves,
explosion, physical explosion, missiles, windage,
boiling liquid expanding vapour thermal radiation,
Unconfined explosion (BLEVE) combustion products
- Vapour cloud explosion
Gas Clouds
Heavy - Jets Asphyxiation, toxicity,
Gases - Evaporation, volatilisation, boil-off flammability, range of
Light Gases concentrations.

9
 Consequence - Fire
TYPES OF FIRES
Jet Fires
• Occurs when a flammable liquid or gas is ignited after it is
released from a pressurized or punctured vessel or pipe.
• The pressure generates a long-flame which is stable under
most conditions
• For a two-phase jet, a part of the liquid may rain-out onto
the floor and give rise to pool fire.
• The duration of the fire is depending on the release rate
and sources capacity and the flame length is directly
proportional to the flow rate.

10
 Consequence - Fire
TYPES OF FIRES
Jet Fires

11
 Consequence - Fire
TYPES OF FIRES
Jet Fires

12
 Consequence - Fire
TYPES OF FIRES
Pool Fires
• Occur on ignition of an accumulation of liquid as a pool on the ground
or on water or other liquid.
• A steady state burning is rapidly achieved as the flame vapor to sustain
the fire is provided by evaporation of the liquid by heat from the flames.

13
 Consequence - Fire
TYPES OF FIRES
Pool Fires

14
 Consequence - Fire
TYPES OF FIRES
Flash Fire
• When a cloud of flammable gas and air is ignited.
• The speed of burning is a function of gas concentration and wind speed.
• The common example is fire due to gas leak and mistakenly ignited using the cigarettes lighter.
• Damage is caused by thermal radiation and oxygen sudden depletion.
• Typical flame propagation is 4 m/s and increased with wind speed.
• Flash fire is transient (short period) in nature and the steady burning period is even shorter and
thus it is difficult to determine its emissive powers and incipient fluxes.

15
 Fire Triangle

Fuels Oxidizers
• Liquids
• Liquids
- hydrogen
- gasoline, acetone,
peroxide, nitric
ether, pentane
acid,
• Solids
perchloric acid
- plastics, wood
• Gases
dust, fibers, metal
- oxygen,
particles
fluorine,
• Gases
chlorine
- acetylene,
Ignition • Solids
propane, carbon
monoxide, sources - metal
• Sparks, flames, peroxides,
hydrogen
static electricity, ammonium
heat nitrate
16
 Application of Fire Triangle

Fires and explosions can be prevented by removing any single leg from the
fire triangle.

Problem: Ignition sources are so plentiful that it is not a reliable control

method.

Robust Control: Prevent existence of flammable mixtures.

17
Application of Fire Triangle

18
 Flammability Characteristics of Liquids
▪ Flash Point Temperature : minimum
temperature at which a liquid gives off
sufficient vapour to form an ignitable
mixture with air near the surface of the
liquid.
▪ One of the major quantities used to
characterize fire and explosion hazard of
liquids.
▪ Determined in OPEN CUPS and CLOSED
CUPS test.
▪ Open cup values are higher than closed cup
values and apply to open vessels, spills etc.
▪ Fire Point : the lowest temperature at
which a liquid will continue to burn, Cleveland Opened Cup Tag Closed Cup Flash
normally slightly above the open cup flash
point. Flash Point Tester Point Tester

19
 Flammability Characteristics of
Vapours and Gases
▪ Flammable / Explosive Limits - Range of composition of material in air
which will burn
▪ Lower Flammability Limit (LFL) or Lower Explosive Limit (LEL)
- minimum concentration of a particular combustible gas or vapor
necessary to support its combustion in air
▪ Upper Flammability Limit (UFL) or Upper Explosive Limit (UEL)
- maximum fuel concentration through which a flame will propagate

▪ Normally expressed as volume percent fuel (% of fuel + air) at

atmospheric temperature and pressure

20
 Flammability Characteristics of
Vapours and Gases
• The mixture will not burn when the composition is lower than the LFL;
the mixture is too lean for combustion.
• The mixture is also not combustible when the composition is too rich,
that is, when it is above UFL.
• A mixture is flammable only when the composition is between the LFL
and the UFL

21
 Flammability Characteristics of
Vapours and Gases
▪ Auto Ignition Temperature (AIT)
• Temperature above which spontaneous combustion can occur
without the use of a spark or flame.
• The value depends on concentration of the vapor, material in
contact and size of the containment
▪ Minimum Oxygen Concentration (MOC)
• Oxygen concentration below which combustion is not possible.
• Expressed as volume % oxygen
• Also called Limiting Oxygen Concentration (LOC)
• Max. Safe Oxygen Conc. (MSOC)

22
Flammability Relationships
Saturation Vapor Pressure Curve

Vapor pressure / concentration

UFL

Flammable
Liquid
Auto-ignition zone
Gas

LFL

Flash Point AIT: Auto Ignition Temp

Temperature → Ambient Temperature
 Typical Values
LFL UFL
Methane: 5% 15%
Propane: 2.1% 9.5%
Butane: 1.6% 8.4%
Hydrogen: 4.0% 75%

Flash Point Temp. (deg F)

Methanol: 54
Benzene: 12
Gasoline: - 40

AIT (deg. F) MOC (Vol. % O2)

Methane: 1000 Methane: 12%
Methanol: 867 Ethane: 11%
Toluene: 997 Hydrogen: 5%
 Flammability Limits of Mixtures
Le Chatelier Rule (1891)

1 1
LFLmix = n
UFLm ix = n
(6-2)
(6-3)
yi yi

i =1 LFLi

i =1 UFLi

yi on a combustible basis only, excluding non flammable species

Assumptions:
1) Product heat capacities constant
2) No. of moles of gas constant
3) Combustion kinetics of pure species unchanged
4) Adiabatic temperature rise the same for all species
25
Details provided in Process Safety Progress, Summer 2000.
 Example - Flammability Limits of Mixtures
Question: Answer:
What are the LFL and UFL The mole fractions on a fuel-only basis are calculated in the following table.
of a gas mixture composed
of 0.8% hexane, 2.0%
methane, and 0.5%
ethylene by volume?

26 26
 HOMEWORK

1. A liquid mixture containing 0.50 mole fraction benzene-

toluene is contained in a storage vessel at 25°C and 1 atm.
The vessel is vented to the atmosphere. Determine if the
vapor in the vessel is flammable or not.

Hint: Benzene-toluene can be assumed to be an ideal liquid-

vapor system.
 Limiting Oxygen Concentration (LOC)

▪ There is a minimum oxygen concentration

required to propagate a flame.
▪ Explosions and fires can be prevented by
reducing the oxygen concentration regardless
of the concentration of the fuel.
▪ This procedure is known as inerting.
▪ LOC is also known as minimum oxygen
concentration (MOC)
▪ The LOC depends on the inert gas species and
has units of percentage of moles of oxygen in
total moles.
▪ If experimental data are not available, the LOC
can be estimated using stoichiometry of the
combustion reaction and the LFL.

28
 Limiting Oxygen Concentration (LOC)

LFL is based on fuel in air. Oxygen is the key ingredient for fire/explosion. There must be a
minimum O2 concentration (MOC or LOC) required to propagate the flame

Example
Estimate the LOC for butane. LFL for butane is 1.9% vol.

Solution
Stoichiometry for this reaction:

C4H10 + 6.5O2 4CO2 + 5H2O

LOC = (Moles fuel/total moles) x (moles O2/moles fuel)

= LFL x (moles O2/moles fuel)
= 1.8 x (6.5 moles O2/1.0 moles fuel)
= 11.7 vol.% O2
The combustion of butane is preventable by adding nitrogen, carbon dioxide or even water
vapour until the oxygen concentration is less than 11.7%. 29
 Flammability Diagram

Useful for:
• Determining if a mixture is flammable.
• Required for control and prevention of flammable
mixtures
Problems:
• Only limited experimental data available.
• Depends on chemical species.
• Function of temperature and pressure.

Flammability diagram can be approximated.

30
 Flammability Diagram

▪ The zone enclosed by the dashed lines represents all CH4

mixtures that are flammable.
▪ The air line represents all possible combinations of fuel
plus air.
▪ The air line intersects the N2 axis at 79% N2 and 21% O2
(composition of pure air).
▪ The UFL and the LFL are shown as the intersection of the
flammability zone boundary with the air line.
▪ The stoichiometric line represents all stoichiometric
combinations of fuel plus oxygen. The combustion
reaction is in the form
Fuel + z O2 ----→ combustion products
▪ The intersection of the stoichiometric line with the oxygen
axis (in volume % oxygen) is given by 100 [z/(1+z)].
▪ The LOC can be estimated by reading the O2
concentration at the intersection of the stoichiometric line
and a horizontal line drawn through the LFL. This is
equivalent to the equation:
O2
N2
LOC = z (LFL)
31
Flammability Diagram

32
 Exercise - Flammability Diagram

Use the same diagram, if you have a mixture

with the following composition, determine if
the mixture is flammable:
Methane : 20%
Oxygen: 60%
Nitrogen: 20%

33
 Flammability Diagram

Methods to Approximate the Flammability Zone

METHOD 1: Given the flammability limits in air, the LOC, and Consider this scenario as an example:
flammability limits in pure oxygen Methane:
LFL: 5.0% fuel in air (Appendix G)
1. Draw flammability limits in air as points on the air line.
UFL: 15% fuel in air (Appendix G)
2. Draw flammability limits in pure O2 as points on the O2
scale (from Table 6-3). LOC: 12% oxygen (Table 6-3)
3. Use expression 100[z/(1+z)] to locate the stoichiometric Combustion Reaction:
point on the O2 axis and draw the stoichiometric line CH4 + 2 O2 --> CO2 + 2 H2O ; z = 2
from this point to the 100% nitrogen apex.
4. Locate the LOC on the O2 axis and draw a line parallel Pure Oxygen: (Table 6-2)
to the fuel axis until it intersects with the stoichiometric LOL: 5.1% fuel in oxygen
line. Draw a point at this intersection. UOL: 61% fuel in oxygen
5. Connect all the points.
 Flammability Diagram: METHOD 1

Methods to Approximate the Flammability Zone

Step 1: Draw LFL and UFL on
air line (%Fuel in air).
0 100
Air Line, all combinations
Air line always extends of fuel + air

FROM: Fuel: 0%, Oxygen:

21% Nitrogen: 79%

TO: Fuel: 100%, Oxygen:

0%, Nitrogen: 0% Flammable
100
0
0 100
Nitrogen
 Flammability Diagram: METHOD 1

Methods to Approximate the Flammability Zone

Step 2: Draw LFL & UFL line 0 100
Air Line, all combinations
Identify LFL & UFL value from of fuel + air
database

LFL & UFL line always extends

UFL (15%)
FROM: Fuel axis
TO: Air line
100 LFL (5%)
0
0 100
Nitrogen
 Flammability Diagram: METHOD 1

Methods to Approximate the Flammability Zone

Step 3: Draw stoichiometric line using z
from combustion reaction. 0 100
Air Line, all combinations
Fuel + (z) Oxygen ---> Products
of fuel + air
CH4 + 2 O2 --> CO2 + 2 H2O

z=2
 z  66.7%
 *100 = 66.7%
UFL (15%)

 1+ z 
100 LFL (5%)
FROM: Fuel: 0%, Oxygen: 100% 0
0 100
Nitrogen: 100% Nitrogen

TO: Oxygen: 66.7%

 Flammability Diagram: METHOD 1

Methods to Approximate the Flammability Zone

0 100
LOC (12%) Air Line, all combinations
Step 4: Draw the LOC line
of fuel + air
Identify LOC value from database

Draw parallel line with fuel line

from LOC value 66.7% UFL (15%)

100 LFL (5%)

0
0 100
Nitrogen
 Flammability Diagram: METHOD 1

Methods to Approximate the Flammability Zone

Step 5: Draw LOL and UOL in pure oxygen, if known (% fuel in pure oxygen).
Connect the dots to get approximate diagram.

0 100
LOC = 12% oxygen

61% Methane

66.7%
O2 UFL = 15% fuel
5.1% Methane 100 LFL = 5% fuel
0 0
Nitrogen 100
 Flammability Diagram

Methods to Approximate the Flammability Zone

▪ Method 2: Given the flammability limits in air and the LOC (Figure 6-11)
• Use steps 1,3 and 4 from Method 1.
• Connect all the points at the nose of flammability zone.
• Only the area to the right of the air line can be determined.

▪ Method 3: Given the flammability limits in air (Figure 6-12)

• Use steps 1 and 3 from Method 1.
• Estimate the LOC using LOC = z (LFL).
• Connect all the points at the nose of flammability zone.
• Only the area to the right of the air line can be determined.

40
 Flammability Diagram

Methods to Approximate the Flammability Zone

Method 2: Given the flammability
limits in air and the LOC

41
 Flammability Diagram

Methods to Approximate the Flammability Zone

Method 3: Given the
flammability limits in air

42
 Flammability Zone

0 100
Apparatus pressure limit
Flammable
Non-Flammable
20 80

40 60

60 40

80 20

100 0
0 20 40 60 80 100

Nitrogen
 Flammability Zone

0 100
Apparatus pressure limit
Flammable
Non-Flammable
20 80

40 60

60 40

80 20

100 0
0 20 40 60 80 100

Nitrogen
 Flammability Zone

Flammability Diagram:
Hydrogen

From Chemical Process Safety, Third Edition,

By Daniel A. Crowl and Joseph F. Louvar
(ISBN: 0131382268)

Figure 6-10 Experimental flammability diagram for hydrogen. This is a different geometry but
still conveys the same information. (Source: Y. D. Jo and D. A. Crowl, Michigan
Technological University, 2006.)
 Consequence Model – Hazard Levels or Effects
(Thermal Radiation Hazard)
FIRE HAZARD TO PLANT
Radiation Protection can be achieved by:
• Radiation is a function of emissivity of the flame, the • Elimination or segregation of combustible
distance from the flame to the target surface, materials
effective fire area, the view factor, absorptivity,
• Use of incombustible materials for construction
atmospheric transmittance and the fraction of the
and insulation of vessels
combustion energy radiated.
• Control of ignition sources
• A tank, which is adjacent to another, can fail due to
the following: • Stringent operational procedures
1. Heat input raising the product surface temperature • Automatic detection and extinguish of fire
above its flash point
• Incombustible and durable insulation for steel
2. A flammable mixture reaching a source of ignition structure
or vice versa
• Compliance to international code of fire protection
3. Over temperature damage to the tank shell or roof and fire fighting
leading to the tank failure
Consequence Model – Hazard Levels or Effects
(Thermal Radiation Hazard)
Consequence Model – Hazard Levels or Effects
(Thermal Radiation Hazard)

Effects of thermal radiation dose on human body

 Consequence Model – Hazard Levels or Effects
(Thermal Radiation Hazard)
▪ Hazard effects can be caused by the release of hazardous material
▪ Hazard level at receptor points shall be estimated for an accident to measure
the severity of the consequence due to the exposure to the hazard.
▪ Modelling Major Fire - Example
 Consequence Model – Hazard Levels or Effects
(Thermal Radiation Hazard)

• The variable responsible for the fire growth hazard is the mass burning rate, m 
(kg/s). Other related extensive variables are energy/heat release rate, Q (kJ/s)
and the heat of combustion, ∆HC (kJ/kg).

• The total heat release by hydrocarbon is obtained via: Q = m

 H C (kJ/s)
A
• The burning rate can be decomposed into the burning rate per unit area , m
(kg/s·m2) and the surface area, A (m2).
 =m
m A A
• The main concern is the exposure to thermal radiation which could be
quantified as Q r = F Q
where F = proportion of heat transferred by radiation
(typically about 0.2)
 Example
Hazard Levels or Effects – Pool Fire

The reactor product is gasoline and the reactor is surrounded by a circular bund
of 10 m diameter. The runaway reaction leads to a gasoline leak which then
occupies the bunded area. Given that the gasoline ignites, determine:

i. The total radiative flux from the flame

ii. The incident thermal radiation on persons standing 15 m from the
circumference of the bund.

Assume the wind speed is negligible, the proportion of heat radiated is 0.4, the
mass burning rate per unit area is 0.1 kgm-2s-1 , the density of air at ambient
temperature is 1.17 kg m-3, the calorific value of the fuel is 45 MJ kg-1 and the
atmospheric transmissivity is 1.0.
 Example
Hazard Levels or Effects – Pool Fire

The mass burning rate is

 kg    10 m 2  kg
m = m A A =  0.1 2  x   = 7.85
 m s  4  s

Total energy released

Q = m H C =  7.85 kg 2  x  45 x 106 J  = 3.53 x 108 W

 s   kg 

Total radiative energy from flame

Q r = F Q = 0.4 x 3.53 x 108 = 1.4 x 108 W

 Example
Hazard Levels or Effects – Pool Fire
0.61
Flame height 𝑚ሶ 𝐴
𝐻𝐹 = 𝐷𝐹 ∗ 42 0.5
𝜌𝑎𝑖𝑟 𝑔 𝐷𝐹
0.61
0.1 kgΤm2 s
= 10 m x 42 x
kgൗ
1.17 3 x 9.81 mΤs ⋅ 10 m 0.5
m
= 23 m
The incident thermal radiation at 15 m from the bund edge
where S2 = (202 +11.52) = 532 m2

𝜏 𝐹 𝑄ሶ 1 x 1.4 x 108
𝐼= 2
=
4𝜋𝑆 4 𝜋 𝑆2
1 x 1.4 x 108 W
𝐼= = 21000 2 Sm
4 𝜋 (532) m
(HF/2) m
(D/2 + 15) m
 Example
Hazard Levels or Effects – Pool Fire