6 Flashcards
Harmful consequences of release of dangerous materials can be split into three main types:
*Toxicity
*Fire
*Explosion
What happens to hazardous material when loss of containment happens?
Source dependent (how much comes out and in what state):
- Phase(s) gas/vapour/ liquid / solid
- Mass flowrate
Dispersion of the material occurs Impacts
- Toxicity
- Thermal radiation from fire
- Overpressure from explosion
What are the factors that influence the dispersion of flammable or toxic vapour when released into the
atmosphere?
- It depends on pre and post release conditions
- Velocity of release
- Buoyancy of release
- Amount/duration of release
- Temperature of release
- Weather conditions
o Wind speed/direction
o Atmospheric stability - Local topography
Consequence modelling
enables:
- The assessment of harm
Insights into reducing the risks by:
. Reducing the frequency of the
release
. Mitigation measures to reduce
release duration or exposure to
people or environmental receptors - Design decisions to be risk based
- Scenario Scenario based emergency response
planning
Toxic definition
a property of substances which, when introduced into or
absorbed by a living organism, destroys or injures health
Toxic effects depend on:
Toxicity of substance(s)
Concentration
Exposure time
Modelling can be used to calculate and predict the concentrations at distances from the release point
Toxic gas plumes can travel a long way because the concentration is often in ppm (parts per million in air)
CFD model toxic dispersion
Simulation
Simulation study
Massive chlorine release from railcar in urban area
60 tons chlorine released over 5 minutes
Shows dispersion for 3000 s ( 50 minutes)
Fire basics
.Process of combustion with generation of heat or smoke or flame or combination of these
.Flame consists of ignited stream of vapour evolved by the flammable material that continue to burn at the interface with the air
.The rate of burning:
- Controlled by rate of oxygen transfer to the burning material
- Affects the height of the flame
.Products can asphyxiate or be toxic.
.Smouldering fires have the same effects
Fire triangle
slides :)
In order to start a fire you need the three things shown in the fire triangle. Without one of these elements there will not be a fire. The oxidant is not always oxygen, it could be another oxidant such as fluorine or chlorine.
What is the significance of the lower flammable limit (LFL) and upper flammable limit (UFL) for a vapour in air?
- Flammable gas or vapour will burn in air over a limited range of concentration
- Lower flammable limit (LFL) or Lower Explosive Limit (LEL) – below this concentration limit the mixture is too lean and will not burn
- Upper flammable limit (UFL) or Upper Explosive Limit (UEL) – above this concentration the mixture if too rich and will not ignite
- Flammable range – the concentrations between the LFL and UFL where ignition can occur
- LFL and UFL are temperature and pressure dependent
What is the significance of flash point for a flammable liquid? Name three common flammable liquids and rank them in order of increasing potential risk.
- Flash point is the lowest temperature at which air saturated with its vapour can be ignited
- It is widely quoted and used to classify materials into different hazard groups for transport and supply
- Liquid handled below their flash point present a reduced fire hazard
There are several solutions for the three common liquids part of this question. Some examples are given in lecture slides. You would be provided with this information in an exam.
Auto ignition temperature (AIT)
The lowest temperature at which flammable gas, vapour or dust, mixed with air will ignite from its own heat or in a heated
environment, in the absence of other ignition sources.
It measures how easily a flammable liquid can be ignited by a hot surface
Name three types of fire and the characteristics of each.
For this question you can describe any of the following types of fire. The characteristics of these can be found in Lecture 6:
- Pool fire
- Jet fire
- Flash fire
- Fireball
- Dust fire
Pool fire
*Occurs when liquid in a pool on the ground or on water is
ignited
*Fire burns steadily as the fuel required to sustain the flames
provided by evaporation of liquid in pool
*Height of the flames about twice diameter of pool
*Storage tank fires have similar characteristics
Jet fire
*Generally from a small hole in pipe or pressure vessel
*Long flame which is stable and largely unaffected by wind
*Induces large amounts of air and burns with intense radiation
*For a liquid or two phase jet, part of the liquid may rain out ’ to form a pool fire
*A gas jet fire should be extinguished by turning off the gas supply otherwise unburnt gas could accumulate and lead to explosion
Flash fire
*Flash fire occurs when a cloud of flammable gas in air is ignited
*Flame travels quickly and engulfs cloud rapidly
*Flash fires can initiate pool or jet fires
*If the cloud contains air, the flame may accelerate and cause an explosion
Fireball
When quantity of flammable liquid suddenly released and ignited immediately
Fuel rapidly burnt as spherical fireball, rising due to momentum of the release
and buoyancy of hot flames
Mass of fuel determines the fireball size
Arise following a BLEVE (boiling liquid expanding vapour explosion) where fire induces heating and subsequent failure
of storage vessel
Dust fire
- Some powdered materials in process, storage or transport can self heat
- Typically produces a smouldering fire
- If the product is then disturbed, fire may break out suddenly
when air comes into contact with hot material - If product raised into a cloud, dust explosion may occur
- Can occur if layers of dust allowed to form on hot surfaces e.g. on electric motor casing
Explosions
A sudden and violent release of energy
*The violence of the explosion depends on the rate at which
energy is released
*Damage caused by the dissipating energy
There are 3 basic types:
*Nuclear (not covered here)
*Physical
*Chemical
Explosion behaviour
- Ambient temperature
- Ambient pressure
- Composition
- Physical properties
- Nature of ignition source
- Geometry of surroundings:
confined or unconfined - Amount of combustible material
- Turbulence of combustible
material - Time before ignition
- Rate at which material released
Explosion damage
*After combustible material consumed, reaction front terminates, but the pressure wave continues moving
*Blast wave composed of pressure wave and subsequent wind
*Blast wave causes most of the damage
*Can cause injury to personnel and damage to structures
*Missiles can be generated during the initial explosion from
fragments of building or plant, loose items such as tools
Types of explosion
Overpressure
Vapour cloud explosion
Confined explosion
BLEVE
Dust explosion
Mist explosion
explosion due to either/or
Chemical explosions:
*Blast wave from rapid expansion of gases
*Caused by
*Thermal heating of reaction
products
*Change in number of moles
Mechanical explosions:
*Reaction does not occur
*Energy obtained is obtained
from the energy content of
contained substance
*e.g. sudden failure of car tyre
Vapour cloud explosion
*Most dangerous and destructive
*Sequence of steps
-Release of large quantity of flammable vapour
-Dispersion of vapour
-Ignition of the cloud
*To control:
-Low inventories of volatiles
-Process conditions to minimise flashing
-Using gas detectors to alert of leaks
-Automated block valves to shut systems down and minimise release
Dust explosions
*Wide range of chemical and foodstuffs, including flour and
sugar, can cause dust explosions
*When particles raised in the air as dust cloud they can be
ignited and explode with violence comparable to vapour cloud
explosion
*The pressure wave raises more dust in the air, and
subsequently more explosions
BLEVE
Boiling Liquid Expanding Vapour Explosion
*When tank containing liquid held at above its atmospheric
pressure boiling point ruptures, resulting in explosive
vaporisation
*Most commonly caused by fire
BLEVE steps
- Fire develops adjacent to liquid
tank - Fire heats walls of tank
- Tank walls below liquid level cooled by liquid. Increase in
liquid T and P
-If flames reach tank walls or roof there is only vapour and no
liquid to remove the heat
-Tank metal temperature rises
until tank loses structural strength
-Tank ruptures! Explosive vapourisation of contents
To prevent fire and explosion
Strategies employed:
-Prevent flammable mixture
-Prevent initiation of fire or explosion
-Minimise damage after fire or explosion occurred
List and describe three ways to prevent or mitigate fire and explosion in a process plant.
You can describe any 3 of the following:
- Inerting
- Flammability diagrams
- Static electricity
- Explosion proof equipment and instruments
- Ventilation
- Sprinklers
Inerting
adding inert gas to reduce the concentration of O2 below the flammable limit. Usually CO2 or N2, although steam sometimes used. Also purging – sweep through method of inerting. Assumes perfect mixing within the vessel and no dead spaces
- Flammability diagrams
tool to prevent existence of flammable mixtures with elimination of ignition source as secondary measure
Static electricity
common ignition source resulting from static charge build up and sudden discharge. It is a very elusive ignition source. Design features are added such as inerting atmosphere around regions where static sparks are unavoidable. Prevent by grounding and bonding.
Explosion proof equipment and instruments
areas or zones are used in potentially flammable designs. Can get equipment rated for those specific zones e.g. spark free pumps, blast proof tanks
Ventilation
there are benefits from having an open air plant but this is not always possible. In buildings ventilation and dilution is very important. There are recommended ventilation rates for processes handling flammable materials
Sprinklers
to contain fires. Activated by a variety of methods. Cannot be stopped unless turned off at the mains. Common to have meltable fusible link holding a plug in place. Can create considerable water damage.
Main causes of chemical reaction incidents:
*Lack of understanding of process chemistry and thermochemistry
*Inadequate design for heat transfer
*Inadequate control and safety back up systems
*Inadequate operational procedures
Preventing an incident requires:
*Background understanding case histories
*Identification of reactive chemical hazards
*Characterisation of hazards, to understand the energy released
*Control of hazards (inherent, passive, active, administrative)
Difficult to predict and identify
Occurs in plants due to:
*Chemicals reacting by design
*Chemicals reacting by accident
Runaway reaction
*Process unable to remove adequate heat from reactor to control the temperature
*Reactor temperature increases
*Results in higher reaction rate and even faster rate of heat
generation
Effect of scale up
Rate of heat production is
proportional to volume
Cooling capacity is proportional
to surface area
As vessels get bigger the volume
increases more quickly than the
surface area
Any changes in batch size need
to be carefully managed
Controlling reactive hazards
Inherent:
*Use pathway that uses less hazardous chemicals
*Eliminate or reduce inventory
*Reduce equipment size to reduce leak rate
Passive:
*Ensure incompatible chemicals always separated
*Adequate separation distances
*Passive fire protection of vessels e.g. fire resistant insulation
Active:
*Properly designed control systems to control reactivity in process
*Properly designed relief systems
*Identify and characterise all possible reactions
Admin:
*Communicate and train on chemical reactivity hazards
*Document chemical reactivity risks and management decisions
*Many any process changes involving reactive species
Common causes of over pressure
External fire
Heat exchanger tube failure
Liquid expansion
Cooling water failure
Electricity failure
Blocked outlet
Control failure
Chemical reaction
Loss of reflux
Rules of thumb
*Pressure relief required for all pressure vessels or pipework
where there may be trapped pressure e.g. even long liquid
pipelines potentially exposed to changes in temperature
*Connect outlet of PRV to vent somewhere safe or collect the
flow
*No restrictions in the inlet or outlet line
*As close to the vessel or line as possible
In a typical batch reactor, exothermic process what are:
There isn’t a single correct answer to this question. You should use your engineering common sense to think about and list the hazards, as well as the potential solutions. You will get marks for making sensible and logical points.
a. the likely hazards and how could you minimise or eliminate the hazards
b. the likely things that can go wrong
c. typical safety systems you would expect to see
Describe the strengths and limitations of the following hazard identification techniques and when you would use them:
The answers for this one can be found directly in the lecture notes. Some additional points below.
a. What-if
Used to focus on defined portions of the process
b. HAZOP
Used once there is a firm design, as it consists of a stage by stage examination of the process.
