Deflagration Ignition Hazards

Summary
Whenever a manufacturing site handles and/or processes combustible or flammable materials, a key to appropriate
safety management is the establishment (and maintenance) of a Basis of Safety (BOS) to prevent connection of
the three elements of the fire triangle. Deflagrations can have particularly dire consequences, and the BOS must
include an assessment of ignition sources and material properties to ensure that incendive ignition sources are
adequately managed. Use a checklist of potential ignition sources under both normal and abnormal operating/
processing/storage conditions to ensure that all are considered. Ensure that flammability properties of handled
materials are well understood and considered in a hazard assessment. Safeguards such as hazardous area
classified equipment, shutdown interlocks, and material containment systems must be included in mechanical
integrity programs to ensure their reliability.
Hazards Control
& AssessDeflagration
Ignition
Hazards
Chilworth Technology has a team of highly skilled process safety specialists that provide independent consulting
advice on PSM and fire and explosion prevention and protection measures, and safety engineering. We have
worked with many clients with regard to these issues and OSHA inspections. We have also been involved in
informal OSHA conferencing with respect to citations that have been written as a result of inspections. We can
assist you in resolving issues and in the citation-abatement process.
David E. Kaelin
David E. Kaelin, Sr., B.S.Ch.E., Mr. Kaelin has over 25 years experience in the specialty chemical manufacturing
industry and 15 years specializing as a Process Safety Engineer. He has participated in the design and
construction of numerous chemical processing facilities and provided support and training in all areas of PSM.
As a Process Safety Engineer he has led process hazard analysis, risk assessments and facility siting reviews.
At the corporate level he has created and taught courses in PSM and hazard recognition methods. Mr. Kaelin
is an expert in the application of hazard recognition techniques including: HAZOP, FMEA. What-If, Fault Tree
Analysis, Risk Screening and Checklist. He is an active member of AIChE, and NFPA.
CHILWORTH TECHNOLOGY, INC.
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DEFLAGRATION IGNITION HAZARDS
David E. Kaelin Sr., Senior Process Safety Specialist
Introduction
A major consideration in appropriate process safety management must be the prevention of fires and
explosions. When processing and handling combustible or flammable materials, the possibility that a
flammable atmosphere will be ignited must be considered. A flammable atmosphere can be created from
the vapors of a liquid heated above its flash point, a flammable gas, combustible mist, or combustible dust.
These fuels if mixed with air or other oxidants in the correct proportions can be ignited with devastating
consequences.
One aspect of the combustion of fuels is the effects of the event. Fires occur when the fuel/air mixture is
created at the combustion zone such as the pyrolysis of a wood log in a fireplace where the off-gases burn in
a gentle fashion. If, however the flammable mixture is pre-mixed such as the dispersed vapors from a liquid
spill or gas leak, then the combustion zone (flame) can propagate through the mixture.
When flame propagation is slower than the speed of sound then the propagation event is called a deflagration. In open
areas, a deflagration will not create pressure effects and a flash fire occurs. But if the deflagration occurs in a confined
space or congested area, significant pressure effects can occur (an explosion). A deflagration in an operating area
can have extreme effects including operator injury or fatality, ignition of secondary fires, as well as building collapse.
An appropriate Basis of Safety (BOS) for the management of deflagrations is prevention of the formation of the
flammable atmosphere. The logic for this BOS is shown below:
If an operation or activity cannot prevent the formation of a flammable atmosphere, then it is critical to identify and
control (eliminate) all incendive ignition sources that might occur under normal or abnormal operating/processing/
storage conditions. An incendive ignition source is an energy source with adequate energy to ignite a specific flammable
atmosphere and is fuel-specific. Process safety information must include data concerning the ignition sensitivity for
combustible and flammable atmospheres (materials). Such information is used by the hazard assessment team to
assess ignition sources and determine which sources must be tightly managed.
Regardless of the primary BOS, it is best practice to always manage ignition sources when handling and processing
ignitable materials. Upsets can occur and materials can be released. In addition, codes and standards including those
for flammable gases, flammable and combustible liquids, and combustible dusts require ignition-source management.
It was once said (by Trevor Kletz) that “ignition sources are free”, meaning it is challenging to control ignition sources to
100% effectiveness, particularly if adequate information on the ignition sensitivity of the flammable atmosphere by the
identified ignition sources is not available. The control of ignition sources is overwhelmingly administrative in nature,
subject to human error and safety culture failings.
In practice, thirteen sources of ignition have been identified as being responsible for the vast majority of deflagration
events in industry. These are listed below.
Table 1
Typical Deflagration Ignition Sources (from British Standard EN-1127-1:2007)
1. Hot Surfaces
2. Flames and Hot Gases
3. Mechanically Generated Sparks (friction and impact)
4. Electrical Equipment
5. Stray Electric Currents
6. Static Electricity
7. Lightning Arcs and Flashes
8. Radio Frequency (RF) Electromagnetic Waves
9. Ultraviolet, Infrared and Visible Radiation
10. Ionizing Radiation
11. Ultrasonic Energy
12. Adiabatic Compression and Shock Waves
13. Exothermic Reactions and Self-Ignition
Each of the ignition sources listed above has some limitation of its potential energy available to cause ignition of a
flammable atmosphere. As a consequence, the incendivity of each source for a given flammable atmosphere can be
assessed if the atmosphere’s ignition sensitivity is known.
Table 2
Ignition Sources
Listed Source
Typical Cause
Level of Incendivity
At Risk Flammable
Atmospheres
Hot Surfaces
Process Equipment and Utility
Systems,
Electrical devices
Low to moderate
likelihood of incendivity,
Easily identified
Powder layers/deposits are
particularly susceptible to ignition
due to self-heating effects. Autoignition of vapors and gases
requires extreme temperatures in
most cases
Flames and Hot
Gases
Mis-managed hot work
Direct-fired process
equipment,
Primary fire or explosion event
High degree of
incendivity,
Easily identified
Essentially all flammable
atmospheres
Friction and Impact
Sparks
Mis-operation of rotating
equipment
Blenders, agitators and mills
(beware of tramp materials)
mechanical equipment
breakdown
Low to moderate
likelihood of incendivity,
Easily identified
Gases, vapors and mists.
Low likelihood of ignition of dusts
if tip speed is restricted to less
than 1 m/sec.
Electrical
Equipment
Inappropriate equipment in
hazardous areas,
Less-than-adequate
maintenance of rated
equipment
Moderate
Easily identified
Essentially all flammable
atmospheres,
Vapors and gases most likely
ignited within devices by sparks,
Dusts most likely ignited by
deposits on surfaces
Stray Electric
Currents
Short-circuits or short to earth
of electrical devices,
Return currents in power
generating systems
High incendivity,
Strictly follow electrical
code and maintain
systems and equipment
All flammable atmospheres
Static Electricity
Ungrounded conductors, use
of plastics in insulating liquid
or powder operations
Lightning Arcs and
Flashes
Primarily an outdoor hazard.
Greater in some specific areas
RF Electromagnetic
Waves
RF generators used for
heating, drying, hardening,
welding and cutting
UV, IR and Visible
Electromagnetic
Waves
Laser light and focused
sunlight
Moderate to high
incendivity,
Low understanding by
many sites of this hazard
High incendivity,
Easily identified,
Manage with grounding
and/or inerting
Moderate to high
incendivity if conductive
parts become receiving
aerials.
Moderate to high
invendivity,
Rare cases
Ionizing Radiation
Ultrasonic Energy
Self heating of a radioactive
source,
Energy transfer to a powder
Equipment used for cleaning,
welding of plastics and
reactant mixing
Moderately incendive
Low incendivity
Vapors and gases, and some
mists and dusts
All flammable atmospheres,
Tank vents are at greatest risk
All flammable atmospheres
All flammable atmospheres,
Powders can be overheated
directly
All flammable atmospheres,
Powders can be overheated
directly
All flammable atmospheres,
Powders can be overheated
directly
Adiabatic
Compression and
Shock Waves
Predominantly within
compressor equipment where
combustible gases or vapors
are compressed (heated)
without intermediate cooling
High incendivity if autoignition temperature can
be exceeded and an
oxidant is present
Vapors, gases, and mists;
Calculations can predict this
phenomena
Exothermic
Reactions and
Self-Ignition
Humidity or water reactions
with water-reactive substances
such as alkali metals.
Reaction of pyrophoric
substances with air,
Self-ignition of powders on hot
surfaces,
Inadvertent mixing of reactive
chemicals
High incendivity if autoignition temperature can
be exceeded and an
oxidant is present
All flammable atmospheres.
Determine the self-ignition
initiation temperature of powders
(not the same as auto-ignition
temperature)