NFPA Journal - Summer 2023 - 62
Codes + Tech
The damage potential of an explosion
depends on the pressure that is
created from the explosion as well as
how quickly energy is released from
the explosion. Explosions can be either
detonations or deflagrations, depending
on their flame speed.
A deflagration is an explosion where
the flame speed is slower than the speed
of sound, which is approximately equal
to 343 meters per second, or about 750
mph. Explosives that deflagrate are
known as low explosives. The actual
speed of the explosion can vary from
1-350 meters per second (2-780 mph).
Peak pressures produced by low explosives
are orders of magnitude lower
than those produced by high explosives,
and the damage inflicted by low
explosives can vary greatly depending
on the fuel and confinement. For example,
black powder merely fizzles if it is
ignited outside of containment, but if it is
confined it creates an explosion that can
propel bullets. Other examples of deflagrations
involving low explosives include
+
The damage
potential of an
explosion depends
on the pressure created
by the blast as well as
how quickly that energy
is released.
the ignition of propane gas for a cooking
grill and the ignition of fuel in a combustion
engine in a car.
A detonation is an explosion where
the flame speed is greater than the
speed of sound. Detonations are louder
and often more destructive than deflagrations.
While deflagration occurs
when a fuel and oxidizer (typically air)
mix, a detonation doesn't always need
an external oxidizer. Explosives that
detonate are referred to as high explosives
and have a detonation speed in
the range of 2,000-8,200 meters per
second (4,500-18,000 mph). High
explosives are typically designed to
cause destruction-often for demolition,
mining, or warfare. Examples of
high explosives that detonate include
dynamite, TNT, and C4, a plastic-based
explosive.
For more information on explosions,
check out the 21st edition of the
NFPA Fire Protection Handbook®
, which
contains several chapters on the topic,
including Chapter 2-8, " Explosions, "
Chapter 6-16, " Explosives and Blasting
Agents, " Chapter 17-8, " Explosion
Prevention and Protection, " and Chapter
18-6, " Deflagration Venting. "
NFPA also publishes several codes
and standards related to explosions,
including NFPA 495, Explosive
Materials Code; NFPA 69, Standard on
Explosion Prevention Systems; NFPA
68, Standard on Explosion Protection
by Deflagration Venting; and NFPA
67, Guide on Explosion Protection for
Gaseous Mixtures in Pipe Systems.
BRIAN O'CONNOR is a technical services
engineer at NFPA.
62 | NFPA JOURNAL * SUMMER 2023
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NFPA Journal - Summer 2023
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