ELA964 Unit 3 Characterization of Explosion Hazards Slides

Copyright ©American Institute of Chemical Engineers 2018. All rights reserved.

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SAChE® Certificate Program

Level 2, Course 4b: Explosion Hazards

Unit 3 – Characterization of Explosion Hazards

Narration:

[No narration]


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Objectives

Narration (male voice):

This is the third of five units in the Explosion Hazards course. By the end of this unit, titled

“Characterization of Explosion Hazards,” you will be able to define and describe potential

hazards associated with explosions.


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SECTION 1: Blasts

Narration:

[No narration]


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Explosion Hazards


In this unit, we’re going to discuss four hazards posed by explosions:

• Blasts;

• Fragments and missiles;

• Fire and thermal radiation; and

• Toxics.

This first section will focus on blast hazards.


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Blast Defined


A blast is a transient change in the gas density, pressure, and velocity of the air surrounding an

explosion point. The initial change can be either discontinuous or gradual. Recall from Unit 1

that a discontinuous change is referred to as a shock wave; a gradual change is known as a

pressure wave. Blast waves move at sonic velocity.


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Blast Wave Formation


If an explosion occurs in a gas, the energy causes the gas to expand rapidly, forcing back the

surrounding gas and initiating a blast wave that moves rapidly outward from the blast source.

The blast wave can cause damage to the surroundings. Much of the damage from explosions is

done by this blast wave.


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Understanding Blast Wave Terminology


If the blast wave pressure front has a very abrupt pressure change as a function of distance at a

fixed time, as shown in this figure, and in the second figure as a function of time at a fixed

location, it is called a shock wave or shock front. A shock wave is expected from highly explosive

materials, such as TNT, but it can also occur from the sudden rupture of a pressure vessel or a

vapor cloud explosion (VCE) with a high flame speed.

These figures show the typical abrupt rise in pressure at the shock front, followed by a decrease

in pressure behind it. The maximum pressure over ambient conditions is called the peak

overpressure.

A blast wave generally refers to both shock waves and pressure waves that do not have an

abrupt pressure change, such as what would generally occur from the combustion of a

flammable gas.


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Blast Wave Propagation – Video


This video illustrates blast wave propagation produced by a condensed phase explosion.


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Understanding Blast Wave Terminology (continued)


An important consideration is how the pressure is measured as the blast wave passes. If the

pressure transducer is at right angles to the blast wave, the overpressure measured is called the

side-on overpressure (sometimes called the free-field or incident overpressure). At a fixed

location, the side-on overpressure increases abruptly to its maximum value (peak side-on

overpressure) and then drops off as the blast wave passes.

If the pressure transducer is placed in the middle of a large wall facing the oncoming shock wave,

then the pressure measured is the reflected overpressure.

A visual representation of these terms is shown on the next slide.


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This illustration shows the location of overpressure, side-on overpressure and reflected

overpressure for an example blast scenario.


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Here are a few additional terms to be aware of…

The blast impulse is defined as the change in momentum and has dimensions of force-time

product. For a blast wave, the area under the pressure-time curve is the impulse per unit of

projected area. The impulse is reported separately for each of the overpressure (positive phase)

and underpressure (negative phase) periods.

The positive phase impulse is often derived more simply as the area of a triangle formed from

the peak pressure and the duration. This value is a measure of the work done by the positive

phase of the explosion at a specific location.


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The higher the explosion energy, the greater the impulse at a given peak pressure. The higher

the impulse associated with the blast wave, the greater potential for damage. The blast wave at

a location is most often characterized by engineers using the pressure and impulse values, or

the pressure and duration values (such as 0.07 bar, 2.1 bar-ms or 0.07 bar, 60 ms).

The negative phase may also contribute to the damage caused by a blast wave, but we will focus

on the positive phase in this unit.


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Blast Wave Scaling – C-4 Example


Empirically derived blast curves for both pressure and impulse, such as the C-4 curves shown

here, are often used to estimate the effects of blast waves. C-4 is a plastic explosive.


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TNT Equivalency


TNT equivalency is a simple method for equating a known energy of an explosive substance to

an equivalent mass of TNT. This approach assumes that an exploding fuel mass behaves like

exploding TNT on an equivalent energy basis. The plots shown here illustrate pressure and

impulse equivalence values for C-4 based on TNT. All current blast scaling techniques were

developed based on this method.

The important basic concepts to pick up from the graphs are the following:

• Blast pressure and impulse both are plotted on log-log plots. This means that there is a

significant reduction in blast as a function of distance from the explosion center.

• Blast pressure and impulse both vary as a function of the distance (R) divided by the

cubed root of the explosion energy (W), shown as (R/W(1/3)

). The physical basis for this is

rooted in the fact that the blast expands equally in three dimensions. Scaling for

explosions at ground level must have the energy multiplied by two to account for

reflection of the blast wave by the ground.


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This method was historically used for VCE blast modeling, but since that time has been replaced

by much more accurate models. It is not a good representation of VCE blast waves, and has not

been recommended by the relevant American Petroleum Institute (API) recommended practice

or current texts on this subject for some time. This approach is only applied within the insurance

risk engineering community as of this writing in late 2018. More recent scaling models use

dimensionless parameters for both axes and revised curves specific to the explosion type;

however, the use of fundamental scaling parameters is consistent with the TNT equivalency

approach.


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Scaling Curves


Scaling curves like the one shown here provide a quick way to estimate blast loads as a function

of distance. Different sets of curves have been developed for condensed phase explosives (such

as TNT), pressure vessel bursts and vapor cloud explosions.

While some basics of consequence analysis for explosions are covered here, refer to the SAChE

Consequence Analysis course for more detailed information to accurately predict the hazardous

effects of explosions.


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Understanding Blast Load


We can use a door to illustrate the effect of small pressures on a surface. A typical door is 76 x

203 cm, or 15,428 cm2. A 0.69 N/cm

2 blast load on 15,428 square cm is equivalent to a weight of

10,700 N! This is enough to cause standard doors to fail.


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Damage Estimates for Common Structures


This table provides damage estimates for common structures based on overpressure. This is a

simplified approach that makes assumptions as to building types and material specifics. It is not

accurate over the full range of materials and structures found in petrochemical facilities. It is

only included in this course to provide a general understanding that blast pressure that seems

small can do significant damage. More detailed analyses of structures include the effect of

impulse. (This topic is covered in more detail in the Consequence Analysis SAChE course.)

[Female voice]

Take a few minutes to scroll down and read the table.


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SECTION 2: Fragments and Missiles

Narration:

[No narration]


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Debris Projection


An explosion occurring in a confined vessel or structure can rupture the vessel or structure,

resulting in the projection of debris over a wide area. This debris, or missiles, can cause injury to

people and damage to structures and equipment.

Unconfined explosions also create missiles by blast wave impact and subsequent destruction of

structures. In a process facility, debris can strike storage tanks, process equipment, and

pipelines, resulting in secondary fires or explosions.

The photographs shown here were taken at a manufacturing plant following a violent explosion

that occurred in a chemical distillation tower. Upon rupture of the top section of the tower,

structured metal packing was ejected from the tower and heavy debris was spread over a wide

area. Three workers in the control room were injured by shattered glass. One nitrotoluene

storage tank at the site was punctured by explosion debris, igniting a fire that burned for several

hours.


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BLEVEs


As discussed earlier, a boiling liquid expanding vapor explosion (BLEVE) has the potential for

projecting missiles great distances because of the large amount of energy released during this

incident.

The photographs shown here are from a case involving a reboiler that catastrophically ruptured,

causing a BLEVE and fire. Two workers died and 167 others were injured.


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Number and Size of Fragments


It is not unusual to see many fragments when detonations occur internal to a vessel. Some of

these fragments can be quite small relative to what occurs in a physical overpressure, where

many fewer large fragments are more common.

This photograph shows the aftermath of an explosion caused by a runaway reaction within a

reactor vessel. The incident resulted in the deaths of four employees and injuries to 32 others,

28 of whom were outside the plant’s fence line when the incident occurred. Debris from the

reactor was found up to 1.6 kilometers away.


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Variables Affecting Projectile Travel Distance


There are a number of factors that need to be considered to determine the distance a fragment

could travel in an explosion. Among others, these include:

• Fragment shape;

• Fragment mass;

• Drag factor;

• Aerodynamic lift (sometimes referred to as “frisbeeing”);

• The fraction of the total explosion energy that is translated into kinetic energy of the

fragment; and

• Trajectory.


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SECTION 3: Fire and Thermal Radiation

Narration:

[No narration]


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Fire


In this section, we’ll explore the effects of fire that can result from an explosion. This

information is considered in more detail in the SAChE Fire Hazards course.

A hazard zone is typically defined as the region extending beyond that of the flammable cloud

itself. This accounts for expansion effects and for lack of perfect mixing.


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Part 2


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Thermal Radiation


The ignition of a flammable material may result in a pool fire, flash fire, jet fire or a fireball. The

energy conveyed to the surroundings due to thermal radiation and convection is dependent on

the release conditions and environment into which the release occurs. The main hazard

associated with large fires is thermal radiation.


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Thermal Radiation Effects on People


The degree to which a person is burned by fire is a function of the thermal dose they receive.

The thermal dose, measured in J/m2

or kW-hr/m2, is obtained by multiplying the thermal

radiation intensity, or flux, (J/s/m2 or kW/m

2) by the exposure time.

Sunburn is an all too common form of thermal radiation burn. In that case, the intensity is

relatively low, but the person is exposed to the sun for too long.

Here are some example thermal radiation values:

• Solar radiation at the earth’s surface is approximately 0.5 kW/m2;

• Radiation at the surface of a pool fire is approximately 100 kW/m2; and

• Radiation at the surface of a fireball is approximately 200 kW/m2.


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Part 2


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Thermal Radiation Effects on People (continued)


This table shows the time for physiological effects on bare skin to occur following exposure to

specific thermal radiation levels.


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Thermal Radiation Effects on Materials


The effect of thermal radiation on process equipment and building materials depends on the

duration and nature of the exposure. Thermal radiation may be reflected or absorbed. It is the

radiation absorbed by a material that produces heat resulting in damage to the material.

The absorbed radiation depends on the color and nature of the material. For example, a larger

proportion of thermal radiation is reflected by white materials relative to black materials, which

absorb a much larger proportion of the incident radiation.


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Thermal Radiation Effects on Materials (continued)


This table shows the effects of thermal radiation on materials. A “piloted ignition of wood”

means that a flame source is impinging on the wood.


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SECTION 4: Toxic Releases


[No narration]


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Toxic Releases


When a vessel or pipe leaks, fails, or explodes, its contents will be released. If those contents are

toxic and are not fully consumed during a fire, facility personnel, emergency responders – and

even people in the surrounding community – may be subject to the hazards posed by the toxic

material.

Byproducts of a fire can also be toxic. If a fire or explosion has occurred in a process area, always

consult with someone who understands the chemistry of combustion to understand when

chemicals may be present as byproducts of combustion that could be toxic and present a hazard

either to the community or emergency responders.

Toxic materials can also be released if debris from an explosion penetrates vessels, piping, and

other process equipment unrelated to the initial explosion if that equipment contains toxic

materials.


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Toxic Releases (continued)


The existence or severity of toxic hazards that result from an explosion often are related to what

is happening in the vessel (such as a reactor) at the time of the explosion. Depending on the

material involved and the process underway, exposure of the vessel contents when released to

the atmosphere may produce significant toxic hazards.

The photograph shown here was taken after an explosion at a refinery. Hazardous catalyst

particles were released into the surrounding community. The CSB investigation reported a

“near-miss” when debris from the explosion nearly led to penetration of tanks containing highly

toxic hydrofluoric acid.


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Unit 3 Summary


We’ve reached the end of the third unit in the Explosion Hazards course. Having completed this

unit, titled “Characterization of Explosion Hazards,” you should now be able to define and

describe potential hazards associated with explosions.

In Unit 4, you will learn about tests used to characterize explosion hazards. But first, please take

the quiz for Unit 3 beginning on the next slide.

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ELA964 Unit 3 Characterization of Explosion Hazards Slides