CSB s Hoeganaes Corporation Case Study 1

Hoeganaes Corporation Case Study December 2011

Case Study

U.S. Chemical Safety and Hazard Investigation Board

Hoeganaes Corporation: Gallatin, TN

Metal Dust Flash Fires and Hydrogen Explosion

January 31, 2011; March 29, 2011; May 27, 2011

5 Killed, 3 Injured

No. 2011-4-I-TN

KEY ISSUES

s Hazard recognition and training

s Engineering controls 

s Fire codes/enforcement

sRegulatory oversight
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Hoeganaes Corporation Case Study December 2011

1.0 INTRODUCTION

This case study examines multiple iron dust flash fires and a hydrogen explosion at the 

Hoeganaes facility in Gallatin, TN. The first iron dust flash fire incident killed two workers 

and the second injured an employee. The third incident, a hydrogen explosion and resulting 

iron dust flash fires, claimed three lives and injured two other workers.

1.1 HOEGANAES CORPORATION

Hoeganaes Corp. is a worldwide producer of atomized steel and iron powders. Headquartered 

in Cinnaminson, NJ, Hoeganaes has facilities in the U.S., Germany, China, and Romania. 

The Hoeganaes Corp. is a subsidiary of GKN, a multinational engineering company headquartered in the United Kingdom. GKN has businesses in addition to powder metallurgy, including 

aerospace and automotive driveline industries. GKN acquired the Hoeganaes Corp. in 1999.

The largest consumer for the powdered metal (PM1

) product is the automotive industry, 

which presses and sinters2

 the powder into small metal parts. 

1.2 FACILITY DESCRIPTION

The Hoeganaes Gallatin facility (Figure 1), located 30 miles northeast of Nashville, Tennessee, 

employs just under 200 employees. Since becoming operational in the 1980s, they have increased 

their manufacturing capability over 550 percent from 45,000 to over 300,000 tons.

TABLE OF CONTENTS

1.0 Introduction 2

2.0 Process Discussion 3

3.0 The Incidents 3

4.0 Analysis 6

5.0 Key Findings 24

6.0 Recommendations 24

 Appendix A: Determination of 

 Iron Powder Explosibility from 

 Pressure Ratio Calculations 28

 Appendix B: Determination of 

 Iron Powder Classification from 

 Explosion Severity Calculation 30

FIGURE 1

Satellite view of the 

Hoeganaes Gallatin facility.

1

PM is the accepted acronym by the powdered metals industry.

2

Sintering is the process of solidifying PM via heat and/or pressure to form a component. 
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Hoeganaes Corporation Case Study December 2011

3

Ductility is the physical property of a material where it is capable of sustaining large permanent changes in shape without breaking.

2.0 PROCESS DISCUSSION

Hoeganaes receives and melts scrap steel. Various elements are added to the molten metal 

to meet customer specifications, but the “workhorse” product, Ancorsteel 1000TM, is over 99 

percent iron. The molten iron is cooled and milled into a coarse powder that is processed 

in long annealing furnaces to make the iron more ductile.3

 The furnaces are called “band 

furnaces,” for the 100 foot conveyor belt, or band, that runs through them. A hydrogen 

atmosphere is provided in the band furnace to reduce the iron by removing oxides and 

preventing oxidation. The hydrogen is supplied to the facility by a contract provider, onsite. 

Hydrogen is conveyed to the furnaces via pipes located in a trench under the floor and 

covered by metal plates. 

 In the process of going through the furnace, the coarse powder becomes a thick sheet called 

“cake.” The cake is sent to a cake breaker and ultimately crushed into the fine PM product. 

The majority of the finished PM product has a particle diameter between 45-150 microns, 

or roughly the width of a human hair (Figure 2).

3.0 THE INCIDENTS

3.1 JANUARY 31, 2011 (TWO FATALITIES) 

PM product is transferred through the plant by various mechanisms including screw conveyors and bucket elevators. Bucket elevators have a tendency to go “off-track” when the belt 

pulling the buckets becomes misaligned. Once sufficiently off-track the strain on the motor 

increases until the torque is too great and the motor shuts down. On January 31, 2011, at 

about 5:00 am Hoeganaes plant operators suspected bucket elevator #12 of being off-track 

and a maintenance mechanic and an electrician were called to inspect the equipment.

FIGURE 2

Fine PM collected from 

the Hoeganaes plant 

(penny shown for scale).
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Hoeganaes Corporation Case Study December 2011

Based on their observations, they did not believe that the belt was off-track and requested, via radio, that the operator in the control room restart the motor (Figure 4). 

When the elevator was restarted, vibrations from the equipment dispersed fine iron 

dust into the air. During a CSB interview, one of the workers recalled being engulfed in 

flames, almost immediately after the motor was restarted. 

City of Gallatin emergency responders arrived 

with ambulances and 

transported the mechanic 

and electrician to the 

Vanderbilt Burn Center 

in Nashville, TN. Both 

employees were severely 

burned over a large 

percentage of their bodies. 

The first employee died 

from his injuries two days 

later. The second employee 

survived for nearly four 

months before succumbing to his injuries in late 

May 2011. 

3.2 MARCH 29, 2011 (ONE INJURED) 

As part of an ongoing furnace improvement project, a Hoeganaes engineer and an outside 

contractor were replacing igniters on a band furnace. The pair experienced difficulty in reconnecting a particular natural gas line after replacing an igniter. While using a hammer to 

force the gas port to reconnect, the Hoeganaes engineer inadvertently lofted large amounts 

of combustible iron dust from flat surfaces on the side of the band furnace, spanning 20 feet 

above him. As soon as the dust dispersed, the engineer recalled being engulfed in flames. He 

jumped and fell from a rolling stepladder in his attempt to escape the fireball. He received 

first- and second-degree burns to both thighs, superficial burns to his face, and scrapes from 

his fall. After seeing the initial flash of the dust igniting, the contractor took evasive action 

and escaped without injury. 

FIGURE 4 (RIGHT)

Scene of January 31, 

2011, incident area.

FIGURE 5

Computer graphic of 

January 31 iron dust 

flash fire.

FIGURE 3 (LEFT)

Computer graphic of 

maintenance workers 

inspecting bucket 

elevator #12 just prior 

to January 31 flash fire.

Elevator Enclosure

Motor
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Hoeganaes Corporation Case Study December 2011

The engineer was wearing the Hoeganaes-designated personal protective equipment, which 

included pants and a shirt that were rated as flame resistant clothing (FRC). He was also 

wearing an FRC rated jacket that provided extra shielding to his upper torso from the flash 

fire.

3.3 MAY 27, 2011 (THREE FATALITIES, TWO INJURIES)

Around 6 am on May 27, 2011, operators near band furnace #1 heard a hissing noise that 

they identified as a gas leak. The operators determined that the leak was in a trench, an area 

below the band furnaces that contains hydrogen, nitrogen, and cooling water runoff pipes, 

in addition to a vent pipe for the furnaces. The operators informed the maintenance department about the hissing, and six mechanics were dispatched to find and repair the leak. One 

annealing area operator stood by as the mechanics sought out the source of the leak. 

Although maintenance personnel knew that hydrogen piping was in the same trench, they 

presumed that the leak was nonflammable nitrogen because of a recent leak in a nitrogen 

pipe elsewhere in the plant and began to try to remove trench covers. However, the trench 

FIGURE 6 (LEFT)

Computer graphic of 

the gas line connection 

involved in the March 29 

flash fire.

FIGURE 7 (RIGHT)

The gas line connection 

involved in March 29, 2011, 

incident.

FIGURE 8

Computer graphic of 

March 29 iron dust 

flash fire.
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Hoeganaes Corporation Case Study December 2011

covers were too difficult to lift without machinery. Using an overhead crane, they were able 

to remove some of the trench covers. They determined that the leak was near the southernmost trench covers, which the crane could not reach. Shortly after 6:30 am, maintenance 

personnel acquired a forklift equipped with a chain on its forks, and were able to reach and 

begin removing the southernmost trench covers.

Interviews with eyewitnesses indicate that just as the first trench cover was wrenched from 

its position by the forklift, friction created sparks, followed by a powerful explosion. 

Several days after the explosion, CSB investigators observed a large hole (approximately 3 x 

7 inches) in a corroded section of piping that carried hydrogen and ran through the trench 

(Figure 11).

As the leaking hydrogen gas exploded, the resulting overpressure dispersed large quantities of 

iron dust from rafters and other surfaces in the upper reaches of the building. Portions of this 

dust subsequently ignited. Multiple eyewitnesses reported embers raining down and igniting 

FIGURE 9 (TOP)

Scene of the May 27 

incident before (left, taken 

during the CSB’s January 

31 incident investigation) 

and after (right). Note 

visible accumulations of 

iron powder on surfaces. 

Circled areas show trench 

cover location.

FIGURE 10 (RIGHT)

Computer graphic of 

maintenance crews 

starting to remove the 

trench covers using a 

forklift just prior to May 27 

explosion.

Feb 07 2011 May 28 2011
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Hoeganaes Corporation Case Study December 2011

multiple dust flash fires in the area. They also reported visibility so 

limited in some instances that flashlights were required; one eyewitness said that even with a flashlight, he could see only 3 to 4 feet 

ahead due to extensive dust and smoke.

The hydrogen explosion and ensuing iron dust flash fires injured four 

of the responding mechanics and the annealing operator.4

 The two 

mechanics near the forklift were transported to a local hospital where 

they were treated for smoke inhalation and released shortly thereafter.

Two other mechanics and the operator who stood by during the 

operation were rushed to Vanderbilt Burn Center. Less than a week 

after the incident, two employees succumbed to their injuries. The 

third seriously injured employee died from his injuries almost seven 

weeks after the incident.

Due to the extensive nature of the injuries, and the abundance of both hydrogen and combustible dust present at the time of the incident, it is difficult to specifically determine which 

fuel, if not both, caused the fatal injuries to the victims. 

3.4 EMERGENCY RESPONSE

The Gallatin Fire Department (GFD) has responded to 30 incidents of various types over 

the past 12 years at the Hoeganaes Corp., including the January 31, March 29, and May 27 

incidents. In June 1999, the GFD responded to a fire caused by iron dust that ignited in a 

baghouse. One person suffered smoke inhalation injuries as a result of the incident. 

Before the GFD arrived at each of the 2011 incidents, Hoeganaes volunteer first responders cared for the injured. Hoeganaes volunteers participate in annual training that covers 

first response, CPR, and first aid. They are instructed to provide care until GFD and EMS 

responders arrive. 

Immediately following each incident, the volunteers provided first aid and comfort to the 

injured by applying water to cool the burns and covering the victims with a burn blanket 

to keep them comfortable. EMS arrived within minutes of the initial 9-1-1 call and transported the injured personnel to hospitals. 

4At the time of incident, two of the mechanics and the operator were standing near the trench while the other two mechanics were positioned and 

possibly shielded by the forklift when the explosion occurred.

FIGURE 11 (TOP LEFT)

Hole in 4-inch piping after 

the May 27, 2011, incident.

FIGURE 12 (TOP RIGHT)

Computer graphic of the 

May 27, 2011, hydrogen 

explosion.

FIGURE 13 (BOTTOM)

Upward disturbance of 

trench covers caused by 

the hydrogen explosion in 

the May 27, 2011, incident.
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Hoeganaes Corporation Case Study December 2011

4.0 ANALYSIS

4.1 COMBUSTIBLE DUST TESTING

According to National Fire Protection Association (NFPA) 484, Standard for Combustible 

Metals, a facility that handles metal dust should commission one of two screening tests to 

determine if a metal dust is combustible and the provisions of the standard apply (Section 

4.4.1). If results from either of the two tests show that the dust is combustible or explosible, 

NFPA 484 would apply to the facility either as a matter of voluntary good practice or as a 

requirement by a relevant regulatory body. 

The first screening test for the determination of combustibility, also known as the “train test,” 

measures the burning rate of a dust layer over the length of a sample.5

 If there is propagation 

beyond the ignition point or heated zone, then the sample is considered combustible.6

The second test, for explosibility determination, serves as a basis to determine if a metal 

powder or dust is capable of initiating an explosion when suspended in a dust cloud. This 

test, performed in a Hartmann apparatus, determines the minimum ignition energy of a 

dust cloud in air by a high voltage spark.7

If either of the screening tests produces a positive result for combustibility or explosibility, NFPA 

484 requires further explosibility testing be conducted in a 20-L sphere. Several values (below) 

from the explosibility test results can be used to characterize the severity of a dust explosion. 

4.1.1 CSB COMBUSTIBLE DUST TESTING

4.1.1.1 Combustibility Demonstration

In order to visually demonstrate the combustibility of the Hoeganaes iron samples, a modified 

“Go/No-Go” test was performed by the CSB. Generally, this test is performed in a closed 

vessel, but the CSB was interested in directly observing any flames the dust may produce.8

EXPLOSIVITY VALUES

KSt: calculated value that compares the relative explosion severity and consequence to 

other dusts (bar m/s). The higher the KSt number, the more energetic the explosion. 

Pmax: maximum explosion overpressure generated in the test vessel (bar)

܎P/܎t: maximum rate of pressure rise, predicts violence of the explosion (bar/s)

Explosion Severity (ES): Index to determine if Class II electrical equipment is required 

as an OSHA requirement.

ES > 0.5, Class II Combustible

ES < 0.4, Combustible but not Class II

5

UN Recommendations on the Transport of Dangerous Goods: Model Regulations – Manual of Tests and Criteria, Part III, Subsection 33.2.1

6

NFPA 484 defines a combustible metal dust as a particulate metal that presents a fire or explosion hazard when suspended in air or the process specific 

oxidizing medium over a range of concentrations, regardless of particle size or shape. 

7

ASTM E2019, Standard Test Method for Minimum Ignition Energy of a Dust Cloud in Air

8

The “Go/No-Go” test is typically performed in a modified one-liter Hartmann tube; also known as the explosibility screening test as described in NFPA 484.
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Hoeganaes Corporation Case Study December 2011

This test dispersed about 30 grams of iron dust—sampled from the baghouse9 associated with 

the bucket elevator from the January 31, 2011, incident—above an 8 inch burner. Upon being 

released, the dust auto-ignites in air due to the heat given off from the burner below (Figure 

14). An intense white flame was produced that reached a peak diameter of 18 inches.

4.1.1.2 20-Liter (20-L) Test Method

CSB investigators collected iron powder samples from various locations in the Hoeganaes 

facility and commissioned testing to characterize its combustibility using two different 

test methods, the 20-liter (20-L) and one-meter cubed (1-m3

) test chambers. The 20-L test 

laboratory used the standard test method, ASTM E1226, Pressure and Rate of Pressure 

Rise for Combustible Dusts, for the selected iron powder samples. Each dust sample was 

injected and ignited in a 20-L spherical test vessel equipped with transducers to record a 

pressure-versus-time profile of the dust deflagration in the sphere.

Table 1 shows data from the CSB’s combustibility tests of the Hoeganaes dust and a comparison to dust testing the CSB commissioned for previous dust incidents at other companies. 

The CSB test data indicate that the iron powder is combustible and is covered by the 

requirements of NFPA 484 (Section 4.4.1). Although values indicate that the dust produces 

a weak explosion relative to other dusts, the dust is considered combustible by the OSHA 

definition10 and can result in a flash fire capable of causing injuries and fatalities. 

FIGURE 14

CSB iron dust 

combustibility 

demonstration, see 

Section 4.1.1.1.

9

Ventilation equipment that removes airborne particulate by forcing air through a specially designed filtration bag. 

10OSHA 3371-08 2009: “a solid material composed of distinct particles or pieces, regardless of size, shape, or chemical composition, which presents a 

fire or deflagration hazard when suspended in air or some other oxidizing medium over a range of concentrations.”
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Hoeganaes Corporation Case Study December 2011

20-L COMBUSTIBLE DUST TEST DATA FROM CSB INVESTIGATIONS11

Hoeganaes 

Iron Dust 

20L test12

Granulated 

Sugar

Aluminum 

Dust

Polyethylene 

Dust

Phenolic 

Resin

Pmax (bar) 3.5 5.2 9.4 8.34 7.58

܎P/܎t(bar/s) 68 129 357 515 586

KSt (bar m/s) 19 35 103 140 165

Explosion 

Severity (ES)

0.077 0.22 1.08 1.38 1.43

Classification Combustible Combustible Combustible, 

Class II

Combustible, 

Class II

Combustible, 

Class II

Frequently, the hazards of different combustible dusts are evaluated by their potential 

explosive capabilities. However, the hazards of combustible dusts are not limited to explosions. The Hoeganaes iron powder propagates an explosion less rapidly compared to other 

dusts, so there is less overpressure damage, consistent with observations by CSB investigators. Dust testing results from Hoeganaes and prior CSB investigations illustrate that dusts 

with low KSt values can cause flash fires that result in deaths and serious injuries. Although 

combustible dusts can lead to explosions, combustible dust flash fires also pose a risk that 

must be addressed in industry.

According to the 20-L standard test method, E1226, a dust sample can be defined as 

combustible or explosible based on a calculated pressure ratio (PR) using the pressure data 

recorded in the 20-L test chamber. For sample concentrations of 1,000 and 2,000 g/m3

, a 

pressure ratio value greater than or equal to 2 is considered explosible. If the pressure ratio 

is less than 2, the sample is considered non-explosible. However, the test method cautions 

that the dust can still burn and a dust cloud may experience a deflagration depending upon 

conditions such as the temperature and particle size.13 The iron dust sample from baghouse 

#4 had a PR of 4.0 and 4.7 at concentrations of 1,000 and 2,000 g/m3 respectively, indicating that the dust sample is explosible. 

4.1.1.3 One-Meter Cubed (1-m3) Test Method

The CSB collected a subsequent sample14 from baghouse #4 and subjected it to combustibility testing using the one-meter cubed (1-m3

) method, ISO 6184-1 Explosion Protection 

Systems: Determination of Explosion Indices of Combustible Dusts in Air. The 1-m3

test vessel is larger than the 20-L vessel, and the dust, along with air and a fuel source, is 

injected into the system differently. 

The iron powder from baghouse #4 underwent an explosibility screening test in the 1-m3

vessel in an attempt to ignite the sample. At several dust concentrations, none of the tests 

produced significant pressure which exceeded the test qualifications for ignition and therefore the dust sample was considered non-explosible according to this method.

TABLE 1

Combustibility data for 

selected materials.

11For a more detailed discussion on characteristic combustible dust values, see CSB 2006-H-1 Combustible Dust Hazard Study.

12CSB investigators collected this iron dust sample from baghouse #4 at the Hoeganaes Gallatin facility after the January 31, 2011, incident. 

13American Society for Testing and Materials (ASTM), E1226-10, Pressure and Rate of Pressure Rise for Combustible Dust, ASTM International, 2010.

14At the time this sample was taken in August 2011, the Gallatin facility had not been fully operational for about three months. As such, the sample 

collected did not contain fines representative of the environment at the time of the 2011 flash fire incidents.
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4.1.1.4 Comparing Dust Testing Methods

Both the 20-L and 1-m3 tests are accepted methods that can characterize dust explosibility; 

however results from the two tests may differ. There are several factors that can contribute 

to varying results among the dust test methods. Dust characteristics, such as particle size, 

moisture content, and degree of oxidation (for metals) can affect the ignitability of the 

sample in the test chamber. 

The 20-L and 1-m3

 test chambers were designed to simulate dust explosions in facility 

settings, but each test has limitations. The main difference between the two tests is the 

chamber size and the dust dispersion mechanism. Since the 1-m3

 test is larger, theoretically 

it can better simulate an open-space dust cloud explosion. However, that larger volume also 

makes it harder to create a uniform distribution of dust within the testing chamber. In the 

smaller 20-L test chamber it is easier to create a uniform distribution; however, it is possible 

that the smaller chamber also creates an “overdriving” effect. Since the 20-L chamber is 

smaller, the energy exerted by the igniters15 may combust enough dust creating the appearance of ignition16 — a situation that would not occur in a facility setting.

NFPA revised the 2012 edition of NFPA 484 to state that explosibility screening tests shall 

be performed in accordance to the 20-L test standard, E1226. However, NFPA added to 

the standard annex that the results of the 20-L test can be conservative and an owner or 

operator of a facility may elect to use a 1-m3

 test for dust explosibility testing as the 20-L 

test may result in false positives for dusts with lower KSt values.

Despite the discrepancies between the two test methods, the empirical evidence from the 

flash fire incidents at Hoeganaes shows that dusts with lower KSt values are capable of 

fueling flash fires with severe consequences. This further suggests that facilities should not 

rely on the 1-m3

 test as a sole determination of dust combustibility hazards. Dusts with 

lower KSt values and characteristics similar to the iron powder at Hoeganaes may not ignite 

in the 1-m3

 chamber but still have the ability to result in fatal flash fires. 

It should be noted that both tests are for explosibility screening, and alone may not convey 

the full combustibility hazard.

4.1.2 HOEGANAES COMBUSTIBLE DUST TESTING

4.1.2.1 Minimum Ignition Energy (MIE) 

In 2010, Hoeganaes contracted to test iron dust samples from the plant for combustibility 

as a result of an insurance audit recommendation. The test had one sample that was similar 

in particle size, moisture content, and location to the dust involved in the 2011 incidents. 

That sample gave the results seen in Table 2. 

The minimum ignition energy (MIE) testing determined that a continuous arc did ignite the 

representative samples from 2010, but a 500 mJ source did not. The conclusion from the 

testing was that the minimum ignition energy was greater than 500 mJ. This information is 

valuable in determining potential ignition sources for each of the incidents. 

15The igniter may increase the temperature in the smaller 20-L chamber, raising the overall temperature of the system and allowing a non-explosive 

system to appear explosive.

16Going, J et al., “Flammability limit measurements for dust in 20-L and 1-m3

 vessels.” Journal of Loss Prevention in the Process Industries. May 2003 
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4.2 IGNITION SOURCES

Witnesses indicated that the May 27, 2011, hydrogen explosion was ignited by sparks generated 

during the lifting of the trench cover. This is reasonable considering that the MIE of hydrogen is 

0.02 mJ, and the energy of mechanical sparks from metal to metal contact can be several mJ.20

The testing contracted by Hoeganaes in 2010 determined that the minimum ignition energy 

for representative iron dust samples was greater than 500 mJ, and that a continuous arc 

would ignite the samples. One witness at ground level reported hearing an “electric sound” 

at the time of the incident. The motor operating bucket elevator #12 was a likely source of 

ignition since it had exposed wiring, was not properly grounded, and was within a few feet 

of the dust cloud source. The wiring was exposed because the electrical conduit supplying 

power to this motor was not securely connected to the motor’s junction box.

HOEGANAES MINIMUM IGNITION ENERGY (MIE) TEST RESULTS

Sample Iron Dust

Particle size (%<75 μm) 99%

Pmax(bar) 3.3

(܎P/܎t)max (bar/s) 51

KSt (bar*m/s) 15

MIE (mJ17, Cloud) >500

MIT18 (°C,Cloud) 560-580

MEC19 (g/m3

) 200-250

TABLE 2

Combustible dust test 

results commissioned by 

Hoeganaes in 2010.

FIGURE 15

Exposed electrical 

wiring on elevator motor 

near January 2011 

incident site (motor 

panel cover was rotated 

post incident by fire 

department).

Motor

Exposed wires

17mJ is an abbreviation for millijoules, which is a unit of energy. One Joule is equal to 1000 millijoules or approximately 0.24 calorie.

18Minimum Ignition Temperature.

19Minimum Explosible Concentration. 

20V. Babrauskas, Ignition Handbook, Fire Science Publishers, Issaquah, WA, 2003.
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Prior to the CSB notifying Hoeganaes that evidence from the incident area needed to be preserved, the company removed and modified evidence from the scene, including the elevator 

motor, wiring, and conduit. However, on examination, there were spots that appeared to be arc 

marks both inside the junction box, and on the outside of the motor housing.

4.3 HOEGANAES

4.3.1 HAZARD RECOGNITION

In general industry the combustibility of metal dust is a well-established hazard, but metal 

dust fires and explosions continue to claim lives and destroy property. The CSB reviewed three 

publications dating back to the 1940s and 1950s that addressed metal dust (including iron dust) 

hazards and explosion protection methods. The National Fire Protection Association (NFPA) 

code for the Prevention of Dust Explosions, published in 1946,21 lists general precautions for all 

types of dusts, including metal powder, and specific provisions for certain types of dusts.

The Building Construction section of the code states, “Avoid beams, ledges or other places 

where dust may settle, particularly overhead.” The Gallatin facility, built in the 1980s, was not 

designed to avoid significant overhead accumulations of dust. The code calls for designing and 

maintaining dust-tight equipment to avoid leaks and, where this is not possible, to enforce good 

housekeeping procedures. 

The code also cautions against sources of ignition in areas containing dust and recommends 

locating dust collectors outdoors or in separate rooms equipped with explosion venting. 

In 1957, the NFPA published the Report of Important Dust Explosions which included a 

summary of over 1,000 dust explosions between 1860 and 1956 in the U.S. and Canada.22 The 

report listed 80 metal dust fires and explosions, including one iron dust incident that resulted in 

a fatality in 1951. 

A 1958 article in an American Chemical Society publication states, “Powdered metals dispersed 

in oxygen or air form explosive mixtures... their flammability and explosibility have been 

reported in considerable detail...”23

FIGURE 16

Mounds of iron dust 

along elevated surfaces 

at the Gallatin plant, 

February 3, 2011.

21National Fire Codes, Vol. II. The Prevention of Dust Explosions 1946. National Fire Protection Association, Boston, MA., 1946.

22National Fire Protection Association, Report of Important Dust Explosions, NFPA, Boston, MA, 1950.

23Grosse, A.V., and J.B. Conway. “Combustion of Metals in Oxygen.” Industrial & Engineering Chemistry 50.4 (1958): 663-72.
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In the 1990s and 2000s, the Pittsburgh Research Laboratory of the National Institute for 

Occupational Safety and Health (NIOSH) conducted a study of the explosibility of various 

metals, including iron. The results of these experiments, published in scientific journals,24

showed the explosibility characteristics of iron powder to aid hazard evaluation in metal processing industries. However, management within the Hoeganaes Corp. and GKN Corp. did 

not commission an analysis of its own potentially combustible PM products and constituents 

until January 2009, as a result of an insurance audit conducted in 2008. This combustible 

dust testing concluded that all three iron powder samples collected from various locations in 

the plant were explosible (Section 4.3.6.1). However, these results did not trigger an effective 

overhaul of the dust containment and housekeeping procedures at the Gallatin facility. 

Representatives from Hoeganaes told the CSB that the dust analysis results did trigger 

an operator training program for the recognition of combustible dust hazards. However, 

Hoeganaes did not mitigate the dust hazard. Since Hoeganaes did not control the combustible 

dust hazard, operators were forced to tolerate the conditions at the facility. Over time, these 

flash fires became normalized, since they did not result in any serious injuries prior to the fatal 

incident on January 31, 2011. 

Operators and mechanics reported being involved in multiple flash fires during their employment at the Gallatin facility. At the time of the incidents, many were aware that the iron dust 

could burn or smolder. However, they were not trained to understand the potentially severe 

hazard when accumulated dust is dispersed in air. Rarely would operators report the minor 

flash fires and near-misses that periodically occurred. 

4.3.2 ENGINEERING CONTROLS

4.3.2.1 Combustible Dust

Thorough hazard recognition is key to effectively managing the risk from combustible dust. 

Once the hazard is recognized, applying the “hierarchy of controls” for fire and explosion 

prevention helps address the fugitive dust issue at the source: the material itself, the processing equipment, and the work procedures. The hierarchy of controls is a safety concept in 

which a hierarchal ordering of control mechanisms is applied to reduce risk. It covers the 

spectrum from elimination at the source, at the top of the hierarchy, through engineering 

and administrative (procedural) controls to personal protective equipment (PPE), at the 

bottom of the hierarchy.25

Installing and maintaining engineering controls to eliminate fugitive dust accumulation 

is the most effective method to prevent dust fires and explosions. Conveyance systems 

and appropriately sized dust collection equipment are examples of engineering controls 

that eliminate or mitigate fugitive dust generation at the source. Engineering controls are 

preferred over housekeeping, but a robust housekeeping program is important to manage fugitive dust accumulations in areas where engineering controls need maintenance or 

improvement. Additionally, administrative controls, such as worker training and operating 

procedures, complement robust engineering controls.

Significant quantities of iron dust escaped from equipment throughout the Hoeganaes facility. 

Enclosures on the conveyance equipment leaked fugitive emissions of iron dust.. In addition, 

the dust collection systems were historically unreliable and did not prevent large amounts 

of combustible iron dust from becoming airborne and accumulating on elevated surfaces 

throughout the processing areas.

24Cashdollar, K., Flammability of Metals and Other Elemental Dust Clouds. Loss Prevention Symposium. AICHE 1994.

 Cashdollar, K., Zlochower, I., Explosion Temperatures and Pressures of Metals and other Elemental Dust Clouds. Journal of Loss 

Prevention in the Process Industries 20 (337-348) 2007.

25Kletz, T., Amyotte, P., Process Plants: A Handbook for Inherently Safer Design. CRC Press, 2010.
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Hoeganaes Corporation Case Study December 2011

4.3.2.2 Hydrogen

The trench involved on the May 27, 2011, incident contains many pipes including nitrogen 

and hydrogen supply and vent pipes for the band furnaces. In addition to housing pipes, the 

trench also acts as a drain for the cooling water used in the band furnaces. At the time of 

the incident this water came out of the furnaces hot and drained directly onto the pipes and 

into the trench (Figure 17).

According to ASME B31.3, design concerns about ambient conditions around process pipes 

focus on environments and changes that can create physical stresses in the piping. Section 

10.5.3 of NFPA 55 requires annual maintenance including inspection for physical damage 

and leak tightness. CGA G-5.4 similarly requires regular inspection for physical damage 

and leak tightness. However, Hoeganaes did not regularly inspect the pipes in the trench. 

The design and maintenance of this trench, should have addressed the issue of slow corrosion over time caused by the hot water runoff and solids accumulation. 

Both NFPA 55 and NFPA 2 state, “Provisions shall be made for controlling and mitigating unauthorized discharges.” NFPA 2 further requires that “the storage, use, or handling 

of [hydrogen] in a building or facility shall be accomplished in a manner that provides a 

reasonable level of safety... from illness, injury or death...” However, the CSB found no 

evidence of a Hoeganaes procedure to inspect piping within the trench to ensure that corrosion had not compromised the piping systems which would allow an uncontrolled release 

of hydrogen. Moreover, Hoeganaes had no written procedure or protocol to mitigate gas 

leaks, and maintenance crews were allowed to begin investigating a suspected leak without 

testing the atmosphere for concentrations of explosive gas.

4.3.3 ADMINISTRATIVE CONTROLS

4.3.3.1 Housekeeping

Observations by CSB investigators at the Gallatin facility shortly after the first incident 

indicated that combustible dust was leaking from equipment and that housekeeping was 

ineffective (see Figures 16, 18, 19, 20 and 21). Combustible iron dust coated almost every 

STANDARDS THAT ADDRESS THE 

HYDROGEN SUPPLY AND VENT PIPES IN 

THE TRENCH INCLUDE: 

s !3-%26 B31.3, Process Piping 

s #'!27 G-5.4-2010, Standard for Hydrogen 

 Piping Systems at User Locations 

s .&0!  (YDROGEN 4ECHNOLOGIES #ODE

s .&0!  #OMPRESSED 'ASES AND #RYOGENIC

 Fluids Code.

FIGURE 17

Water flow (upper left 

of photo) and externally 

corroded pipes in the 

trench involved in the 

May 27, 2011, incident.

26The American Society of Mechanical Engineers (ASME) is a professional body focused on mechanical engineering. The organization is known for setting codes and standards for mechanical devices. The ASME conducts one of the world’s largest technical 

publishing operations through the ASME Press. The organization holds numerous technical conferences and hundreds of professional development courses each year, and sponsors numerous outreach and educational programs.

27The Compressed Gas Association (CGA) develops and publishes technical information, standards, and recommendations for the 

manufacture, storage, transportation, distribution, and use of industrial gases.
CSB s Hoeganaes Corporation Case Study 16

Hoeganaes Corporation Case Study December 2011

surface up to 4 inches deep and was visible in the air. Mitigation of the combustible dust hazard by Hoeganaes was limited to a less-than-adequate vacuuming service, sparsely enclosed 

conveyance equipment, and an inadequate baghouse filtration system. 

Although bucket elevators and some conveyance equipment were enclosed, fugitive dust 

emissions were evident throughout the facility. Moreover, the CSB investigators observed 

leaks of fugitive dust to the atmosphere when the bags used in the baghouse filtration system were pulsed, which allowed dust to escape into the work areas many times each hour. 

The baghouse filters are designed to collect the smallest, and consequently most dangerous, 

dust particles. Yet, the CSB found that the baghouses were often out of service. Employees 

reported that the baghouse associated with bucket elevator #12 was out of service sporadically for the 7 days leading up to the fatal incident on January 31, 2011, allowing fine 

combustible iron dust to remain in the area, from which it was dispersed when the elevator 

was restarted during maintenance. 

4.3.4 PERSONAL PROTECTIVE EQUIPMENT

4.3.4.1 Flame Resistant Clothing (FRC)

Workers in production-related operations wear flame resistant clothing 

(FRC) to reduce risk of thermal injury 

from flash fire incidents. As part of the 

Personal Protective Equipment (PPE) 

Standard (29 CFR 1910.132), OSHA 

requires employers to provide workers 

with FRC in workplaces when flash fire 

or explosion hazards are present. 

FRC can reduce the severity of burn injuries sustained during a flash fire when engineering 

and administrative controls fail. FRC, usually worn as coveralls, is made of treated natural or 

synthetic fibers that resist burning and withstand heat. 

There are two NFPA standards that provide guidance on the design and use of FRC. NFPA 

2112, Standard on Flame-Resistant Garments for Protection of Industrial Personnel Against 

Flash Fire, provides the minimum requirements for the design, testing, and certification of 

FRC. NFPA 2113, Standard on Selection, Care, Use, and Maintenance of Flame-Resistant 

Garments for the Protection of Industrial Personnel Against Flash Fire, provides guidance 

for the selection, use, and maintenance of FRC. The 2009 edition of NFPA 484 included a 

requirement for workers to wear FRC if working in metal dust-handling operations, but it did 

FIGURE 20

Iron dust on structural 

supports, February 3, 2011.

FIGURE 18 (LEFT)

Iron dust on rafters, 

February 3, 2011.

FIGURE 19 (RIGHT)

Iron dust on overhead 

surfaces, February 3, 2011.
CSB s Hoeganaes Corporation Case Study 17

Hoeganaes Corporation Case Study December 2011

not specifically reference NFPA 2112 or 2113 in the standard. The 2012 edition of NFPA 484 

requires that new and existing facilities covered by the standard adhere to the requirements of 

NFPA 2113 for FRC.

Hoeganaes employees were required to wear FRC, and the injured and fatally injured 

employees were wearing the Hoeganaes-designated FRC at the time of the 2011 flash fire 

incidents. Though FRC is intended to reduce the severity of thermal injuries, five severely 

burned employees died following the January and May incidents. The specific FRC worn did 

not provide any significant protection against the combustible iron dust flash fires and the 

hydrogen explosion at Hoeganaes.

4.3.5 1992 INCIDENT

On May 13, 1992, a hydrogen explosion and iron dust flash fire similar to the May 2011 

incident in Gallatin severely burned an employee working at the Hoeganaes facility in 

Cinnaminson, NJ. CSB investigators interviewed the injured employee from the 1992 incident 

and learned that a hydrogen explosion event in a furnace dispersed and ignited significant 

accumulations of iron dust which resulted in thermal burns over 90% of his body. The injured 

worker spent a year in a burn unit and is still recovering from his burn injuries.

4.3.6 INSURANCE INSPECTIONS

4.3.6.1 Allianz

In November 2008, Allianz, a German-based risk insurer, conducted a routine audit of the 

Hoeganaes facility. The audit report noted that improved housekeeping was needed in several 

areas of the facility. In the list of risk improvement proposals, the Allianz report stated, “The 

potential for explosions caused when clouds of powdered metal are aroused in equipment... 

should be analyzed by an independent consultant.” The proposal recommended an independent 

dust hazard analysis and a subsequent hazard study to identify suitable mitigation techniques, 

should the iron dust in the facility be found to be explosible.

In January 2009, Hoeganaes collected samples of base iron dust, furnace-feed dust, and 

baghouse dust and commissioned explosibility testing as Allianz recommended (Table 3). In 

September 2010, Hoeganaes requested another test of various powdered metals. Test results 

showed that 5 of the 9 iron samples had KSt values greater than 1. 

The Allianz audit findings initiated several action items as part of the Hoeganaes Combustible 

Dust Program at the Gallatin facility. The scope of the program was to understand and align 

company practices with the OSHA Combustible Dust National Emphasis Program (NEP) 

(Section 4.5.1). Action items included combustible dust training for employees and understanding relevant NFPA codes at the facility. Although the majority of the Hoeganaes Combustible 

Dust Program action items had planned completion dates prior to the 2011 flash fire incidents, 

the program did not effectively mitigate the combustible dust hazards at the facility. 

HOEGANAES IRON DUST EXPLOSIBILITY TESTING

(20-L)

Sample Base Iron Furnace Feed Baghouse Dust

Pmax(bar) 1.9 2.8 3.8

܎P/܎t(bar/s) 273 63 80

KSt (bar m/s) 74 17 22

TABLE 3

Dust explosibility test 

results commissioned by 

Hoeganaes in 2009.
CSB s Hoeganaes Corporation Case Study 18

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4.4 FIRE CODES

4.4.1 NFPA 484

NFPA 484, Standard for Combustible Metals, an industry consensus standard, applies to 

facilities that produce, process, finish, handle, recycle, and store metals and alloys in a form 

capable of combustion or explosion. NFPA codes involving combustible metal dusts have 

evolved several times since the 1946 The Prevention of Dust Explosions (Section 4.3.1). In the 

1950s, the NFPA divided the 1946 document into several codes for specific materials, such 

as magnesium and titanium. In 2002, all of the NFPA combustible metal dust standards were 

combined into NFPA 484. NFPA 484 describes the tests and methods for determining metal 

dust combustibility and provides guidelines for preventing dust explosions and flash fires for 

all types of metal dusts. 

The CSB commissioned testing similar to the 2009 edition of NFPA 484 explosibility determination test requirements (Section 4.1). The testing concluded that the Hoeganaes metal 

dust sample is explosible and therefore NFPA 484 applies. 

Had Hoeganaes voluntarily followed, or been required to follow, NFPA 484 by the GFD 

(authority having jurisdiction28) the January and March incidents may have been prevented, 

and the effects of the May accident could have been reduced. As with many NFPA standards, 

NFPA 484 has a retroactivity clause for certain 

chapters, stating that requirements for all 

existing equipment, installed prior to the current edition of the code, are not enforceable by 

the authority having jurisdiction, unless it is 

determined that the existing situation presents an 

unacceptable safety and health hazard. 

Neither the City of Gallatin nor the GFD identified combustible metal dust as a concern or 

hazard during previous inspections conducted 

prior to the 2011 incidents (Section 4.4.3).

Chapter 12 of NFPA 484, “Requirements for 

Combustible Metals” includes provisions to control or eliminate dust fires and explosions. 

It requires engineering controls for dust-producing processes such as enclosures and capture 

devices connected to dust collection systems. The standard describes recommended housekeeping practices and frequencies, and how to control ignition sources. 

Practices at Hoeganaes did not conform to the safety recommendations set forth in NFPA 

484. Under “Building Construction,” NFPA 484 requires that floors, elevated platforms, and 

gratings where dust can accumulate be designed to minimize dust accumulations and facilitate 

cleaning.29 The Hoeganaes facility has numerous flat surfaces overhead upon which the CSB 

investigators observed significant accumulations of combustible iron dust. Since Hoeganaes has 

an iron powder-producing operation, specific engineering controls outlined in NFPA 484 apply 

to the machines that manufacture and convey the PM. All machines that produce fine particles 

of iron should be connected to a dust collection system that has the appropriate velocity to 

capture all dust. The CSB investigators observed that some of the PM conveyance equipment at 

FIGURE 21

Photo of equipment 

obscured by airborne dust, 

taken by CSB investigators, 

February 7, 2011.

28The organization, office, or individual responsible for approving equipment, materials, an installation, or a procedure.

29Chapter 12 is not retroactive to existing facilities.
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Hoeganaes Corporation Case Study December 2011

Hoeganaes’s was not enclosed; as such, it was not designed to control significant dust emissions, 

and employees further reported that baghouse dust collectors were often down for maintenance. 

This section of the standard also requires that dust collection systems be located outdoors; at 

Hoeganaes, the baghouses are located inside, posing a serious fire and explosion hazard. 

Chapter 13 of NFPA 484 includes provisions for housekeeping and applies to all new and existing facilities. It requires that accumulations of excessive dust on any portions of buildings or 

machinery not regularly cleaned in daily operations be minimized and that fugitive dust not be 

allowed to accumulate. Hoeganaes used a vacuuming service to reduce quantities of dust that 

had accumulated. However, inadequate dust collection systems and dust leakages from equipment produced accumulations beyond what could be controlled by the limited housekeeping 

service that was being provided. 

4.4.2 ELECTRICAL CLASSIFICATION

The classification of combustible dust hazardous locations is based on the criteria established 

by article 500 of NFPA 70, National Electric Code (NEC). The NEC defines hazardous locations as areas “where fire or explosion hazards may exist due to flammable gases or vapors, 

flammable liquids, combustible dust, or ignitable fibers or flyings.” The classifications are 

broken down into three hazardous material classes:

UÊ
>ÃÃÊÊqÊy>>LiÊ}>ÃÊÀÊÛ>«À

UÊ
>ÃÃÊÊqÊVLÕÃÌLiÊ`ÕÃÌ

UÊ
>ÃÃÊÊqÊwLiÀÃÊ>`ÊyÞ}Ã

Each class is further categorized into one of two divisions, based on operating conditions: 

UÊÛÃÊ£ÊqÊ À>\Ê>Ài>ÃÊÜiÀiÊÌiÊV>ÃÃwi`Ê>â>À`ÕÃÊ>ÌiÀ>ÊÃÊiÞÊÌÊLiÊ

 present under normal operating conditions

UÊÛÃÊÓÊqÊLÀ>\Ê>Ài>ÃÊÜiÀiÊÌiÊV>ÃÃwi`Ê>â>À`ÕÃÊ>ÌiÀ>ÊÃÊiÞÊÌÊLiÊ

 contained and present only through accidental release

With the proper evaluation of electrically classified areas in an operating facility, appropriately 

rated equipment can be installed.

4.4.2.1 Combustible Dust

NFPA 499, Classification of Combustible Dusts and of Hazardous (Classified) Locations for 

Electrical Installation in Chemical Process Areas, specifies the type of electrical equipment acceptable in atmospheres containing combustible dust. It applies to locations where combustible 

dusts are produced, processed, and handled, or where surface accumulations of dust could be 

ignited by electrical equipment. Based on the requirements of NFPA 499 and the NEC, classified electric services and equipment would be required in a facility where combustible metal 

dust was present. Specifically, NFPA 499 states that Division 1 electrical equipment should be 

used in areas where combustible dust can accumulate to 1/8 of an inch (3 mm).

OSHA 1910 Subpart S includes definitions and requirements for hazardous or electrically classified locations. To determine whether classified electrical equipment is needed for a combustible dust, a Class II combustibility test is conducted with dust samples from the facility and 

the explosion severity (ES) ratio calculated. An ES of greater than 0.5 signifies an appreciable 

explosion hazard, which means either that Class II electrical equipment must be installed or 

dust accumulations near electrical equipment must be prevented. ES values less than 0.5 are 

generally considered to be lower explosion hazards, and non-rated electrical equipment in 

those atmospheres is acceptable.
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Hoeganaes Corporation Case Study December 2011

In January 2009, Hoeganaes submitted samples for explosibility testing. Although several 

samples were determined to be explosible, the ES values were low, precluding the need for 

classified electrical installations in the Hoeganaes facility. The CSB tested iron dust samples 

and found an explosion severity ratio of 0.01 to 0.1, significantly less than the ratio that 

would require classified equipment under existing codes. 

4.4.2.2 Flammable Gases

Test results on the iron samples in the vicinity of the Hoeganaes incidents did not require 

the installation of classified electrical services. However, the flammable hydrogen in the 

band furnaces did. NFPA 497, Recommended Practice for the Classification of Flammable 

Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations 

in Chemical Process Areas, lists hydrogen as a Class I flammable gas. Areas with Class I 

materials are further classified as Division 2 if the material is normally contained inside the 

equipment, or Division 1 if the material is normally present in flammable concentrations 

outside the equipment, such as during maintenance. Because hydrogen normally vents into the 

work area at one end of the annealing furnace, the area around the annealing furnace should 

have been designated as Class I Division 1, and the electrical equipment in that area designed, 

installed, and maintained to meet those recommendations. 

Moreover, the standard states that if no physical boundary surrounds a Division 1 area, the 

transitional area between Division 1 and an unclassified area is designated as Division 2. In 

addition, because the hydrogen piping system includes potential leak points, such as valves 

and flanges, these areas should have been designated as Class I Division 2. As such, large 

areas in the annealing building would be required to have Class I Division 1 or Class I 

Division 2 electrical service installed. 

Despite these classifications and their recommendations, the CSB observed inappropriate electrical installations, including large electrical cabinets that were open to the atmosphere and that 

had significant iron dust accumulations, incomplete conduit, and regular 110-volt cord-plug 

outlets instead of ignition-proof electrical devices approved by the NEC for Class I atmospheres.

4.4.3 INTERNATIONAL FIRE CODE

The International Fire Code (IFC) establishes minimum requirements for fire protection 

and prevention systems. The International Code Council (ICC), a membership association 

responsible for developing safety codes in residential and commercial buildings, publishes 

the IFC. Chapter 13 of the IFC, “Combustible Dust-Producing Operations,” (2006 edition) 

briefly addresses the prevention of ignition sources and housekeeping for areas where combustible dust is generated, stored, manufactured, or handled.30 The IFC also references several 

NFPA standards, such as NFPA 484, but language in section 1304 is vague as to whether the 

compliance with the listed NFPA standards is mandatory or voluntary. The IFC authorizes the 

authority having jurisdiction, such as the fire department or municipality, to enforce “applicable provisions” of these NFPA standards; the word “authorizes” does not carry the same 

weight as “shall enforce” and might be interpreted as a discretionary rather than mandatory 

code requirement. 

Companies are required to comply with the IFC only if promulgated through local, state, or 

federal regulations.31 The State of Tennessee Division of Fire Prevention and the City of Gallatin 

both adopted the 2006 version of the IFC into their codes. Though the general precautions for 

housekeeping and ignition sources are required through code adoption of the IFC, the noted 

30Combustible Dust-Producing Operations” is located in Chapter 22 of the IFC 2012 edition.

31In a previous investigation of a serious dust explosion at West Pharmaceutical Services in North Carolina in 2003, the CSB recommended that the state require mandatory compliance with the detailed provisions of the relevant NFPA dust standard (NFPA 654) 

rather than the much briefer and less prescriptive requirements of IFC Chapter 13.
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NFPA standards could be interpreted as voluntary. The Tennessee Fire Code specifically declares 

that the state does not enforce “optional or recommended” standards or practices. 

Practices at the Hoeganaes facility did not conform with the requirements set forth in Chapter 

13 of the IFC. The code prohibits devices using an open flame and the use of spark-producing 

equipment in areas with combustible dust. It also states that accumulated combustible dust 

will be kept to a minimum inside buildings. However, adhering to the much more detailed 

design and engineering requirements of NFPA 484 would have further reduced the likelihood 

that the three serious incidents would have occurred.

At the time of the 2011 flash fire incidents, the Hoeganaes facility was operating under the 

provisions of the 2006 IFC. The GFD has the authority to inspect facilities against the IFC, issue 

violations, and stop-work orders if buildings or operations are declared unsafe based on the 

code’s provisions. All construction and design provisions apply to new or existing structures if, 

in the opinion of the code official, a distinct hazard to life or property exists. All administrative, 

operational, and maintenance provisions apply to new and existing conditions and operations. 

For the City of Gallatin, the fire chief is responsible for enforcement of the IFC. CSB investigators reviewed the GFD’s inspection history at the Hoeganaes facility. The fire department 

conducted three inspections in the previous 12 years, in 1999, 2002, and 2011. The 2011 

inspection was performed just two weeks prior to the May 27, 2011, incident. The report for 

this inspection documented observations at the facility related to fire suppression and emergency egress, but did not mention combustible dust hazards even after the January and March 

2011 incidents. The CSB found no evidence that the GFD inspected Hoeganaes against the 

provisions of the 2006 IFC for the hazards associated with combustible metal dust, electrical 

installation, and operations that use flammable gases.

4.5 REGULATORY OVERSIGHT

4.5.1 OSHA

4.5.1.1 Combustible Dust

In 2006, following three catastrophic dust explosions that claimed 14 lives in 2003, the CSB 

issued its Combustible Dust Hazard Study. The study identified 281 dust fires and explosions in 

the U.S. between 1980 and 2005 that resulted in 119 fatalities and 718 injures. 

The absence of an OSHA comprehensive combustible dust standard was a key finding in the 

2006 study and resulted in a CSB recommendation to OSHA to initiate rulemaking for a 

general industry combustible dust standard. The recommendation remains “open-acceptable”32 

as of the publication of this report. 

A significant reduction in grain dust incidents resulted from a prior dust regulation enacted by 

OSHA for grain handling facilities. In 1987, OSHA promulgated a grain facilities standard 

in response to a series of major grain dust explosions. A 2003 OSHA analysis of grain dust 

incidents showed that fatalities dropped 60 percent after the regulation was enacted.

Another recommendation from the 2006 CSB Combustible Dust Study was for OSHA to 

develop a national special emphasis program33 (SEP) to address dust in industry while the 

comprehensive standard was being developed. In October 2007, OSHA initiated a Combustible 

Dust National Emphasis Program (NEP) to target industries that generate, store, or handle 

combustible dusts. The program provides guidance to OSHA inspectors about how to apply 

32In 2009, the CSB voted to change the status of the OSHA recommendation to “open-acceptable” after OSHA initiated an advance 

notice of proposed rulemaking (ANPR) for the combustible dust regulation.

33SEPs and NEPs are not regulations but rather are enforcement tools that allow OSHA to focus resources on inspections.
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existing safety statutes or standards, such as the General Duty Clause (OSH Act 5(a)(1)) and the 

Walking-Working Surfaces standard (29 CFR 1910.22), to facilities with combustible dust. 

Although the scope of the NEP applies to various types of combustible dusts including metal 

dusts at Hoeganaes, only those facilities assigned to particular North American Industry 

Classification System (NAICS) codes are specifically targeted for inspections by OSHA under 

the NEP. These NAICS codes classify facilities by primary business activity. If a facility that 

handles combustible dust is not included in the NAICS code list, OSHA can initiate an NEP 

inspection only as a result of a complaint, referral, or occupational injury. The NAICS code 

for the Hoeganaes Gallatin facility (331111, Iron and Steel Mills) is not included in the list of 

industries targeted by the NEP, although similar industries that handle metals are. According 

to the U.S. Census Bureau 2007 Economic Census, there were 352 facilities in the Iron and 

Steel Mills industry code (331111) employing over 106,000 workers. 

The CSB 2006 dust study found that 20 percent of documented dust incidents from 1980 to 

2005 occurred in the metals industry. During that period, the CSB documented three iron dust 

incidents that resulted in three fatalities and four injuries. One of those documented incidents 

occurred at the Hoeganaes Gallatin facility in 1996 when iron powder caught fire in a dust 

collector and one worker received a smoke inhalation injury.

In February 2008, a catastrophic sugar dust explosion at Imperial Sugar killed 14 and injured 

36. A month later, OSHA revised and reissued the Combustible Dust NEP to include facilities 

that handle sugar. As a result, OSHA notified all facilities covered by the sugar industry codes 

that they were subject to inspection.

The Combustible Dust NEP is the only national OSHA program to specifically promote effective combustible dust hazard management. OSHA did not initiate rulemaking on combustible 

dust, as the CSB had recommended in November 2006, until April 2009. In its final report on 

the Imperial Sugar disaster in September 2009, the CSB recommended that OSHA “proceed 

expeditiously” with the new dust standard. Although OSHA issued an advance notice of 

proposed rulemaking (ANPR) for combustible dust in October 2009 and has since convened 

various stakeholder meetings, no proposed or final rule has been published. 

Combustible dust fires and explosions have continued to occur at industrial facilities across the 

country; since issuing the 2006 CSB Combustible Dust Study, the CSB has recorded a number 

of significant combustible dust incidents. Until a combustible dust standard is enacted, the NEP 

remains OSHA’s primary tool for addressing combustible dust in the workplace.

4.5.1.2 Process Safety Management

The Process Safety Management Standard, 29 CFR 1910.119 (PSM), is an OSHA regulation 

for processes that contain highly hazardous materials or significant quantities of flammables. 

The intent of PSM, as stated in the standard, is “preventing or minimizing the consequences 

of catastrophic releases of toxic, reactive, flammable, or explosive chemicals.” PSM applies 

to processes using or producing any of 137 listed highly hazardous chemicals at or above 

threshold quantities and processes with flammable liquids or gases onsite in quantities of 

10,000 pounds or more in one location. If applied to Hoeganaes, elements of PSM would 

have required practices and procedures that could have prevented or lessened the severity of 

the May 27, 2011 incident.

In the May 2011 incident, there was a hydrogen leak and subsequent hydrogen explosion and 

dust flash fires that ultimately killed three workers. The hydrogen provided to the Hoeganaes 

facility originates from an onsite generation and storage unit, owned and operated by a 
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Hoeganaes Corporation Case Study December 2011

contractor on land leased from Hoeganaes. Since the hydrogen generation facility’s total 

intended operational capacity exceeds 10,000 pounds of hydrogen, it is covered by PSM.

4.5.2 TOSHA

Tennessee is one of 24 states with a state-specific occupational safety and health plan. It 

develops and operates its own safety and health programs with approval from federal OSHA.

In November 2011, TOSHA issued citations to the Hoeganaes Gallatin Facility for the May 27, 

2011, incident. Hoeganaes received 15 OSHA PSM Standard violations related to the hydrogen 

system. OSHA concluded that the company lacked appropriate procedures to ensure mechanical integrity of the hydrogen piping, failed to develop an emergency response plan for leak 

detection and response, and did not perform a hazard assessment on the hydrogen process.

Although the Combustible Dust NEP encourages but does not require state plans to adopt the 

NEP, Tennessee OSHA adopted the dust NEP in March 2008.

The NEP allows OSHA Area Offices to add NAICS codes to the list of facilities targeted by 

the NEP. However, TOSHA has not added the NAICS code that includes Hoeganaes as of the 

issuance of this study. 

4.5.3 METAL DUST AWARENESS

Since Hoeganaes has been in operation, several opportunities have arisen to increase awareness and address metal dust issues at the facility through technical literature, audits, inspections, and regulatory oversight. These resources have not been effectively used by Hoeganaes. 

Gaps in codes and regulations, inadequate inspections, and poor hazard recognition all 

contributed to the three incidents at the Gallatin facility. 

Table 4 lists a timeline of events from 1956 to the present related to combustible metal dust 

and the lack of effective controls at Hoeganaes until the third incident in May 2011.
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4.6 METAL POWDER PRODUCERS ASSOCIATION

The Metal Powder Producers Association (MPPA) is one of six trade associations that make up 

the Metal Powder Industries Federation (MPIF). MPPA membership is open to manufacturers of 

metal powders, metal flakes, metal fibers, or non-metallic powder additives used with these materials. The stated objective of the MPPA is to “arrange for the collection and dissemination of 

information pertaining to the metal powder producing industries; provide technical facts, data, 

and standards, fundamental to metal powders and to the applications of metal powders...”36

TABLE 4

Timeline of metal dust 

publications, oversight, 

and opportunities to 

address metal dust 

hazards in industry and at 

Hoeganaes.

34OSHA Office of Program Evaluation, Regulatory Review of OSHA’s Grain Handling Facilities Standard (29 CFR 1910.272). 

February 2003.

35OSHA regulated program to prevent noise induced hearing loss (29 CFR 1910.35).

36http://www.mpif.org/aboutmpif/mppa.asp.

KEY MILESTONES FOR COMBUSTIBLE METAL DUST CONTROL IN INDUSTRY AND AT HOEGANAES

Year Action

1946 NFPA publishes Code for the Prevention of Dust Explosions that includes general requirements for metal dusts

1958 American Chemical Society publication discusses powdered metals and iron dust 

explosibility

1980 Hoeganaes Gallatin facility established

1987 OSHA promulgates Grain Dust Standard, which decreases the number of explosions 44% 

and fatalities 60%34

1992 Hydrogen explosion and iron dust flash fire severely burns employee at the Hoeganaes 

facility in Cinnaminson, NJ

1996 Employee of Hoeganes Gallatin facility suffers smoke inhalation in dust collector fire. 

Metal dust ignites inside dust collector (ignited during a cutting operation)

1999 Gallatin FD inspects Hoeganaes; no mention of dust accumulations

2002 Gallatin FD inspects Hoeganaes; no mention of dust accumulations

2002 NFPA 484 is issued which addresses additional combustible metals that would include 

iron dust

2006 The CSB issues recommendation to OSHA to promulgate a comprehensive combustible 

dust standard

2006 City of Gallatin adopts International Building and Fire Codes

2007 OSHA issues Combustible Dust National Emphasis Program (NEP)

2008 Tennessee OSHA adopts federal Combustible Dust NEP (March)

2008 Tennessee OSHA inspects Hoeganaes facility. Conducts respirable metal dust sampling. 

Cites Hoeganaes for hearing conservation35 (Oct.) No observation of a combustible dust 

hazard

2008 Allianz conducts insurance audit at Hoeganaes, recommends combustible dust testing 

and independent consultant (Nov.)

2009 Hoeganaes conducts combustible dust testing of three iron powder samples as recommended by 2008 Allianz audit; all three samples found to be combustible.

2010 Hoeganaes conducts combustible dust testing of 23 powdered metals; 5 of 9 iron samples 

found to be combustible. No substantial actions to mitigate combustible dust hazard

2011 January 31 incident; fatal combustible dust flash fire at Hoeganaes Gallatin facility

2011 March 29 incident; combustible dust flash fire at Hoeganaes Gallatin facility

2011 Gallatin FD inspects Hoeganaes, no mention of dust accumulations (May 11)

2011 May 27 incident; fatal hydrogen explosion/combustible dust flash fire at Hoeganaes 

Gallatin facility
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To achieve this, the MPPA offers a monthly newsletter and bi-annual meetings (fall and spring) 

to promote shared learning within the PM industry. In recent years, the spring meeting, which 

spotlights many safety topics, has begun focusing on the issue of combustible dust hazards in 

the PM industry. The MPPA has sought out external combustible dust expertise and OSHA has 

presented and participated in its safety meetings. 

5.0 KEY FINDINGS

Over the course of investigating the events at the Hoeganaes facility, the CSB made the following key findings:

1. Significant accumulations of combustible iron powder at the Hoeganaes facility fueled 

 fatal flash fires when lofted near an ignition source.

2. Hoeganaes facility management were aware of the iron powder combustibility hazard 

two years prior to the fatal flash fire incidents but did not take necessary action to 

mitigate the hazard through engineering controls and housekeeping. 

Î°Êi}>>iÃÊ``ÊÌÊÃÌÌÕÌiÊ«ÀVi`ÕÀiÃÊqÊÃÕVÊ>ÃÊVLÕÃÌLiÊ}>ÃÊÌÀ}ÊqÊÀÊÌÀ>-

ing for employees to avoid flammable gas fires and explosions. 

4. OSHA did not include iron and steel mills (NAICS code 331111), the industry classification code for Hoeganaes, in its Combustible Dust National Emphasis Program when it 

was first issued in 2007 or when it was re-issued in 2008. 

5. The 2006 International Fire Code Chapter 13, Combustible Dusts, which was adopted 

by the City of Gallatin at the time of the incidents, does not clearly require jurisdictions to 

enforce the more comprehensive and rigorous NFPA standards for the prevention of dust 

fires and explosions.

6. The Tennessee Fire Code and the City of Gallatin do not enforce “optional or recommended” standards or practices of the IFC.

7. The Gallatin Fire Department inspected the Hoeganaes facility after the first two iron 

powder flash fires but did not cite or otherwise address combustible dust hazards present 

at the facility just weeks before the third fatal hydrogen explosion and dust flash fire.

8. The flame-resistant clothing (FRC) supplied by Hoeganaes to its employees did not 

provide any significant protection against the combustible iron dust flash fires and the 

hydrogen explosion that caused the fatalities.

9. GKN and Hoeganaes did not provide corporate oversight to ensure the Hoeganaes 

Gallatin facility was adequately managing combustible dusts prior to and throughout the 

succession of serious incidents at the Gallatin facility.

6.0 RECOMMENDATIONS

6.1 OSHA

2011-4-I-TN-R1

Ensure that the forthcoming OSHA Combustible Dust Standard includes coverage for combustible metal dusts including iron and steel powders.
CSB s Hoeganaes Corporation Case Study 26

Hoeganaes Corporation Case Study December 2011

2011-4-I-TN-R2 

Develop and publish a proposed combustible dust standard for general industry within one year of the approval of this case study. 

2011-4-I-TN-R3 

Revise the Combustible Dust National Emphasis Program (NEP) to add industry codes for

facilities that generate metal dusts (e.g., North American Industrial Classification System,

NAICS, code 331111 Iron and Steel Mills, and other applicable codes not currently listed).

Send notification letters to all facilities nationwide under these codes to inform them of the

hazards of combustible metal dusts and NEP coverage.

6.2 INTERNATIONAL CODE COUNCIL

2011-4-I-TN-R4 

Revise IFC Chapter 2237 Combustible Dust Producing Operations; Section 2204.1 Standards, 

to require mandatory compliance and enforcement with the detailed requirements of the 

NFPA standards cited in the chapter, including NFPA 484.

6.3 TOSHA

2011-4-I-TN-R5 

Revise the state-adopted Dust National Emphasis Program (NEP) to add industry codes for

facilities that generate metal dusts (e.g., North American Industrial Classification System,

NAICS, code 331111 Iron and Steel Mills, and other applicable codes not currently listed).

Send notification letters to all facilities statewide under these codes to inform them of the

hazards of combustible metal dusts and NEP coverage.

6.4 HOEGANAES

2011-4-I-TN-R6 

Conduct periodic independent audits of the Hoeganaes Gallatin facility for compliance with 

the following NFPA standards, using knowledgeable experts, and implement all recommended corrective actions:

UÊÊ *Ê{n{]ÊStandard for Combustible Metals, Metal Powders, and Metal Dusts

UÊÊ *Ê{]ÊRecommended Practice for the Classification of Combustible Dusts and of 

Hazardous Locations for Electrical Installations in Chemical Process Areas

UÊÊ *Ê{Ç]ÊRecommended Practice for the Classification of Flammable Liquids, 

Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in 

Chemical Process Areas

UÊÊ *ÊÓ]ÊHydrogen Technologies Code

UÊÊ *ÊÓ££Î]ÊStandard on Selection, Care, Use, and Maintenance of Flame-Resistant 

Garments for Protection of Industrial Personnel Against Flash Fire

37Combustible Dust Producing Operations, Chapter 13 of the IFC, was moved to Chapter 22 in later editions.
CSB s Hoeganaes Corporation Case Study 27

Hoeganaes Corporation Case Study December 2011

2011-4-I-TN-R7 

Develop training materials that address combustible dust and plant-specific metal dust hazards 

and train all employees and contractors. Require periodic (e.g., annual) refresher training for all 

employees and contractors. 

2011-4-I-TN-R8 

Implement a preventive maintenance program and leak detection and leak mitigation 

procedures for all flammable gas piping and gas processing equipment.

2011-4-I-TN-R9 

Develop and implement a near-miss reporting and investigation policy that includes the 

following at a minimum:

UÊ Ensure facility-wide worker participation in reporting all near-miss events and operational disruptions (such as significant iron powder accumulations, smoldering fires, or 

unsafe conditions or practices) that could result in worker injury.

UÊÊ
ÃÕÀiÊÌ>ÌÊÌiÊi>ÀÃÃÊÀi«ÀÌ}Ê«À}À>ÊÀiμÕÀiÃÊ«À«ÌÊÛiÃÌ}>ÌÃ]Ê>ÃÊ>««À«À-

ate, and that results are promptly circulated throughout the Hoeganaes Corporation.

UÊÊ
ÃÌ>LÃÊÀiÃÊ>`ÊÀiÃ«ÃLÌiÃÊvÀÊÌiÊ>>}iiÌ]ÊiÝiVÕÌ]Ê>`ÊÀiÃÕÌÊvÊ>Ê

recommendations from near-miss investigations

UÊÊ
ÃÕÀiÊÌiÊi>ÀÃÃÊ«À}À>ÊÃÊ«iÀ>Ì>Ê>ÌÊ>ÊÌiÃÊ­i°}°Ê}ÌÃ]ÊÜiii`Ã]Ê

holiday shifts).

6.5 METAL POWDER PRODUCERS ASSOCIATION (MPPA)

2011-4-I-TN-R10 

Communicate the findings of this report to all your members, e.g. through a safety article in 

an upcoming monthly newsletter. 

6.6 CITY OF GALLATIN, TN

2011-4-I-TN-R11 

Require all facilities covered by IFC Chapter 13 (2006 edition) to conform to NFPA standards 

for combustible dusts including NFPA 484.

6.7 GALLATIN FIRE DEPARTMENT

2011-4-I-TN-R12 

Ensure that all industrial facilities in the City of Gallatin are inspected periodically against the 

International Fire Code. All facility inspections shall be documented.

2011-4-I-TN-R13 

Implement a program to ensure that fire inspectors and response personnel are trained to 

recognize and address combustible dust hazards.
CSB s Hoeganaes Corporation Case Study 28

Hoeganaes Corporation Case Study December 2011

APPENDIX A: DETERMINATION OF IRON POWDER

EXPLOSIBILITY FROM PRESSURE RATIO CALCULATIONS

BACKGROUND

The CSB commissioned testing of four Hoeganaes iron dust samples in a 20-L Test Chamber 

(ASTM Standard E1226-10, Standard Test Method for Explosibility of Dust Clouds). The 

test method states that dust explosibility can also be characterized through the calculation of 

a pressure ratio (PR). The pressure ratio calculation is also known as the explosibility screening test (Section 13). The 20-L test chamber records the maximum explosion pressure reached 

during a single deflagration test at a dust concentration of 1,000 and 2,000 g/m3 . 

If PR > 2, the dust is considered explosible

If PR < 2, it is classified as “not explosible” under those test conditions and the standard 

goes on to caution that it is not necessarily “not combustible” and still may be capable of 

deflagrative combustion.

CALCULATIONS

The CSB calculated the pressure ratio based on the 20-L test results from the baghouse dust.

According to E1226-10:

Pressure ratio = PR = (Pex,a − 6Pignitor) / Pignition

Where:

Pex,a = maximum explosion pressure (bar absolute)

6Pignitor = maximum pressure rise in chamber due to igniter = 2.5 (bar absolute)

Pignition = absolute pressure at the time of ignition = 0 bar gauge = 1.0123 (bar absolute)

The test value Pmax,a already corrects for the igniter pressure:

Pmax,a = (Pex,a − 6Pignitor)

Pmax,a values obtained from 20L testing of Hoeganaes dust:

Convert bar gauge to bar absolute:

0 bar gauge = 1.0125 bar absolute

3.1 bar gauge + 1.01325 = 4.1 bar absolute

3.8 bar gauge + 1.0125 = 4.8 bar absolute

Pmax, a DUST CONCENTRATION

3.1 bar gauge 1,000 g/m3

3.8 bar gauge 2,000 g/m3
CSB s Hoeganaes Corporation Case Study 29

Hoeganaes Corporation Case Study December 2011

The equation reduces to:

PR = Pmax,a / Pignition

Thus:

PR1,000 = 4.1/1.01325

PR1,000 = 4.0

PR2,000 = 4.8/1.01325

PR2,000 = 4.7

The pressure ratios at 1,000 and 2,000 g/m3

 are both greater than 2 and the iron dust sample 

is considered explosible based on ASTM E1226.
CSB s Hoeganaes Corporation Case Study 30

Hoeganaes Corporation Case Study December 2011

APPENDIX B: DETERMINATION OF IRON POWDER

CLASSIFICATION FROM EXPLOSION SEVERITY

CALCULATION

BACKGROUND

The CSB commissioned testing of four Hoeganaes iron dust samples in a 20-L Test 

Chamber (ASTM Standard E1226-10, Standard Test Method for Explosibility of 

Dust Clouds). OSHA cites the National Materials Advisory Board (NMAB) 353-3-80, 

Classification of Combustible Dusts in Accordance with the National Electric Code, for 

the determination of a Class II combustible dust location. The NMAB 353-3-80 states that 

Class II dusts can be characterized through the calculation of an explosion severity (ES). 

According to the NMAB 353-3-80: 

If ES > 0.5, the dust is considered an appreciable explosion hazard that requires suitable 

electrical equipment for Class II locations.

CALCULATIONS

The CSB calculated the pressure ratio based on the 20-L test results from the baghouse dust. 

According to NMAB 353-3-80: 

Explosion Severity = ES = 

Where:

Pmax = maximum explosion pressure, (bar gauge)

 = maximum rate of pressure rise, (bar gauge per second)

Reference dust = Pittsburgh coal dust

Pmax values obtained from 20L testing of Hoeganaes dust:

DUST SAMPLE Pmax ,P/,t

max

Hoeganaes Baghouse 3.5 bar gauge 68 bar gauge per second

Pittsburgh Coal 7.3 bar gauge 426 bar gauge per second

Pmax (sample) x

Pmax (reference dust) x

t܎

max (sample)

t܎

max (reference dust)

P܎

P܎

t܎

max (reference dust)

P܎
CSB s Hoeganaes Corporation Case Study 31

Hoeganaes Corporation Case Study December 2011

Thus:

The explosion severity of the baghouse dust is less than 0.5 and the iron dust sample is not 

considered a Class II combustible dust.

238

3,110

ES = 

ES = 0.077

(3.5 bar gauge) x (68 ) s

bar gauge

ES = 

(7.3 bar gauge) x (426 )

s

bar gauge