Mobile Acetylene Trailer Accidents:  

Fire During Unloading in Dallas, Texas, July 25, 2007;  

Fire During Unloading in The Woodlands, Texas  

August 7, 2007; and  

Overturn and Fire in East New Orleans, Louisiana,  

October 20, 2007

Special Investigation Report 

NTSB/SIR-09/01 

PB2009-917002 National

Transportation 

Safety Board 

NTSB/SIR-09/01 

PB2009-917002 

Notation 8071 

Adopted January 13, 2009 

Special Investigation Report 

Mobile Acetylene Trailer Accidents: 

Fire During Unloading in Dallas, Texas, July 25, 2007; 

Fire During Unloading in The Woodlands, Texas, 

August 7, 2007; and 

Overturn and Fire in East New Orleans, Louisiana, 

October 20, 2007 

National 

Transportation 

Safety Board 

490 L’Enfant Plaza, S.W. 

Washington, D.C. 20594
National Transportation Safety Board. 2009. Mobile Acetylene Trailer Accidents: Fire During 

Unloading in Dallas, Texas, July 25, 2007; Fire During Unloading in The Woodlands, Texas, 

August 7, 2007; and Overturn and Fire in East New Orleans, Louisiana, October 20, 2007. Special 

Investigation Report NTSB/SIR-09/01. Washington, DC. 

Abstract: The National Transportation Safety Board investigated three accidents that involved highway 

vehicles transporting bulk quantities of acetylene gas that occurred in 2007 and reviewed reports of a 

2008 overturn accident of another vehicle. The vehicles, called mobile acetylene trailers, carried up to 225 

cylinders that were connected by a manifold system and filled with acetylene. Two of the accidents 

occurred as the vehicles overturned on public highways, and two of the accidents occurred while the 

vehicles were being prepared for unloading. In the two overturn accidents, cylinders were ejected from 

the trailers and damaged, releasing acetylene, which ignited. In one unloading accident, the fire on the 

initial trailer spread to cylinders on an adjacent trailer; in the other, the fire also spread to nearby 

buildings and vehicles. The failures of the cylinders on these trailers and the resultant damage raised 

concerns about the accident protection provided by these vehicles, the adequacy of the minimum safety 

standards and procedures applicable to unloading these vehicles, and the adequacy of fire suppression 

systems at loading and unloading facilities. 

The safety issues discussed in this report are adequacy of mobile acetylene trailer design for protecting 

cylinders during transport, effectiveness and safety of unloading procedures for mobile acetylene trailers, 

and adequacy of fire suppression systems at mobile acetylene trailer loading and unloading facilities. 

As a result of this special investigation, the Safety Board makes safety recommendations to the Pipeline 

and Hazardous Materials Safety Administration and the Compressed Gas Association. 

The National Transportation Safety Board is an independent Federal agency dedicated to promoting aviation, 

railroad, highway, marine, pipeline, and hazardous materials safety. Established in 1967, the agency is mandated by 

Congress through the Independent Safety Board Act of 1974 to investigate transportation accidents, determine the 

probable causes of the accidents, issue safety recommendations, study transportation safety issues, and evaluate the 

safety effectiveness of government agencies involved in transportation. The Safety Board makes public its actions 

and decisions through accident reports, safety studies, special investigation reports, safety recommendations, and 

statistical reviews. 

Recent publications are available in their entirety on the Internet at <http://www.ntsb.gov>. Other information about 

available publications also may be obtained from the website or by contacting: 

National Transportation Safety Board 

Records Management Division, CIO-40 

490 L’Enfant Plaza, SW 

Washington, DC 20594 

(800) 877-6799 or (202) 314-6551

Safety Board publications may be purchased, by individual copy or by subscription, from the National Technical 

Information Service. To purchase this publication, order report number PB2009-917002 from: 

National Technical Information Service 

5285 Port Royal Road 

Springfield, Virginia 22161 

(800) 553-6847 or (703) 605-6000 

The Independent Safety Board Act, as codified at 49 U.S.C. Section 1154(b), precludes the admission into evidence 

or use of Board reports related to an incident or accident in a civil action for damages resulting from a matter 

mentioned in the report. 
Special Investigation Report 

Contents 

Figures............................................................................................................................................ ii

Acronyms and Abbreviations..................................................................................................... iii

Executive Summary.......................................................................................................................1

Acetylene Gas.................................................................................................................................3

Transporting Acetylene....................................................................................................................4 

Western’s Acetylene Unloading Procedures....................................................................................6 

Accident Narratives.......................................................................................................................8

Overturn Accidents..........................................................................................................................8 

East New Orleans, Louisiana ...................................................................................................8 

Lamar, Colorado ....................................................................................................................10 

Unloading Preparation Accidents..................................................................................................11 

The Woodlands, Texas............................................................................................................11 

Dallas, Texas ..........................................................................................................................13 

Earlier Western Accidents .............................................................................................................17 

Other Information .......................................................................................................................19

Regulations and Standards.............................................................................................................19 

Federal Regulations................................................................................................................19 

Industry Standards..................................................................................................................20 

Postaccident Activities...................................................................................................................21 

Analysis.........................................................................................................................................22

Overturn Accidents........................................................................................................................22 

Unloading Accidents......................................................................................................................23 

The Woodlands, Texas............................................................................................................23 

Dallas, Texas ..........................................................................................................................24 

Fire Suppression at Unloading Sites..............................................................................................26 

Conclusions...................................................................................................................................27

Findings..........................................................................................................................................27 

Recommendations........................................................................................................................28

Board Member Statement...........................................................................................................29

Appendix A...................................................................................................................................30

Investigation...................................................................................................................................30 

National Transportation Safety Board i
Special Investigation Report 

Figures 

Figure 1. The Woodlands trailer (a small mobile acetylene trailer). ............................................. 5 

Figure 2. The Woodlands cylinders on front right of accident trailer............................................ 6 

Figure 3. Western unloading procedures. (Pressurizing manifold in step 2 is for leak test. 

Opening cylinder valves in step 12 is pressurizing manifold for unloading.) ................................ 7 

Figure 4. East New Orleans accident vehicle and cylinders.......................................................... 9 

Figure 5. East New Orleans cylinder showing burst near head. .................................................... 9 

Figure 6. Lamar accident vehicle................................................................................................. 10 

Figure 7. Security camera photograph of acetylene gas fire in The Woodlands, Texas.............. 11 

Figure 8. Security camera photograph of black plume coming from a cylinder on trailer in 

The Woodlands, Texas.................................................................................................................. 12 

Figure 9. Acetylene gas fire at Southwest facility in Dallas........................................................ 13 

Figure 10. Dallas accident damage to Southwest facility and vehicles. (Accident trailer and 

cylinders in foreground.)............................................................................................................... 14 

Figure 11. Final phase of fire suppression at Southwest facility in Dallas.................................. 16 

Figure 12. Dallas cylinder damage. ............................................................................................. 16 

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Acronyms and Abbreviations 

CFR Code of Federal Regulations

CGA Compressed Gas Association 

DOT U.S. Department of Transportation 

F Fahrenheit 

NFPA National Fire Prevention Association 

PHMSA Pipeline and Hazardous Materials Safety Administration 

psig pounds per square inch, gauge 

Southwest Southwest Industrial Gases 

Western Western International Gas & Cylinders, Inc. 

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National Transportation Safety Board iv
Special Investigation Report 

Executive Summary 

The National Transportation Safety Board investigated three accidents that involved 

highway vehicles transporting bulk quantities of acetylene gas that occurred between July 25 and 

October 20, 2007, and reviewed reports of a 2008 overturn accident of another vehicle. The 

vehicles, called mobile acetylene trailers, carried up to 225 cylinders that were connected by a 

manifold system1 and filled with acetylene. Two of the accidents occurred as the vehicles 

overturned on public highways, in East New Orleans, Louisiana, on October 20, 2007, and in 

Lamar, Colorado, on June 9, 2008. Two of the accidents occurred while the vehicles were being 

prepared for unloading, in Dallas, Texas, on July 25, 2007, and in The Woodlands, Texas, on 

August 7, 2007. In the two overturn accidents, cylinders were ejected from the trailers and 

damaged, releasing acetylene, which ignited. In one unloading accident, the fire on the initial 

trailer spread to cylinders on an adjacent trailer; in the other, the fire spread to cylinders on 

adjacent trailers and to nearby buildings and vehicles. The failures of the cylinders on these 

mobile acetylene trailers and the resultant damage raised concerns about the accident protection 

provided by these vehicles, the adequacy of the minimum safety standards and procedures 

applicable to unloading these vehicles, and the adequacy of fire suppression systems at loading 

and unloading facilities. To address these concerns, the Safety Board conducted a special 

investigation of mobile acetylene trailers. 

These trailers, with multiple cylinders and a manifold system for loading and unloading 

the cylinders, are the only vehicles authorized to transport and deliver bulk quantities of 

acetylene. Currently, two companies in the United States operate mobile acetylene trailers. Each 

company has about 200 trailers, thus the total number of these trailers is about 400. 

The trailers in the four accidents that the Safety Board investigated and reviewed were 

operated by Western International Gas & Cylinders, Inc. (Western). In the 10 years before these 

four accidents, Western mobile acetylene trailers had been involved in additional accidents; the 

company investigated four of them. The Safety Board reviewed reports of three of these 

accidents that had some relevance to the accidents in this investigation. 

In its investigation, the Safety Board attempted to determine the hazards of the 

transportation and delivery of acetylene gas in bulk. The investigation looked at the protection of 

the cylinders during a vehicle overturn and the standards and procedures for handling acetylene 

during loading and unloading. The following safety issues were identified in this special 

investigation: 

• Adequacy of mobile acetylene trailer design for protecting cylinders during transport 

• Effectiveness and safety of unloading procedures for mobile acetylene trailers 

• Adequacy of fire suppression systems at mobile acetylene trailer loading and 

unloading facilities. 

 1

 A manifold system collects the acetylene gas from multiple cylinders into one pipe or chamber; when the 

cylinders are filled, the acetylene gas is dispersed from one pipe to multiple cylinders. 

National Transportation Safety Board 1
Special Investigation Report 

As a result of this special investigation, the Safety Board makes recommendations to the 

Pipeline and Hazardous Materials Safety Administration and the Compressed Gas Association. 

National Transportation Safety Board 2
Special Investigation Report 

Acetylene Gas 

Acetylene is a colorless flammable gas that has historically been handled relatively safely 

at pressures below atmospheric pressure (about 15 pounds per square inch, gauge2

 [psig]). 

However, at elevated pressures acetylene becomes extremely unstable.3 With the addition of 

energy, acetylene undergoes a decomposition reaction in which it breaks down to flammable 

hydrogen gas and elemental carbon. Carbon generated during this reaction is deposited on piping 

and other components. The decomposition reaction releases tremendous amounts of energy and 

may generate temperatures between 5072° and 5252° Fahrenheit (F). A black plume forms when 

the carbon and hydrogen are expelled from the cylinder. External heating, compression heating, 

an electrostatic spark, or a shockwave can also cause an acetylene decomposition reaction. With 

substantial energy input, a decomposition reaction can occur within a cylinder or pressurized 

system in the absence of air. When combined with air, pressurized acetylene will readily ignite 

with very little energy input and a lower temperature. Such a reaction can result in deflagration or 

detonation.4 

Western has a material safety data sheet for acetylene that states that the flash point5 for 

acetylene is 32° F, acetylene has flammable limits6 between 2.5 percent and 82 percent in air, 

and it is lighter than air. Acetylene also may decompose explosively at elevated temperatures and 

pressures. The potential for a decomposition reaction to occur is proportional to the pressure; the 

higher the pressure, the more likely it is for a reaction to occur.

According to a paper by S. A. Miller and E. Penny,7 the progression from deflagration to 

detonation is dependent on the original pressure and the diameter and length of the pipe. Testing 

showed that an acetylene deflagration in a 1-inch-diameter pipe originally pressurized to 73.5 

psig takes 12 feet to reach detonation. However, when the original acetylene pressure is 294 psig, 

detonation occurs between 2.8 and 3.2 feet. Once detonation occurs, the reaction front travels 

through open piping at a speed greater than the speed of sound. The acetylene in the cylinders on 

the vehicles involved in the accidents in East New Orleans, The Woodlands, and Dallas was 

 2

 The term psig is a unit of pressure relative to the surrounding atmosphere (as in a container). 

3

 Acetylene (C2H2) has a triple bond between the two carbon atoms in its molecular structure. The triple bond is 

highly unstable. Ullmann’s Encyclopedia of Industrial Chemistry, Electronic Release, 7th ed., Wiley-VCH, Weinheim, 

Germany, 2001. 

4 Deflagration is a very rapid exothermic chemical reaction that is propagated through the unreacted material at 

a velocity less than the speed of sound. The propagation is achieved by the spread of flame and hot gases into the 

unreacted material. Detonation is an explosive process in which the extremely rapid chemical reaction is propagated 

through the unreacted material at supersonic velocity by means of a shockwave. The shockwave is associated with a 

compressive zone of very high temperature passing through the unreacted material. 

5

 The flash point of a chemical is the lowest temperature at which a liquid can form an ignitable mixture in air 

near the surface of the liquid. The lower the flash point, the easier it is to ignite the material. 

6

 The flammable limits of a chemical define the concentration range in which a flammable substance can 

produce a fire or explosion when an ignition source (such as a spark or open flame) is present. The concentration is 

generally expressed as percent fuel by volume. 

7

 S. A. Miller and E. Penny, “Hazards in Handling Acetylene in Chemical Processes Particularly Under 

Pressure.” Symposium on Chemical Process Hazards, INSTN Chemical Engineers, 1960. 

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Special Investigation Report 

pressurized to about 250 psig.Residue pressure in the cylinders involved in the Lamar, Colorado, 

accident was less than 15 psig. 

According to Samuel A. Miller’s book, Acetylene: Its Properties, Manufacture and 

Uses,8 testing of acetylene explosions caused by shockwaves in piping showed that for certain 

pipe lengths, pipe diameters, and acetylene pressures, ignition occurred after the shock wave was 

reflected from the closed end of a tube. In postaccident discussions, a representative of Praxair, 

Inc.,9

 stated that testing performed by Union Carbide had revealed that a decomposition reaction 

could occur with the rapid introduction of high-pressure acetylene into a blocked 1-inch-diameter 

pipe as the shockwave was reflected. Union Carbide found that this pipe must be at least 1 1/4 

inch long for the reaction to occur. 

Transporting Acetylene 

Given the extreme reactivity of pressurized acetylene, no bulk or large pressure vessel 

has been developed that is considered safe for its transportation. Therefore, bulk acetylene is 

transported on mobile acetylene trailers, of which one often contains more than 200 U.S. 

Department of Transportation (DOT) specification 8AL cylinders that are manifolded, that is, 

linked together by a manifold system used for loading and unloading. (These cylinders are 

approximately 4 feet tall and 1 foot in diameter.) The trailers were designed so that the cylinders 

are mounted upright, set close together, and braced in groups to control their movement during 

transportation and to prevent them from falling over. (See figure 1.) The cylinders on these 

vehicles are not secured to the trailer to prevent their ejection during overturn, nor are they 

required to be. 

Title 49 Code of Federal Regulations (CFR) 178.60 contains the DOT specifications for 

the 8AL cylinders that are designed specifically to contain acetylene gas during transportation. 

The 8AL cylinders are filled with a porous mass and a liquid solvent. Although acetone typically 

is the solvent used in individual acetylene cylinders, dimethylformamide is the liquid solvent 

used in bulk transportation cylinders because it is less volatile and requires less maintenance. 

These features are designed, in part, to reduce the likelihood that the acetylene will undergo a 

decomposition reaction within the cylinder. Each cylinder has two fusible plugs in the top that 

are designed to melt in the event of an internal decomposition reaction or when exposed to 

external heat and thus relieve pressure in the container. Venting pressure through the fusible plug 

openings reduces the likelihood of a cylinder overpressure failure. However, exposing an 

acetylene cylinder to excessive heat or energy can overwhelm the ability of the fusible plugs to 

vent sufficient gas, and a cylinder failure can occur. 

 8

 Samuel A. Miller, Acetylene: Its Properties, Manufacture and Uses. Vol. 1. (New York: Academic Press, 

1965). 

9

 According to the Compressed Gas Association, Union Carbide Corporation originally designed and tested 

mobile acetylene trailers. In 1992, Union Carbide Corporation’s Industrial Gas Division formed an independent 

company that is now called Praxair, Inc. 

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Figure 1. The Woodlands trailer (a small mobile acetylene trailer). 

Figure 2 shows cylinders on a mobile acetylene trailer. The valve opening on each 

cylinder is connected by spiral tubing, called a pigtail, to the manifold that delivers or receives 

the acetylene gas to or from the cylinder. Attached to the manifold, on the back of the trailer, is a 

cargo transfer fitting, which is not visible in this photograph. This fitting is the connection 

between the trailer and the piping from the plant. The tubing and manifold piping are designed to 

contain the energy of an accidental decomposition reaction. A manual block valve (block valve) 

controls the flow of acetylene to and from the cargo transfer fitting. The piping includes an airactuated valve that shuts off the flow if a trailer pulls away accidentally before it is disconnected. 

A needle valve on the manifold is used to reduce pressure on the manifold after cargo transfer 

and leak testing. Pressure gauges are mounted on the trailer: one on the manifold and one on the 

trailer piping between the block valve and the connection to the plant piping. According to 

Western, filling the cylinders through the manifold connection takes about 12 hours. 

During transportation of the cylinders, each cylinder valve must be closed and the 

pressure in the manifold and pigtails is reduced. Because pressurized acetylene’s potential for 

ignition increases dramatically when it is combined with air, some positive acetylene pressure 

must be maintained in the manifold and pigtails at all times to prevent the introduction of air. 

Although the trailer has strong outer railings and the manifold is mounted above the top 

of the cylinders, neither the railings nor the manifold protects the cylinder valves in case of a 

trailer overturn. The four accident mobile acetylene trailers had no rollover protection for the 

valves, nor were they required to. 

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Special Investigation Report 

Pigtail 

Manifold

Fusible plug

Figure 2. The Woodlands cylinders on front right of accident trailer. 

Western’s Acetylene Unloading Procedures 

Western had standard operating procedures for connecting mobile acetylene trailers to 

piping at customer sites. The procedures that were used at the sites of the two unloading accidents—

the Hughes Christensen (The Woodlands) and Southwest (Dallas) unloading sites—are shown in 

figure 3. 

Operating a mobile acetylene trailer encompasses two jobs: driving the trailer and 

performing the tasks involved in loading and unloading the acetylene cylinders carried on the 

trailer. Although one person can both drive the trailer and load/unload the trailer, mobile 

acetylene trailers are often operated by a team of two people, called “driver/operators,” with one 

driving the trailer and one or both performing the tasks of loading/unloading. In this report, the 

driver/operators will be identified as “operators.” Each operator is required to carry a copy of the 

standard operating procedures and to follow the procedures step by step when preparing a trailer 

for unloading. 

Western’s standard operating procedures require that after connecting the trailer to the 

plant piping (step 9), the manual block valve on the trailer be opened (step 10) before the trailer 

manifold is fully pressurized (step 12). Performing these operations out of sequence—opening 

the block valve after pressurizing the manifold allows highly pressurized acetylene to enter a 

blocked section of piping that contains residual air, which is dangerous because (1) a blocked 

pipe potentially can cause a reflected shockwave that can initiate a decomposition reaction and 

(2) pressurized acetylene that mixes with air inside piping can ignite easily with little energy 

input. Western’s procedure of slowly opening the block valve when the pressure in the manifold 

is at 50 psig (step 10) allows lower pressure acetylene to mix safely with air in that portion of the 

National Transportation Safety Board 6
Special Investigation Report 

piping. This minimizes the potential for an accidental decomposition reaction to occur within the 

piping by reducing the pressure shockwave and by slowly mixing the acetylene and residual air. 

3. Perform a leak check by soaping the entire trailer manifold, B-valves, and cylinders. Do

not try to fix any cylinder or B-valve neck leaks. Identify the leaks using the Trailer Leak

Report. Leaks will be repaired by certified personnel when the trailer is returned to a

Western repair facility. 

4. Open the needle valve on the manifold and vent to atmosphere until the pressure reaches

approximately 50 psi. Then, close the needle valve. Do not close or tighten the needle

valve excessively. Do not use a dummy B valve to vent manifold. 

5. Back the trailer into the empty bay. 

6. Attach the ground wire to the trailer. 

7. Unhook the tractor from the trailer. 

8. When parking a chemical trailer at any location, a glad hand lock must be attached to the

red airbrake glad hand. 

9. Ensure that the O-ring is in place and attach the discharge arm to the trailer and tighten

using a union wrench. DO NOT OPEN THE DISCHARGE ARM VALVE. 

10. Open the trailer manual block valve AND verify that the half union pressure gauge reading

matches the header pressure gauge reading. 

11. Slowly open the discharge arm valve, pause 5 seconds, and close the valve. Repeat this

cycle 3 times. Verify that the pressure gauge at the half union rises and falls. This allows

the gas to slowly pressurize the discharge arm and purge any air from the line. Leave the

valve in the closed position. 

12. Open all cylinder valves four wrist turns. Go over the trailer again and turn one more wrist

turn to verify that all cylinder valves are open. 

13. Verify that the trailer manual block valve AND the half union pressure gauge readings

match the header pressure gauge reading. Attach the airline to the trailer. 

14. Slowly open the discharge arm valve and leave it open at these sites: 

 Airgas Baker-Hughes (Hughes Christensen) 

15. Leave the discharge arm valve closed at these sites: 

 Southwest Industrial Gas 

2. Open two cylinders on the trailer and pressurize the manifold to 150 psi. Upon reaching

150 psi, close the cylinders. 

1. Stop at least 50 feet from any on-site trailer. 

Western International Gas & Cylinders Unloading Procedures 

Figure 3. Western unloading procedures. (Pressurizing manifold in step 2 is for leak test. 

Opening cylinder valves in step 12 is pressurizing manifold for unloading.) 

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Special Investigation Report 

Accident Narratives 

The accidents included in this special investigation are two trailer overturns that occurred 

in East New Orleans, Louisiana, on October 20, 2007, and in Lamar, Colorado, on June 9, 2008, 

and two accidents that occurred before the acetylene was unloaded, in Dallas, Texas, on 

July 25, 2007, and in The Woodlands, Texas, on August 7, 2007. Also included in this report are 

three previous accidents (in 2001 and 2005) involving Western mobile acetylene trailers at 

loading/unloading sites in which cylinders were damaged, releasing acetylene, which ignited, or 

the acetylene decomposed, resulting in fire. 

Overturn Accidents 

East New Orleans, Louisiana 

About 5:00 a.m. central daylight time10 on October 20, 2007, an eastbound Western 

tractor and mobile acetylene trailer carrying 220 manifolded DOT 8AL specification cylinders 

(450 standard cubic feet11 capacity each) filled with acetylene gas overturned on Interstate 

Highway 10 in East New Orleans, Louisiana. As a result of the overturn, the cylinders were 

thrown from the trailer, and acetylene gas from 145 of the 220 cylinders was released and ignited. 

The overturn and resulting fire destroyed the tractor, the mobile acetylene trailer, and many of 

the cylinders. (See figure 4.) Six other vehicles were damaged when they struck or were struck 

by the accident debris. There were no fatalities; the operator sustained minor injuries from the 

accident. Property damage to the vehicle totaled $200,000. 

Just after midnight on October 20, the operator had picked up a load of acetylene from 

the BASF Corporation plant in Geismar, Louisiana, and was to take the load to the Western 

facility in Slidell, Louisiana. He had been off duty since 3:00 p.m. October 17. The capacity of 

all the cylinders was about 100,000 standard cubic feet of acetylene. The operator told the police 

that he had fallen asleep while driving and hit a bridge rail. 

Of the 145 cylinders that released acetylene, 88 were so severely damaged that they were 

discarded. Of the remaining 57 cylinders that released acetylene, 32 had their valves broken off, and 

all 57 had evidence of heat exposure and melted fusible plugs. Significant damage to these cylinders 

included burst heads, a bulging and burst sidewall near the top head weld, extreme localized bulging 

of the heads and sidewalls, and denting. (See figure 5.) 

 10 Except where otherwise noted, all times in this report are central daylight time. 

11 A standard cubic foot is the volume of a gas at a pressure of 14.7 psi (atmospheric pressure) and 60° F. 

National Transportation Safety Board 8
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 Figure 4. East New Orleans accident vehicle and cylinders. 

 Figure 5. East New Orleans cylinder showing burst near head. 

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Special Investigation Report 

Lamar, Colorado 

About 12:15 p.m. mountain daylight time on June 9, 2008, a southbound Western tractor 

and mobile acetylene trailer carrying 225 DOT 8AL specification cylinders (450 standard cubic 

feet capacity each) containing acetylene gas residue overturned on Highway 287 about 6.5 miles 

south of Lamar, Colorado.12 As a result of the overturn, about half of the cylinders were thrown 

from the trailer, and acetylene gas from 86 of the 225 cylinders was released and ignited. (See 

figure 6.) The overturn damaged the tractor and the trailer. There was some fire damage on the 

trailer. There were no injuries from the accident. Property damage to the tractor and trailer 

totaled about $200,000. 

Figure 6. Lamar accident vehicle. 

The trailer had been unloaded at a Western facility in Pueblo, Colorado, and was en route 

to Louisiana to be refilled. There was still a residue of acetylene in each cylinder. The valves of 

all the cylinders were closed during transportation. Before this trip, the operator had been off 

duty since June 6, 2008. The Colorado accident report indicated that the vehicle skidded for 106 

feet, went off the right side of the roadway, and traveled an additional 250 feet before striking a 

delineator pole. The vehicle was then steered to the left onto the roadway, where it rolled over 

completely. The vehicle came to rest perpendicular to and completely across the two-lane road. 

At least half of the cylinders were thrown from the vehicle, and the remainder fell over. The 

valves were broken off of 52 cylinders, and acetylene was released and ignited. The fire caused 

 12 The Safety Board did not investigate this accident. 

National Transportation Safety Board 10
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fusible plugs on other cylinders to melt, which released acetylene that also ignited. About 86 

cylinders were damaged by the fire. The tractor and trailer had extensive collision damage; the 

trailer had some fire damage. 

Unloading Preparation Accidents 

The Woodlands, Texas 

About 2:55 p.m. on August 7, 2007, acetylene gas ignited on a Western mobile acetylene 

trailer containing 62 manifolded DOT 8AL specification cylinders (420 standard cubic feet 

capacity each) resulting in a fire on that trailer and an adjacent trailer at the Hughes Christensen 

plant in The Woodlands, Texas. (See figure 7.) The fire occurred as the operators were preparing 

the trailer to be unloaded. There were no injuries or fatalities. Property damage to the two trailers 

totaled $40,200. 

Figure 7. Security camera photograph of acetylene gas fire in The Woodlands, Texas. 

The trailer had been filled and shipped from the Western plant in Bellville, Texas. The 

total capacity of the cylinders on the trailer was about 26,040 standard cubic feet of acetylene. A 

trainee operator was to make his first delivery of a mobile acetylene trailer. An operator trainer 

accompanied the trainee. The operator trainer had made a few trips in which connections were made 

to facility piping; however, he usually delivered truckloads of filled and palletized cylinders to 

customer sites. He had made two earlier trips to The Woodlands, the latest in June 2007. The team 

said that they had followed the written standard operating procedures when they prepared the trailer 

and connected it at the Hughes Christensen site. Tape from a security camera showed a person on 

the trailer moving back and forth along the length; the person appeared to be opening the 

cylinder valves and pressurizing the manifold. When the accident occurred, the two operators 

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were standing on the ground at the back of the trailer near the trailer’s block valve and 

attachment to the plant’s discharge arm, but the focus of the camera was not adequate to show 

what they were doing. During postaccident interviews, both operators said that they had believed 

that they had completed the connection procedures and had pressurized the manifold. They told 

interviewers that after they had reviewed Western’s standard operating procedures manual, 

however, they realized that they had not opened the block valve before pressurizing the manifold 

as Western’s procedures required. Just after the accident, the trainee operator told a Hughes 

Christensen employee that the trailer had jumped as they opened the block valve. Shortly after 

that, the security camera recorded a black plume coming from one of the cylinders on the trailer. 

(See figure 8.) A similar reaction occurred on other cylinders on the trailer until the released 

gases ignited. Heat from the fire on the accident trailer caused the release of unreacted acetylene 

gas on an adjacent trailer. 

Figure 8. Security camera photograph of black plume coming from a cylinder on trailer in 

The Woodlands, Texas. 

The postaccident examination of the trailer being prepared for unloading revealed that the 

trailer and manifold piping had some fire damage at and above the level of the top of the 

acetylene cylinders. There was little damage to the trailer below that level. The cylinders’ fusible 

plugs were all melted, with some evidence of fusible material remaining on the top of some of 

the cylinders. At least three cylinders in one cluster had visible bulging on their sidewalls just 

below the circumferential head weld. The cylinder valves appeared to be intact; however, the 

hand wheels had been consumed in the fire. The pigtails also had evidence of heat exposure but 

appeared to be intact. The connection fitting from the trailer to the plant piping had a large deposit 

of black powderlike material inside. A large deposit of the same black material was also in the plant 

discharge arm at the Hughes Christensen plant. There was no deposit of a similar material in the 

discharge arm for the second trailer. 

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The only emergency response fire fighting equipment available at the Hughes 

Christensen acetylene unloading location was fire extinguishers. There was no automated water 

deluge system to suppress a fire. 

Dallas, Texas 

About 9:50 a.m. on July 25, 2007, acetylene gas ignited on a Western mobile acetylene 

trailer containing 225 manifolded DOT 8AL specification cylinders (450 standard cubic feet 

capacity each) on the docks at the Southwest Industrial Gases (Southwest) facility in Dallas, 

Texas. (See figure 9.) The fire occurred as the operator was preparing the trailer to be unloaded. 

A video recording made by a Dallas television network did not record the initiation of this 

accident; however, it showed a gradual progression of the fire and explosions from the trailer on 

which the operator was working to adjacent trailers, to Southwest’s building, and to vehicles 

parked in front, one of which contained acetylene-filled cylinders for delivery. The fire and 

explosions destroyed the Southwest facility and many vehicles, damaged several nearby 

buildings, and caused the failure of hundreds of acetylene-filled cylinders from three other 

Western acetylene trailers,13 the delivery truck, and the facility’s docks and storage. (See 

figure 10.) The operator sustained back injuries, and the plant manager and general manager 

were severely burned during the accident. There were no fatalities. Property damages have not 

been fully assessed, but Southwest estimated losses of more than $5 million. 

 Figure 9. Acetylene gas fire at Southwest facility in Dallas. 

 13 The three other trailers were attached to the plant piping and in use at the time of the accident. 

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Figure 10. Dallas accident damage to Southwest facility and vehicles. (Accident trailer and 

cylinders in foreground.) 

The accident trailer was filled and shipped from the BASF Corporation plant in Geismar, 

Louisiana. The capacity of the cylinders was approximately 100,000 standard cubic feet of 

acetylene. The operator typically worked in a two-person team that drove, loaded, and unloaded 

mobile acetylene trailers. The accident occurred during the operator’s third delivery to 

Southwest, which was his first solo delivery to that location. He had made five solo deliveries to 

other facilities. The operator said that he had followed Western’s standard operating procedures 

when he prepared the trailer for unloading at the Southwest facility. He said that after cycling the 

facility’s discharge arm valve, leaving it closed (step 11 of Western’s unloading procedures), he 

checked the pressure gauges on the manifold and both read 0 psig, which he did not identify as a 

problem. Step 4 of Western’s unloading procedures requires that 50 psig of acetylene pressure 

remain in the manifold, and discussions with Western management revealed that at the 

completion of step 11 there should still have been slightly less than 50 psig pressure in the 

manifold. 

The operator said that he had climbed on the bed of the trailer and had begun opening the 

cylinder valves, beginning with the cylinders at the rear of the trailer on the right side of the 

vehicle, when he heard a “boom” and felt a pressure surge that threw him back against the 

support rail for the cylinders on the side of the trailer. He said that he saw black smoke and 

National Transportation Safety Board 14
Special Investigation Report 

flames coming from the top of a cylinder toward the front of the trailer. He then ran from the 

trailer down the dock and around the building. 

A Southwest employee on the facility dock about 8 or 9 feet away from the back of the 

accident trailer said that about 5 minutes after he saw the operator climb on the trailer he saw 

black smoke over a 5-foot-high partial wall. He stated that he saw fire coming from one of the 

two rear cylinders on the right side of the trailer. He said he then heard a loud noise, not an 

explosion, which he described as pressure being released through an open valve. Then he 

grabbed a fire extinguisher, ran into the break room, and shouted, “Fire!” He ran out the door 

toward the trailer as the operator passed him going in the opposite direction. He was not able to 

use the fire extinguisher because, when he ran onto the dock, a cylinder exploded. He said that 

before he evacuated he saw the plant manager and general manager using a fire hose on the fire. 

The plant manager and the plant general manager heard a loud pressure release noise and 

ran to the dock. The plant manager said that he saw a gray haze over the trailer, indicating a gas 

leak. He could not tell where the leak was coming from. The general manager said he saw “a 

mirage of gas, like you see on a hot day on the highway mixed with black eyelashes” over the 

accident trailer. He noted that the gas was coming from the back of that trailer. 

Both managers then helped operate the fire hose, and both saw cylinder explosions, were 

knocked down, and received second and/or third degree burns while they were operating the 

hose. Another employee turned on a permanently mounted water monitor and began spraying 

water onto the trailer. However, because supplying water to the monitor reduced the water 

pressure in the fire hose, the water monitor was shut off. The hose was not effective in 

suppressing the fire, which was extinguished about 9 hours later by the Dallas Fire Department. 

(See figure 11.) The only fire-fighting equipment available at the Dallas plant included fire 

extinguishers, one fire hose, and the manually operated water monitor on the roof. There was no 

automated water deluge system. 

Postaccident examination of the accident site revealed that the discharge arm to the 

accident trailer had broken from the plant’s piping and the trailer connection. The discharge arm 

valve was found in the closed position. No acetylene cylinders were found on the accident trailer. 

The three other Western mobile acetylene trailers that were attached to Southwest’s unloading 

apparatus were destroyed, although some cylinders remained on two of these trailers. More than 

1,000 cylinders from the trailers and from plant inventory were recovered from an area with a 

radius of about 1/4 mile. (See figure 12.) Although the cylinders from the Western trailers could 

be differentiated from Southwest’s cylinders, Western’s records did not identify the cylinders that 

were mounted on each trailer, so no detailed examinations were performed on the cylinders. A 

sampling of cylinders of the same size and construction as those from the trailers were visually 

examined. Damage ranged from a melted fusible plug to a longitudinal tear running the length of 

the cylinder. In some cases, the top or bottom cylinder head had torn off. 

National Transportation Safety Board 15
Special Investigation Report 

 Figure 11. Final phase of fire suppression at Southwest facility in Dallas. 

 Figure 12. Dallas cylinder damage. 

National Transportation Safety Board 16
Special Investigation Report 

An examination of postaccident photographs of Southwest’s equipment revealed 

significant problems with Southwest’s acetylene compressors. These problems included plugged 

or missing pressure safety relief devices, pressure safety relief devices with vent piping that did 

not exit the building, crankcase oil level devices that did not have sight glasses, and an 

automated emergency shutdown device with a shutdown indicator set to about 420 psig, or 20 

psig higher than allowed by the National Fire Prevention Association (NFPA) standards. Also, 

the flash arrestor tower14 between the trailer and the plant lacked about 3 feet of its maximum 

capacity of 13 feet of pall rings or Raschig rings,15 thus about 10 feet of the rings remained in the 

flash arrestor. The Western representatives were concerned that a flame front from a failure 

within the plant may have passed through the flash arrestor and involved the trailers. 

Earlier Western Accidents 

Western investigated four earlier accidents that involved its mobile acetylene trailers in 

the last 10 years; three had some relevance to the four accidents discussed above. 

On April 10, 2001, a mobile acetylene trailer containing 227 manifolded cylinders caught 

fire at Western’s plant in Bellville, Texas. The fire destroyed a tractor and two other bulk 

acetylene trailers. A fourth trailer had minor damage resulting from a tire fire. There were no 

serious injuries. Western’s investigation determined that the fire initiated on the ball of a ball 

valve on the trailer’s manifold system. Western determined that stainless steel shavings had been 

left inside the manifold piping during the construction of the trailer. When high-pressure 

acetylene was introduced into the piping, these shavings were entrained in the flow and ignited 

the acetylene as they struck the ball of the valve. 

The damage resulting from this accident was increased by the force of the reaction, 

causing the failure of several of the pigtails on the manifold, which allowed the fire to escape the 

system. Flames from the broken pigtails touched the sides of adjacent cylinders, causing one or 

more of them to rupture. This was followed by the rupture and ejection of most of the remaining 

cylinders on the trailer. The investigation also determined that at least one cylinder valve was 

open and that the acetylene from this cylinder provided additional fuel to the reaction. The fire 

on the initial trailer caused the melting of fusible plugs on cylinders on two adjacent acetylene 

trailers. This released additional acetylene that also ignited. None of the cylinders on the other 

trailers failed catastrophically. 

As a result of this fire, plantwide water deluge systems were installed in many of 

Western’s facilities. Also, concrete block firewalls were constructed in the trailer gas area at 

Bellville. Valves were removed from the manifolds on all of Western’s trailers, and the system 

was flushed with high-pressure nitrogen to remove any shavings or other material. The ball 

 14 A flash arrestor tower is located on a mobile acetylene trailer between the manifold and the plant piping and 

is designed to stop a decomposition reaction both from occurring and from continuing through the piping if such a 

reaction was not stopped. 

15 Pall rings, or Raschig rings, are types of packing in the shape of a short piece of pipe. These are placed into a 

flash arrestor tower, and because they have a lot of surface area they absorb and dissipate heat from an acetylene 

reaction. 

National Transportation Safety Board 17
Special Investigation Report 

valves were retrofitted with flash arrestors to slow the actuation of the valve, and “Y” strainers 

were installed in the manifolds to prevent material in a manifold from reaching the valve. 

On June 21, 2001, at Dow Olefin’s Seadrift, Texas, plant, a detonation occurred in 

facility piping at a control valve while a mobile acetylene trailer was being prepared for loading. 

Both the trailer piping beyond the block valve on the trailer and the plant discharge arm beyond 

the discharge arm valve were open to the atmosphere before the trailer was connected. Therefore, 

after the trailer was attached to the plant piping, the piping contained residual air. The explosion 

self-extinguished, and the operator was not seriously injured. Western’s investigation of this 

accident determined that air had been trapped between two valves as the fill arm was connected 

to the trailer. Then, as high-pressure acetylene was introduced into the section of piping 

containing air, the heat of compression caused the acetylene to ignite. Contributing to the 

accident was a leakage in a facility control valve on the fill line that raised the internal pressure 

on that line. 

As a result of this accident, Western’s procedures were changed to require the pressure in 

the facility fill line to be reduced after disconnecting a trailer and to require the fill arm to be 

purged with nitrogen after it is connected to a trailer. Also, alarms were installed to detect 

leaking facility control valves. 

On June 8, 2005, a decomposition reaction occurred in the manifold system on a mobile 

acetylene trailer at Western’s Bellville plant that caused the fusible plugs of five cylinders to 

melt, releasing the products of decomposition. The materials released did not ignite before the 

deluge system was manually activated, controlling the incident. The incident started when a 

mobile acetylene trailer, with the cylinder valves open and the manifold fully pressurized, was 

moved into another bay and the block valve was opened, which initiated an acetylene 

decomposition reaction. 

Western’s investigation determined that the accident was caused by a failure to follow 

Western’s procedures that require that all cylinder valves be closed and the manifold pressure be 

reduced. The quick activation of the water deluge system and workers closing other cylinder 

valves on the trailer under the protection of the water deluge limited the consequences of the 

accident. After the accident, Western implemented measures to ensure that its standard operating 

procedures are followed for all trailers. 

National Transportation Safety Board 18
Special Investigation Report 

Other Information 

Regulations and Standards 

Federal Regulations 

Title 49 CFR Subchapter C, Hazardous Materials Regulations, regulates acetylene as a 

Division 2.1 flammable gas. “Acetylene, dissolved” is the proper shipping name for the material, 

and it is assigned United Nations identification number UN1001. Section 173.303 of the 

Hazardous Materials Regulations authorizes acetylene for transportation only in DOT 

specification 8 or 8AL cylinders or in United Nations cylinders conforming to ISO 3807-2. 

Transportation of acetylene in bulk packagings, such as cargo tanks, portable tanks, or rail tank 

cars, is prohibited. 

Section 173.301(g) states that cylinders containing acetylene and other regulated gases 

can be manifolded during transportation. However, the manifolded cylinders must conform to the 

following conditions: (1) manifolded branch lines must be sufficiently flexible to prevent 

damage to the valves; (2) the cylinders must be supported and held together as a unit by 

structurally adequate means; (3) each cylinder must be equipped with an individual shutoff valve 

that is tightly closed in transit and an individual pressure relief device that discharges upward; 

and (4) the valves and pressure relief devices must be protected from damage by framing, a 

cabinet, or other method. 

Section 177.834(h) states that except for cargo tanks and portable tanks, the discharge or 

emptying of a package’s contents before the package is removed from the motor vehicle is 

prohibited. However, as a direct result of the Dallas and The Woodlands accidents, the Pipeline 

and Hazardous Materials Safety Administration (PHMSA) issued a safety advisory in September 

2007 that stated that this prohibition does not apply in all circumstances. For example, PHMSA 

said that it has interpreted the prohibition in section 177.834(h) as not applying to tube trailers, 

which are DOT specification 3AX, 3AAX, and 3T cylinders securely mounted (horizontally) to a 

transport vehicle, because removing them from the motor vehicle before discharging their 

contents is not practicable. PHMSA added that it has long applied the same standard to discharge 

operations involving manifolded acetylene cylinders that are securely mounted to a motor 

vehicle. 

In addition, PHMSA stated the following: 

... because of acetylene’s volatility and instability, we have determined that the safety 

risks associated with removing individual acetylene cylinders from a manifolded system 

that conforms to [the Hazardous Materials Regulations] requirements and industry 

standards are far greater than the safety risks associated with discharging cylinder 

contents while the manifolded cylinders are securely mounted on the vehicle. 

National Transportation Safety Board 19
Special Investigation Report 

Industry Standards

The Compressed Gas Association’s (CGA’s) publication G-1.6, Recommended Practices 

for Mobile Acetylene Trailer Systems, provides safe practices for the design, construction, and 

operation of mobile acetylene trailer systems. The publication also provides recommended safe 

practices for auxiliary equipment used in conjunction with mobile acetylene trailers, including 

piping, regulators, flash arrestors, and meters. The relevant areas addressed by the publication 

are the following: 

In section 4, “Design and construction” 

• Piping must: ... be braced and supported .... 

• Leads between cylinders and manifolds must be sufficiently long and flexible to 

minimize strain on valves and leads. 

• Manifolds must be equipped with a shut-off valve, pressure gauge, and vent. 

• Vents and pressure relief devices must be directed upwards above the acetylene 

piping 

• Cylinders must: conform to the Hazardous Materials Regulations; be vertical, 

supported, and secured; have valves that are capable of being closed in the event 

of an emergency; ... and be arranged in aisles to allow access. 

In section 5, “Operation” 

• Valves must be closed during transportation. 

• Acetylene pressure must be maintained in leads and manifolds during delivery 

and return shipments. 

• During any manual valve operations, or when the trailer is being connected or 

disconnected, a trained person must be in attendance. 

In section 7, “General provisions” 

• Charging and discharging stations for mobile acetylene trailers must be provided 

with conspicuously located and easily accessible fire hoses or fixed spray 

systems and dry chemical fire extinguishers. Nozzles on fire hoses should be of 

the type that adjusts from full stream to a fog pattern. 

The CGA publication G-1.2, Acetylene Metering and Piping, also provides recommended 

practices for use with acetylene trailers and in acetylene processing facilities. Section 14.2.1.3, 

“Design,” specifically addresses the construction and maintenance of flash arrestors: 

National Transportation Safety Board 20
Special Investigation Report 

NFPA 70[ ] 16 recommends 6.5 feet (2 meters) packed height in a wetted tower in low 

pressure service, though successful tests were made with a packed height of 40 inches 

(102 centimeters). The code further recommends that the packed height be doubled for 

dry-packed arrestors. The packing should be examined at intervals no longer than 1 year 

for conditions of settling, corrosion, and plugging. 

Postaccident Activities 

Since the accidents, Western has made a number of changes to its procedures and 

equipment to address problems identified during the investigations. The company has improved 

its procedures for preparing trailers for unloading at customer sites, provided additional 

equipment for trailer and cylinder safety, reviewed its customers’ facilities to insure their safe 

operation, and installed water deluge systems at customer facilities. These measures are 

described below. 

Western operators preparing their trailers for unloading at facilities without computergenerated instructions are now required to call the Western headquarters control room operator 

five times throughout the process to verify that critical steps are performed in the proper 

sequence. During each of the five calls, the control room operator verifies that the current step 

has been performed before allowing the operator to proceed to the next step in the sequence. 

The additional safety equipment Western is installing includes flash arrestors on every 

mobile acetylene trailer line between the manifold and the trailer loading and unloading 

attachments to stop the propagation of an acetylene flame front and decomposition reaction 

between the trailer and the plant. Also, valve protection caps will be installed on all cylinders 

mounted on mobile acetylene trailers. 

Regarding the safety of its customers’ facilities, Western has performed enhanced 

surveys at all customer facilities to ensure they are compliant with all applicable codes, 

regulations, and industry practices. Western’s management has stated that automated water 

deluge systems have prevented escaping acetylene from igniting and would have prevented the 

property damage and injuries that occurred in these accidents had such systems been in place. 

Since these accidents, Western has completed installation of water deluge systems at every 

customer’s unloading site and will require them to be installed at future customer sites. 

 16 NFPA 70 does not address packed height in flash arrestors, and the NFPA has found no reference for this 

section. 

National Transportation Safety Board 21
Special Investigation Report 

Analysis 

Transporting acetylene cylinders poses risks because of the inherent reactivity of the gas. 

Additional risk is incurred when the cylinders are moved at speed on roads because the risk of a 

motor vehicle accident is added. Although the causes of the highway accidents addressed in this 

report include a driver falling asleep, that cause and others such as driver error are not included 

in this analysis. Rather, we have documented and analyzed the results of mobile acetylene trailer 

accidents in an attempt to identify ways to reduce the risk of fire. 

When a mobile acetylene trailer is not on the road but is loading or unloading, the 

reactivity of acetylene gas requires that specific procedures be followed to ensure safety. In these 

accidents, the operators carried standard procedures lists that they were supposed to follow; 

nevertheless, steps were performed out of order or incorrectly. Therefore, we looked at the 

loading/unloading processes in their entirety, focusing on ways to guide and assist operators as 

they perform their complex tasks. 

Overturn Accidents 

In the East New Orleans, Louisiana, accident in which the tractor and mobile acetylene 

trailer overturned, the cylinders mounted on the trailer were thrown from the vehicle. The 

postaccident examination of the cylinders recovered from the scene revealed that 32 of the 

cylinders recovered had their valves broken off during the accident, releasing acetylene that 

ignited. Similarly, in the Lamar, Colorado, accident, about half of the 225 cylinders were thrown 

from the trailer. The postaccident examination of the cylinders recovered from the scene revealed 

that 86 of the cylinders had their valves broken off during the accident, resulting in the release 

and ignition of the residual acetylene. An examination of these cylinders showed that the fire 

from the cylinders with broken valves caused the fusible plugs of other cylinders to melt, 

releasing additional acetylene that ignited. Possible ignition sources include the generation of a 

static charge in the rapidly escaping acetylene and solvent and a spark caused by the steel 

cylinders striking concrete, rocks or stones, or the road surface. 

The Safety Board concludes that the fires in the East New Orleans, Louisiana, and 

Lamar, Colorado, accidents occurred as a result of the ejection of unsecured cylinders during the 

rollovers of the mobile acetylene trailers, resulting in damage to many of the cylinder valves and 

the release of acetylene, which then ignited. 

These two accidents demonstrate the high likelihood of an acetylene release and fire 

when cylinder valves are damaged in rollover accidents. With the extreme flammability of 

acetylene, undamaged cylinders in proximity to burning cylinders are likely to become involved 

in a fire. In overturn accidents in which a mobile acetylene trailer rolls onto its side, unprotected 

or unrestrained cylinders may be ejected from the trailer, as occurred in these two accidents. 

Ejection of cylinders from a trailer in an accident significantly increases the likelihood of 

damage to the cylinder valves. In the event that cylinders remain secured to the trailer in an 

National Transportation Safety Board 22
Special Investigation Report 

overturn, their valves can be damaged by guard rails and other roadside objects and even contact 

with the road surface. With the extreme flammability and reactivity of acetylene, undamaged 

cylinders close to burning acetylene cylinders are also very likely to become involved in a fire. 

The cylinders in these two accidents were not adequately secured to the trailers to prevent 

their ejection during overturn. Current DOT regulations for manifolded cylinder systems on 

trailers, such as mobile acetylene trailers, require only that each cylinder be equipped with an 

individual shutoff valve that is tightly closed in transit and that the valves and pressure relief 

devices be protected from damage by framing, a cabinet, or other method. Current Federal 

regulations do not set sufficient standards for the level of protection that must be provided. To 

reduce the risks of transporting acetylene, cylinders must remain secured to the trailer during a 

rollover accident, and the cylinder valves must be protected from impact and damage. 

Therefore, the Safety Board concludes that the requirements in the Hazardous Materials 

Regulations covering mobile acetylene trailers do not sufficiently address (1) the protection of 

the cylinders, valves, and fittings on the trailers from impact forces that occur during an overturn 

accident and (2) the secure mounting of the cylinders to the vehicles. The Safety Board believes 

that PHMSA should modify 49 Code of Federal Regulations 173.301 to clearly require (1) that 

cylinders be securely mounted on mobile acetylene trailers and other trailers with manifolded 

cylinders to reduce the likelihood of cylinders being ejected during an accident and (2) that the 

cylinder valves, piping, and fittings be protected from multidirectional impact forces that are 

likely to occur during highway accidents, including rollovers. 

Unloading Accidents 

The two unloading accidents occurred when the mobile acetylene trailers were connected 

to piping at loading/unloading plants while the operators were preparing the trailers to be 

unloaded. 

The Woodlands, Texas 

The accident in The Woodlands, Texas, was described in detail by witnesses and 

recorded by a security camera at the plant. According to the statements of the operators and the 

video recording, the team had completed connecting their trailer to the plant’s discharge arm and 

had pressurized the trailer manifold by opening all of the cylinders on the trailer. At that point, 

the team realized that they had not opened the trailer’s block valve. They then opened the valve. 

According to the team’s initial statement to emergency responders, when they opened the block 

valve they heard a loud noise and felt the trailer shake. As noted previously, opening the block 

valve allows highly pressurized acetylene to enter a blocked section of piping that contains 

residual air, which is dangerous because (1) a blocked pipe potentially can cause a reflected 

shockwave that can initiate a decomposition reaction and (2) pressurized acetylene that mixes 

with air inside piping can ignite easily with little energy input. The black plume rising from the 

top of a cylinder seen in the security camera video is an indication that an acetylene 

decomposition reaction had occurred on the trailer. With this evidence of the decomposition 

National Transportation Safety Board 23
Special Investigation Report 

reaction, the black powder found in the fitting between the trailer and the plant piping and in the 

plant discharge arm was determined to be carbon, which is a byproduct of a decomposition reaction. 

The Safety Board concludes that the ignition of the acetylene in the accident in The Woodlands, 

Texas, likely occurred because of the operators’ failure to follow Western’s standard operating 

procedures, which resulted in the introduction of high-pressure acetylene into closed piping that 

contained air, which in turn initiated an acetylene decomposition reaction. 

Dallas, Texas 

The video recording of the Dallas, Texas, accident did not record the initiation of the 

accident, and the operator and other witnesses made contradictory observations about the 

initiation of the accident. In addition, representatives of Western expressed concern that a flame 

front caused by a failure within the plant’s compressor system may have passed through the 

poorly maintained flash arrestor and started the fire on the accident trailer. Several varying 

scenarios of the initiation of the accident were considered and discounted during the 

investigation. 

The first scenario was the catastrophic failure of a cylinder. The operator said that he 

heard an explosion and was thrown back by it and then he saw a black plume coming from one 

of the closed cylinders; such a plume is an effect of the venting of carbon, which is a byproduct 

of an acetylene decomposition reaction. An explosion of sufficient force to throw an adult 

requires a large pressure surge, and an acetylene cylinder failing catastrophically was the only 

source on the trailer that was capable of such a surge. However, the operator was not burned, and 

neither he nor the nearby witness observed a fireball or explosive damage to the trailer during the 

initial stage of the accident, which could be expected in the event of catastrophic failure of an 

acetylene cylinder. Further, the witness did not hear an explosion despite being only about 9 feet 

from the trailer. In addition, there were no known conditions that could have caused a 

catastrophic cylinder failure; the cylinders on the trailer had been filled more than 10 hours 

before the accident, and the closed cylinders had not been handled since the trailer was backed 

into the unloading area. Therefore, it is unlikely that a catastrophic cylinder failure caused this 

accident. 

The second scenario was a fire originating in the plant’s compressor system that resulted 

in a flame front passing through the plant’s piping, through the flash arrestor, and into the 

accident trailer. All the witnesses stated that the first events occurred on the accident trailer: the 

first black plume was observed coming from the trailer, the trailer was surrounded by vapor, and 

the video recordings showed the trailer fully engulfed in fire before the fire progressed to the 

three adjacent trailers. Given the extreme reactivity of acetylene and the explosive propagation of 

a decomposition reaction through open piping, the three other trailers that had valves open to the 

plant piping likely would have been affected immediately had the fire originated in the plant 

rather than at the accident trailer. More important, the plant’s discharge arm valve, between the 

plant piping and the accident trailer, was closed, so there was no channel through which the 

flame front could enter the trailer. Therefore, it is unlikely that a fire that originated in the plant 

passed into the accident trailer and caused the accident. 

National Transportation Safety Board 24
Special Investigation Report 

The most likely scenario was the initiation of an acetylene decomposition reaction on the 

accident trailer. This is supported by witness observations of a black plume over the trailer and 

the operator’s postaccident statement that the trailer’s pressure gauges indicated that the pressure 

in the manifold was 0 psig after the operator had cycled the valve on the plant’s discharge arm, 

leaving it closed. Because Western’s standard operating procedures mandate that 50 psig of 

pressure be maintained in the manifold between the completion of the leak test and the opening 

of the discharge arm valve, a pressure reading of 0 psig indicate that the unloading procedures 

had not been performed correctly. The lack of positive pressure in the manifold likely allowed 

some air to enter the manifold. In addition, had there been 0 psig acetylene pressure in the 

manifold when the operator connected the trailer to the plant’s discharge arm, the procedures 

used to mix acetylene and the air in the connection fittings at low pressure would have been 

ineffective and air could have remained in that area. As the operator began opening the cylinder 

valves and introduced pressurized acetylene into the manifold, the pressure surge likely hit the 

closed discharge arm valve and was reflected, generating heat that was sufficient to ignite the 

acetylene-air mixture in the manifold. The air was quickly consumed, and a decomposition 

reaction propagated through the manifold and into the trailer’s cylinders through their open 

valves. The heat from the reaction inside the cylinders melted the cylinders’ fusible plugs and 

released black plumes. Finally, when the released products of decomposition (hydrogen gas and 

carbon) ignited, the fire heated nearby cylinders, causing their fusible plugs to melt, releasing 

acetylene gas that then ignited, and fire engulfed the accident trailer. The fire then spread by 

radiant heat to the three adjacent trailers. Had the manifold pressure been maintained at 50 psig, 

as required by Western’s standard operating procedures, air could not have entered the manifold 

piping to lower the ignition temperature of the acetylene, and the air between the block valve on 

the trailer and the plant discharge arm valve would have mixed safely with the acetylene entering 

from the manifold. 

Therefore, the Safety Board concludes that the ignition of the acetylene in the Dallas, 

Texas, accident likely occurred as a result of the operator’s failure to follow Western’s standard 

operating procedures, which resulted in the reduction of the pressure in the manifold to 0 psig, 

allowing air to enter the manifold and initiating a decomposition reaction when pressurized 

acetylene was subsequently introduced into the manifold. 

Because the Dallas and The Woodlands accidents likely were initiated by the failure of 

operators to correctly perform Western’s detailed unloading procedures, the Safety Board 

evaluated Western’s unloading requirements. Each trailer operator carried a detailed standard 

operating procedures manual for reference and guidance during loading and unloading. 

However, in both these accidents, the operators did not perform the unloading procedures 

correctly or in the proper sequence, which resulted in the acetylene decomposition reactions. In 

The Woodlands, when the operators realized that they had not performed a required step, they 

performed the skipped step after they realized their omission. In Dallas, the operator did not 

recognize that the lack of pressure in the trailer manifold, at a step in the process in which some 

pressure should be maintained in the manifold, indicated that the unloading procedures had not 

been performed correctly. 

National Transportation Safety Board 25
Special Investigation Report 

Federal regulations and CGA guidance concerning mobile acetylene trailers focus on 

design and are silent concerning trailer unloading procedures other than the recommendation that 

a trained person be in attendance during manual valve operations and when a trailer is being 

connected or disconnected. 

The accidents in Dallas and The Woodlands demonstrate the catastrophic results that can 

occur when the unloading procedures are not followed exactly. Despite the detailed written 

unloading procedures Western developed and implemented, the procedures were not sufficient to 

safeguard against the initiation of a decomposition reaction within the manifold piping and 

cylinders when simple human errors occurred, as they will from time to time. The complexity of 

the unloading procedures and the extreme instability of acetylene together created situations 

having little or no room for human error. The Safety Board, therefore, concludes that because of 

the instability of acetylene, the current acetylene unloading procedures by themselves are not 

adequate to ensure safety. The Safety Board believes that PHMSA should require fail-safe 

equipment that ensures that operators of mobile acetylene trailers can perform unloading 

procedures only correctly and in sequence. 

Fire Suppression at Unloading Sites 

There was no automated water deluge system at either The Woodlands or Dallas. 

Western installed automated water deluge systems at many of its facilities after the 

April 10, 2001, mobile acetylene trailer fire at its Bellville, Texas, facility. According to 

Western, the effectiveness of such systems at controlling the spread of fire to other cylinders on 

the same trailer and from one trailer to the next was proven at the June 8, 2005, decomposition 

reaction that also occurred at Bellville. The actuation of the water deluge system limited the 

spread of the fire to other cylinders on the same trailer. In the Dallas accident, attempts to 

extinguish the fire using a fire hose were ineffective. Had the unloading facilities at The 

Woodlands and Dallas had automated water deluge systems, the fires also may have been 

controlled and the spread of fire between the cylinders and to the nearby mobile acetylene trailers 

may have been reduced or eliminated. After these accidents, Western completed installation of 

water deluge systems at all Western-owned and customer unloading sites, including The 

Woodlands. However, the CGA standards require only conspicuously located and easily 

accessible fire hoses or fixed spray systems and dry chemical fire extinguishers. Automated 

water deluge systems, which appear to be effective on acetylene cylinder fires, are not required 

by the CGA standards. Therefore, the Safety Board believes that the CGA should revise the 

recommended practices in CGA standard G-1.6, section 7, General Provisions, to require 

automated water deluge systems at all mobile acetylene trailer loading and unloading locations to 

control the spread of fire to other cylinders on a trailer and to nearby mobile acetylene trailers. 

National Transportation Safety Board 26
Special Investigation Report 

Conclusions 

Findings 

1. The fires in the East New Orleans, Louisiana, and Lamar, Colorado, accidents occurred as a 

result of the ejection of unsecured cylinders during the rollovers of the mobile acetylene 

trailers, resulting in damage to many of the cylinder valves and the release of acetylene, 

which then ignited. 

2. The requirements in the Hazardous Materials Regulations covering mobile acetylene trailers 

do not sufficiently address (1) the protection of the cylinders, valves, and fittings on the 

trailers from impact forces that occur during an overturn accident and (2) the secure 

mounting of the cylinders to the vehicles. 

3. The ignition of the acetylene in the accident in The Woodlands, Texas, likely occurred 

because of the operators’ failure to follow Western International Gas & Cylinders, Inc.’s, 

standard operating procedures, which resulted in the introduction of high-pressure acetylene 

into closed piping that contained air, which in turn initiated an acetylene decomposition 

reaction. 

4. The ignition of the acetylene in the Dallas, Texas, accident likely occurred because of the 

operator’s failure to follow Western’s standard operating procedures, which resulted in the 

reduction of the pressure in the manifold to 0 psig, allowing air to enter the manifold and 

initiating a decomposition reaction when pressurized acetylene was subsequently introduced 

into the manifold. 

5. Because of the instability of acetylene, the current acetylene unloading procedures by 

themselves are not adequate to ensure safety. 

National Transportation Safety Board 27
Special Investigation Report 

Recommendations 

As a result of its investigation of the mobile acetylene trailer accidents in East New 

Orleans, Louisiana, on October 20, 2007; in Dallas, Texas, on July 25, 2007; and in The 

Woodlands, Texas, on August 7, 2007, the National Transportation Safety Board makes the 

following safety recommendations: 

To the Pipeline and Hazardous Materials Safety Administration: 

Modify 49 Code of Federal Regulations 173.301 to clearly require (1) that 

cylinders be securely mounted on mobile acetylene trailers and other trailers with 

manifolded cylinders to reduce the likelihood of cylinders being ejected during an 

accident and (2) that the cylinder valves, piping, and fittings be protected from 

multidirectional impact forces that are likely to occur during highway accidents, 

including rollovers. (H-09-01) 

Require fail-safe equipment that ensures that operators of mobile acetylene trailers 

can perform unloading procedures only correctly and in sequence. (H-09-02) 

To the Compressed Gas Association: 

Revise the recommended practices in Compressed Gas Association standard 

G-1.6, section 7, General Provisions, to require automated water deluge systems 

at all mobile acetylene trailer loading and unloading locations to control the 

spread of fire to other cylinders on a trailer and to nearby mobile acetylene 

trailers. (H-09-03) 

BY THE NATIONAL TRANSPORTATION SAFETY BOARD 

MARK V. ROSENKER ROBERT L. SUMWALT 

Acting Chairman Member 

DEBORAH A. P. HERSMAN STEVEN R. CHEALANDER 

Member Member 

KATHRYN O’LEARY HIGGINS 

Member 

Adopted: January 13, 2009 

Kathryn O’Leary Higgins, Member, filed the following concurring statement on 

January 9, 2009. 

National Transportation Safety Board 28
Special Investigation Report 

Board Member Statement 

Member Kathryn O’Leary Higgins, concurring: 

I concur in this special investigation report on mobile acetylene trailer accidents and am 

pleased that our hazardous materials investigators looked at several accidents and proposed 

recommendations to the Pipeline and Hazardous Materials Safety Administration to improve the 

handling and securing of acetylene tanks. But I write this statement because I am concerned that 

we do not address the reasons that two of these accidents occurred. 

Two of the four operators overturned their vehicles. I have reviewed the police reports 

for the two rollover accidents that are part of the docket for this special investigation report. The 

report of the rollover accident in East New Orleans, Louisiana indicates driver condition (falling 

asleep) as the cause of the accident. No drug or alcohol tests were administered. The report for 

the Lamar, Colorado accident is much less specific, and the driver’s actions are not explained. It 

indicates that the truck left the road to the right, hit a post, was steered to the left back on to the 

road and rolled over. Nothing was said in the report about the driver’s condition. 

Based on the limited information in the police reports, it seems that these rollover 

accidents would not have occurred but for driver error. I believe we should have looked more 

closely at driver issues, including the experience and training of these drivers and their recent 

history, as part of our special investigation to try to determine what led to these accidents. Once 

we identified our interest in these accidents, I believe we should have asked our highway 

investigators to assist our hazardous materials team so we could answer questions about driver 

performance and company practice. But without such investigation, we do not know what really 

caused these accidents and how they might be prevented in the future. There are only two 

companies that transport acetylene. It should not have been too difficult to take a closer look at 

their operations as part of this investigation to better understand their practices and policies for 

their drivers. 

The NTSB is a small agency and our resources are limited. We cannot do everything, but 

if we decide to look more closely at accidents because of the hazardous materials release and 

subsequent fires, I think we should take the steps necessary to understand what caused the 

accidents in the first place. I also understand that our limited approach is consistent with other 

special investigations. Our recommendations, if adopted, will improve how these canisters are 

mounted, secured and handled – and that will be a solid safety improvement. But our failure to 

look more closely at driver performance is a missed opportunity to understand what led to two of 

these accidents and to make recommendations to prevent such accidents in the future. 

National Transportation Safety Board 29
Special Investigation Report 

Appendix A 

Investigation 

The National Transportation Safety Board was notified of the Dallas, Texas, accident on 

July 27, 2007; however, the initial information did not indicate that it was a transportation 

incident. The Chemical Safety Board launched on the accident. When the National 

Transportation Safety Board received information that vehicle unloading was involved in the 

accident, a highway investigator from the Dallas regional office was dispatched to the site. On 

August 1, 2007, a hazardous materials investigator from the Washington, D.C., headquarters was 

assigned as investigator-in-charge of the National Transportation Safety Board’s investigation 

and dispatched to the carrier’s headquarters. On August 7, 2007, the Chemical Safety Board 

notified the National Transportation Safety Board of a second accident involving the unloading 

of a similar vehicle in The Woodlands, Texas. On August 9, 2007, the hazardous materials 

investigator from the Washington, D.C., office was dispatched to investigate this accident as 

well. On October 20, 2007, the National Transportation Safety Board was notified of the 

overturn of a similar vehicle in East New Orleans, Louisiana. On October 23, 2007, the 

hazardous materials investigator from Washington, D.C., traveled to New Orleans to begin the 

investigation of this accident also. One group was established to investigate all three hazardous 

materials accidents. On January 18, 2008, the Chemical Safety Board announced that it was 

discontinuing its investigations of the Dallas and The Woodlands, Texas, incidents to avoid 

duplication of effort. 

Participating in the investigations were representatives of the Pipeline and Hazardous 

Materials Safety Administration and Western International Gas & Cylinders, Inc. 

No depositions were taken, and there was no public hearing. 

National Transportation Safety Board 30