National Transportation Safety Board

Washington, D.C. 20594

Hazardous Materials Accident Brief

Accident No.: DCA-96-MZ-001

Transportation Mode: Rail

Type of Accident: Tank car failure and release of poisonous and corrosive vapors

Location: Gaylord Chemical Corporation, Bogalusa, Louisiana

Date and Time: October 23, 1995, about 4:45 p.m.

Shipper: Vicksburg Chemical Company

Tank Car Specification: DOT 105A500W

Tank Car Manufacturer: Union Tank Car Company

Injured: 4,710 people were treated at area hospitals,

 of whom 81 were admitted

Evacuated: Approximately 3,000 people

Material Released: Nitrogen tetroxide, a poisonous material (oxidizer)

Type of Failure: Corrosion

Accident Narrative

At 3:55 p.m. on October 23, 1995, at the Gaylord Chemical Corporation plant in

Bogalusa, Louisiana, yellow-brown vapors began leaking from the dome of the DOT class

105A railroad tank car UTLX 82329 that contained a mixture of nitrogen tetroxide, which is a

liquefied poisonous gas and oxidizer, and water. The vapors initially formed a plume between

10 and 15 feet in diameter. Plant personnel notified emergency response agencies and used

two plant fire hoses to spray water into the plume to suppress the vapors. About 4:30 p.m.

Bogalusa fire personnel arrived at the plant and set up fire hoses to help-suppress the vapors.

The head on the B-end of the tank car failed about 4:45 p.m., resulting in one end of the

tank car jacket being torn away and thrown about 350 feet. The tank car was then propelled

35 feet down the track and derailed at a track bumping block. A large reddish-brown vapor

cloud was released from the tank car. Vapors continued to be released from the opening in the

tank car for another 36 hours until the chemical reaction that had occurred within the tank was

brought under control through neutralization and dilution.

Some 3,000 people were evacuated from the area as a result of the vapor cloud. Of 4,710

people who were treated at local hospitals, 81 people were admitted.

Events Preceding the Accident

On September 14, 1995, nitrogen tetroxide vapors had leaked at a valve on the tank car

while it was at the Gaylord plant. A mechanic sprayed water to suppress the vapors and then

directed the water stream onto the valve stem as the tank car was unloaded. When the water
was turned off could not be determined. Another mechanic stated that the water was no longer

being applied over the valve on September 22, when employees of the Union Tank Car

Company (manufacturer and owner of the tank car) arrived at the plant to replace the valve;

they replaced four valves. Postaccident examination of those valves removed from the tank

car disclosed that one valve stem had significant wear.

The tank car was moved to the Vicksburg Chemical Company in Vicksburg, Mississippi,

on September 26, for another load of nitrogen tetroxide. On October 5, the chemical company

recorded that the tare weight of this tank car was 9,500 pounds over the marked weight for the

tank car. A Vicksburg employee noticed the new valves on the tank car and assumed that the

car had been rebuilt and that the tank car’s trucks had been replaced, thus changing the

weight. He did not, however, verify these assumptions. The tank car was then filled with

approximately 10,000 gallons (110,000 pounds) of nitrogen tetroxide. A quality check had

been conducted on the nitrogen tetroxide before it was transferred to the tank car, and it was

found to meet the specifications of the Gaylord Chemical Corporation. No quality check was

performed on the material after it was loaded into the tank car. The loaded tank car was

returned to Gaylord.

On October 12, Gaylord mechanics began transferring nitrogen tetroxide from the tank

car to a storage tank. While this transfer was taking place, the material in the storage tank was

simultaneously being transferred into the plant. At 9 a.m., the process safety interlocks shut

down the chemical reactor. Gaylord’s investigation into the cause of the chemical reactor

shutdown revealed that the nitrogen tetroxide had been contaminated with water. To prevent

corrosion damage from nitric acid, which results when water reacts with nitrogen tetroxide,

Gaylord decided to transfer the contaminated material from the tank car and storage tank into

stainless steel highway cargo tank trailers. A meter was used to measure the amount of

product transferred from the tanks. A pressure gauge with a maximum calibrated pressure of

100 psig was used to monitor the internal pressure of the tank car.

Overnight, between October 12 and 13, the meter indicated that 10,100 gallons of product

were transferred from the tank car into two cargo tanks. No other check was made on the tank

car or the cargo tanks before the accident to determine how much material had been

transferred. When the transfer operation was stopped, a material sample was taken from the

transfer system; the sample contained a small amount of green liquid with foam. At 4 a.m. on

October 13, Gaylord employees believed the tank car was empty, except for a small amount

of residual material that could not be pushed up through one of the eduction pipes, and water

was added to the tank car to dilute this residual material. Postaccident weighing of the

material loaded into the two cargo tanks revealed that only 491 gallons (6,080 pounds) had

been transferred. Postaccident examination of the tank car showed that the carbon steel

eduction pipes had corroded and that only a small section of each, near the top of the tank,

remained.

No work was performed on the tank car for several days while Gaylord employees were

cleaning, testing, and inspecting the plant storage tank. On October 16, Gaylord determined

that the vapors coming from the tank car were excessive, indicating that the material in the

tank car was not dilute enough to permit discharge of the residual material into the plant

sewer system. Plans were made to transfer the residual material into another cargo tank, but

these plans were changed when vapors started leaking from the manway of another cargo tank
that had been filled with material from the storage tank. Gaylord determined that several

cargo tanks had gaskets and valves that were not appropriate for nitrogen tetroxide and

fuming nitric acid and between October 17 and 20 replaced these gaskets and valves.

 On October 19, material samples obtained from the tank car and the three cargo tanks

were taken to the Vicksburg Chemical Company for chemical analysis. Gaylord employees

began the transfer of the material remaining in the tank car on October 20, and by 6 p.m. that

day, the meter indicated that 6,700 gallons of material had been transferred from the tank car

to a cargo tank. At the conclusion of the transfer, a material sample taken from the transfer

system was observed to contain foam and a very small amount of green liquid. Postaccident

weighing of the material loaded into this cargo tank revealed that only about 850 gallons

(10,520 pounds) of material had been transferred.

Water was added to the tank car using a fire hose to purge and clean it; the addition of

water was discontinued when the internal pressure of the tank car rose to between 60 and 65

psig. Gaylord employees vented the vapors from the tank car through the plant scrubber to

lower the pressure in the tank car. Later on the evening of October 20, Gaylord employees

began emptying the material from the tank car into the plant sewer system; after 2 or 3

minutes, a large vapor cloud was released, and the unloading was stopped. On October 21, a

Gaylord employee twice added water to dilute the material in the tank car and vented the

vapors through the scrubber. The internal pressure of the tank car increased each time the

water was added.

On October 23, the gauge on the tank car indicated that the internal pressure was 18 psig.

Plans were made to complete the emptying and the cleaning of the tank car. At 12:54 p.m.,

Gaylord received the results of the chemical analysis of the material sample obtained from the

tank car on October 19. The results indicated that the material was wet nitrogen tetroxide.

These results were unexpected because Gaylord believed that on October 13 the nitrogen

tetroxide had been unloaded from the tank car and that the residue had been diluted with

water. After discussing the apparent discrepancy with the Vicksburg laboratory specialist who

had tested the sample, Gaylord personnel decided the sample may not have been

representative of the material in the tank car.

Between 1:30 and 1:45 p.m. on October 23, Gaylord employees put more water into the

tank car, and the internal pressure rose from 18 to 80 psig in 4 to 5 minutes, at which point the

water was turned off. About 2 p.m., the pressure reached 92 psig and then slowly began to

decline. The plant scrubber was still being used to vent the vapors from the tank car, and an

additional hose was run from one of the tank car valves to a water-filled drum for additional

venting. The internal tank car pressure was 55 psig by 2:30 p.m. When Gaylord employees

reopened the fire hose, the pressure rose to 100 psig, which is the maximum calibrated

pressure on the gauge. The water was turned off, and the vapors were again vented. At 3 p.m.,

the pressure appeared to be falling, but at 3:30 p.m., it was still above the measurement limit

of the gauge and appeared to be rising. At 3:55 p.m., a yellow-brown vapor was observed

leaking from the dome of the tank car. The head on the B-end of the tank car failed about 4:45

p.m. releasing a huge reddish-brown vapor cloud.
Postaccident examination of the safety relief mechanism mounted in the dome revealed

that it had recently activated. (Activation pressure is about 375 psig.) The examination also

revealed an 8-inch-high by 66-inch-wide corroded opening in the B-end head of the tank car

near the top. In addition, at least three distinctive horizontal bands of corrosion were found

near the top of the interior tank wall; one band was at the same level as the opening in the

tank head.

Postaccident Actions

Gaylord Chemical Corporation has discontinued the use of tank cars to transport nitrogen

tetroxide and now uses smaller tanks that hold only 3 percent of the volume capacity of the

tank car involved in the accident. Cargo purity assurance procedures, including the testing of

cargo before and after loading, have been implemented to prevent the transport of

contaminated cargo. The plant has additionally installed equipment to warn residents of any

accidental release and, in coordination with the city of Bogalusa, has developed emergency

procedures and conducted training.

The tank car destroyed in this accident was the last carbon steel tank car in use that had

been approved by the US Department of Transportation for transporting nitrogen tetroxide.

Probable Cause

The National Transportation Safety Board determines that the probable cause of the

accident was the lack of adequate procedures on the part of the Gaylord Chemical

Corporation and the Vicksburg Chemical Company to prevent or detect the contamination of

nitrogen tetroxide with water, resulting in the formation of an extremely corrosive product

and the subsequent failure of the tank car. Contributing to the severity of the accident were

the Gaylord Chemical Corporation’s inadequate procedures for emergency transfer of

contaminated cargo from the tank car.

Recommendations – None

Adopted 1/27/98