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Metal and Nonmetal Mine Safety and Health

Accident Investigation Report
Surface Nonmetal Mine

Fatal Explosion of Vessel Under Pressure

Hunter Plant
Colorado Materials Incorporated
New Braunfels, Comal County, Texas
I.D. No. 41-03430
April 22, 1998


Willard J. Graham
Supervisory Metal and Nonmetal Mine Inspector

Benny W. Lara
Metal and Nonmetal Mine Inspector

James Smiser
Metal and Nonmetal Mine Inspector


Eugene Hennen
Technical Support - Mechanical Engineer

Originating Office
Mine Safety and Health Administration
South Central District
1100 Commerce Street, Room 4C50
Dallas, Texas 75242-0119

Doyle D. Fink
District Manager


Bobby Joe Mendez, serviceman, age 28, was fatally injured at about 1: 15 p.m., on April 22, 1998, when he was struck by pieces of a tire which exploded after heat was applied to the wheel hub/brake assembly. Scott Holmes, welder, and Warren Monteque, diesel mechanic, were also injured. Mendez had about eight years mining experience, all as a serviceman at this operation. Mendez, Holmes and Monteque had not received training in accordance with 30 CFR Part 48.

MSHA was notified at 2:50 p.m. on the day of the accident by a telephone call from the plant manager for the mining company. An investigation was started the same day.

The Hunter Plant, a surface crushed stone operation, owned and operated by Colorado Materials, Inc., was located near New Braunfels, Coma1 County, Texas. The principal operating officials were John R. Janek and Gary L. Brim, Governmental Affairs Officer and Safety Director. The mine was normally operated two, lo-hour shifts a day, five to six days a week. A total of 100 persons was employed.

Limestone was mined by drilling and blasting multiple benches in the pit. Broken stone was trucked to the plant for upgrading. The finished product was sold to various customers for use as construction aggregate.

The last regular inspection at this operation was completed on April 11, 1998. Another inspection was conducted at the conclusion of this investigation.

Physical Factors Involved

The accident occurred behind the main shop building. The equipment involved was a Caterpillar 63 1, series.11 pan scraper, which had been taken out of service for brake repairs. The rear tire and wheel assemblies had been removed to facilitate replacement of the brake drums and linings. The tire was a Type B Michelin, tubeless steel cord radial, size 37.25RX35XR. It was rated two stars which equated to a ply rating of 48. Estimated weight was 2000 pounds. The tire diameter was 89 inches and the tread width was 3 1 inches. The rim diameter was 36 inches. Normal operating pressure was 76 pounds per square inch (psi). Pressure measurements indicated that the opposite tire was inflated to 50 psi. Additional testing and analysis is detailed in Appendix II.

The brake drum was attached to the wheel by 45 steel studs and fasteners, 5/8- inch in diameter. The brake, drum was made of cast iron, approximately 1 - inch thick.

Weather conditions at the time of the accident were 80-85 degrees F, sunny and clear.

Description of Accident

On the day of the accident, Bobby Joe Mendez, (victim) reported for work at 6:00 a.m., his normal starting time. Mendez worked at the main shop under the direction of John Keller, equipment maintenance supervisor. His primary responsibilities were to service mine equipment and to help as needed at the shop. Keller assigned Chris Haseloff, mechanic helper, and Alfonso Peneulaz, equipment operator, to complete the brake repairs on the Caterpillar scraper, a job they had started the previous day.

Haseloff, Peneulaz and Mendez washed debris and mud from the tire hub assemblies. The left rear brake drum fasteners were removed from the wheel assembly and they moved on to the right rear. After removing the fasteners, they hammered and pried on the brake drum without success. The tire assembly was then moved behind the shop and placed on its side with the brake drum up. Penetrating oil was applied to the stud threads and double nutting the stud was attempted. They also drove wedges between the wheel and the brake drum flange in an effort to loosen the drum. After a great amount of effort, Keller decided to split the brake drum.

At about 11:30 a.m., Scott Holmes, welder, drove his maintenance truck, which was equipped with a welding machine, to the rear of the shop near the tire. Holmes attempted to cut a slot in the brake drum with a carbon arc air cutter, butthe carbon arc was ineffective in cutting the cast iron drum, so Holmes decided to use the oxygen/acetylene torch. Holmes climbed atop the tire and cut a one-inch slot on opposite sides of the drum. One side was cut completely in two, while the other side was cut to the mounting flange. Warren Monteque, diesel mechanic, arrived and they all stopped to eat lunch.

Haseloff and Peneulaz finished eating early and resumed work on the wheel. Haseloff dragged a water hose from the shop and wet the wheel hub/brake assembly to cool it. They pried and hammered on the brake drum to no avail. Haseloff and Peneulaz gave up and went into the shop to begin installation of the new brake part for the left rear of the scraper. Holmes, Montegue and Mendez returned about 1:OO p.m. and applied water to the hub assembly. They placed a pry bar in the slot which had been cut in the drum. The drum moved a bit and then the tire exploded. Holmes had walked around the tire to obtain a wedge from the maintenance truck when the tire exploded. He was blown against the truck's dual wheels. Monteque was standing near the tire on the other side and Mendez was at the shop wall. The explosion turned the tire completely inside out and sent pieces of the side wall in all directions. The tire came to rest beside the maintenance truck driver's side door, with the brake drum facing down. The projected pieces of tire were mostly contained between the shop and the warehouse building nearby. Both buildings sustained damage. The maintenance truck was extensively damaged. The force of the blast was detected up to one-half mile from the site.

Many employees rushed to the scene of the accident. Holmes and Monteque were given first aid and airlifted to a medical center in San Antonio, Texas, where they were treated for their injuries. Mendez was pronounced dead at the scene a short time later.


Use of the torch to cut the brake drum transferred sufficient heat to ignite material within the tire, generating combustible vapors and increased pressure. Heat from the burning material ignited the combustible vapors within the tire causing it to explode.


Order No. 4324226 was issued to Colorado Materials, Inc. on April 22, 1998, under the provisions of Section 103 (k) of the Mine Act to ensure the safety of persons until the affected areas of the mine could return to normal.

This order was terminated on June 25, 1998, after it was determined that the mine could safely resume normal operations.

Citation No. 7861378 was issued to Colorado Materials, Inc. on June 25, 1998, under the provisions of Section 104 (d) (1) of the Mine Act for violation of 30 CFR 56.4500:

A fatal accident and two serious injuries occurred at this operation on April 22, 1998, when a tire exploded while three miners were attempting to remove a brake drum from its rim. The equipment manufacturer's operation and maintenance manual warns of the explosive hazards generated by welding or heating rim components of tires. The supervisor engaged in aggravated conduct constituting more than ordinary negligence, in that he instructed one of the miners to cut a slot in the brake drum with an oxygen/acetylene torch. This generated combustible gases inside the tire. Failure to assure safe work procedures is a serious lack of reasonable care and is an unwarrantable failure to comply with a mandatory safety standard.

This citation was terminated on June 25, 1998, after the company changed their wheel/brake maintenance procedures. Tires will be dismounted and removed from the wheel. Additionally, the company has installed a 1500-gallon nitrogen tank for the purpose of replacing compressed air in all tires used on heavy equipment.


I. List of Participants in the Investigation.

II. Executive Summary of Field Examination / Laboratory Analysis

Appendix I.

Colorado Materials, Inc.
Johnny Weisman Chief Executive Officer
Gary R. Janek Plant Manager
Gary L. Brim Government Affairs Officer and Safety Director
Robert Richardson Mine Safety Officer
Bruce Wymer Equipment Manager
John R. Keller Equipment Maintenance Supervisor
William M. Knolle Attorney

Mine Safety and Health Administration

Willard J. Graham Supervisory Mine Safety and Health Inspector
Benny W. Lara Mine Safety and Health Inspector
James Smiser Mine Safety and Health Inspector
Eugene Hennen Mechanical Engineer

Appendix II. - Executive Summary of Field Examination/ Laboratory Analysis for a Fatal Tire Explosion Accident at the Colorado Materials, Inc., conducted by MSHA Technical Support.

A laboratory investigation was conducted at the Approval & Certification Center (A&CC) as part of a technical assistance request to Metal and Nonmetal Safety and Health's South Central District.on the tire explosion accident that occurred at the Colorado Materials surface limestone aggregate facility. The laboratory investigation included literature research, visual and microscopic examination, torch cutting tests on the brake drum of the recovered wheel rim, and ignition and heating tests on the damaged and undamaged liner materials.

After completion of the field examination and the laboratory testing and analysis, it was concluded that the most likely event that caused the accident is that cutting with the torch transferred sufficient heat into the tire to ignite combustible material within the tire. The tire liner ignited and smoldered for a length of time, filling the tire with combustible vapors. The tests show the tire inner liner material is combustible. Physical examination and thermal analysis of the damaged liner material indicates that combustion of the tire liner had actually taken place. Tests indicate the torch used on the drum was capable of generating temperatures on the tire side of the rim sufficient to ignite the tire liner when cuts similar to those made by plant workers were conducted. The laboratory test cuts were conducted without the presence of the tire. The tire would have trapped the heat generated by the cutting operation. Thus, the temperatures generated in the tire prior to the explosion could readily have been higher than measured during testing. The time delay between the completion of hot work and the explosion would have been favorable to create an explosive atmosphere within the tire from gases generated by a smoldering fire.

The magnitude of the accident damage can be explained by a combustion explosion. Since a smoldering fire in the tire would cause some internal heating before the explosion, this increase in temperature would create an initial pressure above the original pressure in the tire. Based upon pressure measurements taken on the opposite tire, the exploded tire probably contained about 50 psig of air prior to work being performed on it. The pressurized air would have contained more total oxygen than at atmospheric pressure, thus allowing for a longer burning time. The pressurized air could also increase the ability of any combustible material to ignite by lowering ignition temperatures below those observed in the laboratory tests. As the tire smoldered, the generation of volatiles and concurrent oxygen consumption would eventually drive the mixture through the explosive range. Given that the tire before the explosion was at an initial pressure of about 50 psig, a maximum explosion pressure of approximately 400 psig could have developed. This explosion pressure would be 4 times greater than would develop from a combustion explosion at normal atmospheric pressure. The rate of explosion pressure rise would have been extremely fast and would be over 3 times greater than would develop from a combustion explosion at normal atmospheric pressure. This magnitude of maximum pressure and rate of pressure rise from an explosion in the tire would be consistent with the physical damage observed at the accident site.

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