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On September 23, 2005, a 1998 Motor Coach Industries, Inc., 54-passenger motorcoach, operated by Global Limo Inc. (Global), of Pharr, Texas, was traveling northbound on Interstate 45 (I-45) near Wilmer, Texas.1 The motorcoach, en route from Bellaire to Dallas, Texas, as part of the evacuation in anticipation of Hurricane Rita, was carrying 44 assisted living facility residents and nursing staff. The trip had begun about 3:00 p.m. on September 22. Fifteen hours later, about 6:00 a.m. on the following day, a motorist noticed that the right-rear tire hub was glowing red and alerted the motorcoach driver, who stopped in the left traffic lane and then proceeded to the right shoulder of I-45 near milepost 269.5. The driver and nursing staff exited the motorcoach and observed flames emanating from the right-rear wheel well. As they initiated an evacuation of the motorcoach, with assistance from passersby, heavy smoke and fire quickly engulfed the entire vehicle. Twenty-three passengers were fatally injured. Of the 21 passengers who escaped, 2 were seriously injured and 19 received minor injuries; the motorcoach driver also received minor injuries.
TO THE PIPELINE AND HAZARDOUS MATERIALS SAFETY ADMINISTRATION: Develop standards for the safe transportation of partially pressurized aluminum cylinders by, for example, requiring the addition of temperature-actuated pressure relief devices or the reduction of residual pressure to safe limits, to ensure that such cylinders do not experience overpressure failure when exposed to a fire.
Original recommendation transmittal letter:
Closed - Unacceptable Action
Wilmer, TX, United States
Motorcoach Fire on Interstate 45 During Hurricane Rita Evacuation
Addressee(s) and Addressee Status:
PHMSA (Closed - Unacceptable Action)
Safety Recommendation History
We are disappointed that PHMSA believes the motorcoach fire in Wilmer, Texas, was not caused by the failure of a PRD and was a unique incident that does not represent standard commercial operations for transportation of compressed gas in an aluminum cylinder. PHMSA further believes that the safeguards provided under the current hazardous materials regulations provide a level of safety consistent with the public interest; accordingly, the agency plans to take no further action to address this recommendation. We respectfully disagree with the findings of PHMSA’s research and the decision of the agency to abandon the development of standards for the safe transportation of partially pressurized aluminum cylinders. As PHMSA plans to take no further action to address Safety Recommendation I-07-1, the recommendation is classified CLOSED—UNACCEPTABLE ACTION.
-From Cynthia L. Quarterman, Administrator: An internal review of the findings of the accident report, incident data collected by our Office of Hazardous Materials Safety, and requirements in the Hazardous Materials Regulations (HMR; 49 CFR Parts 171-180) leads us to conclude that safeguards under the current HMR provide a level of safety consistent with the public interest. Regarding NTSB’s view that “use of aluminum cylinders to transport compressed gas presents a threat to transportation safety,” we note the following: Millions of aluminum cylinders, including over 50 million in oxygen service alone, are safely transported commercially each year either fully or partially pressurized; • The majority of the partially pressurized cylinders are empty or so close to empty as defined in the HMR (25 psig) as to be considered empty; The partially filled state of the cylinders involved in the incident was because of the associated personal use and is not representative of the millions of commercial shipments; The manner in which the cylinders in the incident were loaded and transported in the motor coach is not representative of the manner in which cylinders are commercially transported under the requirements of the HMR; and Most commercially transported aluminum cylinders are shipped on open body trucks; and there are no comparable incident(s) to the Wilmer, Texas incident in our database. Pertaining to NTSB’s concern over the loss of material strength of aluminum in a fire and the associated risk of overpressure failure, the HMR provide safety measures appropriate for this risk through the application of requirements for use of a PRD. The HMR require that a PRD be tested and selected in accordance with Compressed Gas Association (CGA) standards.1 These standards provide for pressure-actuated and temperature-actuated PRDs, and address use of these PRDs based on the gas type and the associated risks in transportation. The PRD must be capable of preventing the rupture of a normally filled cylinder subjected to a fire test conducted in accordance with a CGA standard.2 Additionally, for a cylinder containing certain types of gas (e.g., flammable), the PRD setting must provide a tolerance on the set-to-discharge pressure of ±0 to -10% of the cylinder test pressure. This tolerance ensures that no unintended release of gas will occur during the fast filling or during transportation under high temperature weather conditions. See 49 CFR 173.301(f). PRD requirements help to ensure that a fully charged cylinder does not rupture during filling or an engulfing fire. It is important to realize that normal transportation of gas cylinders include either fully charged or almost empty (partially filled) conditions. The current PRD requirements under 49 CFR 173.301(f) will ensure safe transportation of a full charged cylinder that has a significant amount of stored energy. The worst case failure mode for a fully charged cylinder is rupture of the cylinder and sudden release of energy. Even though a PRD may not properly function on a partially filled cylinder, the amount of stored energy is not significant and the most common failure mode in a fire will be a leak rather than a rupture. The HMR requirements thus balance the need to prevent the dangers from a hazardous material release into the environment through a properly functioning PRD and the need to prevent the dangers associated with the rupture of a cylinder. It is also noted that although a cylinder is equipped with a PRD to prevent a rupture under abnormal conditions, a PRD is not designed to prevent a failure under all conditions. For example, when the heat from a fire transferred to a cylinder is intense, localized (e.g., a torch fire), and away from the PRD, the cylinder may weaken quickly (prior to expansion of the internal gas pressure) and rupture before or while the PRD operates. The HMR establishes the minimum safety standards for conditions normally incident to transportation. In summary, it is our position that: 1) the Wilmer, Texas incident was unique and does not reflect the commercial transportation of compressed gas in an aluminum cylinder; 2) the accident was not related to the malfunction of a PRD; 3) the HMR currently provides for measures to prevent failure of an aluminum cylinder during transportation; and 4) incident data of nationwide transport of aluminum cylinders does not support amendments to the HMR. Therefore, PHMSA plans no further action with regard to Safety Recommendation I-07-1.
The Safety Board is concerned that PHMSA’s response does not address the intent of Safety Recommendation I-07-1. The Board issued this recommendation to address its concerns regarding aluminum cylinders containing any gas under high pressure. Through the investigation of the Wilmer, Texas, accident, the Board found that Federal hazardous materials regulations are designed for the transportation of fully pressurized cylinders and require that a pressure relief device (PRD) prevent the failure of these cylinders. These regulations do not address the safety of partially pressurized cylinders. Although this was not an issue prior to 1981 when all specification cylinders were designed of steel, amendments to the hazardous materials regulations allow the use of aluminum cylinders to transport compressed gases that present a threat to transportation safety. The Board’s investigation found that aluminum cylinders will lose material strength in a fire, and partially pressurized cylinders can fail catastrophically before their PRD actuates. PHMSA’s Associate Administrator and Deputy Associate Administrator for Hazardous Materials Safety and Safety Board staff discussed the above information at meetings held in the fall of 2007. During these meetings, PHMSA staff said they did not believe that a change in the PRD or reduction of pressure in cylinders would be practical, and they believed that such measures would increase the risk of injuries. They further stated that current temperature-activated PRDs would actuate at lower temperatures than that of the current burst disk and would have released oxygen into the fire earlier in the accident. They added that the function of both the PRD and the cylinder should be to delay the release of oxygen as long as possible in order to provide an opportunity to evacuate people from the scene, to extinguish the fire, and to cool the cylinders. They also stated that the potential for danger to the public and emergency responders is significantly less from the failure of an aluminum cylinder than from the failure of a steel cylinder. The Safety Board agrees that providing additional oxygen to a fire presents a hazard and should be delayed as long as possible; however, the failure of a pressure vessel containing several hundred to nearly 3,000 pounds per square inch of pressure (psig) also poses a significant hazard to emergency responders and others. Aluminum cylinders can be transported as cargo in any mode of transportation, and the presence of aluminum cylinders containing any compressed gas may pose a significant hazard in transportation fires. For a PRD to be functional, it must prevent the catastrophic failure of the cylinder under all conditions. PRDs mounted on steel cylinders have been shown to prevent the failure of the cylinder in a fire in all loading conditions. The aluminum cylinders retrieved from the Wilmer accident demonstrated that the PRDs on two cylinders did not activate, and those cylinders failed catastrophically. The catastrophic failure of a cylinder should not be considered a safe method of relieving pressure. PHMSA staff have indicated that under conditions such as those of the Wilmer accident, a partially pressurized cylinder with a currently available temperature-actuated PRD would release sooner than one equipped with a pressure-activated PRD and thereby intensify the fire. While this may be true, Safety Board calculations contained in Materials Laboratory Study 05-124 show that, for a fully pressurized cylinder of the type in service at the time of the Wilmer accident, a pressure-activated PRD disk would actuate at a temperature in the range of 250° F to 320° F. Since the Wilmer accident, the Compressed Gas Association has reduced the pressure activation range for this type of PRD to between 2,700 psig and 3,000 psig. As a result, a fully pressurized cylinder can now be equipped with a PRD that will actuate between 200° F and 260° F. As noted in the Safety Board's accident report, a manufacturer of the cylinders stated that these cylinders lose strength well below their melting point of 1,020° F to 1,206° F, and a fully pressurized cylinder would maintain its integrity for only minutes at temperatures of about 600° F. Therefore, use of a temperature-actuated PRD rated for the same temperature range between 200° F and 260° F would afford partially pressurized cylinders a similar level of protection as that for fully pressurized cylinders. Further, a temperature-activated PRD rated for 350° F delays the discharge of a partially pressurized cylinder without the catastrophic rupture of the cylinder, and by discharging at a higher temperature would thereby surpass the existing level of protection afforded by a pressure-activated PRD on a fully pressurized cylinder. Supplemental information accompanying PHMSA’s January 22, 2008, response indicates that “designing a PRD to release oxygen just prior to an overpressure failure, although a worthy goal, is one that is extraordinarily complex and expensive. The Safety Board disagrees. A relatively simple system of a pressure-activated device in parallel with a temperature-activated device could prevent the premature release of cylinder contents while minimizing the risk of a catastrophic failure for both fully pressurized and partially pressurized cylinders. The Safety Board points out that this Safety Recommendation was not limited to oxygen-filled cylinders. PHMSA’s January 22 response has focused on oxygen-filled cylinders, but many other compressed gases are also currently transported in aluminum cylinders. According to Luxfer, the only domestic manufacturer of industrial-sized aluminum cylinders, Division 2.3 poisonous gases are transported in aluminum cylinders as well. Hazard Zone A poisonous materials, because of their extremely hazardous nature, may not be transported in cylinders containing PRDs; their internal pressures are limited to 1/4 to 1/2 of the marked service pressure to prevent the release of these materials under any circumstance. However, the Safety Board’s investigation of aluminum cylinders, and the relatively low temperatures at which aluminum loses its material strength and fails, is cause for additional concern in the transportation of poisonous and other materials in these cylinders. Because PHMSA’s efforts to date to address the safe transportation of partially pressurized aluminum cylinders do not address the Safety Board’s recommendation, Safety Recommendation I-07-1 is classified OPEN -- UNACCEPTABLE RESPONSE. The Board urges PHMSA to reconsider its position on this recommendation and to provide the Board with information on any alternative solutions it may undertake to address Safety Recommendation I-07-1.
Letter Mail Controlled 1/29/2008 8:57:01 AM MC# 2080033: - From Stacey L. Gerard, Assistant Administrator and Chief Safety Officer: The Pipeline and Hazardous Materials Safety Administration (PHMSA) was pleased to support NTSB’s investigation of the Wilmer, Texas accident, particularly with the metallurgical evaluation of the aluminum oxygen cylinders. As you know, we have extensive expertise in this area. Our examination of the cylinders showed that the oxygen contained in the cylinders was released both through the proper operation of the cylinder pressure relief devices (PRDs) and as a result of cylinder rupture. PHMSA estimates that millions of oxygen cylinders are safely being transported and used every year. In response to I-07-1, we share your concern about the potential safety hazards posed by the transportation of oxidizing gases such as oxygen in pressurized aluminum cylinders. We recently amended the Hazardous Materials Regulations (HMR; 49 CFR Parts 171-180) to require the set pressure for PRDs installed on cylinders used to transport flammable and poisonous gases to be set at the cylinder test pressure with a tolerance of +0/-10%. This action will extend the time before PRDs actuate without compromising the strength of the cylinder or significantly increasing the probability that the cylinder will burst because of extreme pressure build-up. We have an active rulemaking project to consider applying this requirement to cylinders containing oxidizing gases such as oxygen, which should further enhance safety for both partially pressurized and full cylinders. PHMSA met with NTSB on October 26 and November 1 and discussed each of the above NTSB recommendations. PHMSA technical experts provided technical rationale (e.g. test data, charts) describing the operation and performance of aluminum cylinders fitted with PRDs used for the transport of oxygen under various fire scenarios. During our meeting, PHMSA presented test data and charts to demonstrate that the use of temperature-activated PRDs would not have reduced the probability of a cylinder rupture in the Wilmer, Texas incident. Our analysis of the factors affecting the effectiveness of PRDs on partially filled cylinders is provided in the enclosed paper. PHMSA will continue to work with the Compressed Gas Association to explore possible options for enhancing oxidizing and flammable gas cylinder survivability in various fire situations.
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