# Cargo Tanks; Miscellaneous Requirements; Final Rule DEPARTMENT OF TRANSPORTATION

- **operation:** document
- **citation:** Not available
- **title:** Cargo Tanks; Miscellaneous Requirements; Final Rule DEPARTMENT OF TRANSPORTATION
- **source type:** rulemaking
- **agency:** Office of the Federal Register
- **status:** historical
- **official:** true
- **published on:** 1994-11-03
- **effective on:** 1995-01-05
- **summary:** RSPA is amending certain requirements for the manufacture, qualification and maintenance of cargo tank motor vehicles. These regulatory actions are based on petitions for rulemaking, exemptions, National Transportation Safety Board recommendations, and RSPA initiative. The intended effect of these actions is to relax certain regulatory requirements and to reduce unnecessary economic burdens on industry where there will be no adverse effect on safety.
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Federal Register, Volume 59 Issue 212 (Thursday, November 3, 1994) [Federal Register Volume 59, Number 212 (Thursday, November 3, 1994)] [Unknown Section] [Page 0] From the Federal Register Online via the Government Publishing Office [ www.gpo.gov ] [FR Doc No: 94-26625] [[Page Unknown]] [Federal Register: November 3, 1994] _______________________________________________________________________ Part II Department of Transportation _______________________________________________________________________ Research and Special Programs Administration _______________________________________________________________________ 49 CFR Part 171 et al. Cargo Tanks; Miscellaneous Requirements; Final Rule DEPARTMENT OF TRANSPORTATION Research and Special Programs Administration 49 CFR Parts 171, 173, 178, and 180 [Docket No. HM-183C; Amdt. Nos. 171-129, 173-240, 178-105, 180-7] RIN 2137-AC37 Cargo Tanks; Miscellaneous Requirements AGENCY: Research and Special Programs Administration (RSPA), DOT. ACTION: Final rule. ----------------------------------------------------------------------- SUMMARY: RSPA is amending certain requirements for the manufacture, qualification and maintenance of cargo tank motor vehicles. These regulatory actions are based on petitions for rulemaking, exemptions, National Transportation Safety Board recommendations, and RSPA initiative. The intended effect of these actions is to relax certain regulatory requirements and to reduce unnecessary economic burdens on industry where there will be no adverse effect on safety. DATES: Effective date: January 5, 1995. Compliance date: Compliance with the regulations, as amended herein, is authorized immediately. Incorporation by reference: The incorporation by reference of certain publications listed in this final rule is approved by the Director of the Office of the Federal Register as of January 5, 1995. FOR FURTHER INFORMATION CONTACT: Ronald Kirkpatrick, telephone (202) 366-4545, Office of Hazardous Materials Technology, or Jennifer Karim, (202) 366-4488, Office of Hazardous Materials Standards, Research and Special Programs Administration, U.S. Department of Transportation, Washington, DC 20590-0001. SUPPLEMENTARY INFORMATION: I. Background On March 3, 1993, RSPA published in the Federal Register a notice of proposed rulemaking (NPRM) [Docket No. HM-183C; Notice No. 93-7; 58 FR 12316] proposing to amend certain requirements for the manufacture, qualification and maintenance of cargo tank motor vehicles. Most issues raised in the NPRM relate to requirements that were adopted in final rules published under Docket No. HM-183/HM-183A (June 12, 1989, 54 FR 24982; May 22, 1990, 55 FR 21035; September 7, 1990, 55 FR 37028; June 17, 1991, 56 FR 27872). The final rules established three new cargo tank specifications designated as DOT 406, DOT 407 and DOT 412, and revised the structural design requirements for MC 331 and MC 338 cargo tanks. Voluntary compliance for manufacture of cargo tanks to these new or revised specifications was authorized beginning on October 1, 1990. As manufacturers began modifying their manufacturing operations to construct cargo tanks to the new requirements, they encountered certain technical problems which caused them to question changes they had endorsed several years ago. They raised issues relating to structural integrity, accident damage protection, use of dual function pressure relief devices, and certification by the American Society of Mechanical Engineers (ASME). Also RSPA received several petitions for rulemaking addressing certain issues not previously raised. In the NPRM, RSPA pointed out these concerns and other issues based on petitions for rulemaking, exemptions, and National Transportation Safety Board (NTSB) recommendations. The NPRM also announced a public meeting that was held in Chicago on March 24 and 25, 1993, to address issues raised in the NPRM. On March 8, 1993, RSPA published a final rule [Docket HM-183, 58 FR 12904] granting an extension until April 21, 1994, for the continued construction of cargo tank motor vehicles to the MC 306, MC 307, MC 312, MC 331, and MC 338 specifications. This action was granted to allow additional time for RSPA and industry to address certain technical issues concerning the manufacture of cargo tank motor vehicles to the DOT 406, DOT 407 and DOT 412 specifications, and to resolve certain concerns about the structural design requirements in the MC 331 and MC 338 specifications. At the March 24-25 public meeting, several significantly different views of design engineers and cargo tank manufacturers were identified. Additionally, preliminary results were presented of an advanced structural evaluation (using finite element analysis) of the MC 331 cargo tank which raised questions in regard to stress levels in areas of concentrated loadings. On January 12, 1994, RSPA published a final rule [Docket HM-183, 58 FR 1784] granting another extension until August 31, 1995, for continued construction of cargo tank motor vehicles to the MC specifications. The final rule also announced a public meeting in Washington, D.C. on February 7-8, 1994. At the February public meeting, RSPA obtained clarification of certain comments received in response to the NPRM and also obtained additional supporting data on certain alternate proposals offered by industry. In addition, RSPA sought information to resolve the remaining issues on structural integrity, accident damage protection, use of dual function pressure relief devices, and ASME certification of low pressure cargo tank motor vehicle manufacturers. II. Summary of Comments RSPA received over 50 written comments in response to the proposals contained in the NPRM from trade associations, cargo tank manufacturers and repairers, manufacturers of cargo tank parts and equipment, and Federal, State and local agencies. RSPA has considered all comments, public meeting transcripts and petitions in the development of this final rule. In the NPRM, RSPA informed industry that the following items would be open for discussion at the March public meeting: 1. Application of the ASME Code to DOT 400-series specification cargo tank motor vehicles. a. The feasibility of citing all sections of the ASME Code that must be met in construction of DOT specification cargo tank motor vehicles as opposed to citing only those sections that do not apply. b. The development of a consensus standard containing procedures for quality control, welding and design as an alternative to the procedures contained in the ASME Code. 2. The progress of the industry on development and testing of dual function vents, reclosing pressure relief devices capable of reseating with the loss of less than one gallon of lading, and self-closing systems for vacuum-loaded hazardous waste tanks. 3. The regulatory proposals contained in the NPRM. Most commenters supported application of the ASME Code. One commenter stated that his company, which had initially opposed becoming ASME certified, has benefited from the expertise of the National Board inspector. The ASME Code is an internationally recognized consensus standard for the design and construction of pressure vessels. It is also the only proven quality control standard for pressure vessels and cargo tanks. The Cargo Tank Manufacturing Association (CTMA) submitted a draft quality control manual for review. CTMA recommended that RSPA recognize the manual, which includes quality control procedures, as an alternative to requiring manufacturers to have an ASME ``U'' stamp or National Board ``R'' stamp. The use of the alternative standard recommended by CTMA is not equivalent in scope and detail to the ASME Code and is not included in this final rule. It has also been brought to RSPA's attention that some ``U'' stamp and ``R'' stamp holders may believe they are not required to apply all provisions of the ASME or National Board quality control program for work on non-ASME DOT specification cargo tanks. When the regulations requiring cargo tank manufacturers and repair facilities to hold ``U'' and ``R'' stamps were promulgated, RSPA stated in the preamble discussions of the final rules and at public meetings that major provisions of the ASME or National Board quality control programs would apply to all work on DOT specification cargo tanks. Such provisions include welder qualifications, welding techniques, and quality control procedures. For instance, refer to the preamble discussions in the following final rules: ``B. Cargo Tank: Manufacturer Qualification, Registration, Quality Control, and Certification,'' (June 12, 1989, 54 FR 24984); ``Section 180.413 (Repair, Modification, Stretching, and Rebarrelling),'' (September 7, 1990, 55 FR 37044). One commenter stated that specifying all applicable sections of the ASME Code, rather than providing exceptions to sections that are not applicable, would provide greater assurance to engineers and designers that they have not overlooked an obscure section of the ASME Code. The commenter did not provide RSPA with any suggested wording for implementation of the recommendation. The National Propane Gas Association (NPGA) submitted a report on the structural integrity of the MC 331 specification cargo tank. NPGA recommended uniformity in design loading requirements for all DOT specification cargo tanks. RSPA will consider the design loading requirements for MC 331 and MC 338 specification cargo tanks in a future rulemaking. The report is available for review in the public docket. III. Section-by-Section Review This review by section discusses only significant comments received to the proposals in the NPRM, changes made based on alternative proposals offered by commenters, and clarifications to certain provisions based on RSPA's initiatives. For those provisions that are adopted as proposed, readers are referred to the preamble discussion in the NPRM. Section 171.7 The Truck Trailer Manufacturers Association (TTMA) submitted a petition for rulemaking (P-1236) requesting RSPA to incorporate by reference the latest editions of the ASME Code from the 1986 Edition and Addenda through 1985 to the 1992 edition and Addenda through 1993, and to update the National Board inspection Code from the 1983 Edition to the 1992 Edition. RSPA has reviewed these updated standards and agrees the latest editions should be referenced. The table in paragraph (a)(3) has been revised accordingly. Section 173.33 Consistent with changes made in Sec. 180.405(h) in this final rule, a new sentence is added to paragraph (d) stating that the venting requirements of the original DOT cargo tank specification must be met whenever a pressure relief valve is modified to a more recent specification. See preamble discussion for Sec. 180.405(h). Similarly, in view of changes made in Secs. 178.337-11 and 178.338-11 in this final rule, a new paragraph (h) is added to inform shippers that certain MC 331 and MC 338 cargo tanks manufactured after August 31, 1995, must have remotely controlled, internal, self-closing stop valves. See preamble discussion for Sec. 178.337-11. Section 173.225 Commenters supported the proposal to revise paragraph (e)(2) to authorize the use of MC 307 and DOT 407 cargo tank motor vehicles for certain organic peroxides. However, one commenter opposed allowing the use of MC 307 cargo tanks for all organic peroxides. This commenter misunderstood the proposal. Note 14 to the Sec. 173.225(b) Table authorizes bulk packagings for only a few organic peroxides. Therefore, paragraph (e)(2) is adopted as proposed in the NPRM. Section 173.315 For cargo tank motor vehicles in chlorine service, changes in requirements for hose, piping or tubing to be carried on the vehicle and in requirements for testing angle valves, as proposed in the NPRM, are adopted as paragraphs (o)(1) and (o)(2) respectively. Section 178.337-1 Paragraphs (a)(3) and (e)(1) are revised to correct certain section references and paragraph (e)(2) is revised to authorize the use of ceramic fiber/fiberglass insulation for cargo tanks in chlorine service as proposed in the NPRM. Section 178.337-9 As proposed in the NPRM, the restriction is removed against mounting or carrying on a cargo tank motor vehicle any hose, piping or tubing used in loading or unloading; paragraphs (b)(7) (ii) and (iii) are redesignated as (b)(7) (i) and (ii) respectively. Section 178.337-11 In response to NTSB recommendation H-90-91 dealing with release of sulfur dioxide from an MC 331 cargo tank during unloading, RSPA proposed revision of requirements for remotely controlled self-closing stop valves on liquid or vapor discharge lines on newly constructed MC 331 cargo tank motor vehicles. Currently found at paragraph (a)(2), this requirement applies only to equipment intended for transportation of a flammable liquid, a flammable compressed gas, hydrogen chloride (refrigerated liquid) or anhydrous ammonia. RSPA proposed to broaden the requirement to include all compressed gases. This revision also was proposed for MC 338 cargo tanks. Both the NTSB and the Compressed Gas Association (CGA) commented on RSPA's proposal to require remotely controlled internal self-closing stop valves. While supportive of RSPA's proposal, NTSB stated: Although the Safety Board is aware that MC 331 cargo tanks are predominantly used for the transportation of flammable and nonflammable compressed gases, the hazardous materials regulations (49 CFR Parts 171 through 180) do authorize these tanks to be used for the transportation of other classes of hazardous materials such as flammable liquids and poisons. The Safety Board believes that remote controls for internal shut-off valves should be required for any hazardous material that is authorized to be transported in an MC 331 cargo tank. Further the Safety Board believes that all MC 331 and MC 338 cargo tanks currently in hazardous materials service, and not just newly constructed tanks, should also be equipped with remote controls for the internal shut-off valves. The Safety Board has consistently urged the DOT to eliminate ``grandfathering'' clauses that permit hazardous materials to be transported indefinitely in containers or vehicles that fail to meet current minimum safety standards. The Safety Board believes that RSPA should require all MC 331 and MC 338 highway cargo tanks in hazardous materials service to be equipped with remote controls for internal shut-off valves by a specific date. On the other hand, the CGA believes that remotely controlled shut- off valves should not be required for nonflammable ladings. CGA stated that ``operating experience for the nonflammable compressed gases does not warrant the addition of this restriction which would add cost without providing additional safety benefit.'' After consideration of both comments, RSPA has revised the wording in Secs. 178.337-11 and 178.338-11 to require the use of remotely controlled internal self-closing stop valves on any cargo tank motor vehicle certified after August 31, 1995, that is intended for the transportation of hazardous materials other than argon, carbon dioxide, helium, krypton, neon, nitrogen, and xenon. RSPA has excepted these particular gases because they pose a lesser degree of risk to public health, safety, and the environment in the event of their release during transportation. Many of the nonflammable refrigerant gases presently transported in cargo tanks are ozone depleters; some other nonflammable gases are toxic or noxious. RSPA will address the retrofit of existing cargo tanks with remotely controlled internal self-closing stop valves in a separate rulemaking action. Section 178.338-9 As proposed in the NPRM and adopted in this final rule, paragraph (c)(2) is amended by removing the definition of ``same design'' and by adding a reference to the definition of ``same design'' contained in Sec. 178.320. By referencing the definition in Sec. 178.320, minor design variations are allowed for MC 338 cargo tanks. Section 178.338-11 NTSB and CGA submitted comments to this section which were similar to their comments to proposed Sec. 178.337-11. As stated in the preamble discussion to Sec. 178.337-11 above, RSPA has revised the wording in Secs. 178.337-11 and 178.338-11 to require the use of remotely controlled internal self-closing stop valves on any MC 338 cargo tank motor vehicle certified after August 31, 1995, that is intended for the transportation of hazardous materials other than argon, carbon dioxide, helium, krypton, neon, nitrogen, and xenon. Section 178.345-1 In paragraph (c), definitions for ``normal operating loading'' and ``extreme dynamic loading'' are added. These terms are used in revised Sec. 178.345-3 in this final rule. Paragraph (i)(2) requires that the void space within the connecting structure of a cargo tank motor vehicle composed of multiple cargo tanks must be vented to the atmosphere by a drain of at least 1 inch in diameter. In the NPRM, RSPA proposed to remove the drain hole size restriction. RSPA also solicited information on suitable dimensional controls for these drains, how often these areas need to be inspected, the conditions revealed during such inspections, and the availability of equipment for inspecting these areas. Commenters expressed diverse views on the need to inspect these areas. The California Highway Patrol (CHP) strongly supported the need for periodic inspection of the connecting structures on cargo tanks used to transport all hazardous materials. CHP related information about three separate catastrophic failures of molten sulfur trailers that occurred due to fatigue cracking in the void space. All three cargo tank motor vehicles had been insulated, thus preventing any external visual inspection for shell cracks. CHP believes that if an internal visual inspection of the void space had been performed, evidence of cracking may have been detected prior to the catastrophic failures. CHP noted that equipment, such as fiber optics, borescopes and video cameras, is readily available for inspecting closed areas and is being used for pipeline, aircraft, oil well, and boiler inspections. Also, numerous contractors offering inspection and non-destructive testing services are available throughout the U.S. NTSB also concurred that regular and effective inspections of void spaces are essential. NTSB, however, expressed concern that HM-183C did not address the configuration of other appurtenances that could conceal corrosion that might lead to a failure of the tank wall. The NTSB recommendation H-83-30 called for periodic external visual inspection of surfaces obscured by appurtenances, structural members, etc. Several commenters who opposed inspecting these areas stated that double bulkheads and void spaces on MC 306 or DOT 406 cargo tanks in non-corrosive service do not experience the same degree of deterioration as MC 312 or DOT 412 cargo tanks in corrosive service. They stated that cargo tanks in non-corrosive service should not be subject to the same inspections. One commenter submitted to RSPA samples taken from bulkheads adjacent to void spaces of two scrapped MC 306 carbon steel cargo tanks used in gasoline service for 14 and 15 years. The commenter stated that neither tank showed evidence of corrosion at this location; the samples confirm this statement. TTMA stated there is little possibility of corrosion occurring in the void space of any tank, except on carbon steel cargo tanks in corrosive service. TTMA recommended that the connecting structure on self-supporting cargo tanks be thickness tested every two years for signs of corrosion. Several other commenters supported TTMA's comments. One commenter also suggested that the wording be revised to eliminate reference to the inspection opening but continue to provide for a drain. The commenter stated that the minimum acceptable size of any drain opening should be 0.5 inches diameter, which is large enough to insert a borescope or equivalent device. RSPA agrees with commenters that there has been no evidence of corrosion occurring in the void space of any tank, except a carbon steel cargo tank in corrosive service. Corrosion in connecting spaces has serious structural implications for self-supporting cargo tanks. The failure reported in the NTSB investigative report, containing recommendation H-83-30, involved a carbon steel MC 312 cargo tank semi- trailer carrying hydrochloric acid; the failure occurred due to severe corrosion in the tank shell under a circumferential reinforcing ring. Therefore, paragraph (i)(2) is revised to require that the connecting structure in a carbon steel, self-supporting multi-tank cargo tank motor vehicle must have a single drain of at least 1.0 inch diameter, or two or more drains of at least 0.5 inches diameter, 6 inches apart, one of which is located on the bottom centerline. In addition, Sec. 180.407(i) is revised to require thickness testing of these areas as suggested by TTMA. RSPA believes that NTSB recommendation H-83-30, which calls for periodic external visual inspection of cargo tank surfaces obscured by appurtenances and structural attachments, is adequately addressed in current Sec. 180.407(d). Paragraph (d) requires that a periodic external visual inspection must be conducted of all major appurtenances and structural attachments on a cargo tank to detect signs of corrosion or damage. Section 178.345-3 Commenters expressed concern over the lack of flexibility in calculating compressive stresses for non-ASME DOT 400-series cargo tanks. They recommended that RSPA provide alternatives to ASME Code Section VIII, Division 1 UG-23(b) for calculating the maximum allowable compressive buckling stress in tank walls for low pressure cargo tanks. The static design and construction of all DOT 400-series cargo tanks must be in accordance with Section VIII of the ASME Code. Any DOT 400- series cargo tank which is required to be certified to the ASME Code also must be designed in accordance with the Code's requirements for dynamic loading, including UG-23(b). This applies to DOT 407 cargo tanks with a MAWP greater than 35 psig and each tank designed to be loaded by vacuum, and to DOT 412 cargo tanks having a MAWP greater than 15 psig. TTMA stated manufacturers believe the requirements in Sec. 178.345- 3(b) should be modified to allow several methods of analysis as appropriate for the cargo tank under consideration. Using the methods outlined in the ASME Code produces lower allowable compressive stress values, resulting in substantially thicker sheets for the DOT 400- series tanks as compared with the MC 300-series cargo tanks. One commenter stated that while the UG-23(b) calculations may be appropriate for DOT 407 cargo tank with MAWP ratings between 25 and 35 psig, this formula will rarely yield reasonable results for DOT 406 or DOT 412 cargo tanks having a MAWP of 15 psig or less. Commenters recommended two alternatives to the ASME UG-23(b) design calculations, both of which are formulas from engineering texts. One is from the ``Alcoa Structural Handbook,'' 1960, page 156 and Table 23; the other is from ``Formulas for Stress and Strain,'' Fifth Edition, by Roark and Young, pages 554 and 555 and Table 35. The ``Alcoa Structural Handbook'' formula is as follows: TR03NO94.000 where: R i/t s is greater than 200 E = modulus of elasticity of material at design temperature R i = inside radius of the shell (largest radius of non-circular cross-section) t s = minimum thickness of shell less corrosion allowance S bA = critical compressive buckling stress per the Alcoa formula S c = allowable compressive stress due to static bending loads The Roark and Young formula is as follows: TR03NO94.001 where: R i/t s is greater than 10 S bY = critical compressive buckling stress per Roark and Young v = Poisson's ratio Other symbols are the same as in the Alcoa formula, above. Based on the merit of these comments, RSPA is revising paragraph (b) to allow alternative methods for determining compressive buckling stress for DOT 400-series cargo tanks which are not required to be certified in accordance with the ASME Code. This allows manufacturers more freedom in the design of DOT 400-series cargo tank motor vehicles, particularly the DOT 406 cargo tank. RSPA solicited information on the structural integrity of cargo tanks and, in particular, the loading combinations that may be encountered during operation of cargo tank motor vehicles as prescribed in paragraph (c). Information was received from several commenters which indicated that the loadings from normal operating conditions are different from loadings experienced in extreme dynamic events. The normal operating loadings are more frequent in occurrence, but much lower than the extreme dynamic loadings. The requirement contained in current paragraph (c) only specifies extreme dynamic loadings. A cargo tank designer must determine which loadings, if any, should be considered as acting simultaneously. TTMA stated it is unlikely that extreme dynamic loadings will occur and highly unlikely that such loadings will occur at the same time. TTMA reported that if the extreme dynamic loadings are considered by the cargo tank designer as acting simultaneously, the resulting weight of a DOT 406 cargo tank would increase significantly. TTMA went on to state that such an increase in tank weight would have an adverse effect on public safety because it would cause a decrease in the number of gallons delivered each trip, increasing the number of trips and miles driven, thus increasing the probability of more accidents, personal injuries and fatalities. During discussions on structural integrity issues at the February 1994 public meeting, the potential for changes in loading due to liquid movement was addressed. Commenters generally agreed that while significant lading movement can occur during partially loaded conditions and that such movement cannot be disregarded by cargo tank designers and vehicle operators, the variables involved are more than can be comprehensively dealt with at this time. The general agreement was that the highest stress conditions on most cargo tank configurations occur when the cargo tanks are full. Discussions on how to combine the loadings in calculating the structural integrity requirements have been going on for a number of years. RSPA agrees with commenters that the loadings currently in the HMR are based on extreme conditions that would be realized only on a rare occasion, if ever. Thus, based on recent information presented at the public meetings and written comments received in response to the NPRM, RSPA concludes that, for the design and construction of cargo tanks, it is best to consider separately the effects of normal operating loadings, which are known to act in combination, and the effects of extreme dynamic loadings, which are not expected to act in combination with each other. Therefore, in this final rule, RSPA is revising paragraph (c) to provide structural design requirements that will be more reflective of conditions encountered by cargo tank motor vehicles. These revisions will require cargo tank designers to consider normal operating loadings to be acting simultaneously, except that longitudinal acceleration and deceleration cannot occur at the same time. Also, extreme dynamic loadings must be considered in separate calculations; these loadings may be considered to be acting independently. Paragraph (d) prescribes design calculations that should be considered to account for stresses due to impact in an accident. For consistency, the accident damage requirements contained in current paragraph (d) are moved to Sec. 178.345-8(e) where other accident damage protection requirements appear. Also, consistent with other changes made to this section, paragraphs (a) (1) and (3) are amended by removing the reference to paragraph (d) of this section. Paragraph (e) is editorially revised, for clarity and consistency, by changing the word ``wall'' to read ``shell and heads'', and paragraphs (e) through (g) are redesignated as paragraphs (d) through (f). Section 178.345-5 Commenters supported the proposed change in paragraph (b) that all fittings and devices mounted on a manhole cover must withstand the same static internal fluid pressure as that required for the manhole. However, commenters requested a revision to clarify that fitting and device manufacturers are responsible for testing and certifying the structural integrity of their products. RSPA agrees with the commenters that the fitting and device manufacturers should be responsible for ensuring the integrity of their components. Therefore, the proposed provision is revised for clarity and added as new paragraph (f). Another commenter suggested that paragraph (e) be revised to require that each manhole cover must be marked with the date of certification. RSPA will consider this comment in a future rulemaking action. Section 178.345-6 A minor editorial change is adopted as proposed in the NPRM. Section 178.345-8 Commenters recommended several changes to the accident damage protection requirements. They requested that all accident damage protection devices be designed so that calculated stress under the conditions prescribed not exceed the ultimate strength of the material of construction. They pointed out certain inconsistencies in the design criteria specified in this section. For example, in paragraph (d), the design stress for accidents involving longitudinal deceleration is based on ``the lesser of the yield strength or 75 percent of the ultimate strength''; the general requirement for accident damage protection in paragraph (a)(3), is based on ``75 percent of the ultimate strength''; while bottom damage and rollover damage protection, in paragraphs (b) and (c), both are based on the ``ultimate strength.'' RSPA agrees there is merit in using the same criteria whenever possible. Therefore, in this final rule, the design of all accident damage protection devices is based on the ultimate strength of the material of construction. The primary purpose of accident damage protection is to prevent the release of hazardous lading from a cargo tank in the event of an accident. For example, during an accident involving the maximum level of longitudinal deceleration expected, if the front head of a cargo tank experiences stress levels above the yield point of the material of construction, the head will bulge or distort. However, if that deformed head continues to contain the lading, the intent of this requirement has been met. In a practical sense, good engineering practice provides for factors of safety when analytical methods are not well established and when safety considerations call for reducing the probability of failures. When an accident imposes loads on the cargo tank wall, the material of the wall, however, must be stronger than the accident damage protection device. For example, in a rollover accident, the portion of the cargo tank wall to which a rollover protection device is attached should not fail before the rollover protection device fails. The design stresses in the protection device itself can be based on the ultimate strength of the material, but the loads transmitted to the cargo tank wall must be based on a more conservative value. This can be achieved by use of factors of safety. Accordingly, paragraph (a)(3) is revised to base design stresses on the ultimate strength of the material with a 1.3 safety factor (i.e., the reciprocal of 0.75 times ultimate, rounded). Commenters have stated that most impacts on bottom damage protection devices in accidents occur directly from the side of the vehicle. TTMA has stated that any piping at the bottom of a tank is protected fore and aft by the running gear of the cargo tank motor vehicle or its towing vehicle. Contending that reduced forces of impact can be expected from front and rear, TTMA petitioned for a reduction from 155,000 pounds to 27,000 pounds fore and aft along the longitudinal axis of the vehicle. RSPA believes, however, that the possibility of impacts from the front is very real for trailers; for example, during turning maneuvers, or in the event that the towing vehicle rides over an obstacle such as a guard rail. On the other hand, impacts from the rear are less likely on trailers because of the rear suspension. Therefore, RSPA has revised paragraph (b)(1) to recognize that suspension components and structural mounting members can provide all, or part, of bottom damage protection. Additionally, in paragraph (b) introductory text, a second sentence is added to clarify that a single protection device may be used to protect outlets, projections and piping grouped or clustered together. Commenters stated that in the general rollover damage protection requirements, in paragraph (c), the wording ``enclosed inside'' could be misunderstood to require that closures and fittings must be protected from rollover damage on all sides--front, sides, rear and top. They also suggested that the protection devices be located no more than 48 inches from the closure or fitting. RSPA never intended to require that the component being protected be fully enclosed by the protective device. Also, RSPA does not agree with adding a dimensional location requirement. Rather than dimensional controls, one of RSPA's overall objectives is to provide performance requirements when appropriate. Therefore, paragraph (c) is revised to clarify the ambiguous wording. Commenters requested that in paragraph (c)(1), the tangential design load for rollover protection be reduced from 2 ``g'' to 0.5 ``g'' or 1 ``g.'' A commenter stated that ``neither industry nor government have any data to support what this rollover protection device strength should be'', contending that MC 306 accident damage protection has performed well, even considering findings of the NTSB study of overturn accidents. A commenter provided analytical data that indicated the internal bulkheads would be overstressed under 2 ``g'' tangential loads using the current MC 306 design. A February 4, 1992 NTSB investigation report on rollover accidents involving MC 306 and MC 312 cargo tanks recommended several actions by both RSPA and the Federal Highway Administration (FHWA). The NTSB report supported the earlier RSPA decision to increase the rollover design load in the horizontal plane from one-half the weight of the loaded cargo tank motor vehicle prescribed in the MC 306, MC 307 and MC 312 specifications, to twice the weight of the loaded cargo tank motor vehicle prescribed in the DOT 406, DOT 407 and DOT 412 specifications. NTSB also noted that at this time, test results are not available to support this four-fold increase, but limited testing performed under RSPA and FHWA sponsorship for studies of release from dome covers indicate forces can easily exceed 2 ``g''. Additionally, NTSB questioned whether the load specified for the DOT 406, DOT 407 and DOT 412 specifications are adequate in a typical rollover accident. RSPA agrees with comments that in some rollover accidents, cargo tank rotation is limited to 120 degrees or less, so that these horizontal forces do not come into play unless a roadside obstacle is struck. In such incidents, the side of the cargo tank absorbs most of the energy of the rollover. However, in other rollover incidents, cargo tank motor vehicles have rotated 180 degrees or more and rollover protection devices have failed. RSPA performed simple calculations to estimate the forces that would be expected to bring a sliding overturned cargo tank motor vehicle to a halt, at a variety of speeds and stopping distances. From the calculations performed regarding stopping distances, RSPA concludes that the design loads should not be decreased. FHWA will initiate a study aimed at developing a more refined understanding of the forces involved in cargo tank rollover accidents. For these reasons, RSPA rejects requests to lower the tangential design load. In many cases, manufacturers will find it necessary to develop new designs for overturn protection devices, perhaps with associated short-term increased cost, but with enhanced safety benefits. Several manufacturers already have developed satisfactory protection devices which meet these design criteria. In addition, RSPA has made several minor editorial revisions in paragraph (c)(1) to improve clarity. Paragraph (d)(3) prescribes that each cargo tank rear-end protection device and its attachment to the vehicle must be designed to satisfy the conditions specified in paragraph (d)(1) when subject to an impact of the cargo tank at rated payload, at a deceleration of 2 ``g''. Such an impact must be considered as being uniformly applied in a horizontal plane at an angle of 30 degrees or less to the longitudinal axis of the vehicle. Commenters requested elimination of the 30 degree angle for this impact load. They stated that most rear- end collisions of trucks and trailers involve other vehicles and are ``in line'', i.e., the longitudinal centerlines of the two vehicles are parallel at impact. TTMA pointed out that the National Highway Traffic Safety Administration (NHTSA) published a notice of proposed rulemaking [Docket No. 1-11, Notice 9; January 3, 1992], containing a proposal for rear impact guards and protection. The NHTSA proposal specified an impact only in the direction of the longitudinal centerline of the struck vehicle; it did not address angular impact. NHTSA's rear impact requirements are intended for the design of underride guards that will minimize impacts in occurrences where automobiles underride (i.e., slide under) the rear-end of large trucks and trailers. These requirements are intended to protect passenger occupants while RSPA's requirement for cargo tank rear-end protection is intended to prevent impacts to lading retention components that could result in the loss of hazardous material lading. Upon further review, RSPA agrees that requiring rear-end protection devices to withstand impacts at an angle of 30 degrees to the longitudinal axis of the vehicle is excessive. Therefore, paragraph (d)(3) is revised to reduce the angle of impact to 10 degrees. A requirement contained in current paragraph (d) of Sec. 178.345-3 specifying design stress for accidents involving longitudinal deceleration is revised and moved to new paragraph 178.345-8(e) in this final rule. The specified design stress is based on the ultimate strength of the material with a factor of safety of 1.3 (i.e., the reciprocal of 0.75 times ultimate, rounded). The use of 2 ``g'' as a reasonable maximum level for longitudinal deceleration in accident situations generally has been accepted by industry but commenters have stated that the reliability of strain gauge testing and finite element analysis is questionable when structures are loaded above the yield point. For this reason, cargo tank manufacturers who choose to design at this level may use performance testing to prove that tank heads and shell can withstand this 2 ``g'' loading condition. Alternate analytical methods or combinations of test and analysis may be used if they are accurate and verifiable. Section 178.345-10 This section specifies requirements for the pressure relief and vacuum systems on DOT 400-series cargo tank motor vehicles. It also specifies lading retention requirements for the pressure relief system in the event of an overturn. Numerous commenters suggested alternative provisions for pressure relief systems on DOT 400-series cargo tanks. Pressure relief valve manufacturers stated that they have found it very difficult to attain ``no loss of lading'' with valve designs capable of withstanding the characteristic dynamic pressure surge required by the regulation, especially at low design pressures. For these reasons, commenters requested that RSPA allow the loss of one liter of lading. If adopted, a properly 
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