NTSB Safety Recommendation Letter P-98-011 through P-98-012

Page 1Official PDFNational Transportation Safety Board Washington, D.C. 20594 Safety Recommendation Date: A p r i l 30, 1998 In reply refer to: P-98-11 and -12 Mr. James A. Thomas President American Society for Testing and Materials 100 Barr Harbor Drive West Conshohocken, Pennsylvania 19428-2959 Despite the general acceptance of plastic piping as a safe and economical alternative to piping made of steel or other materials, the Safety Board notes that a number of pipeline accidents it has investigated have involved plastic piping that craclted in a brittle-like manner. For example, on October 17, 1994, an explosion and fire in Waterloo, Iowa, destroyed a building and damaged other property. Six persons died and seven were injured in the accident. The Safety Board investigation determined that natural gas had been released from a plastic service pipe that had failed in a brittle-like manner at a connection to a steel main. The Safety Board also investigated a gas explosion that resulted in 33 deaths aid 69 injuries in San Juan, Puerto Rico, in November 1996.' The Safety Board's investigation determined that the explosion resulted from ignition of propane gas that had migmted under pressure from a failed plastic pipe that displayed evidence of brittle-lilce circumferential craclting. The Railroad Commission of Texas investigated a natural gas explosion and fire that resulted in one fatality in L,ake Dallas, Texas, in August 1997,' A metal pipe pressing against a plastic pipe generated stress intensification that led to a brittle-like crack in the plastic pipe A broader Safety Board survey of the accident history of plastic piping suggested that the material may be susceptible to premature brittle-like cracking under conditions of stress intensification. No statistics exist that detail how much and from what years any plastic piping may already have been replaced; however, hundreds of thousands of miles of plastic piping have been installed, with a significant amount of it having been installed prior to the mid-1980s. Any 'For more information, see National Transportation Safety Board Pipeline Accident Report-San ./van GCIY Company, I~?c./€nron Carp, Propane Gas Explo.ii017 in Sa17 J71a1i. Pirerro Rico, 011 No~e117beI 21, 1996 (NTSBIPAR-97IO I ) 'Railroad Commission of Tesas Accident Investigation No 97-AI-055, October 3 I , 1997 6984#

Page 22 vulnerability of this material to premature failure could represent a serious potential hazard to public safety. In an attempt to gauge the extent of brittle-like failures in plastic piping and to assess trends and causes, the Safety Board examined pipeline accident data compiled by the Research and Special Programs Administration (RSPA),. The examination revealed that the data were insufficient to serve as a basis for assessing the long-term performance of plastic pipe. Lacking adequate data from RSPA, the Safety Board reviewed published technical literature and contacted more than 20 experts in gas distribution plastic piping to determine the estimated frequency of brittle-like cracks in plastic piping, 'The majority of the published literature and experts indicated that failure statistics would be expected to vary from one gas system operator to another based on factors such as brands and dates of manufacture of plastic piping in service, installation practices, and ground temperatures, but they indicated that brittle- like failures, as a nationwide average, may represent the second most frequent failure mode for older plastic piping, exceeded only by excavation damage., The Safety Board asked several system operators about their direct experience with brittle-like cracks. Four major gas system operators reported that they had compiled failure statistics sufficient to estimate the extent of brittle-like failures,, Thee of those four said that brittle-like failures are the second most frequent failure mode in their plastic pipeline systems. One of these operators supplied data showing that it experienced at least 77 brittle-like failures in plastic piping in 1996 alone. As an outgrowth of the Safety Board's investigations into the Waterloo, Iowa; San Juan, Puerto Rico; and about a dozen other accidents, and in view of indications that some plastic piping, particularly older piping, may be subject to premature failure attributable to brittle-like cracking, the Safety Board undertook a special investigation of polyethylene gas service pipe. 'The investigation addressed the following safety issues:3 0 The vulnerability of plastic piping to premature failures due to brittle-like cracking; * The adequacy of available guidance relating to the installation and protection of plastic piping connections to steel mains; and * Performance monitoring of plastic pipeline systems as a way of detecting unacceptable performance in piping systems. Almost all of the plastic pipeline accidents the Safety Board has investigated involving brittle-like cracking have been linked to stress intensification generated by external forces acting on the pipe. Examples of conditions that can generate stress intensification include differential earth settlement, particularly at connections with more rigidly anchored fittings; excessive 3For more information, see National Transportation Safety Board Pipeline Special Investigation Report-- Brittle-like Cracking in Plurtic Pipe fot Gus Service (NTSBISIR-98/01)#

Page 3bending as a result of installation configurations, especially at fittings; and point contact with rocks or other objects. The Safety Board’s special investigation determined that much of the available guidance to gas system operators for limiting stress intensification at plastic pipeline connections to steel mains is inadequate or ambiguous. Safety Board investigators contacted representatives of the four principal companies that marketed plastic piping for gas service to determine to what extent plastic piping manufacturers were providing recommendations for limiting shear and bending forces at plastic service connections to steel mains via steel tapping tees Three of these manufacturers had published recommendations addressing these issues. These three manufacturers have historically emphasized heat fusion fitting systems instead of field-assembled mechanical fitting systems. Representatives of these manufacturers indicated that mechanical fittings manufacturers should provide installation instructions covering their systems. Accordingly, one of the manufacturers’ published literature referred the reader to the manufacturers of mechanical fittings for installation instructions. Nonetheless, these three major’ polyethylene pipe manufacturers did, in fact, provide recommendations to limit shear and bending forces, and these recommendations can apply to plastic service connections to steel mains via steel tapping tees. The Safety Board attempted to identifi every U.S. steel tee manufacturer that currently manufactures steel tees with a compression end for plastic gas service connections. None of these manufacturers has published installation recommendations to limit shear and bending forces on the plastic pipe that connects to their steel tapping tees. Based on its review of this guidance and on the history of the plastic pipeline accidents it has investigated, the Safety Board concluded that, because guidance covering the installation of plastic piping is inadequate for limiting stress intensification at plastic service connections to steel mains, many of these connections may have been installed without adequate protection from shear and bending forces. The most recent ASTM standard covering the installation of polyethylene piping (D2774) was revised in 1994. This standard addresses the vulnerability of the point-of-service connection to the main. The standard further recommends the use of a protective sleeve if needed to protect against possible differential settlement. The standard practice additionally advises consultation with manufacturers, which would presumably address designing the sleeve with a proper diameter and length, as well as positioning the sleeve correctly. However, as noted previously, none of the steel tapping tee manufacturers has recommended precautions to limit stresses at the service to main connection; therefore, gas pipeline operators may not realize the importance of determining these parameters. Currently, manufacturers that provide protective sleeves have their own criteria for designing sleeve lengths and diameters for their fittings. Some manufacturers’ criteria are based on limiting stress to a maximum safe value, while one manufacturer has advised the Safety Board that its sleeve is not designed to limit bending, but only to guard against shear forces at the connection point. No standard criteria exist for designing protective sleeves. A published#

Page 44 common criteria would better motivate a wider spectrum of manufacturers and gas operators to apply scientific reasoning to their decisions on protective sleeve use. A published common criteria would provide guidance to gas operators who provide their own sleeves rather than using manufacturer-supplied sleeves. ‘The National Transportation Safety Board therefore malces the following safety recommendations to the American Society for Testing and Materials: Revise ASTM D2774 to emphasize that a protective sleeve, in order to be effective, must be of the proper length and inner diameter for the particular connection and must be positioned properly. (P-98-11) Develop and publish standard criteria for the design of protective sleeves to limit stress intensification at plastic pipeline connections (P-98-12) Also, the National ‘TranspoItation Safety Board issued Safety Recommendations P-98-1 through -5 to the Research and Special Programs Administration; P-98-6 to the Gas Research Institute; P-98-7 through -9 to the Plastics Pipe Institute; P-98-10 to the Gas Piping Technology Committee; P-98-13 to the American Gas Association; P-98-14 and -15 to MidAmerican Energy Corporation; P-98-16 and -17 to Continental Industries, Inc : P-98-18 to Dresser Industries, Inc,; P-98-19 to Inner-’Tite Corporation; and P-98-20 to Mueller Company, The National ‘Transportation Safety Board is an independent Federal agency with the statutory responsibility “to promote transportation safety by conducting independent accident investigations and by fomulating safety improvement recommendations” (Public Law 93-633). The Safety Board is vitally interested in any action taken as a result of its safety recommendations Therefore, it would appreciate a response from you regarding action talcen or contemplated with respect to the recommendations in this letter, Please refer to Safety Recommendations P-98-11 and -12 in your reply. If you need additional information, you may call (202) 3 14-6469. Chairman HALL, Vice Chairman FRANCIS, and Members HAMMERSCHMIDT, GOGLIA, and BLACK concurred in these recommendations. By:#