NTSB Safety Recommendation Letter P-98-010

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-98-10 Mr. John H. Frantz Chairman Gas Piping Technology Committee PECO Energy Company 300 Front Street West Conshohecken, Pennsylvania 19428-2723 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 cracked 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 and 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 migrated under pressure from a failed plastic pipe that displayed evidence of brittle-like circun~ferential cracking. The Railroad Commission of Texas investigated a natural gas explosion and fire that resulted in one fatality in Lake 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--So~i Jiian Gas Company, Inc./Enron Carp , Propone Gas E.sp/aplosion in Sun Jnmi, Prrerro Rico, on November .?I, 19Y6 vTSBIPAR-97IO 1 ) 'Railroad Commission of Texas Accident Investigation No 97-AI-055, October 3 I , 1997 6984#

Page 2vulnerability 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 gas 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. Three 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:' * 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 3 For. more information, see National 'Transportation Safety Board Pipeline Special Investigation Report- ( Brifrle-like Cracking in PlaMc Pipefor Gas Service (NTSBlSlR-98lOl).#

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 coilnections to steel mains via steel tapping tees. With respect to the specific issue of limiting bends, DuPont, in .January 1970, issued recommendations to limit bends for polyethylene pipe. DuPont/Uponor‘ later published bend radius recommendations that differentiated between pipe segments consisting of pipe alone and those with fusion fittings. The recommendations specified much less bending for pipe segments with fusion fittings; however, DuPontllJponor did not provide bend limits for mechanical fittings. Two of the other major manufacturers (Phillips Driscopipe and Plexco) provide bend limits and differentiate between pipe alone and pipe with fittings, without specifying the type of fittings. None of the manufacturers’ literature discusses bending with or against any residual bend remaining in the pipe after it is uncoiled. Of these four major polyethylene gas pipe nianufacturers, only one had no published recommendations for limiting shear and bending forces at plastic service connections to steel mains via steel tapping tees. Although that company does not manufacture steel tapping tees with compression ends for attachment to plastic services, it does manufacture pipe that will be attached to steel tapping tees via mechanical compression couplings. The Safety Board attempted to identify every U.S. steel tee nianufactuier 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. The service involved in the Waterloo, Iowa, accident was installed with a bend at the connection point to the main. The plastic service pipe leaving the tee immediately curved ‘Uponor purchased DuPont’s plastic pipe business in 1991#

Page 44 horizontally,. ‘The pipe was cut out and brought into the laboratory, at which time the bend had a measured horizontal radius of approximately 34 inches. Based on field conditions and photos, MidAmerican Energy (the current Waterloo system operator) estimated the original installed horizontal bend radius to have been about 32 inches. This bend is sharper than that allowed by current industry installation recommendations for modern piping adjacent to fittings. Based on its review of this guidance and on the history ofthe 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. In its investigation of the previously referenced 1971 accident in Texas, the Safety Board determined that protective sleeves were too short to fully protect a series of service connections to a main. The Safety Board noted that a protective sleeve must have the comect inner diameter and length if it is to protect the connection from excessive shear forces. As a result, and in response to a Safety Board safety recommendation,’ the 1974 and later editions of the GPTC Guide for Gas Transmission and Distributiori Pipirig System included guidance that “a protective sleeve designed for the specific type of connection should be used to reduce stress concentrations .” Designing protective sleeves for the specific connection is presumed to include designing the sleeve for the correct inner diameter and length, and may also include positioning the sleeve correctly, since positioning the sleeve affects its effective length. However, i f steel tapping tee manufacturers do not address the parameters for sleeve design and positioning, gas pipeline operators may not realize the importance of determining these parameters. ‘The guidance would be much more useful to gas pipeline operators i f the GPTC included in the guide a specific statement of the need to design protective sleeves so that they will have the correct inner diameter and length, as well as the need to properly position the sleeves. The GPTC Guide does not include recommendations to limit bending in plastic piping during the installation of sewice lines under 49 Code of Federal Regztlarions (CFR) 192.361 Although the Guide references the A G A Plastic Pipe Mcinual for Gar Service, and this manual does provide recommendations on bending limits, the GPTC Guide does not reference this manual in its guidance material under 49 CFR 192.361 The National Transportation Safety Board therefore makes the following safety recommendation to the Gas Piping ‘Technology Committee: Revise the Guide for Gas Transrnirsion and Distribution Piping Systems to include complete guidance for the proper installation of plastic service pipe connections to steel mains. The guidance should emphasize the need to limit pipe 5 Safety Recommendation P-72-64 from National Transportation Safety Board Pipeline Accident Report- Lone Slut Gus Company, For1 Worrlt, Texas, Octobet 4, 1971 (NTSBIPAR-7215), /, i#

Page 55 bending and should include a discussion of the proper design and positioning of a protective sleeve to limit stress at the connection. (P-98-10) Also, the National Transportation 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-98-11 and -12 to the American Society for Testing and Materials; P-98-13 to the American Gas Association; P-98-14 and -15 to MidAmerican Energy Corporation; P-98-16 and -1 7 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 formulating safety improvement recommendations” (Public Law 9.3-6.33) 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 taken or contemplated with respect to the recommendation in this letter. Please refer to Safety Recommendation P-98-10 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 this recommendation.#