NTSB Safety Recommendation Letter P-98-013

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-98-13 Mr. David N. Parker President and Chief Executive Officer American Gas Association 15 15 Wilson Boulevard Arlington, VA 22209 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 Sail 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 circumferential 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 s w e y of the accident history of plastic piping suggested that the material may he 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--Sun .Jiiuit Gas Cunipany, Inc./Eivoii Curp, Propane Gar Exphioil in Suit .Juan Piieria Rico, on November 21, /996 (NTSB/PAR-97/0 1). 'Railroad Commission of Texas 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 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; e The adequacy of available guidmce relating to the installation and protection of plastic piping connections to steel mains; and e 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 lovestigation Report- Brittle-like Ctuckiug in Plusric Pipe for Gus Service (NTSBISIR-98IOl)#

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 tlie manufacturers’ published literature referred tlie 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. Tlie Safety Board attempted to identify every U.S. steel tee manufacturer that currently manufacturers 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 tlie plastic pipeline accidents it has investigated, the Safety Board concluded that, because guidance covering tlie 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 service involved in tlie Waterloo, Iowa, accident was installed with a bend at tlie connection point to the main Tlie plastic service pipe leaving tlie tee immediately curved horizontally. The pipe was cut out and brought into the laboratory, at which time tlie bend had a measured horizontal radius of approximately i 4 inches. Based on field conditions and photos. MidAmerican Energy (the current Waterloo system operator,) estimated tlie original installed horizontal bend radius to have been about 32 inches. This bend is sharper tlian that allowed by current industry installation recommendations for modern piping ad,jacent to fittings, The most recent edition of the A G A Plastic Pipe hfnr?frn/ for Fn, Service4 identifies the connection of plastic pipe to service tees as “a critical junction” needing installation measures “to avoid the potentially high ..stresses on tlie plastic at this point ” Altliougli tlie manual recommends proper support and the use of protective sleeves, no guidance is included on the 4A G A Plastic Pipe Mu/ii/alfo~ Gar Seivice, American Gas Association. Catalog No XR 9401, 1994#

Page 44 importance of a protective sleeve's proper length, diameter, or placement,, Instead, the manual includes a sentence recommending that manufacturers' instructions be followed carefully. Such advice presumes that the manufacturers' instructions address designing the sleeve to have the correct inner diameter and length, as well as positioning the sleeve properly, in order to limit the shear and bending forces at the connection. Unfortunately, since none of the steel tapping tee manufacturers recommend any precautions to limit shear and bending forces at the connection point, gas pipeline operators may not realize the importance of determining these parameters. The manual includes, without elaboration, the following sentence: Installation ofthe tee outlet at angles up to 45" from the vertical or along the axis of the main as a 'side saddle' or 'swing joint' may be considered to further minimize., stresses. This sentence is subject to different interpretations and does not explain how stresses might be reduced. Moreover, many gas system pipeline operators recognize that installing services 90" from the main helps with future locating of the pipe and reduces the likelihood of excessive bending, which could generate excessive stxess. In the view of the Safety Board, this sentence does not provide useful guidance as it is written, and the A,G.A, Plastic Materials Committee would be well advised to either expand on or delete this sentence. Figure 48 from the Distribution Book 0 - 2 of the A.G.A,'s GEOP series shows a steel tapping tee with a compression coupling joint connected to a plastic service. 'The illustration shows a protective sleeve and includes a note to extend the protective sleeve to undisturbed or compacted soil or to blocking. But the figures also show the blocking positioned so that either the edge of the blocking or the edge of the protective sleeve might provide a fixed contact point on the plastic service pipe i f the weight of backfill were to cause the pipe to bend down. Additional illustrations within this GEOP Series book show this same positioning ofthe bloclcing with respect to the plastic pipe. 'The Safety Board notes that ASME B31.8 and ASTM D2774 discourage supporting plastic pipe by the use of blocking. In the view of the Safety Board, these illustrations would provide better guidance if they were revised to eliminate showing the possibility of blocking or other fixed contact point supporting plastic pipe. 'The National Transportation Safety Board therefore makes the following safety recommendation to the American Gas Association: Revise your Plastic Pipe Manual for Gas Service and your Gas Engineering and Operating Practices series to provide complete and unambiguous guidance for limiting stress at plastic pipe service connections to steel mains. (P-98-13) 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-98-10 to the Gas Piping 'Technology Committee; P-98-11 and -12 to the American Society for Testing and Materials; P-98-14 and -15#

Page 55 to MidAmerican Energy Corporation; P-98-16 and -1 7 to Continental Industries, Iiic ; P-98-18 to Dresser Industries, Inc.; P-98-19 to IMer-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 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 taken or contemplated with respect to the recommendation in this letter Please refer to Safety Recommendation P-98-13 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.#