NTSB Safety Recommendation Letter P-82-030

Page 1Official PDFNATIONAL TRANSPORTATION SAFETY BOARD WASHINGTON, D.C. ISSUED: September 8, 1982 Mr G. H. Lawrence President American Gas Association 1515 Wilson Boulevard Arlington, Virginia 22209 \ SAFETY RECOMMENDAT 1 ON ( 5 ) P-82-30 - About 10:30 a.m., e.s.t., on January 28, 1982, at Centralia, Missouri, natural gas at 47 psig entered a low pressure distribution system which normally operated at 11 inches water column (0.40 psig) after a backhoe bucket snagged, ruptured, and separated the 3/4-inch-diameter steel pressure regulator control line at the Missouri Power and Light Company's district regulator station No. 1. The backhoe, which was owned and operated by the city of Centralia, was being used to clean a ditch located adjacent to the pressure regulator station. The high pressure gas entering customer piping systems in some cases resulted in high pilot light flames which initiated fires in buildings; while in other cases, the pilot light flames were blown out, allowing gas to escape within the buildings. Of the 167 buildings affected by the overpressure, 1 2 were destroyed and 32 sustained moderate to heavy damages. Five persons received minor injuries. L/ When the backhoe bucket snagged and broke the regulator control line, the regulator, in an attempt to hold the 11 inch W.C. (0.40 psig) and satisfy what the regulator sensed to be a demand for additional gas, opened wide. The wide open position of the regulator allowed gas at 47 psig to enter the low pressure sistem and overload all the appliances. When the overload occurred, some pilot lights were blown out, permitting gas at over 100 times its normal pressure to fill the building. In structures where the furnaces or stoves were in use, the gas flames intensified and burned flammable building materials within the structures. The purpose of the relief valve in series with the district regulator is to avoid overpressuring the downstream, low pressure distribution system. When the relief valve senses an overpressure, it automatically opens and relieves the pressure on the downstream distribution system by venting the excess gas into t h e atmosphere. If the overpressure is small, the relief valve opens just enough to relieve or vent the excess gas. If the overpressure is large, the relief valve opens wider to vent the excess gas through the vent line. To operate properly, the relief valve must have t h e valves open on the inlet side and on the sensing line. Tests after the accident showed that the relief valve opened properly at its low-pressure set point. Debris was found in the vent line before the test, but had - 1/ For more detailed information, read "Pipeline Accident Report -- Missouri Power and Light Company, Natural Gas Fires, Centralia, Missouri, January 28, 1982," (NTSB-PAR-82-3). 3589#

Page 2-2- been blown out after t h e test. Based on a review of t h e gas company's records, it was determined that during an inspection on September 8, 1981, the relief valve had been turned off and entered as such on the inspection form; there w a s no entry stating that the valve had been returned to operation. The in-place test results indicated that the relief valve was in good working order, but at the time of the accident, either the gate valve under the relief valve was closed or the plug valve in the sensing line was dosed. Therefore, the Safety Board must conclude that the relief valve was inoperative before the accident because it failed to operate. The Safety Board's review of the annual inspection reports for regulator Station No. 1 indicated that during the 5-year period before t h e accident the regulator station's relief valve was variously turned off or not checked. The annual inspection reports for t h e regulator station had been filed and never reviewed or studied by gas company management and the fact that the reIief valve was pIaced in an inoperative mode was not known to management, nor did it know why it was inoperative, why any deficiency was not immediately repaired, or make any analysis of what the consequences might be if the valve WRS left in the inoperative mode. Because such analysis of these inspection records was not routinely performed, gas company management was not alerted to the fact the overpressure protection for the low-pressure system in Centralia had been nullified. The serviceman who performed the last annual inspection for station No. 1 said that he reported the position of the relief valve to his supervisor; however, no records exist to indicate what action the supervisor took to rectify the situation. If, at the time of the control line rupture, the relief valve had been in the open or operational position, this accident would have been avoided because the high pressure gas would have been vented to the atmosphere through the relief valve's vent line and the low-pressure distribution system would have been protected from overpressure. fherefore, the National Transportation Safely Board recommends that the American Gas Association: Notify its member companies of the circumstances of this accident and urge them to inspect their regulator station relief valves or other overpressure protection equipment to make sure that they are set and operating properly. (Class 11, Priority Action) (P-82-30) 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." (P.L. 93-633). The Safety Board is vitally interested in any actions taken as a result of its safety recommendations. Therefore, we would appreciate a response from you regarding action taken or contemplated with respect to the recommendation in this letter. BURNETT, Chairman, and McADAMS, BURSLEY, and ENGEN, Members, concurred in this recommendation. GOLDMAN, Vice Chairman, did not participate. i#