NTSB Safety Recommendation Letter P-76-036 through P-76-038

Page 1Official PDFSPORTATION SAF WASHINGTON, D.C. SAFETY RECOMMENDAT I O N (S) P-76-36 thrmgh P-76-38 A t 11:50 a.m. on June 9, 1976, a house at 1214 oak S t r e e t in N i l e s , Michigan, explcded and burned. The explosion and f i r e de- stroyed the house and critically burned the three occupants. The National Transportation Safety Board's investigation disclosed that natural gas at 3-psig pressure had escaped f r m a broken 3- by 3- by l%-inch cast iron tee, which w a s &ut 40 feet fram the house; the gas, trapped f r m a b v e by the concrete paving of the heavily traveled street, had formed a resemmir under the road, had migrated through the soil to the north and east baserent walls of the house, and had entered the building. The source of ignition could not be determined. The 3-inch threaded steel main was constructed a b u t 1920 and is an unusual type of construction i n the gas distribution in- dustry. Same, i f not a l l of the Len- of steel pipe w e r e joined by threaded cast iron tees; steel bushings were used to connect 1- inch service lines to the 1%-inch tee openings. The cast iron tee which connected the service line for 1213 oak S t r e e t broke circum- ferentially, and the break extended through the 1%-inch outlet. Several blocks, including the 1200 blodk of oak Street, were widened and rebuilt during the sumw: of 1973. The area was excavated to within a few inches of the 3-inch main, a new roadkd was in- stalled, and the surface was paved with concrete. The min, previ- ously at the edge of the road, m is positioned under the mad- way. During construction, heavy machinery traveled over and adjacent to the main and m y have caused secondary stresses to be imposed on the pipe as w e l l as to have disturbed the pipe foundation. 1845#

Page 2-2- On February 27, 1974, a leak at the threads of a 3-inch cast iron tee i n front of 1218 oak Streetwas repaired. On Janumy 7, 1 9 the house a t 1218 oak Street exploded; the Michigan Public Service Cornmission investigated the accident and deteimined that "A broken 3-inch threaded cast iron tee was found under the pavement in front of 1218 oak Street." Thus, there have been two accidents involving failures of this unusual gas min construction w i t h i n 3 years after a traffic l a d was applied. Although witnesses stated that they had detected the odor of ga after the June 9, 1976, explosion, persons who were i n the hime just before it exploded did not. The section of pipe that was re- moved from the 3-inch m h did not smell of natural gas, and sniff tests that w e r e performed on gas coming directly f r m a gas appliance a t 1213 oak Street revealed no discernable odor. In 49 CFR 192.625 (b), "odorization of Gas,'' gas operator; are required t o assure that the intensiw of the odorant added t o natural gas is sufficient to be readily detectable at con- centrations of one-fifth of t?e l m e r explosive limit, wluch is a gas-in-air ratio of a b u t 1 permit. Michigan P m e r Company's policy is to have an exployee select a location at randm once a w e , bleed gas into the atmsphere, and determine the relative odor level without consideration of the gas-in-air ratio. Therefore, the National Transportation Safety Board Michigan Power m a n y : Review its records to determine the extent of the system which m y contain steel pipe joined by cast iron fittings subject to adverse traffic loads or foundation settl-t. Inspect and evaluate a representative saqle of these fittings, and i f further failures or failure conditions are found, take action to prevent additional failures. (P-76-36) (Class I, Urgerit Followup) Conduct odor level tests promptly i n each distribution system downstream of pressure reduction stations ta determine the gas-in-air ratio a t which the odor i n the gas is f i r s t detectable. Based upon the results, take necessary action to assure that dl protions of the distribution systems are cdorized sufficiently to alert the public ta any inadvertmt release of gas. The mthods used to perform these tests, the location of the testpoints, and any corrective action slmuld reviewed by the Michiganpublic Service Canisssion. (P-76-37) (Class I, Urgent Followup)#

Page 3-3- Review prccedures for checking gas odorant concentrations to provide a mtbod that will determine the gas-inair ratio a t which the odor in gas is first detectable, establish tests point(s) in each piping system downstream of pressure reducing stations that can be expcte.3 to ke representative of the odor level i n that piping system, and perform tests a t frequent intervals. (P~76-38) ( C l a s s I, Urgent Follmrp) TODD, cbzdnnm , McNXMS, HCGUE, and BIJKZSS, MFsnbers, concurred in the W e recmmendations. HALEY, MErmber, did not participate.#