NTSB Safety Recommendation Letter P-78-044

Page 1Official PDFNATIONAL TRANSPORTATION SAFETY BOARD WASHINGTON, D.C. ISSUED: July 31, 1978 --_________----_____-”------------------- Forwarded to: M r . L. D. Santman Director Materials Transportation Bureau Department of Transportation Washington, D . C . 20590 SAFETY RECOMMENDAT I ON(S) P-78-44 A t 9:00 a . m . , c.d.t., on J u l y 30, 1977, a low-order explosion followed by f i r e destroyed a house i n Cherokee, Alabama. One person i n t h e house suffered third-degree burns, was hospitalized, and died on August 3, 1977. The National Transportation Safety Board’s i n v e s t i g a t i o n of t h e accident revealed t h a t n a t u r a l g a s , leaking from a broken 2 1/4- inch cast-iron g a s main, migrated i n t o the house through a r e c e n t l y b a c k f i l l e d sewer trench and was i g n i t e d by a c i g a r e t t e l i g h t e r . The sewer l a t e r a l s were perpendicular t o and below the gas main. Earth settlement of t h e sewer b a c k f i l l , aided by recent heavy r a i n f a l l , caused t h e f a i l u r e of t h e cast-iron main, which was owned by t h e Cherokee Water and Gas Board (Cherokee). A recorded increased r a t e of gas flow and concurrent pressure drop shown on a 7-day chart at t h e Alabama-Tennessee Natural Gas Company’s Cherokee City Gate S t a t i o n indicated t h a t the main broke a t 9:OO p.m. on J u l y 29, 1977. Metering pressure w a s 4 p s i g below normal. I n t h e 1 2 hours between the break and the explosion, approximately 200 Mcf* of gas escaped through a 1/4-inch f r a c t u r e . The c i t y of Cherokee has a small single-gate s t a t i o n pressure regulating d i s t r i b u t i o n system serving 2 1 1 r e s i d e n t i a l and commercial customers. The average summer load is only 12 Mcf/day; a maximum hour demand of 14.5 Mcf was sustained during a peak d a i l y demand of 346 Mcf on December 1, 1976. The l e a k under discussion occurred during a minimal load period, but would have been apparent on t h i s system at any t i m e of t h e year. There a r e many small systems comparable t o Cherokee’s throughout t h e United S t a t e s . *Mcf = Thousand Cubic Feet 2402#

Page 21 - 2 - Federal regulations (49 CFR 192.741) require all but single-gate pressure-regulating distribution systems to telemeter or record gas pressure in the system. On systems that are supplied by a single-gate pressur&-regulating station, the Federal regulations are ambiguous and leave it up to the operator as to whether telemetering or pressure gauges are necessary in the system. However, in either case, the standards do not require that a pipeline operator continuously monitor the gas pressure to insure early detection of any problems. As in the case at Cherokee, the 7-day chart readily displayed the problem, but the chart was not looked at until after the accident and therefore was of no use in the early detection of the leak. Had Cherokee been aware of the pressure drop or rate-of-flow increase immediately, corrective action could have been taken. The use of telemetering equipment, although not specifically required, is a practical consideration for small, single-gate systems if it can be monitored continuously. However, most small systems do not have enough personnel for this. Cherokee has only two full-time gas system employees. Because it is expensive and inefficient for small systems like Cherokee to provide personnel for 24-hour monitoring, many distribution operators, although not required to do so, have telemetering devices with a high/low alarm system in operation for each station. If the pressure or the flow rate changes beyond preset limits, an alarm sounds. In this case, an alarm at the gas company's Cherokee City Gate Station could be telemetered to a continuously monitored location such as the city's emergency desk or the pipeline supplier's station. The party receiving the alarm could then have alerted gas company personnel to the problem and initiated corrective action. Therefore, the National Transportation Safety Board recommends that the Office of Pipeline Safety Operations of the Materials Transportation Bureau: Amend 49 CFR 192.741 to require that pipeline monitoring equipment be installed on single-gate pressure regulating distribution systems and the information be transmitted to a continuously manned location. (Class 111, Longer Term Action) (P-78-44) KING, Chairman, McADAMS, HOGUE, and DRIVER, Members, concurred in the above recommendation.#