NTSB Safety Recommendation Letter P-78-043

Page 1Official PDFT*P- 9 2. NATIONAL TRANSPORTATION SAFETY BOARD WASHINGTON, D.C. I S S U E D : July 31, 1978 -----------__"_____________I____________ Forwarded to: M r . Lowell Elder, Chairman American Society of Mechanical Engineers * Gas Piping Standards Committee United Engineering Center 345 E a s t 47th Street New York, New York 10017 SAFETY RECOMMENDAT I ON (s) P-78-43 A t 9:00 a . m . , c.d.t., on J u l y 30, 1 9 7 7 , a low-order explosion followed by f i r e destroyed a house i n Cherokee, Alabama. One person i n the 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 gas, leaking from a broken 2 1/4-. inch cast-iron gas main, migrated i n t o t h e house through a r e c e n t l y b a c k f i l l e d sewer trench and w a s 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 laterals were perpendicular t o and below t h e 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 the 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 c h a r t a t 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 at 9:00 p.m. on J u l y 29, 1 9 7 7 . Metering pressure was 4 p s i g below normal. In the 12 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 211 r e s i d e n t i a l and commercial customers. The average summer load is only 1 2 Mcf/day; a maximum hour demand of 14.5 Mcf w a s sustained during a peak d a i l y demand of 346 M c f 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 U n i t e d S t a t e s . * Mcf = Thousand Cubic Feet 2402#

Page 2Federal regulations ( 4 9 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 pressure-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. A s 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 * Had Cherokee been aware of the use in the early detection of the leak. 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 s o , 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 Gas Piping Standards Committee of the American Society of Mechanical Engineers: Develop guidelines for the installation and operation of pipeline monitoring alarms on applicable single-feed systems, such as Cherokee, which will promptly alert operators to emergency conditions such as linebreaks which are evidenced by abnormally high gas flow rates or pressure reductions. (Class 111, Longer Term Action) (P- 78-43) KING, Chairman, KcA.DAMS, ROGUE, and DRIVER, Members, con#