NTSB Safety Recommendation Letter P-77-034 through P-77-036

Page 1Official PDFB- g i i NATIONAL TRANSPORTATION SAFETY BOARD WASHINGTON, D.C. ISSUED: October 31, 1977 Acting D i r e c t o r M a t e r i a l s Transportation Bureau Washington, D.C. 20590 i Department of Transportation P-77-34 through P-77-36 A t 11 a . m . , c.s.t., on December 7 , 1976, a n a t u r a l gas compressor operating a t 1,000 p s i g p r e s s u r e f a i l e d and then exploded and burned a t an Exxon Gas System, I n c . , s t a t i o n near Robstown, Texas. An emergency shutdown system w a s a c t i v a t e d by a worker a t t h e s t a t i o n , b u t t h e four automated f i r e g a t e v a l v e s on t h e two p i p e l i n e s t h a t supplied t h e s t a t i o n d i d n o t c l o s e . Another emergency c o n t r o l system a l s o f a i l e d t o automatically c l o s e t h e shutoff valves leading t o two of t h e f i v e compressors i n t h e s t a t i o n . I-/ A t 11:30 a . m . , a second explosion occurred w i t h i n t h e burning b u i l d i n g i n one of t h e compressors t h a t had n o t s h u t down automatically. P i p e l i n e valves upstream and downstream of t h e s t a t i o n had t o be closed manually. Gas burned f o r 3 hours u n t i l the p i p e l i n e p r e s s u r e decreased t o 250 p s i g and t h e f i r e g a t e valves could be approached and closed manually. The f i r e k i l l e d one person, i n j u r e d two persons, and destroyed three engine-driven compressors and t h e compressor b u i l d i n g . Property damage and gas l o s s was estimated to be $5 m i l l i o n . In t h e i n v e s t i g a t i o n of t h i s accident t h e S a f e t y Board h a s learned of p a s t f a i l u r e s of s t u d s and r e l i a b i l i t y problems with shutdown devices. Several gas transmission companies have s t r i n g e n t torquing and t e s t i n g procedures f o r s t u d s on compressors. T i t l e 49 CFR 1 9 2 does n o t have any minimum s a f e t y standards concerning t h i s maintenance a c t i v i t y which is a common occurrence i n a l l compressor s t a t i o n s . Also, s e v e r a l gas transmission companies i n s p e c t and test t h e i r remotely c o n t r o l l e d shutdown devices as f r e q u e n t l y a s every 1 t o 3 months. The S a f e t y Board b e l i e v e s t h a t t h e c u r r e n t l y allowable 1-year period between i n s p e c t i o n s is too long f o r such c r i t i c a l c o n t r o l equipment. 1 - / For more d e t a i l e d information about t h i s accident read: "Pipeline Accident Report, Exxon Gas System, I n c . , Natural Gas Explosion and F i r e , Robstown, Texas, December 7 , 1976" (NTSB-PAR-77-3). 2198#

Page 2- 2 - There are no requirements o r mention i n 49 CFR 192 of instrument air c o n t r o l s use i n conjunction with emergency shutdown devices. I f instrument a i r i s used f o r t h i s a p p l i c a t i o n , t h e r e should be provision f o r backup o r emergency pneumatic f a c i l i t i e s so that one f a i l u r e event w i l l not make t h e emergency c o n t r o l system inoperative. Therefore, t h e National Transportation Safety Board recommend t h e M a t e r i a l s Transportation Bureau of t h e U.S. Department of Tran Review compressor s t a t i o n a c c i d e n t s t o determine i f there have been s i m i l a r problems with remote-control shutdown devices. If t h e r e have been r e l i a b i l i t y problems, make a survey t o determine t h e optimum time between inspections and amend 49 CFR 192.731(c) by decreasing t h e time i n t e r v a l between i n s p e c t i o n and t e s t i n g from t h e c u r r e n t minimum of 1 year t o r e f l e c t these findings. (Class 111, Longer Term Followup) (P-77-34) Add t o 49 CFR 192 a requirement f o r pneumatic-operated compressor s t a t i o n equipment, similar t o t h e requirement i n 49 CFR 192.167(3) f o r e l e c t r i c - o p e r a t e d equipment, t o i s o l a t e instrument a i r supply t o automatic f a c i l i t i e s , and t o provide backup o r s e p a r a t e emergency pneumatic f a c i l i t i e s . (Class 111, Longer Term Followup)(P-77-35) Add t o 49 CFR 192.729 a s e c t i o n t o r e q u i r e t h e proper torquing procedures f o r s t u d s , as s p e c i f i e d by t h e compressor manufacturer, when reassembling compressors a f t e r maintenance work. Include p e r i o d i c t e s t i n g of t h e s e s t u d s , by u l t r a s o n i c o r o t h e r means, t o i n s u r e t h e i r i n t e g r i t y during operation. (Class 111, Longer Term Followup)(P-77-36) BAILEY, Acting Chairman, McALlAMS and HOGUE, Members, concurred i n the above recommendations. HALEY, Member, d i d n o t p a r t i c i p a t e . 4 By: Kay Bailey Acting Chairman#