NTSB Safety Recommendation Letter P-77-027 through P-77-033

Page 1Official PDFf- 8 NATIONAL TRANSPORTATION SAFETY BO WASHINGTON, D.C. ISSUED: October 31, 1977 M r . Randall Meyer P r e s i d e n t Exxon Company, U.S.A. P . a. BOX 2180 Houston, Texas 77001 _I i SAFETY RECOMMENDAT I O N ( 5 ) P-77-27 through P-77-33 A t 11 a.m., c . s . ~ . , 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 was 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 the 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 auto- m a t i c a l l y c l o s e t h e shutoff v a l v e s leading t o two of the f i v e compressors i n t h e s t a t i o n . - l/ A t 11:30 a.m., a second explosion occurred w i t h i n t h e burning building in one of t h e compressors t h a t had not s h u t down automatically. P i p e l i n e valves upstream and downstream of the 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 t h e 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 v a l v e s 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 t h r e e engine-driven compressors and the compressor b u i l d i n g . Property damage and gas l o s s w a s estimated t o be $5 m i l l i o n . I n v e s t i g a t i o n a f t e r t h e a c c i d e n t revealed t h a t many of t h e s t u d s that secured the 10-inch s u c t i o n v a l v e covers t o t h e compressor c y l i n d e r s were tightened approximately t h r e e times t h e amount recommended by t h e manufacturer of the compressor. Metallurgy i n d i c a t e d that s e v e r a l s t u d s had been overstressed and experienced fatigue-type f a i l u r e s . - 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 , Robstom, Texas, December 7 , 1976" (NTSB-PAR-77-31. 2198#

Page 2There had been no t o t a l systems review of t h e c o n t r o l components t h a t operate the emergency shutdown valves. For example, an equipment- air l i n e t h a t f a i l e d a s a r e s u l t of t h e f i r e may have caused a loss of p r e s s u r e i n an instrument-air l i n e t h a t operated t h e shutoff valves. There had been no a n a l y s i s of t h e e f f e c t of t h i s type of f a i l u r e and, consequently, no dual o r emergency backup air system had been provided. Although t h e remote-control equipment and t h e f i r e g a t e valves w e r inspected y e a r l y i n accordance with 49 CFR 192.731(c), t h e time i n t e r v a l between inspections and operations w a s too long t o uncover p o t e n t i a l equipment malfunctions. The linkage i n a s m a l l 4-way p i l o t gas c o n t r o l valve had become s t u c k and prevented power gas from c l o s i n g the two f i r e g a t e valves on the 36-inch p i p e l i n e . An i n s e c t and mud had clogged t h e 1/8-inch-diameter pressure r e l i e f vent l i n e on a 3-way solenoid valve. The clogged v e n t made t h e two f i r e g a t e valves on the 30-inch p i p e l i n e i n o p e r a t i v e . The u n i t blowdow,n valves were designed t o vent t h e gas trapped w i t h i n t h e compressor building piping only, and were sequenced t o open after a l l of t h e o t h e r shutdown valves had closed. Because some of t h e v a l v e s d i d n o t c l o s e , gas was vented only from a minor p o r t i o n of t h e s t a t i o n ' s piping. There was no means of remotely blowing down a l l of t h e piping w i t h i n t h e compressor s t a t i o n . The operating personnel from o t h e r s t a t i o n s and from neighboring operating d i v i s i o n s of t h e company responded immediately t o t h e emergency; however, i f they had been t r a i n e d on t h e equipment t h a t they might b e expected t o operate i n an emergency, and had there been s i g n s t h a t marked v a l v e s t h a t were o u t of t h e i r normal p o s i t i o n s , t h e f i r e probably could have been put out i n less than 3 hours. Therefore, t h e National Transportation S a f e t y Board recommends that t h e Exxon Gas System, Inc.: Take necessary s t e p s t o i n s u r e t h a t s t u d s on compressors components are tightened t o t h e number of foot-pounds of torque recommended by t h e equipment manufacturer. (Class 11, P r i o r i t y Followup) (P-77-27) Inspect emergency shutdown v a l v e s and t h e i r components a t compressor s t a t i o n s a t more frequent i n t e r v a l s than 1 year (monthly o r q u a r t e r l y ) u n t i l equipment of proven r e l i a b i l i t y has been i n s t a l l e d , t e s t e d , and shown t o be responsive f o r longer periods of i n a c t i v i t y . (Class 11, P r i o r i t y Followup)(P-77-28) 2198#

Page 3- 3 - Make a t o t a l systems review of e l e c t r i c , a i r , and gas-operated emergency equipment, with p a r t i c u l a r emphasis on interconnected a i r systems and backup o r dual-feed air systems i n compressor s t a t i o n s . (Class 11, P r i o r i t y Followup)(P-77-29) Include, i n emergency shutdown systems, a s e p a r a t e c o n t r o l t o remotely operate valves t h a t can independently blow down t h e s t a t i o n piping. P r i o r i t y Followup) (2-77-30) (Class 11, I n v e s t i g a t e more dependable items of c o n t r o l equipment and r e p l a c e e x i s t i n g solenoid and 4-way valves a t f i r e g a t e valves with t h i s equipment. (Class 11, P r i o r i t y Followup)(P-77-31) Designate c r i t i c a l valves on c o n t r o l l i n e s (gas, a i r , and hydraulic) a s t o whether they should be normally open o r closed, and place s i g n s on these valves whenever t h e l i n e s a r e s h u t down f o r maintenance o r when the valves a r e n o t i n t h e i r normal p o s i t i o n s . Such changes should be authorized, logged, and reported t o a l l personnel responsible f o r t h e operation of t h e system. (Class 11, P r i o r i t y Followup) (P-77-32) Train operating personnel from o t h e r s t a t i o n s and o t h e r nearby operating d i v i s o n s of t h e company on t h e operation of emergency transmission l i n e valves and emergency s t a t i o n f i r e c o n t r o l valves. Furnish each nearby company o f f i c e t h a t could be expected t o help i n an emergency with a contingency plan book and drawings of a l l of t h e f a c i l i t i e s they might be expected t o operate. (Class 11, P r i o r i t y Followup) (P-77-33) BAILEY, Acting Chairman, McADAMS and HOGUE, Members, concurred i n the above recommendations. HALEY, Member, did not p a r t i c i p a t e . By: K,ay Bailey Acting Chairman 2198#