Toxic Chemical and Explosives Facilities - American Chemical Society

9 Explosion Suppression of Large Turbulent Areas

Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 31, 2018 | https://pubs.acs.org Publication Date: April 6, 1979 | doi: 10.1021/bk-1979-0096.ch009

WILLIAM A. CROSLEY Detector Electronics Corp., 7351 Washington Avenue South, Minneapolis, MN 55435

Explosions have been occurring since the world was formed. In the last century explosions of man-made origins have been more common and most costly. To many people it is a disaster such as an earthquake or a tornado, and the only protection is to hope it doesn't happen - and feeling there is no way to stop the disaster once it starts. There is enough knowledge and equipment available today to react and stop certain explosions even after they have begun. An explosion that is preceeded by an ignition is a candidate for an explosion that can be suppressed before major harm is done. Explosion suppression systems are and have been used in all parts of the world for some time, but these systems are usually protecting very small and unobstructed areas, where a mechanical pressure sensor can be used successfully to detect the initial pressure from an explosion. Large-scale suppression tests in obstructed areas had never been tested until the U.S. Coast Guard conducted tests in the tanker S.S. Texaco at Mobile Bay, Alabama utilizing ultraviolet detection to actuate the suppression systems. So that you can better visualize the types of explosions we were attempting to suppress, we have a movie film of several unsuppressed explosions. These are slow motion pictures taken through the port hole at a camera speed of 64 frames per second, looking at the tanker's pump room where 12 pounds of liquid propane were allowed to vaporize and mix with the air to form a stoichiometric mixture. The fuel was then ignited by means of an electric arc or a hot wire. Note the blue flame front i n d i c a t i n g the air f u e l mixture was t r u l y a s t o i c h i o m e t r i c one. The flame f r o n t reaching the obstruct i o n s moves f a s t e r around the o b s t r u c t i o n s than the unobstructed p o r t i o n s of the flame f r o n t . This v i s u a l l y d i s p l a y s the f a c t that the flame front of an explosion moves f a s t e r when obstructed than i n an empty volume. We will see a s l i d e a f t e r the movie showing the over-pressure developed f a s t e r i n t h i s obstructed area opposed to the pressures developed i n a considerably l e s s obstructed area. 0-8412-0481-0/79/47-096-179$05.00/0 © 1979 A m e r i c a n C h e m i c a l Society

Scott; Toxic Chemical and Explosives Facilities ACS Symposium Series; American Chemical Society: Washington, DC, 1979.


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The s i z e of the compartment was approximately 32 feet wide, 16 feet long and 40 feet high. The t o t a l volume was approximately 18,000 cubic f e e t . The over-pressure of the unsuppressed explosions was 12 p s i . These unsuppressed t e s t s were conducted to develop data such as over pressures developed when a s t o i c h i o m e t r i c mixture of propane i s i g n i t e d as w e l l as how f a s t the flame f r o n t advanced i n an obstructed area. These next scenes are not sequences of an e x p l o s i o n , but of a burning torch being thrown i n t o an open c u b l i c l e with an open pan of g a s o l i n e on the f l o o r . In the c u b i c l e i s a UV f i r e detector and a 10-pound b o t t l e of Halon 1301, an e x t i n g u i s h i n g agent. The purpose of t h i s scene i s to b e t t e r acquaint you with the speed of the system v i s u a l l y , so that you can b e t t e r understand the r e s t of t h i s speech today. The UV detector i s i n the upper l e f t corner of the c u b i c l e , viewing only the area i n s i d e the c u b i c l e . The 10-pound b o t t l e of Halon i s i n the opposite corner. When the flames from the t o r c h enter the 90° viewing cone of v i s i o n of the detector i n s i d e the c u b i c l e , the detector i n s t a n t l y s i g n a l s a r e l a y i n i t s c o n t r o l l e r which closes and causes a detonator cap to rupture the d i s c on the Halon b o t t l e - r e l e a s i n g the Halon. The Halon suppresses the flames on the torch and the u n l i g h t e d torch f a l l s harmlessly i n t o the pan of g a s o l i n e . The purpose of the explosion-suppression t e s t s at Mobile Bay was to determine (1) i f i t was p o s s i b l e to suppress an e x p l o s i o n i n such a large volume, (2) to determine the most s u i t a b l e type of d e t e c t i o n and (3) to evaluate various types of e x t i n g u i s h i n g agents. The t e s t s proved an explosion could be suppressed i f detected i n i t s e a r l y stages with e x t i n g u i s h i n g agents i n s u f f i c i e n t quant i t y , reaching the flame f r o n t i n l e s s than 100 m i l l i s e c o n d s a f t e r ignition. Pressure sensors were used i n i t i a l l y but were not s u c c e s s f u l because of t h e i r slow response and the r a p i d pressure buildup due to the speed of the flame propagation. U l t r a v i o l e t (UV) detectors were determined to be the only type s u i t a b l e f o r t h i s purpose. Of the e x t i n g u i s h i n g agents, Halon 1211, 2402 and 1301 were s u c c e s s f u l , but i n f a i r l y high concentrations. Water was p a r t i a l l y s u c c e s s f u l and dry chemical was the most s u c c e s s f u l on a concentration basis. I w i l l now describe how the t e s t s were conducted and the i n formation obtained from these t e s t s . F i r s t , l e t ' s discuss the detector p o r t i o n of the system. The f o l l o w i n g c o n s i d e r a t i o n s are of utmost importance f o r the d e t e c t i o n p o r t i o n of the system: 1. In a l a r g e obstructed area where there w i l l be turbulence, the only type sensor a v a i l a b l e i s a r a d i a t i o n type sensor because of the increased speed of the flame propagation. It was proven during these t e s t s that pressure sensors


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Continued. are not s u i t a b l e . 2. Once the detector s i g h t s the f i r e b a l l , i t must respond i n milliseconds. 3. To suppress an e x p l o s i o n , the detectors must be p o s i t i o n ed so that a f i r e b a l l o c c u r r i n g anywhere w i t h i n the protected space w i l l be detected by at l e a s t one detector i n l e s s than 75 m i l l i s e c o n d s a f t e r i g n i t i o n . I f i t takes longer, the explosion w i l l be f u l l y developed before the e x t i n g u s i h i n g agent can reach i t , and the r e s u l t w i l l be a p a r t i a l suppression, or very l i k e l y , no suppression. 4. The d e t e c t i o n system must be protected from f a l s e or spurious s i g n a l s . 5. The d e t e c t i o n system must incorporate safeguards against system f a i l u r e and provide an e a r l y warning system to i n d i c a t e i f the detector i s becoming l e s s s e n s i t i v e , due to an e l e c t r o n i c malfunction or contamination of the viewing window. The detector best s u i t e d to meet these conditions i s a r a d i a t i o n type sensor. U l t r a v i o l e t (UV), i n f r a r e d and v i s i b l e r a d i a t i o n are generated when combustion produces a flame and a l l three types of r a d i a t i o n sensors respond to the r a d i a t i o n from the flame. The v i s i b l e and i n f r a r e d sensors are f a s t , but are subject to many f a l s e s i g n a l s from a r t i f i c i a l l i g h t , s u n l i g h t , hot bodies and other heat producing bodies. The u l t r a v i o l e t (UV) sensor i s f a s t i n response and i s a f f e c t e d by few extraneous s i g n a l s which are c o n t r o l l a b l e . The UV sensor must be the type that only responds to a narrow band of UV, from 1850 Angstroms to 2450 Angstroms. This i s considerably below the UV r a d i a t i o n wavelengths from the sun and a r t i f i c i a l l i g h t s . A b a s i c UV d e t e c t i o n system c o n s i s t s of these three b a s i c components: A UV detector having a 90° cone of v i s i o n . The UV generated i n that viewing area w i l l cause the UV detector to send a voltage pulse to the s i g n a l i n g process s e c t i o n o f the e l e c t r o n i c amplifier/controller. Once UV from the exploding f i r e b a l l enters the sensor's 90° cone of v i s i o n , the s o l i d - s t a t e switch w i l l c l o s e i n l e s s than 10 m i l l i s e c o n d s , which w i l l actuate the e x t i n g u i s h i n g system. The UV d e t e c t i o n system l i k e any man-made product, i s subject to f a i l u r e . These f a i l u r e s could be detector tube f a i l u r e s , e l e c t r o n i c or w i r i n g f a i l u r e s , or o p t i c a l system f a i l u r e or contamination. There are UV f i r e d e t e c t i o n systems a v a i l a b l e that have the c a p a b i l i t y of self-examination f o r f a i l u r e s . This supervisory c i r c u i t i s r e f e r r e d t o as Automatic O p t i c a l I n t e g r i t y . Let's examine a b a s i c UV system that incorporates t h i s Automatic self-examination f e a t u r e . UV from an exploding f i r e b a l l w i l l enter the o p t i c a l surface of the UV sensor and generate a voltage s i g n a l to cause the c o n t r o l l e r ' s r e l a y to actuate and



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r e l e a s e the e x t i n g u i s h i n g agent to suppress the explosion. Also, a UV s i g n a l generator, i n t h i s case a very small UV source lamp, screened from the sensor w i l l pulse b r i e f l y every few seconds. This small l e v e l of UV r a d i a t i o n can only reach the sensor by passing out through the viewing window and r e f l e c t i n g o f f a beveled r i n g and then r e - e n t e r i n g the detector housing i n the same l o c a t i o n where the UV from a f i r e b a l l would enter. The momentary r e a c t i o n of the sensor from the UV source lamp i s a s i g n a l to the system that everything i s i n operation from the detector window through the UV sensor and through the e l e c t r o n i c s . I f the UV from the source lamp f a i l s to pass through or the sensor f a i l s to respond, a f a u l t r e l a y de-energizes, warning the operator of a m a l f u n c t i o n which w i l l normally be a contaminated viewing window. Once the window i s wiped clean the f a u l t r e l a y can be c l e a r e d i n d i c a t i n g the system i s no longer sub-marginal. This type of self-examination i s known as Automatic O p t i c a l I n t e g r i t y . This i s a cut-away view of the detector i n s i d e i t s e x p l o s i o n proof housing. The UV source lamp generates UV which i s i s o l a t e d by t h i s b a r r i e r from the detector tube. The UV cannot pass through t h i s b a r r i e r but must pass through the quartz viewing window, and at that point s t r i k e the UV detector sensor. I f the quartz window e v e n t u a l l y becomes contaminated, a l l of the UV from t h i s source w i l l not be able to penetrate the surface of the quartz window, thus reducing the s i g n a l to the detector, causing the system to go i n t o a f a u l t alarm c o n d i t i o n . Now I w i l l d e s c r i b e the U.S. Coast Guard t e s t s i t e and the e x p l o s i o n suppression systems used. The pump room measured 40 feet from the b i l g e l e v e l to the top hatch, and 32 feet from port s i d e to starboard, and 16 feet from fore to a f t . The t o t a l volume, i n c l u d i n g two f i r s t l e v e l wings, minus the space consumed by the pumps and other o b s t r u c t i o n s , was 18,000 cubic f e e t . Two explosion suppression systems were t e s t e d . They were based on the f o l l o w i n g p r i n c i p l e s : 1. UV detector sees i n c i p i e n t explosion. 2. I t s i g n a l s the c o n t r o l c i r c u i t r y which f i r e s an e l e c t r i c a l l y actuated b l a s t i n g device. 3. The b l a s t i n g device ruptures a r e s t r a i n i n g diaphragm, causing the r e l e a s e of suppression agent which i s under 600 p s i dry n i t r o g e n pressure. 4. The d r i v i n g force of the n i t r o g e n forces the agent out of the container and propels i t toward the flame f r o n t . 5. The flame i s extinguished; the explosion suppressed. One system c o n s i s t e d of c y l i n d r i c a l cannons to contain the agent. The d r i v i n g force was provided by p r e s s u r i z i n g the charged cannon to 600 p s i with dry n i t r o g e n . Again, UV detectors were used to sense the f i r e b a l l , and the suppression agent used was the dry chemical known as Purple K. The other system consisted of s p h e r i c a l high-rate discharge e x t i n g u i s h e r s to contain the agent. The d r i v i n g force was provided by p r e s s u r i z i n g the charged e x t i n g u i s h e r to 325 p s i with



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dry n i t r o g e n . The UV sensor was used f o r the d e t e c t i o n of the f i r e b a l l , and various suppression agents were used i n these ext i n g u i s h e r s . They included water, Halon 2402, Halon 1211 and Halon 1301. Nine UV detectors were i n s t a l l e d throughout the e n t i r e pump room, the same as would be necessary i n an a c t u a l i n s t a l l a t i o n . I f any one of these detectors sees UV r a d i a t i o n , i t w i l l f i r e a l l of the cannons. The c i r c u i t r y was designed to i n d i c a t e at any time i f there was an e l e c t r i c a l malfunction i n the w i r i n g to any of the cannon detonators. The e x t i n g u i s h i n g cannons or spheres were l o c a t e d throughout the e n t i r e pump room above and below the deck p l a t e s as were the nine UV d e t e c t o r s . In t h i s manner a growing f i r e b a l l could be detected at any point w i t h i n the e n t i r e pump room w i t h i n m i l l i seconds a f t e r i g n i t i o n . Two cannons were l o c a t e d i n the top hatch p r o p e l l i n g dry chemical down. At the t h i r d deck l e v e l there were two cannons on the port s i d e and two on the starboard bulkheads. At the second l e v e l there were four a d d i t i o n a l cannons with the nozzles at a 45° angle f i r i n g d i r e c t l y over the pumps i n the pump room. There were s e v e r a l cannons on the bottom l e v e l f i r i n g below the deck and j u s t above the b i l g e water. The pump room was obstructed with the normal equipment, pumps, v a l v e s , pipes and l a d d e r s , the same as when i t was i n service. I f the e n t i r e pump room were f i l l e d with a s t o i c h i o m e t r i c mixture of propane and a i r throughout the e n t i r e volume, unsuppressed explosions would produce pressure of approximately 120 p s i , enough pressure to blow the ship out of the bay. N a t u r a l l y , we could not run t e s t s that could develop these pressures. Based on the design l i m i t a t i o n s of the pump room bulkheads, i t was decided that the bulkheads could e a s i l y withstand 15 p s i . C a l c u l a t i o n s show that i f the s t o i c h i o m e t r i c mixture of a hydrocarbon f u e l were placed i n 10% of the volume and i g n i t e d , expans i o n of the e x p l o s i o n i n t o the remaining 90% of the volume would l i m i t the t h e o r e t i c a l maximum pressure to 12 p s i , and not a f f e c t the bulkheads. With t h i s reasoning 12 pounds of propane were used. A l s o , a s u c c e s s f u l suppression should hold the maximum exp l o s i o n pressure to l e s s than 1 p s i . Thus, i t was reasoned that the t o t a l volume would produce r e a l i s t i c r e s u l t s f o r e x p l o s i o n suppression purposes. The a c t u a l s u c c e s s f u l suppressions that followed were a l l l e s s than 1 p s i and as low as .2 p s i . The f i r s t t e s t s used pressure sensors f o r the d e t e c t i o n s y s tem, but they f a i l e d to respond f a s t enough to suppress the explosion. I t was suspected that the o b s t r u c t i o n s i n the pump room increased the speed of the flame propagation beyond the c a p a b i l i ty of a pressure sensor.



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To prove t h i s , the b i l g e water was allowed to r i s e above the top of the pumps and other major o b s t r u c t i o n s . A d d i t i o n a l unsuppressed t e s t s were conducted and the pressure r i s e was much slower as suspected i n an unobstructed area. This proved that pressure sensors can be used i n empty spaces but not i n obstructed l a r g e spaces. At t h i s point the pressure sensors were no longer used and the UV f i r e d e t e c t i o n systems were now used. During the explosion suppression t e s t s , the UV detector r e sponded between 18 and 25 m i l l i s e c o n d s a f t e r i g n i t i o n . The f i r s t contact of the e x t i n g u i s h i n g agent w i t h a f i r e b a l l occurred i n l e s s than 100 m i l l i s e c o n d s a f t e r i g n i t i o n , and the peak of the suppressed curve occurred w i t h i n 150 m i l l i s e c o n d s at an over-pressure as low as .2 p s i . In analyzing these t e s t s , the f o l l o w i n g f a c t o r s were cons i d e r e d : Size of f i r e b a l l at agent contact, degree of agent breakdown, amount of burning a f t e r the i n i t i a l suppression, and maximum pressure developed. The a n a l y s i s i n d i c a t e s that successf u l suppression requires the l i s t e d minimum a p p l i c a t i o n d e n s i t i e s f o r the suppression of explosion of a propane/air mixture i n obs t r u c t e d spaces s i m i l a r to a ship's pump room. In summary, the amount of agent required to s u c c e s s f u l l y suppress the explosion i n the pump room or s i m i l a r 18,000 cubic foot obstructed area, i s as f o l l o w s : Water r e q u i r e d a concentration of more than .15 pounds per cubic f o o t . Halon 2402 r e q u i r e d more than .12 pounds per cubic f o o t . Halon 1211 required between .06 and .09 pounds per cubic foot. Halon 1301 required between .05 and .08 pounds per cubic foot. Purple K (dry chemical) r e q u i r e d more than .007 pounds per cubic f o o t . This i l l u s t r a t e s that the dry chemical was more than 10 times as e f f e c t i v e as the most e f f i c i e n t Halon. Water was the l e a s t e f f i c i e n t and was the only t e s t that was not completely s u c c e s s f u l . There were not enough containers to be able to reach a .15 pounds per cubic foot d e n s i t y , but there was a p a r t i a l suppression at .066 pounds per cubic foot d e n s i t y . The .15 pounds per cubic foot was determined by e x t r a p o l a t i o n . When the Halon density was not s u f f i c i e n t to suppress the e x p l o s i o n , there was agent breakdown evidenced by the observation of orange-yellow smoke and severe a f t e r b u r n i n g . In c o n c l u s i o n , these t e s t s i n d i c a t e that i t i s p o s s i b l e to suppress c e r t a i n explosions i n l a r g e obstructed areas. Pressure sensors are not s u i t a b l e as explosion suppression systems i n l a r g e obstructed areas, but the b a s i c u l t r a v i o l e t (UV) d e t e c t i o n system a v a i l a b l e today as a f i r e detector i s capable of being used as an explosion detector, p r o v i d i n g the exploding f i r e b a l l i s i n the cone of v i s i o n of one of the detectors w i t h i n the f i r s t 75 m i l l i s e c o n d s . The e x t i n g u i s h i n g agent used must make


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contact with the fireball within 100 milliseconds after ignition. The density of the extinguishing agent required will vary with agent used. In these particular tests, dry chemical was far superior on a weight per unit volume basis by a magnitude of 10 times or greater. The movie film that now follows shows several unsuppressed tests where data was recorded and then a suppression by each of the 5 agents used for suppression. Abstract Explosion suppression for large volumes in which turbulence can be expected is one of the greatest challenges in the fire protection field today, and the need for such a system is great. There are explosion suppression systems in service throughout the world, but in almost every case they are in very small volumes of unobstructed areas, where the mechanical pressure sensor can be used successfully to detect the explosion. Large-scale suppression tests in obstructed areas have never been tested until recent U.S. Coast Guard tests were conducted on an ocean oil tanker utilizing ultraviolet detection systems to actuate dry chemical, Halon and water suppression systems. The purpose of these tests was to determine the most suitable type of detection, and to evaluate various types of extinguishing agent. Pressure sensors were tried initially, but were not successful because of their slow response, and ultraviolet (UV) dectors were finally determined to be the only type that were suitable for this purpose. Of the extinguishing agents, Halon 1211, 2402 and 1301 were successful, but in fairly high concentrations. Water was partially successful and dry chemical was the most successful on a concentration basis. Emphasis should be made that this was a research program to gain additional information for the design of a complete suppression system with a capability of suppressing explosions in large areas as well as small enclosures. Work is continuing in our company to develop these systems. The ultraviolet explosion detectors and the suppression systems are commercially available but must be engineered and designed for each hazardous application. RECEIVED

November 22,

1978.


Toxic Chemical and Explosives Facilities - American Chemical Society

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