Chapter 8
Thermal Effects: An Overview W. R. Herrera and L. M. Vargas
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Southwest Research Institute, 6220 Culebra Road, Post Office Drawer 28510, San Antonio, TX 78284
The design of a facility to withstand the thermal effects of an accidental ignition and subsequent combustion must address a number of safety related design requirements and considerations including thermal safety requirements. It is an accepted fact that 80 to 90 percent of the combustible contents of building compartments are consumed during the period of a fully developing fire. Fire safety design considerations must therefore be selected to lessen the danger of spread of fire, smoke and toxic materials beyond the confines of the fire compartment. Special design features as well as detection and suppression devices can be selected to provide control and even extinguishment of potential fires. Unfortunately, a fire can reach a fully developed stage (according to experts, one in twenty incidents) thus requiring that the design of all facilities perform satisfactorily during a full fire scenario. Once a fire in a compartment has reached the fully developed stage, chances of saving personnel trapped in the compartment or equipment within the compartment are very low. The principal design effort must therefore be directed toward providing life safety and minimization of property loss in communicable areas within the facility. In order to accomplish these goals, the designer must identify the potential damage mechanisms and their effects as well as estimating the magnitude of these threats. The designer can then develop the necessary corrective actions to protect personnel and equipment be it through safe siting of facilities or through personnel protection schemes. A very helpful tool to a designer for identifying both the potential hazards and controls is the hazards/risk assessment analysis. This chapter of the book presents papers dealing with the identification and mitigation of potential hazards to personnel and facilities, with the development of personnel protection schemes and with the safe siting of facilities. The following brief paragraphs have been included to provide the reader with a more detailed explanation of the types of safety related analysis techniques that a designer must address. 0097-6156/87/0345-0148$06.00/0 © 1987 American Chemical Society
Scott and Doemeny; Design Considerations for Toxic Chemical and Explosives Facilities ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
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R i s k Assessment f o r O p e r a t i o n s A d e s i g n e r , as p a r t of h i s f a c i l i t y d e s i g n a n a l y s i s , s h o u l d perform a hazards a n a l y s i s or r i s k assessment o f the v a r i o u s p r o cesses which w i l l be conducted w i t h i n the f a c i l i t y i n o r d e r t o determine what p o t e n t i a l thermal dangers or t h r e a t s e x i s t t o p e r sonnel and equipment. A hazards a n a l y s i s or r i s k assessment w i l l p r o v i d e f o r the i d e n t i f i c a t i o n of p o t e n t i a l hazards and of t h e n e c e s s a r y c o r r e c t i v e a c t i o n s / m e a s u r e s t o prevent or c o n t r o l the hazard. E a r l y i n the d e s i g n of a f a c i l i t y , the p r o c e s s e s and equipment may be c o n c e p t u a l and at t h i s s t a g e , a preliminary hazards a n a l y s i s can be performed. I t i s e a r l y i n the d e s i g n t h a t a p r e l i m i n a r y hazards a n a l y s i s can be most h e l p f u l because i t s i m p l e m e n t a t i o n w i l l have l i t t l e impact on schedules and w i l l p r o v i d e the l a r g e s t p o t e n t i a l f o r c o s t s a v i n g s . As the d e s i g n s are m o d i f i e d and r e f i n e d , the hazards a n a l y s e s s h o u l d r e f l e c t a l l changes i n o r d e r t o i n s u r e t h a t a l l p o t e n t i a l hazards and r i s k s have been i d e n t i f i e d and t h a t the c o r r e s p o n d i n g c o n t r o l s have been implemented. T h i s i t e r a t i v e loop s h o u l d be c o n t i n u e d throughout the d e s i g n and c o n s t r u c t i o n phase of the f a c i l i t y as w e l l as throughout the i d e n t i f i c a t i o n of processes and the i n s t a l l a t i o n o f the process equipment. I t must be emphasized t h a t throughout t h e performance of the hazards a n a l y s i s or r i s k assessment the p r i m a r y emphasis i s on personnel s a f e t y . Processes or equipment i n d e n t i f i e d as p o t e n t i a l hazards s h o u l d be m o d i f i e d , r e - d e s i g n e d o r r e e v a l u a t e d i n o r d e r t o i n s u r e a safe s y s t e m . Expected E f f e c t s and Damage Mechanisms Design p r a c t i c e s stem from s t a n d a r d f i r e t e s t procedures i n which the temperature h i s t o r y o f the t e s t furnace i s regarded as an index o f the d e s t r u c t i v e p o t e n t i a l of a f i r e . Thus, the p r a c t i c e of d e s c r i b i n g the expected e f f e c t s and damage mechanism i s based on temperature h i s t o r i e s . T h i s standard d e s i g n p r a c t i c e i s c o n v e n i e n t but l a c k s a c c u r a c y i n terms o f s t r u c t u r a l performance. The s e v e r i t y of a f i r e s h o u l d address the expected i n t e n s i t y of the heat f l u x t h a t w i l l impact the s t r u c t u r e and the d u r a t i o n o f heat penetration. A s i m p l e a n a l y s i s o f the expect n a t u r e of an unwanted f i r e can be based on the heats of combustion and p y r o l y s i s of t h e p r i n c i p a l c o n t e n t s i n the f a c i l i t y . The heat o f combustion w i l l i d e n t i f y the d e s t r u c t i v e nature of the f i r e , w h i l e the heat o f p y r o l y s i s w i l l i d e n t i f y the s e v e r i t y o f the f i r e w i t h i n the compartment i t s e l f and w i l l a l s o i d e n t i f y the d e s t r u c t i v e p o t e n t i a l o f the f i r e i n adjacent s p a c e s . P r e d i c t i o n of Thermal Exposure Magnitude Harmathy [I) p r o v i d e s a c o n v e n i e n t way of c h a r a c t e r i z i n g " r e a l w o r l d " f i r e s i n terms o f t h r e e f i r e s e v e r i t y p a r a m e t e r s : 1)
The o v e r a l l penetration flux, q (Watts/meter^), i.e., heat f l u x absorbed by the compartment boundar i e s , averaged s p a t i a l l y over t h e boundary s u r f a c e s , and t e m p o r a r i l y over the p e r i o d of f u l l development;
Scott and Doemeny; Design Considerations for Toxic Chemical and Explosives Facilities ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
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TOXIC CHEMICAL AND EXPLOSIVES FACILITIES 2)
The d u r a t i o n o f a f u l l y developed f i r e , and,
Τ (seconds);
3)
The average temperature of the compartment g a s e s , Tg (average " f i r e " t e m p e r a t u r e ) , °K, averaged over t h e compartment volume and t e m p o r a l l y over the p e r i o d of f u l l development.
E n e r g e t i c m a t e r i a l s such as p y r o t e c h n i c s and p r o p e l l a n t s undergo r a p i d exothermic d e c o m p o s i t i o n or r e a c t i o n i n c o n t r a s t t o i n d u s t r i a l m a t e r i a l s t h a t are c l a s s e d as f i r e h a z a r d o u s . Design c o n s i d e r a t i o n s have t o address p r o c e s s i n g c o n d i t i o n s t h a t i d e n t i f y t h e chemical and p h y s i c a l s t a t e s o f the i g n i t i b l e m a t e r i a l , m a t e r i a l o f f a b r i c a t i o n w i t h the i g n i t a b l e m a t e r i a l c o n t a c t s , q u a n t i t i e s and temperatures i n v o l v e d , and the l i k e l i h o o d t h a t t h e s e c o n d i t i o n s w i l l promote t r a n s i t i o n t o an e x p l o s i v e r e a c t i o n a f t e r i g n i t i o n . I f one p r e c l u d e s the p o t e n t i a l f o r an unwanted e x p l o s i v e r e a c t i o n , the nature o f the f i r e w i l l be such t h a t the p e n e t r a t i n g f l u x , (J, w i l l be a f u n c t i o n o f the q u a n t i t y of m a t e r i a l present i n the com partment. The d u r a t i o n of the f i r e , T , w i l l be i n the range o f f r a c t i o n s of a second t a seconds w i t h the average temperature o f the compartment g a s e s , T g , r e a c h i n g a ^ s a t u r a t i o n l e v e l t h a t i s dependent on the r a t e o f heat r e l e a s e , q , ( w a t t s / s e c / s e c ) and t h e mass b u r n i n g r a t e , M ( k g / s e c ) . H e r r e r a , V a r g a s , et a l . (2) r e p o r t e x p e r i m e n t a l measurements o f the b e h a v i o r o f e n e r g e t i c m a t e r i a l s b u r n i n g i n a compartment. The r e s u l t s i n d i c a t e t h a t as the c r i t i c a l l o a d i n g d e n s i t y , M (kg/rrr) i n c r e a s e s , the mass b u r n i n g r a t e i n s i d e the compartment reaches a steady s t a t e c o n d i t i o n and unburned m a t e r i a l i s c a r r i e d out i n the plume. Burning o f the unburned m a t e r i a l then t a k e s p l a c e o u t s i d e the compartment, t h e r e b y c o n t r i b u t i n g t o the d e s t r u c t i v e p o t e n t i a l of the f i r e i n adjacent s p a c e s . c
Thermal P r o t e c t i o n S i t i n g
Criteria
The s i t i n g o f f a c i l i t i e s housing hazardous processes or mate r i a l s i s another method t h a t a d e s i g n e r can use t o improve p e r s o n nel s a f e t y . It i s the i n t e n t of every d e s i g n e r t o d e s i g n a s a f e f a c i l i t y , however, a d e s i g n e r should c o n s i d e r t h e p o t e n t i a l f o r a c c i d e n t o c c u r r e n c e and d e s i g n the f a c i l i t y such t h a t the q u a n t i t y of m a t e r i a l s t h a t c o u l d p o t e n t i a l l y become i n v o l v e d i s m i n i m i z e d . L i m i t i n g involvement can be accomplished by safe s i t i n g o f b u i l d i n g s and process bays w i t h i n the b u i l d i n g s . S i t i n g of f a c i l i t i e s f o r s t o r a g e of m u n i t i o n s , p r o p e l l a n t s and e x p l o s i v e s has been r e g u l a t e d f o r some t i m e and s i t i n g c r i t e r i a i s w e l l documented i n r e p o r t s such as AMC-R-385-100 ( 3 ) . T h i s c r i t e r i a i s geared more f o r fragment impact and b l a s t l o a d i n g of s t r u c t u r e s than f o r thermal l o a d s s i n c e the p r i m a r y t h r e a t s t h a t would r e s u l t from an i g n i t i o n i n t h i s type of a s t o r a g e f a c i l i t y would be fragments and b l a s t . In s i t i n g a f a c i l i t y f o r thermal l o a d s , the d e s i g n e r must concern h i m s e l f w i t h a number of a d d i t i o n a l f a c t o r s i n c l u d i n g : t h e flame spread r a t e , the p o t e n t i a l f i r e s u r f a c e a r e a , the e f f e c t o f c o n finement on the f i r e , and the f i r e b r a n d s t h a t c o u l d develop and would c o n t i n u e t o propagate the f i r e t o adjacent b u i l d i n g s . Once
Scott and Doemeny; Design Considerations for Toxic Chemical and Explosives Facilities ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
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the d e s i g n e r has e s t i m a t e d t h e s i z e , d u r a t i o n and i n t e n s i t y o f t h e p o t e n t i a l f i r e b a l l , then he can e s t a b l i s h " s a f e " d i s t a n c e s between buildings. Another t e c h n i q u e f o r r e d u c i n g t h e p o t e n t i a l f o r f i r e communication and spread i s t o i d e n t i f y t h o s e areas w i t h i n a f a c i l i t y where the p o t e n t i a l f o r f i r e s e x i s t s and then i s o l a t i n g t h e s e areas or b a y s . For example, i f a m i x i n g bay has the p o t e n t i a l f o r a l a r g e f i r e , then s i m i l a r m i x i n g bays s h o u l d not be l o c a t e d a d j a cent t o one another t o prevent p r o p a g a t i o n from one bay t o the next. I f t h e bays must be p l a c e d adjacent t o one a n o t h e r , then p r e c a u t i o n s s h o u l d be taken t o i s o l a t e each bay from the o t h e r using f u l l s i z e d i v i d i n g w a l l s , f i r e proof d o o r s , deluge systems, water c u r t a i n s , e t c .
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Personnel
P r o t e c t i o n Requirements
The d e s i g n o f a f a c i l i t y which w i l l handle hazardous m a t e r i a l s r e q u i r e s t h a t t h e d e s i g n e r concern h i m s e l f w i t h the p o t e n t i a l t h r e a t s t o p e r s o n n e l w o r k i n g i n the f a c i l i t y . These t h r e a t s can be of a thermal n a t u r e , i . e . , f i r e s o r e x p l o s i o n s , o r t h e t h r e a t s can be o f a chemical o r t o x i c n a t u r e . Personnel can be p r o t e c t e d from t h e s e v a r i o u s t h r e a t s i n s e v e r a l ways: t h e process o p e r a t i o n s can be m e c h a n i z e d , t h e r e b y e l i m i n a t i n g any o p e r a t o r exposure t o the h a z a r d s ; t h e p r o c e s s o p e r a t i o n i t s e l f can be d e s e n s i t i z e d , t h e r e b y making an a c c i d e n t l e s s p r o b a b l e ; f i r e d e t e c t i o n and s u p p r e s s i o n systems can be i n s t a l l e d i n hazardous a r e a s ; and t h e o p e r a t o r s can be equipped w i t h t h e n e c e s s a r y p r o t e c t i v e c l o t h i n g , a i r s u p p l i e s , e t c . needed t o s h e l t e r or i s o l a t e t h e o p e r a t o r from a dangerous situation. Of the aforementioned p r o t e c t i o n schemes, personnel p r o t e c t i v e c l o t h i n g i s t h e e a s i e s t " f i x " t o implement. Recently, g r e a t s t r i d e s have been made i n the development o f both thermal and chemical p r o t e c t i v e c l o t h i n g w i t h t h e c l o t h i n g b e i n g not o n l y s a f e but a l s o f a i r l y c o m f o r t a b l e t o wear.
Literature Cited 1. Harmathy, T. Z. ASTM Special Technical Publication No. 685 1979, DBR Paper No. 854, 78 p. 2. Herrera, W. R., et al. "A Study of Fire Hazards from Combustible Ammunition: Effects of Scale and Confinement (Phase II)," SwRI Final Report No. 01-7327 for DOD Explosives Safety Board, Contract MDA903-82-C-0526 (December 1984). 3. AMC Regulation, AMC-R 385-100, Safety Manual, HQ, U.S. Army Material Command, (August 1985). RECEIVED March
23, 1987
Scott and Doemeny; Design Considerations for Toxic Chemical and Explosives Facilities ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
