title: Improved fire protection for underground fuel storage and fuel transfer areas
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Horton, Director 4* {V Library of Congress Cataloging in Publication Data: i — 1 Pomroy, William H Improved fire protection for underground fuel storage and fuel trans- fer areas. (Information circular / United States Department of the Interior, Bureau of Mines; 9032) Bibliography. Supt. of Docs.: 128.27 1. Mine fires— Prevention and control. 2. Petroleum products— Underground storage — Fires and fire prevention. I. Johnson, Guy A. II. Title. III. Series: Information circular (United States. Bureau of Mines; 9032) TN295.U4 [TN315] 622s [622's.8] 84—600269 CONTENTS Abstract Introduction Analysis of fueling system fire hazards Methodology Results Guidelines for safe fueling system design Fuel storage area Fuel transfer area Discussion Generic fire sensing and suppression system. .. Design concepts Fuel transfer area Suppression Controls Detection Fuel storage area Suppression Controls Detection Additional design considerations System reliability and maintenance. Reliability of individual components. System complexity  suppression system. 10 General description 10 Suppression 10 AFFF subsystem 10 Dry chemical subsystem 11 Detection and control 11 Tests 12 Laboratory component testing 12 Suppression 12 Detection and control 12 Laboratory full-scale fire testing of complete system 12 Field test 12 Installation, inspection, and pretests. 12 Fire testing 15 Studies of alternative system 'designs 18 Cost-effectiveness analysis 18 System cost versus fire cost 18 Cost-performance tradeoffs between systems. 18 Summary 19 ILLUSTRATIONS 1. Recommended fueling system design 3 2. Recommended fuel transfer area fire sensing and suppression system 6 3. Recommended fuel storage area fire sensing and suppression system utilizing AFFF suppressant 6 4. Recommended fuel storage area fire sensing and suppression system utilizing high-expansion foam suppressant 7 5. Recommended fuel storage area fire sensing and suppressant system utilizing Halon 1301 suppressant 7 6. Recommended fuel storage area fire sensing and suppression system utilizing dry chemical suppressant ... 8 7. Recommended fuel storage area fire sensing and suppression system utilizing twin-agent suppressant 8 8. Idealized failure rate curve 9 9. Prototype fire sensing and suppression system for underground fuel storage and transfer area 11 10. Configuration of system elements for mockup testing 13 11. Twin-agent discharge during mockup testing 13 12. System control panel 14 13. Ultraviolet flame detector head 14 14. Twin-agent suppression subsystem 14 15. Dry chemical nozzle with blowoff cap 14 16. Foam-water sprinkler nozzle 15 17. Vehicle mockup in fuel transfer area 15 18. Igniting test fire beneath vehicle mockup 16 19. Test fire burning under mockup 16 20. Twin-agent discharge onto test fire 17 21. Test fire fully extinguished 17 TABLES 1. Hazard ranking of potential leak and spill sources 2 2. Hazard ranking of potential ignition sources 2 3 Fueling subsystem hazard indexes 2 4. System cost comparisons 10 5. Test site description 14 6. INTRODUCTION From the early 1950's through the mid-1960's, underground mining was revolutionized by the introduction of mobile rubber-tired diesel-powered equipment. Entirely new and highly productive mining systems were developed based on the concept of "trackless" mining. The increasing use of trackless equipment, however, was accompanied by an increasing frequency of diesel equipment fires. Fires in- volving rubber, oil. hydraulic fluid, grease, and diesel fuel in- creased 557 pet from the 1950-64 period to the 1965-79 period. Mobile equipment was involved in less than 10 pet of all underground metal and nonmetal mine fires from 1950 to 1967 but was involved in about 40 pet of all such fires from 1968 to 1979. In response to this growing hazard, the Bureau initiated a research program aimed at improving fire protec- tion technology for underground metal and nonmetal trackless mining operations. The Bureau's research to develop automatic fire protection systems for underground mobile equipment is described in Information Circular 8954. 2 In addition to fire hazards on vehicles, mobile equipment servicing facilities such as underground shops, maintenance bays, and fuel storage and transfer areas also present a significant fire hazard. These areas are characterized by the presence of combustible materials such as lubricating oils, greases, starting fluids, solvents, hydraulic oils, and diesel fuel; ignition sources such as hot engine surfaces and elec- trical, cutting, and welding equipment; and constant vehicular traffic. Fuel storage areas, which provide bulk storage in both tanks and drums, and transfer areas, where vehicle refuel- ing takes place, are particularly critical because of their poten- tial for the involvement of very large quantities of diesel fuel, lubricating oils, and hydraulic fluids in a fast-growing, high- energy fire. Although no large-scale fires in underground fuel storage or transfer areas have yet been reported, the potential seriousness of such fires prompted the Bureau to embark on research to upgrade the overall level of fire safety technology for these areas. The research began with a comprehensive fire hazards analysis. The results of this analysis were then used to establish (1) guidelines for the design of safe and efficient underground fuel storage and transfer areas and (2) specifications for automatic fire sensing and suppression systems for fueling areas. Prototype fire sensing and sup- pression equipment was fabricated, then laboratory and field tested. All work was accomplished under a contract with the Ansul Co. of Marinette, Wl. 3 ANALYSIS OF FUELING SYSTEM FIRE HAZARDS Guidelines for the construction of safe fuel storage and transfer areas and design specifications for fire protection systems for these areas were based on data acquired through a comprehensive and detailed analysis of fueling area fire hazards. Data sources included Bureau experts and experts employed by the Mine Safety and Health Administration rervisory mining engineer. Twin Cities Research Center. Bureau of leapotis. MN 'Johnson. G A Automatic Fire Protection for Mobile Underground Mining Equipment. BuMines IC 8954. 1983. 12 pp. 3 Ansul Co. Improved Fire Protection System for Underground Fueling Areas (contract H0262023) Volume 1: BuMines OFR 120-78, 1977, 325 pp ; NTIS PB 288 298/AS Volume 2: BuMines OFR 160-82, 1981, 111 pp .; NTISPB83 114744. (MSHA); private consultants; management, safety, and maintenance personnel from several underground mines; equipment manufacturers; and insurance industry represen- tatives. In addition, researchers visited 16 underground mines to observe fuel storage and transfer area designs and safety practices. METHODOLOGY Although the designs of existing fuel storage and transfer areas are based on the unique experiences of their designers and the specific needs of individual mining operations, cer- tain features were common to many of the systems observed. These features, or subsystems, are listed below. 1 . Surface storage 2. Surface transport 3. Shaft transport 4. Borehole transport 5. Transport to underground storage 6. Underground storage 7. Transport to fuel transfer area 8. Fuel transfer An expert panel was convened to rank the hazard associated with each subsystem. The panel consisted of (1) an experienced mine fire hazards analyst who had visited six of the mines in the study, (2) an experienced mine fire hazards analyst who had visited all of the mines in the study, (3) a mining engineering consultant, (4) a petroleum systems design consultant, and (5) a hydraulic systems research and design specialist. The panel evaluated the eight generic subsystems listed above as well as the design of each system, subsystem, and major component observed at the 16 mines studied. The fire hazard of each subsystem was evaluated in terms of (1) leak or spill potential and (2) ignition potential. Potential sources of leaks and/or spills and potential ignition sources were iden- tified and ranked by each panel member. The individual rank- ings were averaged to yield overall hazard rankings for each leak or spill and ignition source. The potential range of hazard rankings was from 1 (most hazardous) to 13 (least hazardous). RESULTS Results of the analysis of the potential for fuel leaks and spills are shown in table 1. The three components most likely to leak or cause a spill are the hose to the vehicle, the pump to the vehicle, and the piping to the vehicle fueling pump. All three of these com- ponents are situated in the fuel transfer area. Shaft and haulageway piping follow as the next most hazardous com- ponents in terms of leaks and spills. Results of the analysis of the potential ignition sources are shown in table 2. Six of the thirteen ignition sources identified, including the three most hazardous sources, occur in the fuel transfer area. A hazard index reflecting the relative fire hazardousness of each fueling subsystem was derived by combining the leak and spill and ignition potential rankings. Hazard indexes for the eight generic fueling subsystems are shown in table 3 in decending order of hazardousness. As might be expected from the analysis of leak and spill and ignition source potentials, the fuel transfer area was TABLE 1 . — Hazard ranking of potential leak and spill sources Fueling system component Hazard ranking 1 Hose to vehicle Underground pump to vehicle Piping to fueling pump Shaft piping valves Haulageway piping Haulageway valves Surface transfer pump Shaft piping Urn rground storage tanks Surface storage tanks Surface piping Haulageway pumps Nozzles 1 1 = most hazardous; 13 = least hazardous. TABLE 2. — Hazard ranking of potential ignition sources 2.3 4.5 5.1 6.5 6.7 6.8 7.0 7.1 7.3 8.3 8.4 8.5 12.8 Source Hazard ranking 1 Electrical sparks from vehicle Vehicle engine heat Electrical sparks from refueling area Smoking Impact sparks (from vehicle-rib impacts) .... Welding Vehicle exhaust Human error Hot hydraulic pumps Drive shaft brakes Unusual occurrences Trash build-up Spontaneous combustion 1 1 = most hazardous; 13 = least hazardous. TABLE 3. — Fueling subsystem hazard indexes 2.2 2.6 3.6 4.4 5.3 7.5 8.6 8.6 8.6 8.7 8.7 8.8 8.8 Index 1 278 217 101 77 59 50 48 43 Subsystem Fuel transfer area Underground storage area Surface storage area Shaft transport Surface transport Borehole transport Transport to fuel transfer area Transport to underground storage area. . .. 1 Highest values indicate most hazardous subsystems. judged to be the most hazardous. Both the probability of a fuel spill and the probability of ignition are highest in this area. Underground fuel storage was ranked as the second most hazardous subsystems. However, although the likelihood of a fire occurring in the storage area is less (than in the fuel transfer area), the magnitude of a fire in the storage area would be far greater. Fuel transfer areas where fuel tanks and drums are pres- ent are more hazardous than areas where only a fueling noz- zle is present. The panel agreed that the most likely underly- ing cause of a fire is operator error, including such unsafe practices as smoking while refueling, overfilling fuel tanks, insufficient precautions during welding in the refueling area, unsafe or careless operation of vehicles, and failure to prop- erly maintain and inspect vehicles and/or refueling equipment. GUIDELINES FOR SAFE FUELING SYSTEM DESIGN Fueling system designs should be aimed at mitigating the conditions identified in the hazards analysis as contributing to the occurrence or severity of fueling system fires. Designs that require a minimum of operator skill are preferred because their use can help minimize operator error. Designs that physically separate the vehicles being refueled from the fuel storage area are preferred because they minimize contact between the fuel source and the ignition source. Designs that limit line pressures and have a low risk of tank overfilling are preferred because three features can help minimize leaks and spills. The recommended fueling system design is shown in figure 1 . and the design features illustrated in this figure are discussed in the next two sections. FUEL STORAGE AREA Surface storage tanks should be designed and constructed in accordance with applicable industry standards. Combusti- ble liquids should not be pumped underground from a large storage tank; instead they should be pumped from a batch tank with less capacity than the underground tank being filled. Borehole transport is preferred over shaft transport. Exhaust from the fuel storage area should be directed to a return. Automatic fire sensing and suppression systems (which are discussed in detail in the next main section of this report) are recommended, as are hand portable fire extinguishers. A major concern in underground fueling system design is whether to use a wet or a dry transfer line between surface storage and the underground location. A wet line, with fuel in it at all times is advantageous because it makes underground storage of fuel unnecessary. A dry line, used to periodically resupply underground tanks, has the advan- tage of placing far less stress on piping and fittings. A hazards analysis indicated that the overall hazard was generally slight- ly less with the dry-line system; however, both systems are acceptable. When choosing between wet and dry transfer lines, local conditions such as type of piping used, length of drop, length of horizontal pipe run from borehole to fuel storage and/or transfer area, frequency of fuel transfers, and the design and layout of the fuel storage and transfer areas should be considered. Underground storage areas should be fully enclosed and constructed of materials having a fire resistance rating of at least 2 h. Windows should close automatically with fusible links, and the door should remain closed when not in use. The floor should be impermeable (rock or concrete) and have a sump for holding spilled fuel. Lights and wiring should be explosion proof. The tanks should be of good quality and supported on con- crete saddles. An overflow vent pipe large enough to release fuel at the maximum possible delivery rate is recommended to avoid rupture during a possible overfill. This vent should also release pressure in the event of a fire. The overflow vent should feed into an overflow tank equipped with an alarm. Tanks should have a float or sight glass with the refill level clearly marked to reduce human error. Mam storoge tank Impoundment^^ * Batch transfer tank Explosion-proof or air pump Positive-displacement pump -■atic-^ Sloped concrete pod shutoff >. r spill trough nozzle iump pump and tank Fire-resistant bulkhead with steel door, self-closing windows, ond automatic-shutof f ventilation fan FIGURE 1.— Recommended fueling system design (fuel storage area, top and lower right, and fuel transfer area, lower left; fire sensing and suppression system not shown). FUEL TRANSFER AREA There should be no fuel or oil tanks or drums within 50 ft of the fuel transfer area (by shortest accessible route) unless such tanks or drums are enclosed in a fire-resistant struc- ture. Emergency pump controls should be located so they can be quickly reached in the event of fire. The area should be well lighted with explosion-proof lamps. As was recom- mended for fuel storage areas, exhaust from the fuel transfer area should be directed to a return. Again, automatic fire sensing and suppression systems and hand portable fire ex- tinguishers are also recommended. Diking or some other form of drainage control should be provided to collect spilled fuel into a container with minimum surface area. A means for absorbing spilled fuel should be located near the nozzles. Fuel transfer areas should not be located where spills would drain toward an underground storage area or shop. Pumps should be located outside the underground storage area to remove that potential ignition source from the stored fuel. The pump should not keep constant pressure in the fuel lines; it should instead be actuated by the operator during the fueling operation. Automatic shutoff nozzles are recom- mended to reduce the incidence of overfills. An excess-flow valve should be installed downstream of the pump. The area should be kept as clean and orderly as possible. DISCUSSION The preceding guidelines for safe fueling system design represent an optimum rather than a minimum approach. They are based on certain assumptions regarding aspects of mine design that are considered commonplace in the North American metal and nonmetal mining industry. Alternative systems and subsystem elements may result in the same or a higher level of safety depending on local conditions. As ex- amples, the transport of fuel in portable tanks instead of fix- ed piping is warranted in adit mines, and fuel storage and fuel transfer areas situated near an exhaust shaft and ven- tilated directly to the return need not be enclosed or protected by suppression systems. GENERIC FIRE SENSING AND SUPPRESSION SYSTEM DESIGN CONCEPTS The development of generic fire sensing and suppression system design concepts was guided by the analysis of fuel- ing system fire hazards. Designs were developed for the fuel transfer area and the underground storage area, which were ranked as the first and second most hazardous fueling sub- systems (table 3). Many different fire suppression systems are currently used to protect underground fuel transfer and storage areas against fires. Due to the many variations in fueling systems designs, no single fire suppression system can be recommended for all applications. Each hazard requires its own analysis and a design concept developed to specifically address that hazard. However, certain general principles of fire protection, such as the suitability of suppressant agents on various fuels and the properties of various detection devices, apply regardless of the hazard under consideration. Therefore, these general principles were applied together with the guidelines for safe fueling system design to develop the generic design concepts for fueling area fire protection systems. The resultant design concepts are discussed in the following sections in the context of these principles and guidelines. Fuel Transfer Area The generic fuel transfer area fire protection system con- sists of three elements: suppression, controls, and detection. Suppression Five suppressants were considered for possible use in the fuel transfer area fire suppression system: aqueous film- forming foam (AFFF), water, high-expansion foam, Halon 1301 4 halogenated fire extinguishing agent, and multipurpose dry chemical. Agents eliminated from consideration includ- 4 Reference to specific products does not imply endorsement by the Bureau of Mines. ed carbon dioxide and Halon 121 1 , because of possible safety hazards to personnel who might be exposed to the agents in a confined area; protein and synthetic foams because they are incompatible with dry chemical, require special air- aspirating equipment, have limited stability, and require higher discharge rates than AFFF; and ordinary dry chemicals because they are unable to extinguish ordinary combustibles such as paper, rags, and wood. A tradeoff study of the capabilities and limitations of the five suppressants was conducted. Based on this study, no single suppressant emerged as having optimum fire ex- tinguishing capabilities in all possible situations. The selec- tion of a suppressant for a particular application is influenced by all of the following factors: fueling area design, enclosure integrity, physical dimensions, and location in the mine; airflow and air velocity through the area; types of fires ex- pected; and possible effects on personnel. The factor that has the greatest impact on suppressant effectiveness is ven- tilation. In completely enclosed areas with no ventilation, AFFF, high-expansion foam, and Halon 1301 can be used to effectively extinguish a typical combustible-liquid spill fire and secure the fuel against reignition. However, under the conditions of moderate-to-high ventilation, which are typical of most fuel transfer areas, only AFFF can extinguish and secure a spill fire. Another important factor influencing suppressant selection is the type of fire expected. In addition to spill fires, combustible-liquid pressure fires and running-fuel fires, as well as fires involving ordinary combustibles, could occur in a typical fuel transfer area. No single agent is completely effective in extinguishing and securing all of these types of fires, especially when the area is subject to moderate-to-high ventilation. Under this worst- case condition, a combination of suppressants is required for total protection. The most effective combination of agents is AFFF and multipurpose dry chemical. The dry chemical achieves quick "knockdown" of the initial flame. The AFFF forms a fast- spreading film over the spilled fuel, preventing the escape of flammable vapors and thereby eliminating the possibility of reignition. This type of "twin-agent" system, utilizing AFFF and multipurpose dry chemical, is commonly used at airports, petrochemical plants, and in other high-hazard facilities where flammable and combustible liquids are present. Controls System control options include manual operation, automatic operation, and automatic operation with manual override. A manually operated system could use hand por- table extinguishers, hose reels, and/or an overhead network of fixed piping and nozzles for suppressant distribution; whereas automatic and automatic-with-manual-override systems would use only fixed piping and nozzles. However, installation of a fixed suppression system does not eliminate the need to provide hand portable extinguishers in the area. The most important criterion in selecting a system control is extinguishing response time. Rapid extinguishment is critical for several reasons: 1 . The longer the preburn time (elapsed time between ig- nition and attempted extinguishment), the more difficult ex- tinguishment becomes. Fuels are heated to temperatures above their flashpoints, and involved surfaces become poten- tial sources for reignition (after fire-suppressant discharge). Secondary combustibles, such as wood or rags may be ig- nited and the fire may grow in size and area beyond the limits of suppression system coverage. 2. The fire may block safe egress of personnel, or person- nel may be injured and threatened by the fire directly. 3. Finally, as long as the fire is permitted to burn, copious amounts of toxic combustion products, such as carbon monoxide, are generated. Manually operated suppression systems depend on the prompt and appropriate response of attending personnel. If personnel are not present, or if they panic or are injured, ac- tivation of the suppression system may be delayed to the point where extinguishment cannot be assured. Worse, the system may not be activated at all. Automatic system operation offers a higher level of pro- tection than manual operation. Automatic control with the added security of a manual override capability is the recom- mended option. Although the potential for false alarms exists with automatic operation, the seriousness of an uncontrol- led fueling area fire justifies this remote risk. Detection Automatic system operation requires the use of a detec- tion system to trigger discharge of the suppressant. Detec- tion options considered included heat, smoke, and flame. Heat, or thermal, detectors, which depend on convected thermal energy for alarm activation, have a proven history of high reliability and low maintenance requirements, even when used in harsh environments. However, the response of thermal sensors may be unacceptably slow, especially in areas subject to moderate-to-high ventilation. Smoke detector responses are typically much faster, but as with thermal detectors, response is affected by local air currents. In addition, nuisance alarms can be expected to result from dust accumulations, humidity changes, and ex- posure to blasting fumes and diesel exhaust. Flame detectors offer the fastest response possible. They are designed to respond to the infrared, visible, and/or ultraviolet light emitted by a fire. Ultraviolet and infrared detec- tors are routinely used in rugged industrial settings such as offshore oil platforms, petrochemical plants, and aircraft hangars. Maintenance is generally confined to cleaning the optical lenses. Typical sources of nuisance alarms include lighting and arc welding for ultraviolet detectors and hot sur- faces or gases for infrared detectors. Ultraviolet detection is recommended for fuel transfer area fire protection systems. Thermal detection was ruled out because it is inherently slower than the other methods. In moderately to highly ventilated areas, detection delays would permit excessive preburn times. Smoke detection was ruled out because frequent nuisance alarms would result from con- stant vehicular traffic. Infrared detectors were rejected because false alarms could be triggered by hot engine sur- faces and exhaust gases. Ultraviolet detection therefore pro- vides the fastest and most reliable fire signal. Although arc welding within the optical field of view of an ultraviolet detector could cause a false alarm, such an occurrence could be avoided by disabling the detection system while the welding operation is performed. Figure 2 illustrates the recommended design concept for the fuel transfer area fire sensing and suppression system. This concept features twin-agent suppression, ultraviolet flame detection, and automatic-with-manual-override control. Fuel Storage Area The storage area fire protection system also consists of suppression, control, and detection subsystems. Suppression As discussed previously, AFFF, high-expansion foam, and Halon 1301 are all effective in extinguishing and securing a typical combustible-liquid spill fire in unventilated areas such as a completely enclosed fuel storage area. Although dry chemical is not capable of securing combustible liquids against reignition, its overall suppressant rating is higher than those of AFFF and high-expansion foam because of its greater extinguishing effectiveness on combustible-liquid pressure and running-fuel fires. All four agents are considered acceptable for this application; however, for optimum protec- tion, an AFFF and dry chemical twin-agent system is required. Controls The same system control options as were discussed for the fuel transfer area— manual, automatic, and automatic with manual override — exist for the storage area. The same selec- tion criteria apply as well. Again, the overall level of safety is higher with automatic operation than with manual opera- tion, and the optimum system would provide the added securi- ty of a manual override discharge capability. Such an op- timum system will minimize preburn time, heating of involv- ed surfaces, ignition of secondary combustibles, fire spread, and the generation of toxic products of combustion. Detection Because the fuel storage area is completely enclosed, fewer constraints are imposed on detector selection. Although thermal detection is inherently slower than smoke or flame detection, acceptable response times could be achieved with a properly designed thermal detection system in this area. Smoke detection would also be acceptable, however, op- timum system performance requires flame detection. Either infrared or ultraviolet detection could be used. Figures 3 through 7 show recommended fuel storage area fire sensing and suppression system layouts utilizing AFFF, high-expansion foam, Halon 1301, dry chemical, and twin- agent suppressant, respectively. Each system depicted in- KEY / Nitrogen cylinders 2 Dry chemical tank 3 Diaphragm foam tank 4 Control box 5 Dry chemical nozzle 6 Detector 7 Detector wiring 8 Foam-water sprinkler head 9 Balanced distribution piping 10 Emergency power supply // Visual and audible warnings 12 To remote warnings 13 Pneumatically operated main valve control 14 Manual actuation 15 Rat io control ler 16 Mine water supply line FIGURE 2.— Recommended fuel transfer area fire sensing and suppression system. KEY / Diaphragm foam tank 2 Pneumatically operated main control valve 3 Rat io control ler 4 Balanced distribution piping 5 Foam-water sprinkler head 6 Mine water supply line 7 Manua I actuation 8 Control box 9 Emergency power supply IO Visual and audible warnings // To remote warnings 12 Detector 13 Detector wiring FIGURE 3.— Recommended fuel storage area fire sensing and suppression system utilizing AFFF suppressant. // KEY / Diaphragm foam tank 2 Pneumatically operated main control valve 3 Foam generator 4 Mine water supply line 5 Manual act uot ion' 6 Control box 7 Emergency power supp iy 8 Visual and audible warn i ng 9 To remote wa r nings 10 Detector V // Detector wiring FIGURE 4.— Recommended fuel storage area fire sensing and suppression system utilizing high-expansion foam suppressant. KEY iced distribution piping nozzle let manifold 4 Halon tank 5 Manual actuation 6 Control box 7 Emergency power supply 8 Visual ond audible warnings 9 To remote warnings 10 Detector // Detector wiring FIGURE 5— Recommended fuel storage area fire sensing and suppression system utilizing Halon 1301 suppressant. KEY / Balanced distribution piping 2 Dry chemical nozzle J Nitrogen cylinder 4 Dry chemical tank 5 Manual actuation 6 Control box 7 Emergency power supply 8 Visual and audible warnings 9 To remote warnings 10 Detector / / Detector wi r ing FIGURE 6.— Recommended fuel storage area fire sensing and suppression system utilizing dry chemical suppressant. 12 13 KEY en cylinders emical tank agm foam tank actuation I box ency power supply and audible warnings mote warnings atically operated main rol valve control ler emical nozzle ed distribution pi ping or or wiring water sprinkler head 16 Mine water supply line FIGURE 7.— Recommended fuel storage area fire sensing and suppression system utilizing twin-agent suppressant. eludes ultraviolet flame detection; however, as noted above, thermal, smoke, or infrared flame detection would also be acceptable. For AFFF. dry chemical, and twin-agent suppres- sion, direct application of suppressant is most effective. Thus, numerous nozzles are distributed throughout the hazard area. However, Halon 1301 and high-expansion foam quickly ex- pand to fill the available enclosure space, making fewer discharge points necessary. Additional Design Considerations The foregoing analysis specifies the basic elements that should be included in fire protection systems for underground fuel transfer and storage areas. Additional design considera- tions relating to reliability, maintenance and system complex- ity are discussed below. In a survey of 18 mine managers, reliability, maintenance, and costs were identified as the three most important criteria for equipment selection. In later sec- tions of the report, comparative cost data are presented and cost-effectiveness analysis of fire protection systems is discussed. System Reliability and Maintenance Where the proper function of a system depends on the pro- per function of all of its components, overall system reliabili- ty is expressed as follows: RsW = (Rd(t)) x (R c2 (t)) x (R c3 (t)) ... (R cn (t)) where R s (t) = system reliability over time interval t and R c (t) = component reliability over time interval t. Thus, system reliability is maximized when the reliability of individual components is maximized and when the number of individual components is minimized. Reliability of Individual Components Individual component reliability generally follows the pat- tern shown in the idealized failure rate curve (fig. 8). The in- itial period is characterized by a relatively high failure rate due to manufacturing and installation defects or applications that exceed the recommended duty cycle or environmental restrictions of the component. It is sometimes referred to as infant mortality. The middle period is characterized by a low FIGURE 8.— Idealized failure rate curve. failure rate. Failures during this period are the result of ran- dom events. It is sometimes referred to as the useful life or prime of life. The final period is characterized by a high and asymptotically increasing failure rate, which results from com- ponents wearing out. This period is sometimes referred to as the wearout or burnout phase. Careful selection of components, with particular attention to manufacturing quality control, rated duty cycles, and en- vironmental restrictions can help minimize infant mortality. The performance of similar components operated under similar conditions can also guide equipment selection. In ad- dition, components that are listed or approved by one of the various nationally recognized independent testing laboratories can be specified. Components are listed or ap- proved by these laboratories only after they have been thoroughly tested and found to meet or exceed the re- quirements of rigorous environmental and operational standards. Periodic inspection is necessary to detect random failures that may occur during the component's useful life. How often inspections are necessary is determined through long-term observation of the system in operation. The fire sensing and suppression systems described above require weekly visual checks. Typical inspection items include inspecting nozzles for obstructions, checking pressure gauges for pressurized components, noting general appearance of components for mechanical damage and corrosion, and checking the posi- tion of main water-supply valves. If component failure is to be avoided, preventive maintenance (PM) is necessary to ensure that components will be repaired or replaced as necessary before they wear out. Manufacturer-recommended PM programs are designed to extend the useful life of a component and/or preempt com- ponent failure by replacement before wearout failures occur. Various components of the fire sensing and suppression systems require PM at 6-month, 1-yr, 2-yr, and 5-yr intervals. Typical maintenance items include testing suppression system actuation mechanisms for proper function and pneumatic actuation lines for leaks (no agent discharge re- quired); weighing pressurized components and comparing to fill levels indicated by pressure gauges; testing detectors with test flames; testing all visual and audible alarms and other system control features; checking fill levels of non- pressurized suppressant-agent containers; and hydrostatic testing of pressure vessels. Component repair or replacement should be performed when necessary. In addition, conditions causing undue wear to components should be corrected if possible. System Complexity The fire sensing and suppression systems discussed are as simple and straightforward as could be designed and yet still perform as required. As few individual components as possible were included in each design. Each system consists of predesigned and manufactured modules to simplify assembly, installation, inspection, maintenance, and operation. COSTS Costs of detection, suppression, and control equipment would vary according to the nature of the hazards that might be expected and the size of the area covered by the system. Table 4 contains estimated costs for five alternate fire pro- tection systems for a typical enclosed fuel storage area 65 10 TABLE 4.— System cost comparisons (Fire sensing and suppression systems for fuel storage and fuel transfer areas using various suppressants; estimated costs in 1983 dollars) Suppressant High-expansion Dry Twin AFFF foam Halon 1301 chemical agent Equipment $10,600 $10,900 $10,300 $10,300 $16,500 Installation 4,400 3,000 3,000 5,200 6,100 Operation 1,500 1,200 2,600 370 3,000 Maintenance, annual 300 300 370 370 500 Total 16,800 15,400 16,270 17,370 26,100 AFFF Aqueous film-forming foam. ft long, 20 ft wide, and 1 3 ft high, containing 1 ,250 gal of com- bustible liquids. A fuel storage area was chosen for this ex- ample over a fuel transfer area because only one system type (twin agent) was recommended for the transfer area, whereas all five types (twin agent, high-expansion foam, Halon 1301 , dry chemical, and AFFF) were judged acceptable for the storage area. Included in each estimate, as a part of equip- ment cost, is $4,400 for detection (ultraviolet flame) and con- trol (automatic with manual override). The installation cost estimates in table 4 are for installa- tions by a fire protection contractor. Significant savings could be realized if mine personnel installed the system. Overall system cost-effectiveness is discussed in a later section of this report. ALTERNATIVE SYSTEM DESIGNS Like the guidelines for fueling system design, the recom- mended conceptual designs for fuel transfer area and fuel storage area fire protection systems represent optimum rather than minimum approaches. Also, these concepts are based on certain assumptions regarding the layout of the fueling area, such as fuel transfer area ventilation, storage area enclosure, etc. Alternative system elements may result in a higher level of safety depending on local conditions. However, the rationales presented in this report for selecting system elements can provide useful guidance if alternatives are considered. PROTOTYPE FIRE SENSING AND SUPPRESSION SYSTEM A complete fire sensing and suppression system was designed, fabricated, and tested under laboratory conditions and in an underground mine to evaluate the feasibility, prac- ticality, and overall effectiveness of the system under simulated and actual mining conditions. GENERAL DESCRIPTION The prototype fire sensing and suppression system (fig. 9) was designed in accordance with the generic design con- cepts discussed previously. Each system element is dis- cussed in detail below. Suppression Since the site selected for in-mine tests was not enclosed and combined both storage and transfer of fuel, a twin-agent AFFF and multipurpose dry chemical suppression system was specified. AFFF Subsystem The AFFF subsystem contains a main control valve, a water pressure and flow control unit, a concentrate bladder tank, a water-concentrate proportioned and a distribution system of pipe and nozzles. The pneumatic pressure from the control unit operates the main control valve of the AFFF subsystem, initiating water flow from the mine water supply line. The flow and pressure of th'rs line are maintained at constant levels through the use of a pressure-reducing valve. This flow of water is used to fill the volume of the bladder tank between the bladder and the tank wall, pressurizing the concentrate inside the blad- der. The concentrate is then forced out of the tank and into the proportioned where it is mixed with the main water flow to yield a solution containing concentrate. The solution then flows through the piping to the nozzles, where air is drawn into it to form the foam that is discharged onto the hazard. Eight standard nozzles, each providing 100 sq ft of coverage, are required to cover the fueling area. At the re- quired minimum delivery rate of 0.1 gpm/sq ft of protected area, the 800-sq-ft fuel storage area would require 80 gpm of foam discharge. A minimal 10-min discharge would require a 24-gal AFFF concentrate tank. Increasing the pressure of the foam-water sprinkler nozzle to 30 psi to provide a higher quality foam would yield 16 gpm per nozzle or 0.16 gpm/sq ft of area. The 800-sq-ft area would then require a 128-gpm discharge and a 40-gal AFFF concentrate tank for a 10-min discharge. Utilizing a 70-gal concentrate tank and operating eight nozzles at 16 gpm per nozzle, the actual foam flow time would be 18.2 min. The distribution piping was hydraulically calculated and sized to provide nearly equal nozzle pressures to all eight nozzles. The AFFF agent used in this system is a 3-pct-type con- centrate intended for use by dilution or proportioning at a 3:97 volume ratio with water. At present, there are no Federal or military specifications covering this type of concentrate. Specification MIL-F-24385A, 2 May, 1977 (U.S. Navy), covers 6-pct AFFF concentrates and requires performance at 3-pct, 6-pct, and 50-pct concentrations before and after aging for 10 days at 150° F of both the concentrate and premixed fresh and salt water solutions. The concentrate used in this system meets all of the fire- performance requirements specification of MIL-F-24385A. It also has been tested and found to perform equally well in water having a hardness (calcium and magnesium) of 500 ppm. The AFFF concentrate is a mixture of fluorochemical surfactants, hydrocarbon surfactants, and solvents. It is specifically formulated to have low corrosion characteristics on most common metals and a low environmental impact. The nozzles are aspirating-type, upright foam-water sprinkler nozzles with 1/2-in NPT connections and a 3/8-in throat. They provide good-quality foam during discharge. 11 FIGURE 9.— Prototype fire sensing and suppression system for underground fuel storage and transfer area. The AFFF system is designed for an operating temperature range of 32 : to 1 20 ■ F. This system should not be used where temperature may drop below 32° F because the AFFF solu- tion will freeze. Corrosion protection is accomplished with bronze valves and fittings and brass piping, and the entire unit has a special epoxy coating. Dry Chemical Subsystem The dry chemical subsystem consists of three major com- ponents: the nitrogen power supply, the dry chemica; storage container, and the distribution system composed of piping and nozzles. Pneumatic pressure from the control-unit car- tridge opens all nitrogen cylinders simultaneously, releasing the nitrogen through pressure regulators to pressurize the storage container and fluidize the dry chemical. When the storage container reaches a predetermined pressure, a frangi- ble disc in the outlet piping bursts, releasing the dry chemical into the piping. Pre-aimed stationary nozzles discharge the dry chemical onto the hazard. The dry chemical tank provides for the storage of 425 lb of agent. The agent is suitable for use on fires involving cellulosic-type fuels such as wood, paper, plastics, etc., as well as flammable and combustible gases and liquids. Federal Specification 0-D-1380A. July 12, 1968 (General Services Ad- ministration), details the composition and physiochemical properties of the type of agent used. The agent is a finely divided solid cnsisting of about 90 wt pet monoammonium phosphate. The remaining 10 pet consists of materials to im- prove flow and packing characteristics and reduce hygroscopicity such as clays, colloidals, silicas, and polymeric siloxzines. This agent is compatible with AFFF in a separate or simultaneous application. The four 1-1/2-in dry chemical nozzles provide a fan-shaped pattern for a large-area sweep and will distribute the dry chemical at a flow rate of about 12 Ib/s. The distribution pip- ing for the dry chemical subsystem is schedule 40 hot-dipped galvanized pipe. A balanced piping arrangement is used to distribute the dry chemical evenly to each of the four nozzles. The dry chemical subsystem is designed for and can be operated in the temperature range of -40° to 120° F. All components have a heavy external epoxy coating for protection against highly humid or corrosive atmospheres. The system is completely sealed to prevent internal corrosion. Detection and Control The detection and control subsystem contains ultraviolet detectors monitored by a control panel that provides a pneumatic output to the suppression subsystem when flames are present within the cone of vision of the detectors. The detectors are self-contained 24-V dc units that provide both instantaneous and time-delayed contacts as outputs. The control system utilizes 5-s-delay contacts as a zone-alarm input, leaving the instantaneous contacts available for addi- tional remote annunciation on an individual-detector basis if required. The detectors are equipped with an "optical integri- ty" feature that allows remote testing of the condition of their 12 optical lenses. The detectors are housed in an explosion-proof enclosure and are easily installed with integral swivel mounts and brackets. Two detectors are required for this system. They are posi- tioned in the fueling area so their 90° fields of vision overlap, with each detector covering the entire area. The two detec- tors are cross-zoned within the control unit— meaning that both detectors must sense the fire before the fire signal passes to the actuation device— to minimize false alarms. The fire signal operates a mechanical device that releases an internally stored pressurized gas to operate the suppres- sion subsystem. The control unit has an internal adjustable time delay to delay actuation of the suppression system un- til a predetermined time has elapsed after the alarm input is received. It also has an abort function to permit system maintenance without nuisance alarms. TESTS Laboratory Component Testing All system components were analyzed for their suitability for use in a harsh underground mine environment. For com- ponents with insufficient histories of performance in underground environments, tests were performed in the laboratory under simulated mining conditions. Suppression AFFF subsystem components were selected with corrosion resistance as the major consideration. Components not nor- mally used in AFFF systems, such as the main control valve and pressure-reducing valve, were chosen based on materials of construction and simplicity of operation. Since little data was available on the performance of the main control valve and pressure-reducing valve under the conditions anticipated in the mine, specialized performance tests were devised and conducted for these components. Operational testing on the pneumatically operated main control valve consisted of obtaining the torque output from the actuator before and after prolonged corrosion testing in a salt spray. The result of the salt spray was a decrease in torque output significant enough to cause concern. Therefore, a new actuator was obtained and modified to prevent cor- rosive atmospheres from affecting the internal parts of the actuator. Gaskets and plugs were used to seal joints, and 0-rings were added to the actuator shaft to eliminate seepage between the shaft and body. A second salt-spray test on the redesigned actuator gave satisfactory results. The objective of the pressure-reducing valve tests was to determine pressure setting versus flow and pressure. No operational problems were encountered during any of the tests. The pressure-reducing valve provided leak-free opera- tion and consistently reduced pressures downstream. Due to previous testing and past performance in actual use, the dry chemical subsystem was considered to be rugged and corrosion resistant enough to withstand the atmosphere and hard usage it would encounter underground. Com- ponents of the system such as cylinders, valves, hoses, gauges, and other equipment have all undergone salt-spray, shock, vibration, and operational tests to obtain listings by Underwriters Laboratories, Inc., and Coast Guard marine ap- proval. Dry chemical installations on offshore platforms are exposed to severely corrosive environments, yet they func- tion for extended periods without failure. Detection and Control All of the component parts of the detection and control sub- system have been extensively tested and are listed by Under- writers Laboratories. The ratings and limitations of the detec- tion system and the capabilities of the emergency power supply and control equipment are predefined by Underwriters Laboratories. Laboratory Full-Scale Fire Testing of Complete System As a last step prior to in-mine installation and testing, the complete prototype system was subjected to full-scale fire tests in a specially designed fire-test fixture. The fire hazard consisted of a 50-sq-ft pan fueled with 2 in of heptane. Heptane was used instead of diesel fuel for ease of ignition and also because it has a much hotter flame and is more difficult to extinguish. A vehicle mockup was situated directly over the pan to simulate a fire under a vehi- cle during the refueling operation and to provide an obstacle to fire-suppressant agent coverage. The AFFF subsystem was installed with four nozzles spaced 10 ft apart in a grid and suspended 10 ft directly over the hazard. The dry chemical unit was installed with two of the four nozzles aimed at the hazard. System actuation was provided by pneumatic actuation lines running from the AFFF and dry chemical subsystems to the control unit. The con- figuration of system elements for mockup testing is shown in figure 10. The heptane was ignited and allowed approximately a 15-s preburn, which gave total fuel involvement. At 15 s, one detec- tor was aimed at the fire; it instantaneously actuated the con- trol unit. Flow from the AFFF subsystem began immediately— pure water for the first 5 s and, once stabilized, foam thereafter. The cooling effect of the foam decreased flame intensity slightly. When the AFFF control valve was opened, the nitrogen cylinder valves on the dry chemical unit simultaneously opened, beginning pressurization of the dry chemical tank. A 12-s delay was built into the dry chemical unit by use of the frangible disc, allowing pressure to build in the tank and fluidize the dry chemical The time span between the bursting of the disc and com- plete extinguishment (fig. 1 1) was approximately 3 s. The total time from detection to extinguishment, during which foam flow had already begun, was approximately 15 s. Additional tests were conducted to determine the effec- tiveness of AFFF and dry chemical used separately on spill fires obstructed from the overhead nozzles by the vehicle mockup. Neither AFFF nor dry chemical alone was able to extinguish such a fire. Field Test The site selected for field tests was a fuel dock in Union Carbide Corp.'s Pine Creek tungsten mine near Bishop, CA. 