Fixed Water-Spray Systems

The suitability of a water spray for application to releases composed of water-soluble materials is specific to the particular material in question. If applied to cryogenic materials, a water spray can cause the material to heat and violent boiling can result. Therefore, it is prudent to understand the properties of the material in question. Key considerations include the flash point, specific gravity, viscosity, and solubility of the material, the temperature of the water spray, and the temperature of the hazardous material (NFPA 15, 1990). 

The water spray system must also be designed to ensure that water does not come in contact with live electrical equipment. Adequate clearances should be maintained under all anticipated weather conditions. 

Inspection and Preventive Maintenance As with all water-based extinguishing systems, particularly those installed outside, inspection and preventive maintenance are critical to reliable operation. Detection devices are typically inspected, tested, and maintained quarterly or at a frequency deemed more appropriate for a particular location. The water-spray system itself is typically flow-tested at least annually (NFPA 25, 1995). Because of the nozzles small orifice size, scale and corrosion buildup can compromise flow and spray patterns. The use of strainers and the conduct of flow tests will help minimize this potential problem. As stated earlier, preventive maintenance of the overall water supply and distribution systems is an integral part of a reliable system. 

---

The usual protection for large installations is to provide a water-spray system. For small bulk storage, fire hoses or monitors are often adequate. However, for installations over 50 tons of storage (and all major cylinder-filling plants) it is accepted that a fixed water-spray system needs to be provided which is automatically initiated by a system capable of detecting a fire threatening the vessels... 

National Fire Protection Association (NFPA), NFPA 15. Fixed Water Spray Systems. NFPA, Quincy, MA., 1990. 

Bare pressure vessels in a process unit protected by vapor depressuring usually do not require protection by fixed water spray systems. 

For example, if LPG vessels are considered to be within a fire-scenario envelope, they require fireproofing unless protected by a fixed water spray system. API Standard 2510, Design and Construction of Liquefied Petroleum Gas Installations (API, 2001) recommends fireproofing pipe supports within 50 ft (15 m) of the LPG vessel, or within the spill containment area. 

The time required to apply adequate, reliable cooling from fixed water spray systems or fixed monitors, including response time for personnel to operate them. 

Structures with equipment on two or more levels, not protected by water spray or deluge systems, should be counted as double the grade area beneath the multilevel structure plus the areas as noted above. Where fixed water spray systems are provided, their water requirements should be added to the fire water demand. Water demands for other areas within a plant may include ... 

Deluge protection can be applied over the entire hazard area with open sprinkler heads located at various floor levels or fixed water spray systems can be applied specifically on the equipment to be protected. Specific water spray application is preferred because it provides better equipment cooling and reduces the water wasted due to wind and fire draft. 

Fixed water spray systems are most commonly used to protect flammable liquid and gas vessels, piping and equipment, process structures and equipment, electrical equipment such as transformers, oil switches, rotating electrical machinery, and openings through which conveyors pass. The type of water spray will depend on the nature of the hazard and the purpose for which the protection is provided. 

The required protection may be obtained by active, passive, or a combination of both protection systems. For example, steel support located in a fire exposed area within process unit battery limits may be protected by either a fixed water spray system or the application of fire resistant insulating material to the steelwork or possibly both. Note Passive protection is generally the preferable method for protecting structural steel. 

Fixed water spray systems designed on an area coverage basis may also be used to wet/cool structural steel supports. In this case, the placement of discharge nozzles should be close, usually within 4 ft (1.2 m) of the steelwork being protected. Alternatively additional nozzles or a separate system may be provided. 

Fire protection can be provided to process vessels with either manual firefighting or fixed water spray systems. Manual firefighting with monitors and hoses can be used to protect process vessels against exposure to fire. See Chapter 7, Section 7.4.3. If water spray is used, it should be applied to all outer surfaces at a rate of 0.25 gpm/ft (10 Ipm/m ) of projected (surface) area. See Chapter 7, Section 7.4.8. Multiple nozzles are typically required. Where water spray from upper nozzles can flow down the sides of the vessel, the nozzles or... 

Activation of layers of protection such as relief valves, interlocks, rupture disks, blowdown systems, halon systems, vapor release alarms, and fixed water spray systems... 

Fundamental to the use of fixed water-spray systems and other water-based extinguishing systems is a water supply that is adequate in terms of capacity, pressurization, and reliability. The capacity available may come from bodies of water, domestic supplies, and/or tank storage. A system s adequacy must be judged on the basis of its projected maximum flows, the duration of its flows, and the volumes necessary to meet industrial needs. The latter is relevant in cases where there is no dedicated supply. 

Because there are no reasonable methods for mitigating risk following the occurrence of a BLEVE, all practical measures should be taken to prevent the causes of a BLEVE. This could include fixed water-spray systems over process vessels containing flammable liquids or which could be exposed to flammable-liquid fire, ensuring that relief devices do not direct vented flammable vapors onto the container, systems for diverting spilled flammable liquid to locations that would not expose process vessels, and pubhc-address systems to warn persons to evacuate locations where loss of process control could lead to a BLEVE [14]. 

American Petroleum Institute (API). RP 2030, apphcation of fixed water spray systems for fire protection in the petroleum and petrochemical industries. 3rd ed. Washington, DC API 2005. 

---