Water mist: A wise recipe for extinguishment
Water mist systems are used as fire protection in buildings with sensitive equipment or materials.
Smart and safe applications for water mist systems are no longer reserved for cruise liners. The first real applications of these systems took place in the 1940s for the protection of passenger ferries from fire. Today, water mist systems are implemented to protect electrical equipment rooms, flammable liquids storage areas, aircraft cabins and galleys, sprayed liquid fuel applications, gas turbines, power generation utilities, underground railway systems, oil and fuel pump rooms, museums, libraries, historic structures, and data centers.
Water mist systems represent an advantageous fire safety system selection for any situation where an abundant amount of applied water is not desired. Because they use less water to suppress a fire, they typically employ high-pressure (and low-volume) pumps to discharge water in the form of numerous small droplets (measuring anywhere from 30 to 200 microns in diameter, or one-millionth of a meter in diameter) through clean, high-performing nozzles.
By blanketing the fine-spray water droplets over a wide surface area, an activated system will maximize heat absorption while displacing oxygen. The emphasis is on value rather than volume. Rapid cooling is a key component of the system’s fire mitigation and fire blocking (of radiant heat) quality, essential for areas where machinery cannot quickly be shut down.
Just as with gaseous agents, water mist systems provide “total protection” of an enclosure. This occurs because the atomized droplets,* along with room air, are drawn to the base of a fire. As the water mist turns into steam (expanding in volume), it forces away from the flame the oxygen necessary for continued combustion. The total protection effect achieves a quick wetting and cooling of the burning surface. This formula for extinguishment also prevents re-ignition. These systems efficiently absorb and remove both smoke and toxic gases from a fire, another feature not present with alternative protection options. The degree to which smoke and gases will be absorbed and removed depends upon the specific system design parameters applied, and also on the volume and composition of what is burning. For certain problematic scenarios, an in-place mode of protection that is diverse with regard to what it will deliver is definitely in order. And the specific design objectives of water mist are fire control, temperature control, exposure protection, suppression, and extinguishment. Fire is simply not sustainable without heat and oxygen.
For active fire protection, a water mist system may be preferable to the traditional fire sprinkler system in cases where the potential damage (from both water and smoke) to a highly decorative or historic structure must be held to a minimum. Although water mist is a more expensive option, the systems disperse only 30% to 40% of the water that a sprinkler system would deliver when in combat with the same fire. Because the mist nozzle** openings are so fine, it is absolutely essential that the water’s purity level is pristine and stays that way over time. For that reason, all piping must be entirely corrosion-resistant. Piping options are limited to copper tube (K, L, or M) or stainless steel pipe—galvanized steel is not allowed. The systems require frequent servicing, at least twice annually. Maintenance practices must ensure that nozzles remain clog-resistant, a precept that is fundamental to the application.
NFPA 750, Standard on Water Mist Fire Protection Systems, requires that a filter or strainer be provided downstream of each nozzle, with significantly more pressure required at the nozzles than would be for a garden-variety sprinkler system. Most water mist systems are limited to a maximum enclosure volume of 60,000 cu ft. The standards also call out limitations that may prohibit installations in areas with a ceiling height in excess of 15 ft. Water mist systems require a water supply of the highest quality, ideally potable water (this reality often presents the greatest obstacle). A water supply consisting of a tank filled by well water is unsuitable and would sabotage effective system function.
Because the presence of accumulated particulates or any corrosion at all represents a virtual cancer for water mist systems, they are not expected to last even half as long as a traditional fire sprinkler system. These systems will probably outlive your car, but from a reasonably optimistic standpoint their expected life is somewhere between 15 and 20 years.
Learning as we go
Water mist for fire protection is still in its early stages of industry adoption, and potential usages are considered only after full-scale fire tests offering visible proof of successful operation for a replicated application have been conducted. That being the case, manufacturers of water mist components represent the virtual horse pulling the cart that updates the codes. NFPA 750 is mainly a performance-based document that relies on demonstrated system performance established through fire tests (tens of thousands have been run) and third-party approvals. While considerably less specific than NFPA 13, and palpably thinner in scope, the NFPA 750 standard contributes relevant guidance with respect to requirements for water mist pumps, piping materials and joining methodology, air or pressurized gas, power bending tools, automatic detection, hazard classification, hangers, strainers, nozzle spacing, and maintenance protocol. The 2010 edition, for example, contains basic principles such as Section 7.3.6: “All system piping and fittings shall be installed so that the entire system can be drained,” and Section 220.127.116.11: “Bending of the pipe shall be permitted as provided by the listing.”
Due to their absence of familiarity and the shortage of specific code criteria, inexperienced authorities having jurisdiction (AHJs) are at a veritable loss when attempting to review water mist installation plans. The standard not-so-subtly allows for a wide array of design possibilities provided that the system design adheres to the listing requisites of pumps, nozzles, and related equipment. The good news is that NFPA 750 (first published in 1996) has evolved into a more prescriptive standard as water mist systems have become increasingly popular. Respect for the standards is a given, since they are eventually adopted by building codes. The following additions were first introduced to the 2006 edition:
- 10.5.2.3: The discharge piping for water mist pumps and pump assemblies for high-pressure or intermediate-pressure water mist systems shall be equipped with a valved test connection and provisions for the installation of a flow metering device to permit accurate measurement of the pump performance during the acceptance test and during annual testing.
- C.4.9: ANSI/UL 2167 requires the nozzle manufacturer to prepare a design and installation manual containing detailed information concerning the intended use and limitations associated with nozzles.
- C.4.8: ANSI/UL 2167 contains a requirement for the marking of water mist nozzles. In the field, these markings provide a method of verifying that the proper nozzles have been installed.
- 18.104.22.168 (4) and (5): Hanger components shall be metal. Plastic inserts shall be permitted in tube clamps to avoid dissimilar metal reactions or dampen vibrations.
Playing by the rules
Property loss prevention data sheets prepared by FM Global concerning water mist systems were released in September 2006. These contain numerous recommendations for design and installation including the following:
- 22.214.171.124.3: Provide documentation showing hydraulic and pneumatic calculations. Ensure hydraulic calculations contain the following information: (a) location identification; (b) description of hazard; (c) design area of water application or volume of space protected; (d) total water requirements as calculated.
- 126.96.36.199.1: Design the system to be automatically actuated. Provide a method for manual actuation.
- 188.8.131.52.2: Automatically shut off or close interlocked devices, such as exhaust fans or doors, before operation of the water mist system.
- 184.108.40.206.5: Ensure the length of an unsupported arm over to a nozzle does not exceed 2 ft for pipe, or 1 ft for tubing.
- 220.127.116.11.7: Provide corrosion-resistant piping from the system strainer to the nozzle. Ensure the material has corrosion resistance equivalent to drawn or seamless copper tube or stainless steel. This is to reduce the possibility of blockage of nozzles due to corrosion.
- 18.104.22.168.5: Use ordinary temperature ratings for automatic nozzles. If in unventilated areas such as attics, use intermediate-temperature rated nozzles. If located within 10 ft of hot surfaces, such as exhaust pipes or steam pipes, use high-temperature rated nozzles.
- 22.214.171.124.1: Provide FM Approved strainers and filters at all water supply connections.
- 126.96.36.199.3: Keep a stock of spare strainers and filters to service nozzles for the largest single hazard or group of hazards protected simultaneously.
- 188.8.131.52.1: Provide FM Approved pumps sized to 120% of the required system water flow rate, at minimum system operating pressure.
- 184.108.40.206.4: Clean or replace filters and strainers after each system operation due to fire.
- 3.1.2 (1): FM Approved water mist systems are pre-engineered systems for enclosure protection. The enclosure volume, ceiling height, and ventilation are considered critical parameters.
- C.2.1.4: Galvanized steel piping is no longer recommended for water mist systems.
Across-the-board acceptance and approval of water mist fire suppression systems is currently limited to those occupancies and operations where thorough fire testing can be documented by testing agency laboratories. While the benefits of water mist are clear to interested professionals, the cart does continue to pull the horse, in a cautiously progressive fashion. It is anticipated that future testing of the effectiveness of water mist on fire will include “fuel” applications such as flammable liquid storage rooms and telecommunications/telephone equipment rooms. In addition, experimental fire testing of water mist systems in conjunction with a labyrinth of exhaust fans to be tripped by a zoned system of detection within storage occupancies is currently being pioneered. What’s in the stars for these more complex water mist systems will be dictated by their actual field test successes or failures, engineered trial-and-error adjustments, and more test trials.
A comprehensive source for water mist information and product testing requirements is available from FM Global in the form of its “Approval Standard for Water-Mist Systems” (5560). It also is a work in progress; it is well-organized and exhaustively researched. The standards represent substantial bodies of work that are keeping up with new techniques and innovations as the water mist concept moves forward.
Case study: Water mist to protect computers and electronic equipment
Thus far, water mist systems have been slow in “taking off” to capture a big share of the special hazards market in the United States. Currently, water mist systems are used to protect smaller applications such as machinery that processes ink, adhesives, and similar flammables. Water mist also claims a small share of the electrical and generator room market. Because the mist systems do deliver water, usage over an extended period will damage equipment and can cause operational downtime as a result. For that reason the lion’s share of new switchgear rooms are protected with FM-200 systems, or Inergen. What follows is a capsulated study of one structure containing numerous water mist systems to protect valuable computer equipment.
In comparison to gas-based suppression agents, water mist is more effective at extinguishing fires involving common electrical materials. Of equal importance, testing has demonstrated that water mist will not harm computers even after the system has discharged for 30 min. Where computer downtime can translate into $1 million per day, it’s easy to comprehend the expanding commercial market for water mist systems. The photographs contained here depict a water mist preaction system designed to protect a large computer server farm. An early-warning Vesda piping system installed throughout the facility provides the necessary detection to sound alarms and to simultaneously trip a solenoid (preaction) valve, filling the system with water. The fire detection system continually draws air into a network of ¾ in. CPVC piping (containing small drilled holes) with a high-efficiency aspirator, passing an air sample through a filter before it enters a laser detection chamber that answers as a high-efficiency smoke detection unit. There are other acceptable supplemental detection devices available for water mist systems, but all must be capable of reliably releasing the deluge or preaction valve priming water pressure via their own activation.
The nozzles (not pictured) used for this system look like ordinary water mist nozzles, small and narrow. But they contain both heat-sensitive glass bulbs, and filters (strainers). They are referred to by the manufacturers as mist nozzle sprinklers and consist of brass housing with a nickel (or chrome) plating. An interior spool valve holds back system air pressure until thermal activation of the compact component. The available temperature ratings for these nozzle types range from 135 to 286 F.
This particular water mist system for the data center comprises relatively small stainless steel piping, fabricated in the field, that will hold pressures exceeding 1000 psi supplied by a positive displacement pump. The pump is equipped with a cleanable suction strainer. System mains, branch pipe, and fittings are all metal-to-metal, with threaded ends on piping. This system uses only 38, 16, and 12 mm pipe. Installers hand-bend the smaller pipe (12 and 16 mm pipe are close matches to ½ and ¾ in.) and use a hydraulic bench to bend the 38 mm pipe (similar in size to 1.5 in.). Integral to the system are the branchline tees (called ports) that all have male outlets (in contrast to the black cast iron tees used in sprinkler systems). The tees are joined to the pipe with a threaded steel nut containing a very small metal disk which is pressed on in the field using a minimal amount of oil. The seal is to be extremely tight—the nuts are cranked hard using a 24-in. wrench. To keep the system interior as pristine as possible, there is no pipe dope whatsoever (or Teflon tape) applied to the threads. All pipes are flushed prior to installation with an air gun that propels a foam plug (that answers as a cleaning projectile), which is shot through the pipe twice, once from each end.
The pipe purifying operation is more demanding than just “cleaning a rifle.” With water mist systems, there is much riding on the integrity of installation. Field personnel must adopt a fetish for exactitude; there is little tolerance for error. The systems are sensitive to a plethora of parameters, and if care is not exercised in any one portion of the methodology, the entire system may run aground. Training is vital; piping is “cold cut” on the job using a solvent that runs continuously with a metal cutting plate. Thread cutting must be level and straight. The system branchlines (Figures 1 and 2) terminate in brass adapters that hold the nozzle, each containing its own strainer. The 200 F rated sprinkler nozzles used in this case will thermally activate when room temperatures elevate high enough to shatter the quick-response frangible bulb. Assuming that smoke has already been detected, thereby releasing the preaction valve, mist will be dispersed and the low-volume fire pump will begin to operate.
What makes the water mist system so suitable for this server farm application is its reduced water supply requirement. It’s a system that debunks the notion that water and electronics don’t mix, and the final engineering appraisal is that a water-mist system provides a promise of computer room protection while serving as an effective safeguard against expensive computer downtime.
Bromann is principal of Rally Fire Protection Services, and the author of two books: “Fire Protection for Commercial Facilities” and “The Design and Layout of Fire Sprinkler Systems.” Bromann has more than 30 years of experience in the field of automatic fire suppression systems.
*Atomizing media is defined by NFPA 750 as “compressed air or other gases that produce water mist by mechanical mixing with water.”
**The water mist nozzles must be listed (UL 2167, the U.S. Coast Guard, Lloyd’s Register, and/or FM Global Standard 5560).
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