Fire protection in MCFs (part 3)

A three-part series on fire protection in mission critical facilities will help you understand the value of your facility, evaluate its risks, and investigate the protection options.

04/26/2008


Fire protection for mission critical facilities (MCFs) can be a complex and daunting topic. We’ve broken up the task into several topics so that you can create manageable assignments out of each one. This three-part series will cover each topic in depth.

* Part 1 : Understand the value of the facility in question. Value can be defined in a number of ways from asset value, to operational value to historical or sentimental value.
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Part 2 : Evaluate the level of risk in the facility in question. What has been done to mitigate these risks? What can be done? What should be done in order to adequately protect against a potential hazard, including fire, in the facility?
* Part 3 : Investigate the options available from water-based systems to waterless systems and determine what is right for your facility and your business. Understand the unique and varying levels of protection your facility will get from each option. It is the business owner’s responsibility to fully understand what each system will provide for his or her business.


Waterless fire protection systems

If the evaluation process above yields the need for waterless fire protection, additional choices and evaluation will be necessary. Myriad options are available in the three major system categories:

1. System controls.
The control system is the “brain” of the fire protection system. Fire alarm system control panels that are used to initiate fire extinguishing systems must be listed as releasing panels. These panels are available as intelligent or conventional style panels.

Conventional control systems involve the collection of multiple circuits or “zones” of protection. Various panels can accommodate different numbers of zones. Each zone is a circuit with detection devices located on each circuit. When a detection device goes into alarm, the circuit is closed, the panel recognizes the signal, and proceeds to the appropriate action (initiation of a suppression system, initiation of audible or visible alarms, etc.). Conventional systems are inherently labor-intensive requiring individual maintenance and testing for each device. Additionally, when in alarm or trouble, there is no way of determining which device has been triggered without investigating each individual device in the zone. Conventional control panels provide a base level of functionality and are the most cost-effective options for a control system.

Intelligent systems provide state-of-the-art controls for a given fire protection system. Intelligent systems allow the owner and servicing personnel to identify each circuit device in trouble or alarm, at the control panel, without surveying each individual device. Each device has a unique address on a given circuit, enabling this identification ability. Intelligent systems offer an enhanced level of functionality and typically will cost more than a conventional panel system.

Fire suppression panels are generally designed and installed specifically for the release of a fire suppression system (such as a sprinkler valve or clean agent container). Suppression panels are fully functional with various methods of detection and devices such as manual pull stations, abort switches, monitoring switches, etc., and come in both conventional and intelligent varieties, as mentioned above.

Fire alarm panels generally include a more broad level of functionality than a suppression-releasing panel, and include several key features like voice evacuation systems and ability to communicate with third-party central monitoring stations (typically referred to as “central station”). In some cases, panels may act as both a fire alarm panel as well as a fire suppression-releasing panel.

2. Hazard detection.
Detection of a hazard within the protected space can come in a variety of forms, including water, heat, and smoke detection. For the purposes of this discussion, we will focus on the most common method of hazard detection when dealing with clean agent fire protection systems: smoke detection.

There are several technologies available for the detection of smoke within a MCF.

Spot detection. Spot smoke detection is the most common variety of smoke detection available. Technology in this area has improved in the past 5 to 10 years, making detectors more reliable and more sensitive and less apt to false alarms. While completely eliminating false alarms is virtually impossible, the latest technology significantly reduces this risk.

The two most common varieties of spot smoke detection are ionization and photoelectric.

* Ionization detection is based on the detection of smoke through a reduction in current across a charged surface. Smoke particles, passing through an ionization chamber, will attach themselves to ions in the chamber created by small amounts of alpha radiation and disrupt the current being generated. The electronics of the detector sense this drop in current, and when sufficient, will initiate an alarm condition. Ionization detectors will respond most quickly to large flaming fires, which produce smaller particles of combustion.
* The most common type of photoelectric detector use a light beam source to detect how much light is being scattered, through the size and frequency of smoke particulates present. When the light is scattered by the particles sufficiently onto a photocell, a current is generated and an alarm condition results. Photoelectric detectors will respond most quickly to smaller, smoldering fires, which produce relatively larger particles of combustion.

Both technologies are commonly used around the world. Both are reliable and proven methods for spot smoke detection. In a cross-zoned strategy of detection, as mentioned above, a designer will typically use one of each type to create a rapid detection scenario for any potential fire hazard.

The second method of smoke detection used in MCFs is air sampling smoke detection or high-sensitivity smoke detection (HSSD). HSSD systems typically are magnitudes more sensitive than conventional spot detectors for several reasons. First, HSSD uses small but powerful fans to draw air from the protected space, through small “sampling points” back to a central detection unit. This enables the detector to “see” the potential products of combustion substantially faster than with spot detectors, which rely on interior air currents to bring the smoke to the detector.

Second, HSSD systems generally rely on technology that can discriminate between particle types, thus evaluating both particle size and number prior to an alarm condition. Finally, HSSD systems can be programmed to accommodate a variety of sensitivity requirements.

HSSD systems are applicable as a replacement for spot type detectors, but are used more commonly as a supplemental level of early warning, in a highly sensitive environment.

Both spot type and HSSD type detection will interface with any type of panel discussed above. Spot detectors are recommended for initiation of a system discharge, while HSSD systems are recommended for early-warning detection only, but can be used to initiate a system discharge if desired.

3. Suppression systems.
If the control system is the “brain” of a fire protection system, the suppression system is the muscle. The suppression system is deployed for the early extinguishment of a potentially catastrophic fire event, in an operationally or monetarily critical space.

The suppression system is made of a multitude of components from valves, piping, nozzles, and cylinders to the suppression agent itself. It is important to recognize the importance of the agent, as well as the system as a whole, and how the agent is delivered to the protected space. We will address each of these topics independently:

* Suppression agents
As mentioned above, Halon 1301 is the standard by which all others are compared. The extinguishment characteristics of halon are outstanding, and its safety for both people and the protected space is exceptional. However, its production ban has led the industry to a number of halon alternative agents that are available today.

Note: Brand names are used below due to their universal recognition. Many different chemical formulas, names, and other designations can be used for each of these compounds. See the chart below for additional designations.

FM-200. FM-200 is the brand name given by a specific manufacturer for the compound HFC-227ea, or heptafluoropropane. FM-200 is a hydro-fluorocarbon (HFC) compound and is the most widely used and recognized halon alternative on the market today. It has been accepted/approved, tested, installed, and extinguished fires around the world. FM-200 extinguishes fire through the absorption of heat and does not significantly reduce the oxygen concentration within the space. An FM-200 discharge does not leave a residue or harm people or equipment in the protected space. FM-200 is the most common choice for applications like data centers, telecommunications facilities, record storage, and clean rooms. The extinguishment times and storage space required are similar to halon. Use concentrations for FM-200 are typically 6.25% to 8% by volume.

FE-13T. FE-13 is also an HFC compound and is a lesser-used, niche agent on the market today. FE-13 has several unique characteristics including a very low boiling point (allows it to vaporize when discharged at very low temperatures) and a high vapor pressure (allows the discharge to be quite energetic). These two features make FE-13 a good choice for low-temperature applications (both storage and protected space) as well as particularly high-ceiling applications. Use concentrations for FE-13 are typically around 20% by volume.

FE-25. FE-25 had long been used for unoccupied spaces, but is now applicable to all traditional occupied space applications through the use of physiologically based pharmacokinetic (PBPK) modeling. PBPK is a simulation methodology used to predict human response and blood stream absorption in the exposure to these types of compounds. The two major standards organizations that establish guidance for use of these systems (ISO and NFPA) have both adopted the PBPK method for the determination of toxicology thresholds at which these agents may be used safely in an occupied space. Prior to the adoption of the PBPK model, FE-25 had a use concentration slightly above the first threshold of toxicology, called the no adverse effect level. Presently, use concentrations are slightly below the thresholds determined using the PBPK methodology. Applications of FE-25 are very similar to those mentioned for FM-200 above. The compound is also an HFC and extinguishes fire in much the same mechanism as both FM-200 and FE-13. Also, physical properties of FE-25 are more similar to Halon 1301 than any other alterative. Unfortunately, current system design guidance provided by the system manufacturer makes it difficult to realistically apply in a halon retrofit application. Use concentration for FE-25 is typically 8% to 9% by volume.


3M Novec 1230 Fire Protection Fluid. Novec 1230 fluid is the most recently developed agent on the clean agent market. Novec 1230 fluid is a fluorine-based compound, but not an HFC like FM-200, FE-13, and FE-25. Novec 1230 fluid extinguishes fire by the absorption of heat and thus does not reduce oxygen concentrations within the protected space, similar to all three HFC compounds mentioned above. The unique characteristic of Novec 1230 fluid is that it has both zero ozone depletion potential and also has an extremely short atmospheric lifetime, which contributes to its low global warming potential. HFC compounds typically have a moderate atmospheric lifetime measured in tens of years, while Novec 1230 has an atmospheric lifetime of approximately 5 days. In applications with end users who are particularly sensitive to environmental considerations, Novec 1230 is a good selection. Use concentration for Novec 1230 fluid is typically 4% to 6% by volume.

Notes on fluorine-based agents:
* All of the agents listed above are considerably more environmentally friendly than Halon 1301. Also, in most jurisdictions, clean agent fire suppression is considered a “non-emissive” use of these compounds and, as such, the end user evaluating the various options typically determines environmental considerations.
* Fluorine-based agents also will break down when exposed to high heat, including open flame. The primary decomposition product in this interaction is hydrogen fluoride, an acidic and dangerous compound at moderate concentrations. For class A hazards, fire sizes are typically small (less than 30 kW), which limit HF production.

Argonite. Argonite is different from the above four agents in that it is a blend of naturally occurring gases, not a fluorine-based compound. Argonite is referred to as an inert gas compound. Argonite is a 50/50 blend of argon and nitrogen. Argonite extinguishes a fire through the reduction of oxygen in the protected space such that combustion can no longer sustain itself. This oxygen reduction does not, however, preclude the use of Argonite in an occupied space. This is because the oxygen concentration is reduced to a point at which combustion cannot be sustained, but human safety is maintained for a 5 min exposure period, per NFPA 2001. Argonite systems require the employment of dedicated venting systems because the quantity of agent needed to reduce oxygen levels sufficiently is significant.

Inergen. Inergen is also a blend of naturally occurring gases, similar to Argonite. Inergen is a blend of approximately 40% argon, 52% nitrogen, and 8% carbon dioxide. The added CO 2 in Inergen serves one primary purpose: CO 2 increases the breathing rate in humans. In a reduced oxygen atmosphere, it is helpful to increase breathing rates to take in more oxygen. Inergen is also subject to venting requirements as mentioned above for Argonite.

Typical use concentrations for both Argonite and Inergen are 35% to 40% by volume, resulting in an oxygen concentration of 12% to 14%.

* Agent delivery systems
Discussion of and evaluation of the various agents available today can be time-consuming and exhausting. Many pages have and can be written on a comparison of the various agents. What is presented above is simply a brief introduction into the most common halon alternatives.

What typically is not discussed at great length is a comparison of the equally important delivery of the agent to the protected space.

Traditionally, the fluorine-based agents are stored in a steel, welded container, and pressurized with nitrogen. The nitrogen provides added pressure/energy to the container, in order to propel the compressed liquid through the piping network when discharged. Nitrogen is super pressurized into the containers at either 360 or 600 psi, depending on local standards and specific application. MCF applications will almost always use 360 psi containers.

In the case of halon, the nitrogen is not overly soluble in Halon 1301, and therefore provides a significant energetic “push” of agent through a piping network. Thus, halon was used in many piping networks that most standard delivery halon alternative systems cannot accommodate today. The elusive “drop in replacement” is something the special hazard fire protection industry continues to search for. Most halon alternative systems today have rough guidelines upon which to base preliminary designs including piping lengths and number of nozzles.

Inert gas systems are not as limited by discharge characteristics, but rather storage space. While Argonite and Inergen systems can be several hundred feet from the protected space, storage area for the cylinders is 10 to 20 times that of halon and halon alternatives.

However, new technology in the delivery of fluorine-based agents allow very close to a true drop in replacement for halon systems. One manufacturer offers a system in which the nitrogen used to “push” the agent, is separated from the agent itself, in a completely separate container, at the point of installation. When the system is activated; the nitrogen flows into the agent storage container and literately pushes the agent through the piping network, like a piston. The system is commonly referred to as piston-flow. The advantage of this type of system is a greater level of nitrogen pressurization, typically up to 400 psi, but more importantly, minimizing the level of mixing between the nitrogen and the agent itself.

The result of this type of system is a considerable improvement in flow performance, which can rival the flow of halon and “fit” into existing halon systems.

If interested in this or other specific agent delivery system characteristics, you are encouraged you to seek out manufacturers of these systems and discuss what is available.





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