Clean agent fire suppression systems

The next edition of NFPA 2001, Standard on Clean Agent Fire Extinguishing Systems, will be published later in 2011. Updates will capture advancements in technologies and the latest thinking on the subject.

05/06/2011


NFPA 2001, Standard on Clean Agent Fire Extinguishing Systems, which is published by the National Fire Protection Assn. (NFPA), serves as the principal (primary) standard covering clean agents and the systems used to store and discharge the agents for fire protection purposes.

The next edition of NFPA 2001 is set for publication in late summer 2011. While the standard was set for an earlier publication date, certain issues could not be resolved during the document’s initial revision cycle timetable, and in accordance with NFPA’s Codes and Standards Making Process, these issues have been scheduled for consideration at NFPA’s 2011 Association Technical Meeting in Boston, June 12-15, 2011, during the NFPA Conference and Expo.

In addition to highlighting the issues that caused NFPA 2001’s delay in publication, this article summarizes key technical changes for the next edition of the standard. These changes will affect how engineers, designers, and facility operators need to consider future installations of clean agent fire suppression systems. Specific details regarding the changes to NFPA 2001 and pending issues to be considered by the NFPA membership in June can be found in the 2010 Fall Revision Cycle Report on Proposals, 2010 Fall Revision Cycle Report on Comments, and the Report of the NFPA Motions Committee on Fall 2010 Revision Cycle Documents. The changes discussed below are presented in the order in which the relevant sections appear in NFPA 2001, and not in any order of importance or priority.

Background

Certain kinds of facilities and operations present unique fire protection challenges. Facilities like telecommunication and data centers, process control rooms, high-value medical facilities, and museums and libraries, to name a few, require not only rapid extinguishment of a fire, but also the use of suppression agents that are nonconductive, do not pose cleanup difficulties, and are not hazardous to personnel. Since the phaseout of halogenated agents was initiated in 1994 due to the detrimental effect of halons on the environment, clean agent fire suppression systems have been used to meet the fire protection needs for many of the above types of applications.

Clean agents are nonconducting fire extinguishants that vaporize readily and do not leave a residue upon evaporation. Clean agents consist of halocarbons or inert gases, and are subject to specific evaluation with regard to their hazards to personnel and their effect on the environment prior to being officially classified as a clean agent. Halocarbons serve to extinguish fire primarily by cooling, whereas inert gases serve to extinguish fire by diluting the concentration of oxygen in the vicinity of the combustion reaction zone. Clean agents are normally stored under high pressure as a liquid and readily vaporize when released to the atmosphere upon discharge from a properly designed system.

Hazards to personnel

The release of a clean agent can pose a health hazard to personnel in the vicinity of the discharge. The health hazard can be presented by the agent itself or by the products of decomposition that result from exposure of certain agents to the fire or hot surfaces. The standard advises against unnecessary exposure of personnel to the agent discharge or the decomposition products.

In light of the health and safety concerns, the standard only addresses those agents that have first been evaluated in a manner equivalent to the process used by the U.S. Environmental Protection Agency’s (EPA) Significant New Alternatives Policy (SNAP) program. Additionally, agents proposed for inclusion in NFPA 2001 must first be evaluated in a manner equivalent to the process used by the EPA’s SNAP program. The SNAP program requires consideration of toxicity data, exposure assessments to personnel, and flammability in addition to atmospheric effects and other environmental impacts.  

The evaluation required under the SNAP program is to consider the agent in its end-use application. Specific to clean agent systems, NFPA 2001 addresses total flooding systems, where a concentration of agent is discharged and held within an enclosure for a period of time; and local or streaming application systems, where a concentration of agent is discharged directly over the burning material in close proximity to the fire source.

For streaming application systems, concentrations of agent in the vicinity of the discharge may exceed the maximum permitted exposure limits determined through the SNAP program in order to achieve the agent concentration necessary for fire extinguishment. Consideration of personnel exposure to agent discharge from local application systems can vary greatly, and is normally a more complicated assessment than for total flooding systems.

It has been noted that the predominant method for assessing exposure and toxicological effects of an agent as part of the SNAP program may not accurately estimate the actual exposure to an agent in certain instances. The predominant assessment method takes into consideration a specific volume of space in which the agent is used in addition to other factors. As such, the SNAP program does not specifically consider an assessment of an agent’s exposure for local application systems, and additional methods, such as personal monitoring tests, can be employed in completing the exposure assessment. The standard has been revised to clarify this, and will require that a clean agent be evaluated in a manner equivalent to the SNAP program specifically for total flooding agents.  

Environmental impact and climate change

In determining which type of clean agent to use for a particular fire hazard, the standard requires the engineer to consider the effects on the environment by the discharge of the agent. Prior editions of NFPA 2001 provided some general guidelines and noted that many factors impact the environmental acceptability of a fire suppression agent. The  next edition will include more specific information on how such an assessment can be undertaken, and provide guidance on a specific agent’s ozone depletion potential (ODP) and global warming potential (GWP).

As defined by the EPA, the ODP is a value that refers to the amount of ozone depletion caused by a substance such as a fire suppression agent. The ODP is the ratio of the impact on ozone of a clean agent, compared to the impact of trichlorofluoromethane (CFC-11). CFC-11 is a compound of chlorine, fluorine, and carbon with a value of 1.0. Other CFCs have ODPs that range from 0.01 to 1.0. Halons, on the other hand, have ODPs ranging up to 10. The clean agents identified in NFPA 2001 have ODPs ranging from 0 to 0.048.

GWP is a measure of how much a given mass of a substance is estimated to contribute to global warming. It is a relative scale that compares the agent in question to that of the same mass of carbon dioxide, CO2. Thus, the GWP of CO2 is defined to be 1.0. CFC-11 has a GWP of 5,000. Water, a substance in numerous end uses, has a GWP of 0. The clean agents identified in NFPA 2001 have GWPs ranging from 0 to 14,800.

Table 1 summarizes the ODP and GWP values for clean agents addressed by NFPA 2001, and will be included in the next edition of the standard

In addition to providing ODP and GWP values for the specific agents, the standard will include guidance on the meaning of such data and how these values can be interpreted. Including information on a particular agent’s GWP and ODP will enable the design engineer to make a more informed assessment in the selection and design of a clean agent system for the specific facility and fire hazard under consideration.

Terminology

When designing a clean agent system, the necessary concentration of a particular agent is determined and specified for the fire hazard under consideration. Two new terms have been introduced to facilitate the system design and approval process.

The adjusted minimum design concentration (AMDC) is defined as the target minimum design concentration after the safety factor and the design factors have been taken into account. The term design concentration is currently used throughout NFPA 2001 but has not been specifically defined as such. When the term design concentration is used in the standard, it is intended to be AMDC. For example, the standard will now clarify that when determining the duration of protection to be provided by a clean agent system discharge, at least 85% of the AMDC is to mean held at the highest level of combustibles for the minimum specified period of time.

Additionally, the term final design concentration (FDC) has been introduced, and it is defined as the actual concentration of agent discharged into an enclosure. The FDC needs to be equal to or greater than the AMDC.

Supervision of system actuating devices

Clean agent systems generally have an electric actuator attached to one or more agent storage containers or selector valves. The electric actuator can be either solenoid or squib operated. A signal from the fire system releasing control unit causes the electric actuator to operate, which in turn initiates rapid operation of the discharge valve(s) and release of the agent.

During system maintenance, it is a common procedure to remove the solenoid-operated actuators from the discharge valve to prevent accidental discharge of agent and permit functional testing of the actuator. Some systems, which incorporate selector valves, also have electric actuators attached to the selector valves to control their operation by electrical signal from the control panel. These electric actuators may also need to be routinely removed from their selector valves during maintenance.

The electrical connection between the solenoid and the system control panel is normally maintained during servicing activities. A means to provide an indication of system impairment at the releasing control panel when the electric actuator is physically removed from the valve has not been previously required. As such, there have been inadvertent impairments of systems at the conclusion of maintenance activities because the technician failed to reinstall the actuator.

With the exception of systems intended for certain marine applications, the standard will now require that when an electric actuator is removed, an audible and visual signal of system impairment be received at the system releasing control panel. As products necessary to complete the functions described above are not yet commercially available, the provisions concerning supervision of electric actuators do not go into effect until January 2016. Squib actuators are covered by this requirement only if the manufacturer’s maintenance instructions require physical removal of the squib operated device from the valve that it controls.

Unwanted system operation

Accidental system actuation can be a significant factor in unwanted discharge of the clean agent. Such accidental actuation could occur when servicing or testing the clean agent system, or when servicing activities associated with other building and process systems in the vicinity of the clean agent system trip the sensors associated with the control of the clean agent system. New provisions pertaining to equipment lockout and disconnect switches have been introduced for the next edition. These provisions are consistent with NFPA 72, National Fire Alarm and Signaling Code, and introduce enhanced control functions for the system.

More specifically, the disconnect switch will be required to be listed and must cause a supervisory signal at the releasing control unit. Furthermore, the switch needs to be located inside a lockable fire alarm control panel or inside a lockable enclosure, or require a key for activation of the switch. This arrangement provides for a more secure configuration and a more obvious visual indication of the system’s status. Where a key is employed, it will not be permitted to be removed while the system is disconnected. This will allow the suppression system to be quickly returned to its operational condition if a fire occurs during servicing operations. The standard will also indicate that suppression system disconnect achieved via software programming is not an acceptable alternative for a physical disconnect switch.

Protection under raised floors

Certain enclosures protected by total flooding clean agent systems, such as control rooms or telecommunication facilities, often include raised floor configurations. This raised floor arrangement consists of a platform under which are installed cables or ventilation equipment used to cool the enclosure and the associated equipment. The current edition of the standard does not specifically address protection of the space below the raised floor.

Unless the space underneath the raised floor is specifically protected, discharged clean agent in the main enclosure will eventually leak underneath the floor and dilute the design concentration necessary for extinguishment of the fire in the main enclosure. Furthermore, if a fire occurs under the raised floor, and the agent is a halocarbon, the amount of agent migrating into the underneath space will unlikely be of a concentration sufficient for extinguishment. Rather than extinguish the fire, these lower concentrations of agent in either the main enclosure or the area below the raised floor will interact with the fire to produce undesirable products of decomposition that can be detrimental to sensitive materials or equipment.

In other types of installations where a clean agent system only protects the space underneath the raised floor, a fire in the space above might activate the clean agent system below. During and after agent discharge, some of the agent from the space under the raised floor can migrate into the room above the raised floor. The discharged agent, which might be below the extinguishing concentration, could be exposed to the fire. If the agent is a halocarbon, thermal decomposition of the agent will occur if flame is present and have a detrimental effect as described above. A similar decomposition scenario also could occur where a halocarbon agent protects the under-floor space, and some other type of fire protection system protects the room above. If the system above does not extinguish the fire in the room, the clean agent system below the raised floor could activate. A concentration of released agent from the under-floor space could migrate to the space above and come in contact with the flames, causing the formation of potentially harmful decomposition products.

Because of the associated concerns with inadequate fire protection and the effects of halocarbon agent decomposition products, systems installed in accordance with the next edition of NFPA 2001 will now be required to simultaneously protect the main part of the enclosure as well as the space below the raised floor. Additionally, if only the space under the raised floor is protected by a total flooding system, an inert gas rather than a halocarbon agent is to be used to protect the space, as inert gases do not form decomposition products. These changes also provide for better correlation with NFPA 75, Standard for the Protection of Information Technology Equipment.

Class A and C fires

The means by which the minimum design concentrations for total flooding systems are determined for certain types of fire hazards have been proposed for the next edition of the standard. The proposed revisions pertain to Class A fires that consist of ordinary combustibles such as wood, paper, cloth, and many plastics, and Class C fires that consist of energized electrical equipment. In both cases, it was initially suggested that the associated safety factor values applied to the extinguishing concentrations for Class A and Class C fire be increased. Further details with regard to cable arrangements and power supply continuity for Class C fire hazards were also outlined.

The Class C issue dates back to several editions of the standard. Since the publication of the last edition of NFPA 2001, the Fire Protection Research Foundation (FPRF) completed a study entitled “Clean Agent Suppression of Energized Electrical Equipment Fires.” The proposed changes on determination of agent concentration are based in part on this study, which can be obtained from the FPRF.

Revision cycle delay

In response to the initial recommendations concerning safety factor increases for Class A and Class C fire hazards, further revisions on how the appropriate design concentrations should be determined were submitted during the second phase, or what is referred to as the comment phase, of the document’s revision cycle. The proposed changes to design concentration determination ultimately caused a delay in the document’s publication date. A motion to return the standard to its current wording for design concentration determination has been tentatively placed on the agenda for NFPA’s 2011 Association Technical Meeting as previously noted. Interested parties should contact the NFPA Codes and Standards Administration department for procedures regarding participation in the Association’s Technical Meeting.

Design concentrations for deep-seated smoldering fires

When combustion of solid materials occurs, two types of fires can occur: flaming combustion in which combustion generally occurs on the surface of the fuel, and smoldering combustion in which combustion generally occurs within the mass of the fuel. The two types of fire often occur concurrently, although one type can precede the other. Flaming combustion, because of its location and ready exposure to system discharge, can be extinguished with relatively lower concentrations of clean agents, and in the absence of deep-seated smoldering combustion is less likely to re-ignite. On the other hand, smoldering combustion, especially if it is deep-seated, is not normally subject to the same rapid extinguishment.  

Smoldering fires exhibit lower rates of heat release but also slower rates of heat loss from the combustion reaction zone within the fuel. Therefore, the fuel remains hot enough to react with oxygen to maintain combustion even though the process is at a much slower rate. Smoldering deep-seated combustion can cause serious damage to surrounding building contents, materials, and equipment, and will normally require higher design concentrations of clean agents and longer hold times so that a sufficient concentration of agent can penetrate the fuel. Furthermore, an inadequate clean agent system design concentration and hold time can result in the rekindling of a deep-seated smoldering fire after the system has operated.

New provisions for the standard will require that the minimum design concentration for a smoldering deep-seated fire hazard be determined by an application-specific test. Advisory text has also been added to the annex to elaborate upon this and facilitate improved system performance where deep-seated smoldering fires are anticipated.

Inert gas systems

The standard specifies a maximum time period by which an agent must be discharged from a total flooding system: 10 seconds for halocarbon agents, and 60 seconds for inert gas agents. A number of factors influence the optimum discharge time for complete agent release. These factors include limitation of agent decomposition products, limitation of fire damage and its effects, enhanced agent mixing, limitation of compartment overpressure, and secondary nozzle effects. Such secondary effects can include formation of projectiles caused by very high discharge velocities, loud noise levels, and lifting of ceiling panels. The likelihood of these secondary nozzle effects increases if the maximum discharge time is too low.

As previously noted, inert gases do not form decomposition products and therefore do not require discharge time limitations on this basis, thus the difference between discharge time limits for halocarbon agents and inert gas agents. In certain applications, a longer duration discharge time might be desirable provided the risk to life and property is not increased, that is, the increase in combustion products and reduced level of oxygen concentration are adequately considered.

It has been argued that advantages for longer discharge times for inert gas agents include smaller system pipe and valve sizes, reduced system costs, less room venting to maintain enclosure integrity, and less turbulence and noise upon agent discharge. In light of these potential advantages, the next edition of the standard will reference a maximum discharge time of 120 seconds for inert gas agent systems protecting Class A surface fire hazards and Class C hazards. The 60-second maximum discharge time will be maintained for Class B hazards that include ignitable liquids and flammable gases.  

Enclosure integrity procedure

Especially for total flooding systems, enclosure integrity is a key design consideration. The standard currently contains a dedicated annex that outlines a method that can be used to equate enclosure leakage, as determined by a physical door fan test procedure, to the worst-case amount of clean agent leakage from the room. Equations included in the annex provide a means to predict the time it will take for the clean agent interface to descend to a certain height, or for continually mixed gases, the time for the concentration to reach to a given percentage. For the next edition, the annex has undergone significant revision and will include new calculation procedures and agent retention algorithms.  

Keeping up-to-date

The next edition of NFPA 2001 will be published in late summer 2011. Changes to the standard serve to capture advancements in the associated technology and the latest thinking on the subject. As engineers, designers, and facility operators, it is imperative that we keep abreast of changes to industry standards. However, technology and best practices do not necessarily move in concert with industry standard revision cycles.

To keep current and well-informed we must continuously consult with clean agent manufacturers, system installers, listing laboratories, and governmental agencies to maintain our best understanding of the associated issues and practices. As professionals working in the field of fire protection and life safety, we must remember that our decisions and recommendations reach beyond our paying clients and serve the broader needs of our society.

Puchovsky is professor of practice and director of corporate and professional education at Worcester Polytechnic Institute. He focuses on fire and life safety system design and regulation, serves as vice president for the Society of Fire Protection Engineers, is a member of several NFPA technical committees, and is the former secretary to NFPA’s Standards Council.



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