The Evolution of U.S. Fire Alarm Systems
Fire alarm systems have been an integral part of society since the late 1800s. The goal of fire alarm systems has remained consistent throughout history: Reduce the loss of life and limit property losses from fire. The fire alarm industry has evolved through time to ensure that these objectives are met.
Fire alarm systems have been an integral part of society since the late 1800s. The goal of fire alarm systems has remained consistent throughout history: Reduce the loss of life and limit property losses from fire. The fire alarm industry has evolved through time to ensure that these objectives are met. Research and development has occurred in system technology, design practices, emergency response methods and system interface applications.
During the past 5 years, international events have caused the industry to reconsider fire alarm system planning, design, installation, testing and maintenance. Fire alarm systems in the modern age must consider events caused by fire, weather, nature, terrorist attacks, biological and hazardous chemical release. Such systems are on the forefront of the fire alarm industry and will change the way systems are designed, configured, manufactured, installed, tested and maintained.
Fire alarm systems: present day
Before we take a look at the latest potential trends, the existing landscape must be analyzed. Fire alarm systems requirements are dictated by the building code that governs the particular jurisdiction where the building resides. Once the determination has been made that a system is required, then the fire protection engineer would refer to National Fire Protection Assoc. (NFPA) 72; National Fire Alarm Code. NFPA 72, from one of the world's largest governing body of codes and standards dictates how systems are designed, implemented, tested and maintained.
Although there are a number of fire alarm system manufacturers today, the majority of systems are similar in type and operation, due to limitations on creativity imposed by NFPA and product listing agencies. The two major types of products on the market are conventional and addressable fire alarm systems. Conventional systems are simple systems where initiating devices are not point addressed and are grouped by zone, a method that was commonplace until about 20 years ago. These systems typically are used in smaller applications, in which a designer or owner would deem that an addressable system would be too costly because of the minimal labor required for system installation, i.e. a fire alarm system used solely to monitor the status of a sprinkler system or a system used solely for elevator recall.
Addressable systems, introduced in the late 1980s, have tremendous benefits including lower cost of system installation and maintenance, ease of compliance testing, and the ability to quickly diagnose and resolve problems. These benefits exist because addressable systems are software-driven, unlike conventional systems of the past. Addressable systems monitor the status and sensitivity of all circuits and devices. The system also can be programmed to activate alarm zones independent of initiating and notification circuit configuration and can control specific building systems without the need for excessive relay wiring.
A typical building fire alarm system would employ an addressable fire alarm control panel, which could be networked to multiple panels depending on building size and configuration. Initiating devices, used to start a condition, include a variety of smoke and heat detectors, duct smoke detectors, flame detectors and manual pull stations. Initiating devices are wired to reside on a signaling line circuit (SLC) that handles between 100 to 350 initiating points depending on the selection of a manufacturer. In situations where the system requires more devices than the allotted SLC, a system for multiple SLC cards is chosen. In the event that a point was needed to be added to the system after it is installed, the SLC is modified to accept the new point and a new device address would need to be assigned.
Another integral element of the fire alarm system is the notification system and how it is accomplished. This is defined by the building code and is accomplished through audible and visual means and is labeled notification appliance circuits. Audible notification generally is pre-recorded via voice messaging or Standard Temporal Code 3 Horns. Visual notification is accomplished via strobe appliances with a variety of light intensity ratings and candela, based on room/corridor geometry and configuration. Circuiting would be accomplished in two manners:
Strobe only and horn/strobes is circuited from a power source either banked at the panel or distributed throughout the facility. Addressable NACs are offered by certain manufacturers and instrumental in retrofit design.
Speaker appliances are circuited from an amplifier either banked at the panel or distributed throughout the facility.
The next element is system interfacing. It is affected by the transition from conventional systems to addressable systems and is accomplished through system contact monitor modules and control relay modules. Although various system manufacturers try and differentiate themselves, all products offer monitor and control modules, as part of the SLC.
Monitor modules are used as a blank point ID to monitor the status of conventional fire alarm or non-fire alarm points. These include sprinkler systems, fire pump controllers, special hazard suppression points, status points for fire doors and conventional fire alarm points such as projected beam smoke detectors. The modules reside on the SLC and are wired into a normally closed or open contact, as required by the interface. Programming of the point generally is dictated by building or fire alarm code requirements.
Control modules are an addressable point as well, used to simply control a building interface in an alarm, supervisory or trouble situation. These typically include HVAC unit shutdown, elevator control or door control. These modules also reside on the SLC, but require an additional 24-volt power circuit in most cases and perhaps an interposing relay when dealing with a control point that requires a high contact rating. Control modules can be programmed to perform when one initiating device changes state or a group of devices or zone is alarmed.
The last element that needs addressing is emergency response signaling and communications. Typically, fire alarm systems report to a 24-hour monitored station that is either located on premise—in the case of a campus environment—or located remotely. Communications to these monitoring stations occurs in many different fashions, however most typically through phone lines used to dial out. Addressable systems provide point condition information to the station which allows for more detailed descriptions to be given during dispatch of emergency personnel.
Modern day conditions have brought a new landscape of risks and hazards that force the fire alarm and fire protection engineering industry to take a different look at how it accomplishes its two main objectives: reduce the loss of life from fire, and limit property losses from fire. The theories lean toward creating systems that will perform in various types of emergencies including fire, weather, nature, terrorist attacks, and biological and hazardous chemical releases. These systems need to provide clear directions in emergency situations through different building system vehicles that increase emergency management effectiveness. These systems are known as mass notification systems.
Mass notification systems manage people's actions during and after an emergency. These systems are designed to provide information and instructions to people in a building, facility, campus or larger geographic area using intelligible voice communications, visible signaling, and textual and graphical information. Events that will utilize these systems include any event requiring control of the movement of a large group of people. Mass notification systems initially were proposed by the U.S. Dept. of Defense as one facet of its force protection strategy. As this focus began to change to include all types of civilian and government facilities, life safety professionals saw the need to develop installation standards for these systems.
The first attempt at standards was undertaken by NFPA as part of revisions to NFPA 72. It is expected that this revision (NFPA 72-2007) will be voted on and published by late 2007. As proposed, Annex E of NFPA 72-2007 will contain recommended standards for mass notification systems. These new standards will change the way safety, security and building systems are integrated to save lives.
So how do we take our present-day addressable fire alarm systems and evolve into mass notification?
The answer lies in segregating certain traditional fire alarm functions while allowing other building systems to perform. One example is voice messaging in a system that requires voice evacuation audible notification. How many times have you been in a hotel when the fire alarm system has been activated and you are the only one who proceeds to an exit? The reason for this may be because of the technology that exists for fire alarm speaker appliances. Because of stringent requirements for agency listing, the quality of today's speaker appliance is below average. If you mix this with the latest design trends by today's architects, you could possibly end up with two scenarios: (1) A building with one speaker every 5 sq. ft. or (2) a system that due to echo and reverberation cannot deliver a clear and intelligible message to building occupants. For this reason mass notification systems may look to utilizing the voice messaging component through an existing public address system that ensures an increase in response time from building occupants.
Another key element that needs consideration is visual signaling of non-fire emergencies. Many system manufacturers are creating amber-lensed strobes in anticipation that non-fire emergencies may require an alternate color. Although no standard has dictated such a requirement, most building owners, designers, and authorities may prefer to distinguish visible signaling for non-fire emergencies. In a case of a building retrofit project, in which the voice evacuation component is shifted to a public address system, existing speaker cabling could be used to accomplish amber strobe signaling. Another option to consider is interfacing to visual paging systems for textual information, if the systems are in place at the particular building, campus or geographical area that is in consideration.
Similar to all new concepts and technologies the following must be considered when considering mass notification:
What systems should be integrated to form the complete mass notification system?
Who will manage and operate the facility's mass notification system: police, fire, building operations, others?
How are priority levels of potential events assigned, for both emergencies and non-emergencies?
How are false activations and inconveniences eliminated or reduced for building personnel in a 24-hour facility such as an airport?
What paging capabilities will the building operators and tenants have during the different types of events?
Are fire code variances required in order to interface the fire and other non-fire systems?
The continuing evolution of the fire alarm industry is very evident by the recent changes to NFPA 72. With these changes, engineers, designers, contractors, manufacturers, authorities having jurisdiction and owners will be instrumental in shaping how mass notification systems take shape as we head into a future that presents risks of different proportions. On the horizon, mass notification systems will help manage emergency situations more effectively hence maintaining our two main objectives. Technological advances will occur and allow the industry to answer some the questions and challenges that have arisen about this new concept. While implementing a mass notification system, it presents challenges, which can be cost-effectively overcome through a design and implementation process that combines the existing infrastructure with the controls and interfaces necessary to create one integrated system.
CFD Methods as Fire Safety Tool
Computational Fluid Dynamics (CFD) is a powerful technique that provides an approximate solution to the coupled governing fluid flow equations for mass, momentum and energy transport. The flexibility of the technique makes it possible to solve these equations in complex spaces, unlike simpler modeling methods that are sometimes used to predict smoke movement. CFD is increasingly being used in fire engineering to predict the movement of smoke from postulated fires in complex enclosed spaces such as atria, shopping malls and warehouses.
Until recently, CFD modeling has not been evaluated carefully against reliable data for complex smoke movement applications. The Health and Safety Executive (HSE) committee, as part of Britain's leading Health and Safety Laboratory, decided to fund a project to address this issue. The main aim of the project is to quantify the advantages and limitations of CFD for predicting smoke movement in complex enclosed spaces. The project began with real scenario modeling of interest to HSE. Investigators checked the sensitivity of CFD results to a range of modeling approaches widely employed by the fire engineering community. Small-scale experiments were performed in order to focus on areas identified by the initial simulations as potentially challenging for CFD. Then the small-scale experiments were modeled with CFD and the results were compared against the experimental results.
The CFD results for the subway station scenario show that in the 5-minute period before forced ventilation is initiated, smoke transported throughout most of the ticket hall and extended to the main exit routes. Other emergency routes existed to enable passengers and staff a means to escape without going through the ticket hall. Following the startup of forced ventilation, smoke is cleared from the large parts of the paid side of the ticket hall, the part past the ticket barrier, by being convected toward the exits and into the dome. Analysis of the results in the offshore accommodation module shows that at approximately 60 seconds after ignition, smoke makes its way into the adjoining corridor, and 120 seconds later smoke has risen halfway up the nearest staircase. In the building under construction, smoke spreads as a ceiling layer within the third floor open plan office. Shortly after one minute, smoke has entered the atrium and 120 seconds later it has risen five stories. At four minutes after ignition, it has reached the highest floor and also is rising in the stairwell.
Overall, the CFD simulations captured many of the observed gross flow conditions. In some cases details of the temperature field also were predicted. However, the simulations were sensitive to the wall heat transfer boundary condition. For example, the CFD simulations of hot gas flow in the horizontal and inclined tunnel configurations tended to over-predict the measured temperature. This over-prediction is pronounced with an adiabatic wall boundary condition. The CFD-predicted flow behavior for the booking hall configuration generally matched the experiments but the temperatures were too high as the smoke enters the booking hall. In the atrium configuration, the simulated hot layer rose immediately when it entered the atrium while the experimental plume propagated across the atrium and rose along the far wall.