Smoke Control for Fire-Protection System Designers
January: Smoke Control<br/> Advice for fire-protection system designers, including the major components of a smoke-control system and the special problems to look out for.
Mike Prowse, Siemens Cerberus
The simple issue with smoke-control systems is to be able to control smoke during a fire incident so that the danger is contained and the building can be safely evacuated. Well at least it sounds simple. The fact is it has recently been managed by the building-automation system. But the trend over the last few years has been to shift that control to the fire-protection system. That shift has created new fire-alarm functions and performance requirements.
Additionally, since Underwriters Laboratories (UL) has a listing category for smoke-control equipment, the fire-alarm manufacturers who have obtained that listing are forcing the requirement into job specifications. Therefore, the growing number of specifications for smoke-control systems are wrapped around some largely unknown performance standards. The customer often does not understand what he or she has asked for, nor what he or she really wants. This places more pressure on the designer to be the subject matter expert.
There are, as one may expect, a variety of types of smoke-control systems. National Fire Protection Association (NFPA) Standards 92A and 92B cover all requirements for all types of smoke-control applications. Some building-system manufacturers produce other resources that help to understand the various applications. Siemens' "Smoke Control Systems Application Guide" is one of these documents.
Smoke-control systems are broken into two major categories: dedicated and non-dedicated systems. Dedicated systems are simply those that don't perform any other functions. The fans and dampers are not used for everyday ventilation, only for smoke-control events. These are often found in stairwells and elevator shafts. Typically, these areas are pressurized to prevent the spread of smoke through exit passageways in the building. In atria, these are typically used for smoke exhaust in order to control the smoke layer.
Non-dedicated systems are those that provide heating, ventilation and air conditioning (HVAC) in the building every day, but are captured by the smoke-control system in the event of a fire. There are many different types of non-dedicated systems and they are based upon the HVAC design. In other words, a building designed with single exhaust and pressurization fans to cover multiple floors or areas is a different type than one with fans for each floor. The basic principles are the same. The smoke-control system captures the fans and dampers in order to control the smoke. However, the actual design of the smoke-control system changes dramatically. In the first example, there are far fewer fans to control per fire zone than in the second.
One basic rule by which we operate every day remains the same. The building is broken up into fire zones. Sometimes this basic building block of life safety is forgotten. However, fire-zone definition is the cornerstone of the various documented smoke-control applications.
What makes up a smoke-control system?
The front end of every smoke-control system is the firefighters' smoke-control station. This station includes annunciation of every fan, damper and other component of the smoke-control system. It also provides manual overrides for every fan and damper to be used by the fire department. It therefore must be in an accessible location.
The two major authorities, UL and the Uniform Building Code, require that the station include a graphic representation of the building. Of course, all systems are not designed to those standards. But if compliance to either is written in the job specification, a graphic annunciator must be provided.
Inputs to the system come from the basic fire initiating devices. Smoke detectors and/or manual stations initiate a smoke-control sequence. As always, these devices are connected to the fire-alarm system.
At the other end of the system are the fans and dampers to be controlled. They are typically connected to some portion of the building-automation system. The challenge is to take the information from the initiating devices to the fans and dampers, and process the incoming information to meet the specified control methodology. In other words, turning the correct fans and dampers on and off at the right times.
The next piece of the system is the interface that takes the fire information to the building-automation system. Not well known is a requirement for positive feedback, at the smoke-control station, that the specific fan or damper has reached the intended state.
Who is in control?
Either the building-automation system provider or the fire-protection system provider must assume a lead role in making the smoke-control operation work properly. Both are providing hardware for the operation. But only one will provide the firefighters' smoke-control station, and only one will process the necessary system logic.
Usually the same company provides the station and processes the logic. This is not a strict requirement, but it makes sense when you consider the large number of lights and switches in the station. It is not practical to transmit a large number of switch input signals through one system to be processed in the other.
Either system can monitor the smoke-control station switch inputs, fans and dampers that return feedback information to the station. But this only covers manual inputs by the firefighter. Automatic initiation of a smoke-control sequence only comes from fire-alarm detectors and manual pull stations. For this reason, a growing number of smoke-control systems are being supplied by fire-alarm installers. However, no matter which system is providing smoke control, the fact remains that data has to transfer from the fire-alarm system to the building-management system.
There are two ways to get information from the fire system to the HVAC system. The first is for the fire system to monitor all inputs, process the smoke-control logic and then operate fans and dampers through relays that capture the device controllers. The other is for the fire system to communicate fire events and conditions to the building-automation control system. That system will then process the logic that operates the proper fans and dampers. This second style of communication is provided through a high level interface, with either a proprietary or open communications protocol between the fire and building-automation systems.
One of the most frequent questions I hear is: "If my fire-alarm system is interfaced to a building-automation system for the express purpose of smoke control, and if my job specification calls for the smoke-control system to be UL listed under their category UUKL, then whose product(s) have to be listed? Fire, HVAC or both?"
As a matter of fact, most major fire-alarm companies have equipment listed for smoke control, as do the building automation companies. Therefore, this issue will likely disappear in the future. However, a good rule of thumb is that the system processing the smoke-control logic must be listed for smoke control.
The firefighters' smoke-control citation must also be listed. In other words, the UL mark must be displayed on the graphic annunciator that supplies the system annunciation and manual override switches The station is often manufactured by a third party—an annunciator manufacturer. That manufacturer must have a listing for his product under the UUKL category.
For example, the fire-system installer is providing the annunciator and engineering the logic for smoke control. He is controlling HVAC devices through his own outputs, i.e. relays. The fire system is providing smoke control and therefore must be UL listed specifically for that purpose. This is in addition to the annunciator, which the fire-alarm installer is purchasing from an annunciator manufacturer. The building-automation system gives up device control during a fire event and until the systems are returned to normal.
The engineer for the construction project will develop a smoke-control strategy for the building. This will be in the form of a document often called a rational analysis. It tells you how this smoke-control system is supposed to work and will include at least three key items. One is the sequence of operation. It defines, step-by-step, how the fans and dampers should operate&$151;shut off fan A, close damper 32, turn on fan F. Typical sequence of events for different applications can usually be found in the manufacturers' smoke-control application guides. Please note that the engineer may also specify a sequence of operation for restoring the entire system to its normal operations after a fire event has ended.
The second key item is detailing the action caused by the various fire-alarm initiating devices. For instance, manual stations will very often start purge or pressurization sequences in stairwells, atria, etc. Smoke detectors may initiate a floor-by-floor sequence designed to contain the smoke at the floor of incidence.
The third key to the rational analysis has to do with positive feedback of the smoke-control operations. The strategy will usually provide some level of detail as to how quickly fans and dampers should operate. This will be specified in terms of actual operation and/or feedback at the firefighters' smoke-control station. The Uniform Building Code has some very specific time limits for both the operation and the feedback. The engineer should also specify what the system should do if feedback is not received in the allotted time.
What to watch out for
The system response time element can place a large burden on the operation of the controlling system and is often the most difficult of criteria to meet. Typically, fire-alarm systems process alarms as a priority. The smoke-control operations are treated as status events. The change of a device's status, and turning on an LED to annunciate the completion of that change, are usually at the lowest priority level in the system.
If the fire system is managing to many other functions, the smoke-control operations may not occur within the required time. This would usually be found at the end of the project when any problem can become very costly. Consult the system manufacturer for recommendations on how to best configure their system to meet the response requirements in your project.
With this information, the engineer should be able to talk intelligently about smoke control. If one is just starting a smoke-control project, be sure to use all of the resources at one's disposal while designing the system. The it will be possible to avoid costly delays and changes as the job approaches completion. What one learns along the way will be preparation for the growing number of smoke-control systems future buildings will demand.
Written by Mike Prowse of Siemens Cerberus