What you need to know about smoke detectors when renovating buildings.
Tenants have always been and will be. Today these squares are home to a labyrinth of cubicles, and tomorrow the office of a private company will open here. Since each company adapts its workspaces to suit itself, the principles of fire protection for a building can change significantly. In other words, the versatility of space, while highly desirable for renters, can be a challenge when it comes to fire safety measures.
In order to protect against the dangers that are necessary for the repurposing of areas, it is necessary first of all to provide for the most complete coverage of the building by the fire protection system. According to the current document NFPA (Fire) 72-02: National Fire Alarm Code issued by the National Fire Protection Association (NFPA stands for National Fire Protection Association), full building coverage is achieved by “specific placement of the proper type of sensors.”
Where to install the sensors?
The purpose of a fire detection system is to provide reliable and early warning of fire in any part of the building. Even “pockets” that are never occupied require protection, since detectors may not immediately respond to the development of a fire near the far wall or behind a closed door, which greatly increases the damage from fire.
Proper placement of sensors is also required for reliable operation of a fire protection system. In general, if only one sensor is installed in a room, then it is better to install it under the ceiling, as close to the center of the room as possible. If central placement of the sensor is not possible, for example due to wiring problems, it is best to install it in a way that provides an open space for it. In this case, the edge of the sensor should not be located closer than 10 cm to the wall. Also, the sensor installed on the wall should be placed so that there is from 10 to 30 cm to the ceiling (Fig. 1).
When ensuring full coverage, it is important to take into account the proximity of the installed detectors to ventilation and air conditioning systems. The document discusses possible reading failure situations if the detector is installed in the path of air currents. Test filling a room with smoke allows you to determine the direction of movement of microparticles, which is necessary for the subsequent correct placement of sensors. The same test identifies the potential for false alarms. An example would be the accumulation of dust in the sensor due to the specific distribution of air currents.
Dust in the sensor changes the level of sensitivity.
How to install sensors?
The National Fire Protection Association standard calls for sensors to be installed at 9.14 m intervals for rooms with an area of 84 square meters. The ceiling must be smooth and there must be no physical obstruction between the top and bottom of the room. An example of such an obstruction would be racks filled with materials.
The effect of ceiling height on sensor placement is also calculated based on space requirements based on the amount and nature of combustible materials in the room.
To determine whether sensor coverage meets the stated standard spacing of 9.14 m, draw a diagram of the room as shown in Figure 1. Then draw a circle with a radius of 6.40 m. It is assumed that the sensor can protect any square or rectangle and the object within it , which fits within the boundaries of this circle (Fig. 2).
This technique shows that in a 3 m wide corridor, two sensors can protect an area up to 25 m long (Fig. 3).
What type of sensors should I install?
To answer this question, you need to understand the purpose of a specific room and its location. For example, ionization smoke detectors detect fire (typical of chemical warehouses) at an earlier stage than smoldering (typical of offices). Ionization sensors immediately recognize fire by particles of burning substances ranging in size from 0.01 to 0.3 microns. However, their capabilities are very limited if the height of the sensor is significant, or if there is a source of high humidity nearby, such as a kitchen or shower.
If we consider photoelectric smoke detectors, they respond faster to smoldering fires with their characteristic combustion particles from 0.3 to 10 microns. This type of sensor immediately detects thick white smoke, but is slower to respond to volatile black smoke produced by plastic and rubber.
A common solution to the selection problem is to install multi-criteria detectors that work in tandem with thermal sensors. Signals from these sensors are processed by a microprocessor, which eliminates false alarms, but the reaction time when a real fire occurs increases. By monitoring the readings from each sensor, as well as monitoring changes in the resulting readings (increasing temperature or decreasing photoelectric response), multi-criteria sensors actually “learn” their environment, allowing them to maintain increased sensitivity while filtering out false alarms.
Smoke Detection in Ventilation
National and local fire safety standards recognize and provide recommendations for the transfer of smoke, toxic gases and flames through the ventilation system from one part of the building to another.
Often in various fire scenarios, indoor smoke is such that it poses a serious danger even to those several floors away from an open flame or fire.
This threat is countered by smoke detectors installed in ducts.
Operator understanding is important here buildings the main purpose of installing smoke detectors in the air ducts. It is to prevent panic and the resulting possible casualties among people, as well as damage to property. This is achieved by reducing the spread of smoke through recirculation.
A ventilation smoke detector is one or more devices used to detect the presence of smoke in various parts of the ventilation, air conditioning and heating system (HVAC) installed in public buildings.
Detecting the presence of smoke is not only for the purpose of protecting itself system from smoke and fire, but also to protect other equipment.
For example, a ventilation duct can lead to rooms where the main computer and storage devices are located.
Also, detecting smoke in ventilation ducts can be the first line of defense.
When a signal is received, the wind blowing systems of the installation are turned off and, if a fire is detected, the humidifiers are activated. For example, if the HVAC system's electric motor suddenly overheats, smoke detectors installed in the ventilation duct give a smoke signal.
The smoke detector is equipped with an auxiliary relay that immediately turns off power to the motor.
A scenario cannot be ruled out in which a fire starts on the second floor of the building (and the HVAC system serves the first through fourth floors). The smoke, accordingly, spreads to the remaining floors. If areas are not equipped with smoke detectors, the only means of fire detection will be detectors installed in the exhaust ventilation ducts of each floor. The amount of smoke in the ventilation system will eventually exceed the alarm limit of the second floor sensor, and a signal will be transmitted to the fire protection system.
The evacuation alarm will sound and the auxiliary relay will turn off the HVAC system. In this and other situations, smoke detection devices installed in ventilation will prove effective in avoiding property loss and casualties.
Installation, maintenance, testing
When The HVAC system, which provides air flow to all parts of the building, is turned on, smoke entering the ventilation duct can end up in all areas of the building.
Since we are talking about smoke detectors intended for installation in ventilation systems, correct installation, as well as timely inspection and maintenance are very important.
NFPA 90A Standard for the Installation of Air Conditioning and Ventilating Systems requires that duct-mounted smoke detectors be installed in the main vent after the air filters. With this configuration, it is possible to automatically turn off the system fan that produces more than 57 cubic meters per minute. 15,000 rpm fans require additional detectors in the exhaust ducts on each floor. A detector installed in this location provides full coverage of the entire room.
Smoke detectors are designed to be maintenance-free for as long as possible. However, dust, dirt and foreign objects can accumulate within the sensor, changing its sensitivity. This is especially true for sensors installed in air ducts, the degree of contamination of which can either increase or decrease the frequency of false calls, which will reduce the degree of security. Both are undesirable. As amended by the document standard from 2002. it is stated that ventilation smoke detectors are verified immediately after acceptance of the facility, and then annually. To determine whether a shorter calibration interval is required, check the regulations in your area.
Under normal operating conditions, sensors require verification at least twice a year, and more often in dirtier conditions. When a sensor system is undergoing maintenance or other maintenance and is temporarily out of service, notify the appropriate authorities. Also, to avoid false alarms, it is necessary that the system in which these sensors operate is turned off.
It is important to check the sensitivity of each sensor. If the sensor sensitivity is within specification, no additional maintenance measures are required. If it is outside the specification, the sensor requires either cleaning or replacement (depending on the manufacturer's recommendation). After testing or maintenance has been completed, ensure that the system is operational and notify the appropriate authorities.
When inspecting, please note that ventilation smoke detector problems that require service typically include:
• holes or cracks in the box body close to the sensor.
• air leakage in the places where the sensor is located, or the presence of sampling tubes.
• tension in the wires to the sensor.
To protect property and save lives in case of fire It is very important to understand the purpose of ventilation smoke detectors, and therefore strictly follow the rules for their installation and maintenance.
Based on materials from the magazine
”Consulting-Specifying Engineer”