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There have always been tenants and there always will be. Today, these spaces are a labyrinth of cubicles, and tomorrow, a private company will open its office here. Since each company adapts the workspace to suit itself, the principles of fire protection of the building can change significantly. In other words, the versatility of the space, which is very desirable for tenants, can be a problem when it comes to fire safety measures.
To protect against the hazards that are inherent in repurposing, it is necessary to first provide the most complete coverage of the building with a 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), complete coverage of a building is achieved by “the proper placement of the proper type of detectors.”
Where to install detectors?
The purpose of a fire detection system is to provide reliable and early warning of a 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 a far wall or behind a closed door, which greatly increases the damage from the fire.
For reliable operation of the fire alarm system, the sensors must also be correctly positioned. In general, if only one sensor is installed in a room, it is better to install it under the ceiling, as close to the center of the room as possible. If it is not possible to place the sensor in the center, for example, due to wiring problems, it is better to install it so as to provide an open space for it. In this case, the edge of the sensor should not be closer than 10 cm to the wall. Also, the sensor installed on the wall should be placed so that there is 10 to 30 cm left to the ceiling (Fig. 1).
An important factor in ensuring full coverage is the proximity of the installed detectors to ventilation and air conditioning systems. The document discusses possible situations of failure in readings if the detector is installed in the path of air flows. Test filling of the 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 reveals the potential for false alarms. An example can be the accumulation of dust in the sensor due to the specific distribution of air flows.
Dust in the sensor changes the sensitivity level.
How to install sensors?
The National Fire Protection Association standard calls for detectors to be spaced 30 feet (9.14 m) apart for a 2,000-square-foot (84 m2) space. The ceiling must be smooth and there must be no physical barrier between the top and bottom of the space. An example of such a barrier would be shelving filled with materials.
The effect of ceiling height on detector placement is also calculated based on the space requirements based on the amount and nature of combustible materials in the space.
To determine whether the sensor coverage corresponds to the declared standard interval 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 inside it that fits within the boundaries of this circle (Fig. 2).
This technique shows that in a corridor 3 m wide, two sensors can protect an area up to 25 m long (Fig. 3).
What type of sensors should I install?
To answer this question, it is necessary to understand the purpose of the specific room and its location. For example, ionization smoke detectors detect a fire (typical of chemical warehouses) earlier than smoldering fires (typical of offices). Ionization detectors immediately recognize fire by combustion particles sized from 0.01 to 0.3 microns. However, their capabilities are very limited if the sensor is located at a significant height 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 typical combustion particles from 0.3 to 10 microns. This type of detector immediately recognizes thick white smoke, but reacts more slowly 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 heat sensors. The signals from these sensors are processed by a microprocessor, which filters out false alarms, but the reaction time in the event of a real fire increases. By monitoring the readings from each sensor, as well as tracking changes in the readings received (increase in temperature or decrease in photoelectric response), multi-criteria sensors actually “learn” the environment, which allows maintaining increased sensitivity while filtering out false alarms.
Smoke Detection in Ventilation
National and local fire safety standards provide for the possibility of transferring smoke, toxic gases and flames through the ventilation system from one part of the building to another and give their recommendations.
Often, in various fire scenarios, the smoke in the premises is such that it poses a serious danger even to those who are separated from the open fire or the fire by several floors.
This threat is countered by smoke detectors installed in the air ducts.
Here it is important for building operators to understand the main purpose of installing smoke detectors in the air ducts. It is to prevent panic and the associated possible loss of life among people, as well as damage to property. This is achieved by reducing the spread of smoke through recirculation.
A vent smoke detector is one or more devices used to detect the presence of smoke in various parts of a ventilation, air conditioning, and heating (HVAC) system installed in a public building.
Detecting the presence of smoke serves not only to protect the system itself from smoke and fire, but also to protect other equipment.
For example, a vent duct may lead to rooms where the main computer and storage devices are located.
Smoke detection in ventilation ducts can also be the first line of defense.
When a signal is received, the wind-blowing systems of the installation are switched off and, if a fire is detected, the humidifiers are activated. For example, if the electric motor of the HVAC system suddenly overheats, then the smoke detectors installed in the ventilation duct give a signal about smoke.
The smoke sensor is equipped with an auxiliary relay, which immediately switches off the power supply to the motor.
It is possible that a fire starts on the second floor of a building (and the HVAC system serves floors one through four). The smoke then spreads to the remaining floors. If the areas are not equipped with smoke detectors, the only means of detecting a fire will be the detectors installed in the exhaust ventilation ducts of each floor. The amount of smoke in the ventilation system will eventually exceed the detection limit of the second floor detector, and a signal will be transmitted to the fire protection system.
The evacuation alarm will sound and the auxiliary relay will shut down the HVAC system. In this and other situations, smoke detection devices installed in the ventilation will be effective in preventing 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 be distributed throughout the building.
Since we are talking about smoke detectors intended for installation in ventilation systems, their 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. This configuration can automatically shut off a system fan that produces more than 57 cubic meters per minute. Fans with a rotation speed of 15,000 rpm require additional detectors in the exhaust ducts of each floor. A detector installed in this location provides complete coverage of the entire room.
Smoke detectors are designed to last as long as possible without maintenance. However, dust, dirt and foreign objects can accumulate in the detector, changing its sensitivity. This is especially true for detectors installed in air ducts, the degree of contamination of which can either increase the frequency of false calls or decrease it, which will reduce the level of protection. Both are undesirable. The 2002 edition of the standard document states that ventilation smoke detectors are verified immediately after acceptance of the facility, and then annually. To find out whether a shorter inter-verification interval is required, you should pay attention to the regulations in force in your area.
Under normal operating conditions, sensors require testing at least twice a year, and more often in dirtier conditions. When a sensor system is undergoing preventive 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 switched off.
It is important to check the sensitivity of each sensor. If the sensor sensitivity is within the specification, then no additional maintenance is required. If it is outside the specification, then the sensor requires either cleaning or replacement (depending on the manufacturer's recommendation). After testing or servicing, ensure that the system is back in working order and notify the appropriate authorities.
When performing an inspection, note that ventilator smoke alarm problems that typically require servicing include:
• holes or cracks in the enclosure near the alarm.
• air leaks where the alarm is located or the presence of sampling tubes.
• strain on the wiring to the alarm.
Understanding the purpose of ventilator smoke alarms and following proper installation and maintenance procedures is essential to protecting property and saving lives in the event of a fire.
Based on materials from the magazine
”Consulting-Specifying Engineer” |
