Fire alarm placement: problematic cases.

Fire alarm placement: problematic cases.

Requirements for the placement of fire alarms are provided in NPB 88-2001* «Fire extinguishing and signaling installations. Norms and rules for designing». However, this document regulates only the basic options for placing alarms for relatively simple cases. In practice, there are often rooms with sloping floors, with decorative suspended lattice ceilings, with supply and exhaust ventilation, etc., which must be properly protected, despite the lack of specific instructions in NPB 88-2001*. For all non-standard cases, there is a general requirement in paragraph 3 of NPB 110-03 «List of buildings, structures, premises and equipment subject to protection by automatic fire extinguishing systems and automatic fire alarms»: «The type of automatic extinguishing system, extinguishing method, type of fire extinguishing agents, type of equipment of automatic fire extinguishing systems are determined by the design organization depending on the technological, design and space-planning features of the protected buildings and premises, taking into account the requirements of current regulatory and technical documents.» NPB 88-2001* also contains general requirements, for example, in paragraph 12.19 «the placement of point heat and smoke fire alarms should be carried out taking into account air flows in the protected premises caused by supply or exhaust ventilation», however, the criteria for optimizing the placement of alarms are not given, it is only indicated that «the distance from the alarm to the ventilation opening must be at least 1 m».In many complex cases, it is possible to avoid serious design errors by using additional materials, for example, the European standard BS 5839-1:2002 on fire detection and alarm systems for buildings, part 1 «Standards and rules for the design, installation and maintenance of systems», where each section and paragraph first sets out the physical processes, and then the requirements arising from them, which allows you to be sure of the correctness of the chosen solution in a particular case. For example, when arranging automatic fire alarms, it is necessary to take into account the specifics of their operation depending on the type:
The effectiveness of an automatic fire detection system will be affected by obstructions between the heat or smoke detectors and the combustion products. It is important that the heat and smoke detectors are not installed too close to obstructions to the flow of hot gases and smoke to the detector. Near the junction of a wall and ceiling there is a «dead space» in which heat or smoke detection will not be effective. Since hot gas and smoke flow horizontally parallel to the ceiling, there is also a stagnant layer near the ceiling; this precludes installation with the sensing element of the heat or smoke detector flush with the ceiling. This limitation may be less important in the case of an aspirating system, since this system actively draws air samples from a moving layer of smoke and hot gases. The possible air flow pattern in the room should be considered when installing heat and smoke detectors. Air conditioning and ventilation systems with high air exchange rates may adversely affect the detectors' capabilities by introducing fresh air into them and removing hot air, smoke and combustion gases, or by diluting smoke and hot gases from the fire. Smoke detectors may be installed in ventilation ducts to monitor smoke. In general, such detectors should help prevent the spread of smoke by the ventilation system, any recirculation should be stopped in the event of a fire. These detectors may be connected to a fire alarm system, but if smoke detectors have normal sensitivity, they cannot be a satisfactory means of detecting a fire in the area from which the air comes, since the smoke is diluted by the extracted clean air….
From the above physical model, two basic principles follow that are taken into account when placing smoke and heat detectors:
in the case of flat ceilings, in the absence of interference and obstacles, smoke and heat detectors protect an area in the form of a circle in the horizontal plane;
it is necessary to regulate the minimum and maximum distance of the detectors from the ceiling. Fig. 1. The simplest diagram of the placement of a smoke and heat detector

According to BS 5839-1:2002, the protection radius for smoke detectors is 7.5 m, for heat detectors — 5.3 m in horizontal projection. Thus, it is easy to determine the placement of detectors in a room of any shape: the distance from any point in the room to the nearest smoke detector in horizontal projection should be no more than 7.5 m, from the heat detector — no more than 5.3 m. These radii of the protected area determine slightly larger distances between detectors when arranged in a square grid (Fig. 1) compared to the requirements of NPB 88-2001*. Significant savings in the number of detectors (approximately 1.3 times) are achieved in large rooms when using the placement of detectors in a triangular grid (Fig. 2). Fig. 2. Arrangement of detectors in large rooms

At present, in practice, these provisions can only be applied when using aspirating detectors. The Recommendations of the Federal State Institution VNIIPO EMERCOM of Russia on the design of fire alarm systems using aspirating smoke fire detectors of the LASD and ASD series indicate that «when protecting rooms of arbitrary shape, the maximum distances between air intake openings and walls are determined based on the fact that the area protected by each air intake opening has the shape of a circle with a radius of 6.36 m (Fig. 3).

Distance to the ceiling
According to the British standard BS5839, fire alarms must be installed on the ceiling so that their sensitive elements are located below the ceiling within the range of:
1) 25 mm — 600 mm for smoke detectors;
2) 25 mm — 150 mm for heat detectors.
A layer of clean air remains directly next to the ceiling, which determines the minimum distance from the sensitive element of the smoke and heat detector to the ceiling, equal to 25 mm. For the same reason, flush mounting of detectors is prohibited. In NPB 88-2001*, such a requirement is specified so far only for a linear smoke fire detector, clause 12.29. «the optical axis passes at a distance of at least 0.1 m from the ceiling level» and for linear heat fire detectors, clause 12.37: «the distance from the detector to the ceiling must be at least 15 mm». According to NPB 88-2001* clause 12.18*, for all point fire detectors «when suspending detectors on a cable, their stable position and orientation in space must be ensured. In this case, the distance from the ceiling to the lowest point of the detector must be no more than 0.3 m». BS5839 specifies different maximum distances from the ceiling for smoke and heat detectors. Smoke detectors provide early fire detection, at the stage of smoldering materials, and can be placed at a distance of about 300 mm from the ceiling even in the absence of the stratification effect. Unlike smoke detectors, heat detectors do not detect smoldering fires, and at the stage of open fire there is a significant increase in temperature, accordingly, the stratification effect is absent and an increase in the distance between the ceiling and the heat-sensitive element by more than 150 mm will lead to an unacceptably late fire detection, i.e. will make them practically inoperative.

Perforated ceilings
In airports, large shopping centres etc decorative grilles are often used to cover air ducts and cables located under the ceiling. For example, «Greeeeee» type ceilings. How should fire detectors be installed in this case? BS 5839-1:2002 states that detectors installed in the main ceiling can be used to protect the area below a perforated false ceiling if the following conditions are simultaneously met:
1) the perforation area is more than 40% of any 1m x 1m section of the ceiling;
2) the minimum dimension of each perforation in any section is not less than 10 mm;
3) the false ceiling thickness is not more than three times the minimum size of each perforation cell.
In all other cases, sensors should be installed below the false ceiling, and if protection of the ceiling void is required, additional sensors should be installed on the main ceiling in the ceiling void.
When the above conditions are met, there is practically no division of the room into two spaces, the smoke passes through the perforation of the false ceiling and is detected by detectors installed on the ceiling. These conditions are met with a large margin for the «Grilyato» type ceiling, for greater persuasiveness it is recommended to consider it as a decorative grille, which practically does not create an obstacle to the spread of smoke.

Inclined ceilings
The absence of the concept of a sloping, non-horizontal ceiling in our standards can lead to serious design errors. The maximum permissible distance from the detector's sensitive element to the ceiling determines the criterion for assessing the horizontality of the ceiling, without using any values ​​for the angle of inclination. If the difference in ceiling heights when using smoke detectors does not exceed 600 mm, then the smoke accumulates in the upper part of the room and the ceiling is considered horizontal regardless of the area of ​​the room. Similarly, for heat detectors, if the difference in heights does not exceed 150 mm, the ceiling is also considered horizontal regardless of the size of the room. With large differences in height, smoke with warm air flows up the slope in the direction of the ridge, and the upper part of the volume is filled. In this case, the first row of fire detectors is installed along the ridge, and the remaining rows are parallel to the first along the slopes. It is possible to place detectors at a lower level, while the sensitive elements of the smoke detector should be located no lower than 600 mm from the top of the ceiling, and the thermal ones no lower than 150 mm (Fig. 4). Fig.4. Protection of a room with slopes at different angles BS 5839-1:2002

In addition, a sloping section of the ceiling usually increases the rate of rise of the smoke and warm air flow towards the top, thus reducing the delay time before the detector is triggered. Accordingly, BS 5839-1:2002 allows for an increase in the distance between detectors in the top row: for each degree of slope angle, the distance between detectors may be increased by 1%, but by a maximum of 25%. If the ceiling slopes have different slope angles, the distance between detectors installed along the ridge is selected based on the smaller value determined by the smaller slope angle (Fig. 4). In this example, the distance between detectors along the ridge may be increased by 18%, i.e. up to 12.39 m. The remaining detectors are installed based on the standard value of the radius of the protected area, equal to 7.5 m in horizontal projection. It is recommended that special attention be paid when determining the location of subsequent rows of detectors in order to avoid gaps between the circles of detectors of different rows and different radii.
Of course, we cannot use these nuances in practice, but the criterion of a sloping ceiling is quite applicable. According to NPB 88-2001*, clause 12.18*, already mentioned above, for all point fire detectors, “the distance from the ceiling to the lowest point of the detector must be no more than 0.3 m.” Thus, in a room of 9 x 9 m with a height difference of about 0.6 m, it is possible to install the detector in the center of the room, and with a greater height difference, it is recommended to place it on a higher part of the ceiling. In this case, the requirement specified in clause 12.18* should be met: “When installing point fire detectors under the ceiling, they should be placed at a distance of at least 0.1 m from the walls.” Note that in BS 5839-1:2002 this distance for horizontal ceilings is 0.5 m.
Similar to the requirements for point smoke detectors, when installing linear smoke detectors in BS 5839-1:2002, it is necessary to ensure a distance from the beam to the horizontal ceiling within the range of 25 mm to 600 mm. In a room with a non-horizontal ceiling, i.e. with a difference in ceiling height of more than 600 mm, it is necessary to protect the space along the roof ridge. In this case, according to BS 5839-1:2002, the distance between the optical axes of linear detectors can also be increased by 1% for each degree of inclination up to a maximum value of 25% (Fig. 5). Fig. 5. Protection of a room with a sloping ceiling

In our practice, the distance between the optical axes not only cannot be reduced, but can hardly be measured in horizontal projection, since Table 6 of NPB 88-2001* specifies the maximum distances directly between the optical axes of the detectors without taking into account their possible placement on an inclined ceiling. Fig. 6. Protection of premises at an average level

If it is not possible to install linear smoke detectors under the ceiling, for example, in atriums with a glass domed roof, BS 5839-1:2002 allows their placement at a level below 600 mm from the ceiling. However, with such placement of detectors, the protected area is significantly reduced and amounts to 12.5% ​​of the installation height in each direction from the optical axis (Fig. 6.) Smoke disperses over a larger area with increasing height, therefore, it is more economical to install linear optical detectors at the highest possible height. For example, when installed at a height of 4 m, for reliable detection of the source, the distance between the optical axes should be no more than 1 m, when installed at a height of 20 m — accordingly no more than 5 m.
Floors with beams
Large industrial premises usually have beams of considerable height on the ceiling. In this case, the detectors must be installed in accordance with paragraph 12.20. NPB 88-2001*: “Point smoke and heat fire detectors must be installed in each ceiling section with a width of 0.75 m or more, limited by building structures (beams, purlins, slab edges, etc.) protruding from the ceiling by more than 0.4 m. If building structures protrude from the ceiling by more than 0.4 m, and the sections formed by them are less than 0.75 m in width, the area monitored by fire detectors, specified in Tables 5 and 8, is reduced by 40%. If there are protruding parts from 0.08 to 0.4 m on the ceiling, the area monitored by fire detectors, specified in Tables 5 and 8, is reduced by 25%.”It is not specified along which axes the distance between the detectors should be reduced. The beams prevent the spread of smoke in the transverse direction, and, therefore, it is necessary to reduce the distances in this direction, ensuring the specified reduction in the monitored area. It makes no sense to reduce the distances between the detectors along the beams, since the smoke spreads even faster between the beams, since the effect of limiting the space is manifested as in a corridor, where the distances between the detectors are allowed to increase by 1.5 times. Fig. 7. Ceiling with beams, M — distance between detectors

BS 5839-1:2002 considers two options in more detail: linear beams (Fig. 7) and honeycombs (Fig. 8). Fig. 8. Honeycomb ceiling

The requirements of BS 5839-1:2002 for permissible distances between detectors across beams depending on the ceiling height and beam height are given in Table 1.
Table 1

Where, H is the ceiling height; W is the cell width; D is the beam height.

For a honeycomb ceiling, depending on the ratio of the beam height to the cell width, fire alarms are installed either on the ceiling or on the beam (Table 2). Here, the beam height limit of 600 mm appears (as opposed to our 400 mm), but the relative beam height is also taken into account — an additional limit of 10% of the room height.
Table 2

Where, H is the ceiling height; W is the cell width; D is the beam height.