Aspiration smoke detectors — a familiar stranger.
Introduction
With the release of Federal Law No. 123 «Technical Regulations on Fire Safety Requirements», more and more interest has been shown in aspiration smoke detectors.
More and more global manufacturers have begun to supply their aspiration systems to the Russian market.
Although the type of these detectors is not new to the Russian market of fire alarm systems (FAS), but before May 1, 2009, interest in such systems was small.
First of all, this was due to both the price of these detectors and the absence of this type of fire detector in the Russian regulatory framework.
Those manufacturers who saw the prospects of aspiration systems on the Russian market, developed recommendations for the use and design of such systems together with VNIIPO, so that designers could at least have something to justify their choice with.
But all this is in the past, at present the need for such recommendations has disappeared and the main documents for the design of aspiration systems are the national standard GOST R 53325-2009 «Fire-fighting equipment. Technical means of fire-fighting automation.
General technical requirements.
Test methods”, which establishes technical requirements and methods of certification tests and the set of rules SP5.13130.2009 “Fire protection systems.
Automatic fire alarm and fire extinguishing systems.
Norms and rules for design”, which sets out the design requirements.
In this article, we will try to understand how aspiration detectors can differ from each other, what you should pay attention to when choosing and designing a system based on this type of detector.
Operating principle
A fire smoke aspiration detector is a detector in which air and smoke samples are forcibly taken from the protected room through a system of pipes with holes and transported to a smoke chamber located in the same block with an aspirator, for example, a turbine, fan or pump, providing a stable air flow.
The pipe system is located in the controlled area, and the aspiration device — the central unit, can be installed in a place convenient for control and maintenance in the same or another room.
Fig. 1 Aspiration detector
In many cases, this method of control – constant forced air extraction through a system of pipes from a controlled volume – provides significant advantages over traditional point detectors, which under certain conditions may simply not be reached by smoke.
The aspirator ensures air flow through each opening from a sufficiently large volume of the room, which compensates for the influence of air flows from supply and exhaust ventilation, air conditioning systems, etc., which distort the “standard” distribution of smoke in the room.
Aspiration also reduces the effect of air stratification (layering) in a high room, when a layer of warm air under the ceiling prevents smoke from entering the upper part of the room.
In addition, the simultaneous entry of smoke through several openings in the pipe (cumulative effect) compensates for the decrease in smoke concentration under the ceiling in a high room.
Such parameters as the design and shape of the smoke inlet of the smoke chamber of a point detector, which, if poorly designed, significantly increase the response time, are not relevant for an aspiration system at all.
Design Features
With all its advantages, an aspiration detector also has a major drawback — this is the high price, which is still the fundamental factor limiting their use.
Although everyone understands perfectly well that an aspiration detector is a technically complex device and is actually a separate autonomous system that connects to the main fire alarm system of the building, and therefore cannot be cheap.
But there are aspiration detectors of the so-called PIB class, interest in which is growing every year.
According to their design features, aspiration detectors can be conditionally classified into two main classes.
The VESDA class – Very Early Smoke Detection Apparatus (equipment for very early smoke detection) includes detectors in which the optical density meter of the environment is built into the smoke chamber of the aspiration unit and is its integral part.
For example, aspiration detectors FAS-420-TT2 from Bosh, Vesda VFT-15 from Xtralis.
The PIB class – Point In the Box (point detector in the box) includes aspiration detectors in which the optical density meter of the environment is a point smoke detector located in the smoke chamber of the aspiration unit.
For example, aspiration detectors LASD and ASD-Pro from System Sensor.
It is the aspiration detectors of class PIB that allow to significantly reduce the price of an aspiration detector, while maintaining all the positive features of an aspiration system.
The capabilities of class PIB detectors are largely determined by the type of smoke detector used, installed in the central unit.
For example, the LASD series of aspiration detectors uses a 7251 laser smoke detector with a miniature laser (Fig. 2).
The brightness of the laser radiation is approximately two orders of magnitude higher (100 times) compared to the LED, and the beam focusing ensures almost complete absence of reflections from the walls of the smoke chamber. As a result, such a laser detector ensures the earliest possible detection of a fire hazard at smoke levels of 0.001 dB/m.
Aspiration systems based on such a detector are capable of generating preliminary alarm signals, which the service personnel can use to eliminate the source of smoke with minimal damage.
At the same time, the price of such a system will be relatively low.
Fig. 2 Smoke chamber of the 7251 point detector
The main parameters to consider when choosing an aspirating detector are its sensitivity, the method of filtering air samples, and air flow control.
Sensitivity and transportation time
Sensitivity is probably the main characteristic of an aspirating detector, as well as any smoke detector.
Sensitivity can be considered as the minimum value of the specific optical density in one of the samples, at which the detector generates a «Fire» signal.
It depends on the smoke detection technology used, as well as on the design of the sampling device, the number, size and location of the holes, etc.
Aspiration systems typically use laser smoke detection technology (Fig. 2) or the LED method (super-sensitive optical mechanism).
The use of laser smoke detection technology allows detecting smoke with a specific optical density of less than 0.001 dB/m, which is hundreds of times higher than the sensitivity of point smoke LED detectors.
This takes the level of fire protection to a qualitatively new level. By detecting such concentrations of smoke, it is possible to prevent a fire long before it develops using improvised means.
In accordance with GOST R 53325-2009, aspiration detectors are divided into three classes by sensitivity:
class A – high-sensitivity detectors (specific optical density of the medium less than 0.035 dB/m);
class B – high-sensitivity detectors (specific optical density of the medium from 0.035 to 0.088 dB/m);
class C – standard-sensitivity detectors (specific optical density of the medium more than 0.088 dB/m).
Tests to determine sensitivity according to GOST R 53325-2009 are carried out only using the «Smoke Channel» stand.
Moreover, a part of the air intake pipe with one hole, the most distant from the aspiration detector block, is located in the smoke duct.
The remaining part of the pipe with open holes should be located outside the smoke duct.
Clean air enters through these holes, and, accordingly, the optical density of the medium entering the aspiration detector block decreases.
Fig. 3. Reducing smoke concentration
When choosing an aspiration detector, it should be taken into account that manufacturers, as a rule, indicate in the technical specifications the maximum sensitivity value of the aspiration unit itself.
It is possible to implement such sensitivity only when using one air intake opening in the pipe.
If there are several air intake openings in the pipe, the concentration of smoke in the air sample will decrease proportionally to the volume of clean air entering the pipe through the remaining openings (Fig. 3).
For example, there are 10 air intake openings in the pipe.
Let's assume that the same volume of air passes through each hole and smoke with a specific optical density of 2%/m3 (0.088 dB/m3) enters the pipe through one air intake hole. Clean air enters through the remaining 9 holes. Thus, in the pipe, the smoke is diluted with clean air by 10 times and its density upon entering the central unit will already be 0.2%/m3 (0.0087 dB/m3).
Therefore, for a preliminary analysis of the sensitivity of the hole itself (since it is the air intake hole that is equivalent to a point smoke detector in accordance with SP5.13130.2009), the following assumption can be used: the sensitivity of the hole is equal to the ratio of the sensitivity of the aspiration unit to the assumed maximum number of holes in the pipe.
In reality, the calculation of smoke dilution with clean air is much more complicated than described above.
It is necessary to take into account the size, number and location of air intake openings, the presence of corner joints, tees and capillaries in the pipe system, the diameter of the pipes, etc.
In addition, to align the air flows through the holes, and accordingly, the sensitivity, a plug is installed at the end of the pipe, as a rule, with a hole, the area of which is several times larger than the air intake holes, which should also be taken into account in the calculation. All these points can be taken into account by the calculation program, which is supplied with the aspiration detector in the kit.
The next important parameter to consider when designing an aspiration system is the transportation time. According to GOST R 53325-2009, this time is given for each class and should not exceed 60 sec for class A detectors, 90 sec for class B, and 120 sec for class C.
Here, this time is understood to mean the time it takes to transport an air sample from the farthest opening in the pipe to the smoke-sensitive element in the unit.
Filtration and air flow control
It is logical to assume that, having high sensitivity, aspiration detectors should be very critical to such interfering effects as dust.
The first aspiration detectors were developed specifically to protect ultra-clean rooms and hermetic zones.
Modern microprocessor aspiration detectors have a built-in filtration system and adapt well to various operating conditions.
For use in dusty and heavily polluted areas, external filters are usually installed on the pipes to capture airborne particles.
To protect areas with high humidity, many manufacturers provide additional devices to protect the central unit from condensation (Fig. 4).
The probability of false triggering of the device in dusty rooms is minimized by programming the appropriate response levels and stabilizing the measurement range.
Fig. 4 Moisture protection device
It is equally important to ensure control of the air flow passing through the smoke chamber of the aspiration detector.
A decrease in air flow indicates clogging of the holes in the pipe system, an increase indicates a leak in the pipe connection or mechanical damage to the pipeline.
In these cases, there is a malfunction — a decrease in sensitivity.
Monitoring changes in the air flow level in an aspiration detector is equivalent to monitoring the loop status when using point fire detectors.
In addition, the values of the «normal» air flow must be stored in non-volatile memory in case of a power outage.
According to GOST R 53325-2009, if an air leak or blockage leads to an increase or decrease in the air flow volume by 20% or more, the aspiration detector must generate a malfunction signal.
The same document provides a requirement for the heat resistance of the air pipeline: it must ensure the performance of its functions under fire conditions for the time required to transport air samples to the processing unit and analyze the condition of the monitored samples using technical means for detecting smoke.
Placement of aspiration detectors
How to place aspiration detectors is prescribed in the set of rules SP5.13130.2009. Let us note the main points.
Class A aspiration detectors can be used to protect objects up to 21 m high, class B – up to 15 m, class C – up to 8 m.
The distances between air intake openings should not exceed 9 m, from the opening to the wall – 4.5 m.
Moreover, these distances do not depend on the height of the protected room and the class of the detector. This is explained by the above-mentioned cumulative effect.
This document also notes that for the protection of large open spaces and rooms with a height of more than 8 m, such as atriums, production facilities, warehouses, retail spaces, passenger terminals, sports halls and stadiums, circuses, museum exhibition halls, art galleries, as well as rooms with a high concentration of electronic equipment (server rooms, automatic telephone exchanges, data processing centers), it is recommended to use aspiration detectors of class A and B.
Conclusion
In conclusion, I would like to note that today, aspiration detectors with laser smoke detection technology are the most effective type of smoke fire detectors, capable of detecting a smoldering source at an extremely early stage of the development of a fire hazard situation, when it is possible to eliminate this source using improvised means without significant material losses.
Aspiration detectors of the conventional class PIB are more affordable and at the same time have all the main advantages of aspiration systems.
And the appearance of aspiration detectors as a class in our regulatory framework will contribute to the ever wider use of aspiration systems at various facilities, taking fire safety to a qualitatively new level.
Single-channel aspiration detector MG-Laser (MaviGard)
The detector has a large dynamic range at the input and an algorithm for adapting to external conditions of the working environment (LDD — laser dust separation technology), which allows it to successfully cope with the task of detecting smoke both in «clean rooms» and in dusty rooms.
The alarm signal can be issued both via relay outputs and via the interface board.
Maximum channel length is 100 m.
Dynamic range — 0.6-25% darkening per meter.
Detectable particle sizes – 0.003–10 µ.
The detector has two alarm levels, relays “Attention”, “Fire”, “Malfunction”.
Laser camera maintenance interval – 10 years.
Power supply: 21.6 – 24.4 V DC. Current consumption: 350 mA.
Overall dimensions: 190 x 230 x 110 cm. Weight – 1.2 kg.
LASD (System Sensor) series aspiration detectors
Designed for use in ultra-clean rooms, containment areas (microelectronics production), processing shops (woodworking, paper, tobacco factories), buildings with non-standard architecture, glass atriums, museums, libraries, archives.
A good solution for long and high rooms (up to 15 m), warehouses, areas with electronic equipment (server rooms, data centers, telecommunications systems).
They are also used to equip spaces behind suspended (stretch) ceilings, under double floors, in hard-to-reach and dusty areas.
They ensure the fastest possible detection of a fire hazard and a reduction in the impact of air flows, the absence of plumes and electronic devices in the protected room. They are easily built into decorative elements and building structures.
Have a convenient design program.
Easy to install, program and maintain.
Minimal costs are required when upgrading a fire alarm system.
Titanus MicroSens FAS-420-TM-RVB (Bosch)
The compact detector from the Titanus MicroSens series monitors an area of up to 400 m² and is connected to fire panels via the LSNi addressable loop.
The detector uses a pipeline of various configurations with a total length of up to 50 m and having up to 8 holes.
The detector response threshold is adjusted in the range of 0.5–2%/m (0.02–0.09 dB/m), classes A and B according to GOST R 53325-2009.
The detector has a 10-segment smoke level indicator, displaying values from 0.05%/m (0.002 dB/m).
The function of monitoring 5 separate zones ROOM IDENT allows the detector to determine the location of the fire with an accuracy of up to the opening of the pipeline.
The detector also monitors the pipeline for contamination and possible damage.
For use in explosive, damp and dusty rooms, special nozzles for the pipeline are used.
VESDA ECO by Xtrali (Xtralis)
VESDA ECO by Xtralis is an aspiration system that combines the capabilities of a highly sensitive smoke detector with the function of monitoring the environment for the presence of hazardous gases.
VESDA ECO uses the VESDA aspirating smoke detector's air sampling pipe system to continuously monitor changes in the concentration of gaseous environmental products in the protected area.
ECO identifies gases (H2, CO, CH4, C3H8, SO2, NO2, O2, NH3) in air samples that could normally cause false alarms of smoke detectors.
Installation of the ECO unit does not require additional changes to the VESDA system design or the installation of additional electrical lines.
FDBZ292 (Siemens)
The air sampling smoke detector is a passive monitoring system without its own pump.
Uses the existing pressure difference in the ventilation system to continuously move air through a bypass duct from the ventilation to the air sampling smoke detector.
Works if the ventilation system is on and air is circulating. The monitored air must be clean, i.e. free of dust particles and aerosols.
The device can use smoke detectors of the Sinteso, AlgoRex and Synova 600 series.
The body of the sampling chamber is made of ABS and has protection category IP54.
Permissible air flow speed – 1–20 m/s.
Ambient temperature:
from -20 °С to + 50 °С. Relative humidity ≤95%.