Very Early Warning Smoke Detection

8/3/2019 Very Early Warning Smoke Detection

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Smoke detection technologies are basically classified into EWSD

(Early warning Smoke Detection) and Very Early Warning Smoke Detection (VEWSD). EWSD

detectors are of ionization or photoelectric type. Ionization detectors are designed to detect very

small particles such as the type produced by flammable liquids. Photoelectric detectors detect

larger particles such as those produced by non- natural materials like PVC.

An EWSD provides detection of a fire condition prior to the time that

it becomes threatening to the occupants of the building. Generally this is the time that smoke is

visible. Let us use the example of paper catching fire within an office. Seconds after the paper

has ignited, smoke will generate and rise to the ceiling. This visible and hot smoke will

eventually enter the smoke detection chamber and trigger the alarm to alert the occupants that a

fire has commenced.

The EWSD are passive in the sense that the smoke has to find its

way to the detector. These detectors wait for smoke and rely on the airflow to transport the

smoke to the detector. Therefore their performance is affected by airflow.

If a computer terminal within a room had a fault in its electronics

resulting in a thermal event, it may smoulder for hours before a flame ignites. This smouldering

stage is the incipient stage to a fire. During this incipient stage the human eye will not see the

particles but may smell them. EWSD are not sensitive enough to detect smoke at the incipient

stage of an electrical type fire. Only a VEWSD will detect an incipient fire and thus the term

VERY EARLY WARNING. This stage of a fire could last for hours or even days.

Since the rate of smoke generation in a smouldering fire is relatively

small, and the airflow velocity in the room is quite high, the movement of smoke is dominated by

the airflow of the mechanical systems. Furthermore the smoke generated during the incipient

stage is not hot therefore there is very little thermal lift. This often restricts smoke movement

directly to the ceiling, where spot type detectors are located, causing the smoke to dissipate more

widely.

The aspirated smoke detection or VESDA is active , constantly

sampling the air from multiple points throughout the environment and therefore is not

totally dependant on thermal energy to transport smoke to enter into the smoke detector.


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AAAssspppiiirrraaattteeeddd DDDeeettteeecccttt iiiooonnn::: Aspirated smoke detection (ASD) systems are quite different from

conventional spot type smoke detectors. Aspirated systems typically comprise a number of small-

bore pipes laid out above or below a ceiling in parallel runs, some metres apart. Small holes, also

some metres apart, are drilled into each pipe to form a matrix of holes (sampling points),

providing an even distribution across the ceiling. Air or smoke is drawn into the pipework throughthe holes and onward to a very sensitive smoke detector mounted nearby, using the negative

pressure of an aspirator (air pump).

HHHooowww mmmuuuccchhh sssmmmoookkkeee ssshhhooouuulllddd wwweee dddeeettteeecccttt??? Obscuration as a unit of measurement has

become the standard definition of smoke detector sensitivity used in the industry today.

Obscuration is the effect that smoke has on reducing visibility. Higher concentrations of smoke

result in higher obscuration levels, lowering visibility.


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Typical Smoke Detection Ratings for smoke detectors:

Photoelectric : 3.0 % 12 % obscuration per metre

Beam : 3.0 % 7.0% obscuration per metre

VESDA : 0.005 % - 20 % obscuration per metre


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Note that the obscuration level at which VESDA detects smokeis lesser (0.005 %) compared to the lower limit of 3.0 % obscuration level for Photoelectric or

Beam detector.

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The following have to be considered when designing a VEWSD system

1. The coverage area per detector or sample point

2. The sensitivity required per sampling point

3. The airflow characteristics and the air change rate within the room.

4. The room size and characteristics raised floor, tall ceilings etc.

5. The annunciation of emergency response systems.

6. The activation of mechanical control systems such as air extraction and suppression systems.


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CCCooovvveeerrraaagggeee AAArrreeeaaa ooofff AAASSSDDD::: The maximum coverage area of ASD detector is 2000 sq. meters

(this is the maximum area coverage permissible within the codes, BS, AS, NFPA). The sample

point of an ASD detector is treated the same as a spot type detector.

The area of coverage for a sampling point is effectively around

100-sq. metre. For ASD applications in high airflow environments, we can decrease the area of

coverage for the sample point by adding more holes and making the distance between each pipe

less.

SSSeeennnsssiiitttiiivvviiitttyyy ooofff AAASSSDDD:::The smoke that reaches the detector is the cumulative smoke, which has

entered all the sampling points in the network. Take the example of a 200 square metre room

with 10 sample points on the ceiling. If the detector sensitivity is set to 0.1 % Obscuration / m this

effectively makes each sample points sensitivity as 0.1 X 10 = 1.0 % Obscuration / m. That is, if

only one sample point was exposed to smoke it would require 1.0 % Obscuration / m to trigger an

alarm. This is because we take into account the dilution caused by the other holes.

Taking the same example, If smoke enters three holes then the

effective sensitivity required to trigger an alarm is 0.1 x 10 divided by three = 0.33 % Obscuration

/ m per sampling point. Clearly, cumulative sampling allows much lower levels of smoke, and

thus enable very early detection. If the same room was designed with EWSD and each detector

was rated at 5 Obscuration /m, the alarm would only trigger once the smoke density has reached

this point throughout the room or at one detector.

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SSSyyysssttteeemmmsss: In the diagram given below, The sampling pipes on the ceiling and within the floor

void are used for detection where the CRAC is out of service. The pipe used to detect smoke

across the return air path is used for detection where CRAC is operational. The enabling and

disabling of the sampling pipes helps to achieve this.

VEWSD techniques are preferred in areas where Air Handling Units

(AHUs) and other DX systems are installed because the air changes which happen in theserooms will dilute the smoke, thus making smoke detection difficult for EWSD.


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hazardous areas like petrochemical plants and also warehouses storing solvents and alcohol.

These are areas where explosive mixtures of gases or vapours can accumulate. These gases if

ignited would cause an explosion.

Equipment installed in hazardous areas must be CE marked and have

appropriate Ex ratings. Flameproof enclosures (Exd) will hold ignition sources within them in such

a way that any ignition of the hazard inside the enclosure will not be transmitted to the

atmosphere outside the enclosure. The VESDA model VESDA smoke detector type VLX - 100

has been specially designed for use in areas where there is a high possibility of explosion.

The VLX 100 consists of a VLC (Vesda Laser Compact) detector enclosed

in a flameproof enclosure made of cast aluminium. The VLX-100 smoke detector draws air

from the monitored area through a piping network. The sampled air passes through a flame

arrestor before reaching the detector. Similarly the exhaust air from the detector passes through

the flame arrester before discharging to a designated area. The VLX 100 model can be used in

Zone 1(refer glossary) areas


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Scheme C & D where the Detector is mounted in an enclosure and the exhaust is

released into the hazardous area is recommended.


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Hazardous area: An area in which flammable substance in the form of gas, vapour or dust when

mixed with air, is present in such proportions that it can explode when in contact with an ignition

source.

Explosion Proof Apparatus: This apparatus is enclosed in a case that is capable of

withstanding an explosion of a specified gas or vapour that may occur within it .It also prevents

ignition of a specified gas or vapour that may occur within it. It also prevents ignition of a

specified gas or vapour surrounding the enclosure by sparks, flashes, or explosion of the gas or

vapour within and that operates at an external temperature that a surrounding flammable

atmosphere will not be ignited.

Flame Proof: The enclosure of the equipment will withstand an internal explosion, and prevent

passage of flame to the surrounding atmosphere.

ExD: Abbreviation for Eex d method for flameproof protection. This includes the use of

explosion-proof enclosure and flame arresters. This protection level is deemed suitable for

electrical equipment in Class I, Zone 1 or Zone 2 environments.

Class I: Locations where flammable gases or vapours are or may be present in the air in

quantities sufficient to produce ignitable mixtures.

Class II: Locations, which are hazardous due to the presence of combustible or electrically

conductive dusts in sufficient quantities for a fire or explosion hazard to exist.

Class III: Locations, which are hazardous due to the presence of easily ignitable fibres of flyings.

However, the material is not suspended in the air in quantities sufficient to produce ignitable

mixtures.

Zone 1: Where ignitable concentrations of flammable gases, vapours or liquids can exist some of

the time under normal operating conditions. (IEC adds: Typically between 10 to 100 hours per

year)

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Very Early Warning Smoke Detection