Fire, Flame & Smoke Detectors

Presented ByAkaash Sahoo

Roll No- 210114001M.Tech in Process Control & Instrumentation

Contents

• Phases of fire• Smoke detection• Flame detection• Heat detection• Other methods of fire detection• References

Phases of fire

• Incipient Phase: Warming causes the emission of invisible but detectable gases like- CO2 , CO

• Smoldering Phase: Smoke is formed in this phase.

• Ignition Phase: Ignition temperature is reached and flames are present and therefore they emit radiation: mainly in UV or IR region.

• Heat release Phase: heat is released; the temperature of the space starts to rise rapidly.

Smoke Detection

Two types: Ionization Chamber Sensor• Used in the incipient stage for early warning.• It’s inexpensive & better at detecting smaller amount of smokes produced

by flaming fire.• Response is very fast but also causes a lot of false alarm.

Photoelectric Sensor• It is effective in the smoldering stage.• It must be maintained so that dust and dirt accumulation doesn’t cause

false alarm.

Ionization Chamber Detectors

•It contains two electrodes at different potential & alpha particle source that ionizes air in the chamber.

•The sensing part of the detector consists of two chambers - an open, outer chamber and a semi-sealed reference chamber within.

• A low activity radioactive foil of Americium 241 is mounted in the reference chamber which is an emitter of alpha particles.

• This enables formation of ion-pairs & causes current to flow between the inner and outer chambers when the detector is powered up.

• As smoke enters the detector, particles become attached to the ions, causing a reduction in ion-pairs and consequently current flow in the outer chamber.

• the drop in the ionization current is constantly monitored and an alarm is triggered when it reaches a set point.

•The ionization current reflects air composition & rises as combustible gas concentration rises.

Photoelectric Detectors

In the normal case, the light from the light source on the left shoots straight across and misses the sensor

When smoke enters the chamber, however, the smoke particles scatter the light and some amount of light hits the sensor.

Flame detection• Basis of detection: Spectral band analysis, Flickering frequency, Radiation

intensity threshold and detection algorithm.• Flame flickering frequency (5-25cps) is different from ambient light

sources and bulbs, tube (120cps) which helps in discrimination of the radiations.

• Mounting location should be carefully selected so that it has an unobstructed vision.

• Flame detectors are used when combustible gases or flammable liquids are present & ignition is instantaneous.

• The cone of vision of detectors and the fuel source is specified generally so that it does not detect potentially background noises.

Advantages/disadvantages

• Detection distance• Sensitivity• Speed of response• Range of applications

• False alarms• High Cost• Blinded by thick smoke,

vapors, grease and oil deposits on the detector’s window

Types of Flame detector UV Detectors- Detection is based on Geiger-Muller tube. When the counted

pulses exceeds the threshold value fire alarm is activated.1. Good for H2 and CH4 fuelled flames. It’s unaffected by hot objects.

2. It’s prone to false alarm and it’s blinded by thick smoke, oil droplets etc.

IR Detectors- Good for hydrocarbon based flames.> Some IR detectors have flicker and statistical analysis algorithms to minimize the effects of black body sources, a false alarm source.

UV/IR Detectors- Both UV “AND” IR sources must be present and exceeding their threshold levels to activate the alarm in one configuration.> In other, UV “OR” IR source presence can trigger the alarm.

• Dual IR- Have longer detection ranges than UV or IR sensors and are more fuel specific in their applications.

> Desensitized by high background levels of IR, reducing their ability to detect a fire

• Multispectrum IR- These detectors offer greater detection ranges and give fewer false alarms.> Different manufacturer uses different flame detection algorithm.

• Closed Circuit Television- Only sensitive to the red-green-blue spectrum and are not suitable for blue/translucent flames from such fuels as hydrogen and methanol.User is able to verify the presence of a fire before taking any action

.

Heat Detection• Oldest type of automatic fire detection.• They have the lowest false alarm rate but they are slow in response.• They can’t differentiate between heat of fire and that of furnace.• Used in confined place where rapid fire is expected or in places where

other methods fail.

Heat Detection

Fixed temperature type Rate of rise type

Fixed temperature heat detectors• Detectors are designed to alarm when the temperature of the operating

elements reaches a specific predefined air temperature.• They cover a wide range of temperature• Types of fixed temperature heat detectors: Electro-mechanical heat detectors- Contains a bi-metallic strip as a part of

electric circuit, that completes the circuit when a particular temperature is reached.

Fusible link type- An eutectic alloy is used, which is fixed on a spring. It changes from solid to liquid at a particular temperature and acts as a solder.> This enables the spring to release & actuate alarm.

Opto-mechanical type- Modern variation of electro-mechanical type.>It contain one or more fiber optic cables separated by a heat sensitive insulator. > A focused light signal is passed through the fiber optic cable. When exposed to heat, the heat sensitive insulator changes state from a solid to a molten state which has the effect of discontinuation of the focused light signal.> This actuates the alarm.

Rate of rise heat detector• As an effect of flaming fire the air temperature rises rapidly• The detector functions when the rate of rise in air temperature exceeds a

preset value; generally (12-15)F/minute.• Rate-of-rise detectors are designed to compensate for the normal changes

in ambient temperature that are expected under non-fire conditions.

Electro-pneumatic type heat detector

•It’s a rise-of-rate type heat detector.

•Electro-pneumatic heat detectors comprise a controlled vented chamber containing a diaphragm that moves due to a pressure differential according to the rate of change of the ambient temperature.

•When the ambient temperature changes faster than the calibrated rate which the vent has been designed to release, the diaphragm moves sufficiently to create an electrical circuit to indicate an alarm. •Electro-pneumatic heat detectors operates at a range of temperatures because they respond to the rate of change in temperature, not at a fixed temperature only.

Other detection methods

• Air aspirating system- It draws air draws an air sample into the detection chamber, via the pipe network.1. The sample is analyzed for the existence of smoke, and then returned to atmosphere. 2. If smoke is present in the sample, it is detected and an alarm signal is transmitted to the main fire alarm control pane

• Gas Sensors- New systems are being developed that can analyze the concentration of particular type of gases like- CO, CO2

> When the concentration exceeds a limit the alarm is triggered.

References• Process Measurement and Analysis- Volume1, by Bela G. Liptek• Heat Detectors – Principle and Operationhttp://firewize.com/page/training/heat-detectors-principle-operation• Techniques of fire detection - Richard W. Bukowski, Center for Fire

Research, National Bureau of Standards• Flame detection types (ppt)