The fundamental role of a fire detection system is to detect fires at a stage sufficiently early to allow safe evacuation, and to initiate an automatic protective response based on the characteristics of the incident. notice that whereas most process protective functions are intended to prevent incidents from occurring, an incident already happens when the fire & gas system detects fire or gas-Fire Alarm and Detection System.
The fire detection system works by monitoring conditions throughout the installation by means of appropriate detectors and provides a continuous display of conditions to the control room operator. in the event of an incident being detected, it is alarmed at the control panel and also locally, and an automatic protective response is initiated according to nature, location, and verity of the detected hazard. for many fires, the response will also include the activation of fire protection systems ( note that not all fire protection systems are meant for extinguishing and some may be required for fire control, maintaining structural integrity or even tenability on escape routes ) For escapes of toxic gas, a typical response consists of alarms and evacuation of personnel.
Automatic fire detection and principles of operation :
The automatic detection systems are required for the detection of fire in its early stage to minimize the heavy fire losses. there are many types of detectors are used in fire services for the detection of fire. they are as follow
- Smoke detector
- Heat detector
- Flame or radiation detector
- Line detector
- Beam detector, etc
Principle of operation :
It works on the principle to detect the change in the protected areas due to fire and to indicate the development of fire condition automatic fire detection works on the following principle- invisible products at combustion ( fire): Heat, temperature, radiation, etc – visible products at combustion (fire) smoke, flame, etc-Fire Alarm, and Detection System.
1. Smoke detectors :
- Smoke detectors are identified according to their operating principles, namely ionization, and photoelectric type smoke detectors.
- ionization smoke type detectors: ionization detectors operate by using a small amount of radioactive material to ionize the air within a sensing chamber in the detector-Fire Alarm and Detection System.
- as smoke enters the chamber, the smoke particles become ionized and reduce the conductivity of the air between the electrodes. generally, these detectors are the lowest cost and respond to smaller smoke particles. these detectors also have the greatest false alarm rate of smoke detectors from cooking and other non-fire sources.
- Photoelectric type detectors: Photoelectric detectors operate on one of the three different principles
- Light obscuration principle
- Light scattering principle
- Cloud chamber principle
- Light obscuration principle photoelectric detectors operate by projecting a light beam into a photosensitive device. most light obscuration detectors are the beam type and are used for smoke detection of large open areas.
- Light scattering principle photoelectric detectors operate with light sources and a photosensitive device. However, unlike the light obscuration detector, the photosensitive device is not in the light beam path. as smoke particles enter the sensing chamber, light is reflected from the smoke particles into the photosensitive device causing the detector to respond.
- Cloud chamber principle detectors operate by drawing an air sample from the protected area into a high humidity chamber within the detector. if the smoke particles are in the chamber, a cloud will form. the density of this cloud is measured with a photoelectric device. when the cloud reaches a predetermined density, the detector will respond with an alarm. cloud chamber detectors are normally used where early detection is required such in computer rooms.
2. Heat detector :
- A heat detector’s sensitivity to a given fire situation depends on the gas temperature which is related to the ceiling height, the radial position of the detector, add the fire’s heat release rate. Heat detectors are identified according to their operating principles and are classified into the following types :
- 1. Fixed temperature detectors
- 2. Rate-of rise detectors
- 3. Rate compensated detectors
Fixed temperature heat detectors are the simplest type of heat detector and are designed to alarm when the sensing element reaches a predetermined temperature.generally, fixed temperature heat detectors are constructed with fusible element type, continuos line type, and bimetal type-Fire Alarm and Detection System.
Rate-of Rise heat detectors are designed to function when the rate of ambient temperature increase exceeds a predetermined value, usually 12f-15f per minute. one type of rate-of-rise detector employs pneumatic filled with air with a relief vent. when the air is heated, the air will expand with the excess volume exhausted through the vent port before the pressure can build.
Rate compensated heat detectors are designed to initiate an alarm when the temperature rises. the detector is essentially constructed with temperature-sensitive contacts within a stainless steel shell. the coefficient of expansion of the shell is different than the internal contacts, producing a signal.
3. Flame or radiation detector :
- These detectors rely on the visible and invisible radiation given off from a heat source and are usually applied in high hazard areas such as fuel loading platforms, industrial process areas, hyperbaric chambers, vaults, high ceiling, and atmospheres in which explosions or very rapid fires can occur basically three are three general types of radiation detectors :
- 1. Infrared flame detectors
- 2. Ultraviolet flame detectors
- 3. Combination IR/UV detectors
Infrared flame detectors are normally used to protect large open areas where an immediate, flame- producing fire is expected such as in the protection of flammable liquid hazard. IR detectors are constructed essentially of a lens and filter system that screens out unwanted wavelengths and focuses the incoming energy on a photovoltaic or another type cell that is sensitive to infrared energy.
Ultraviolet flame detectors are designed to respond to optical radiation in the ultraviolet wavelengths primarily emitted by higher intensity flames. one drawback is that solar radiation can extend to as low 2900 angstrom, while the detector must be able to respond to fire-induced optical radiation below 2900 angstrom. most detectors manufactured are effective in discriminating between solar and fire-induced radiation.
Combination IR/UV detectors are using both of the flame-sensing principles described above for greater discrimination between fire and non-fire radiation sources.
4. Line detector :
The line-type consists of metal tubing, in a loop configuration attached to the ceiling or sidewall near the ceiling of the area to be protected.lines of tubing are normally spaced no more than 30ft apart, not more than 15ft from a well, and with no more than 1000ft of tubing on each circuit. also, a minimum of a least 5 percent of each tube circuit or 25ft of the tube, whichever is greater, must be in each protected area.
5. Beam detector :
- An optical beam detector is a device that uses a projected beam of light to detect smoke across large areas, as an indicator of fire. they are used to detect fires in a building where standard point smoke detectors would either be uneconomical or restricted for use by the height of the building. optical beam smoke detectors are often installation in warehouses as a cost-effective means of protecting large open spaces.
- optical beam smoke detectors work on the principle of light obscuration, where the presence of smoke blocks some of the light from the beam typically through either absorbance or light scattering. once a certain percentage of the transmitted light has been blocked by the smoke, a fire is signaled. optical beam smoke detectors are typically used to detect fires in large commercial and industrial, as components in a large fire alarm system.
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