Basics of Fire Detection System

Fire Detection

In general, the fire detection system shall have the following objectives:

-          To detect fire in very early stage of it’s form.

-          To alert operator at control room and give them the location.

-          To alert any personnel at location of the fire event so that they are aware and could take any    

            necessary action.

-          To activate the fire fighting equipment.

Based on these, there are four main types of the fire detection devices (fire detector):

-          Smoke detector.

-          Heat detector.

-          Flame detector (detect the UV or IR radiated by fire).

-          Multi-detector (combination of smoke and heat).

Each detector has a unique purpose and suitability for a specific application. Thus the detector type should be selected based on the safety (loss prevention) study. It will depend on the speed of response required, consequences to facility, and the area of the fire events.

Smoke detectors

The most used smoke detectors are ionization type and photoelectric type. The ionization type utilizes a constant current that produced by electrode from an ionization of some isotope. Any particle that goes through the chamber and interrupt the ionization will make a reduction of constant current. Thus alarm will be activated. The photoelectric type utilizes a scattered light from smoke particles to measure the smoke present. Any particles that interrupt the light line (it will be scattered) will cause the detector activated.

The smoke detector (both ionization and photoelectric type) are very sensitive and shall not be used at dirty environments, smoky atmosphere, and open areas where there is an unpredictable air flows. For example, smoke detector shall not be used at kitchen, mechanical workshop, etc.

Another type of smoke detectors that most used at offshore oil & gas industries are High Sensitive Smoke Detectors. This detector usually placed on the very critical areas such as cable ducting, control room, and electrical room. This detector take a sample of each sample location by an air-sampling tubes and then draw the sample to a centralized detector which analyze any small present of smoke. It will allow the system to give very fast and earliest alert to an operator of fire event (event it still on the smoke forming stage).

Flame Detectors

Flame detectors are categorized as fast response devices. It senses the flame on a line of sight. A fire consists of three part, smoke, heat, and flame. This detector is utilizing at application when flame is the first part of fire that formed, such as a hydrocarbon leak area.

There are three major types of flame detectors:

-          Ultra-Violet (UV) Flame Detectors

-          Infrared (IR) Flame Detectors

-          Combination of UV and IR Flame Detectors

Ultra-Violet (UV) Flame Detectors

UV type flame detectors are detectors that sense the UV light emitted from a flame. It’s sensitive to a sun UV light, welding arcs, x-rays, and lightning. Therefore the use of UV flame detector is limited on the closed area where the disturbance from sun, lightning etc is absent such as a turbine compressor closure, turbine generator closure etc.

UV flame detectors are also shouldn’t use to detect fire that have heavy smoke (i.e. crude oil) or fires that doesn’t have visible flame (i.e. alcohol etc).

Infrared (IR) Flame Detectors

Infrared flame detectors are detectors that sense the hot light (infrared) emitted by a CO2 product of flame. Its very reliable than UV flame detector because of more immune to UV light, welding arcs, etc. It can be used in open areas or closed areas. The infrared flame detectors signal shall be filtered from hot human body and set it to insensitive to hot body.

Combination of UV & IR Flame Detectors

UV/IR Flame Detectors use both UV and IR sensor to sense a flame. It should be selectable to be UV only or IR only or both sensors active. It should be detect any flame whether indoor or outdoor applications. If both UV and IR sensor are active, it will only send alarm signal when both of it sense a fire. It will reduce the false alarm that caused by using UV only or IR only.

Heat or Thermal Detectors

Heat detector is slower device compare to smoke detector and flame detector. This detector utilize a heat sensitive element (usually are thermistor) to sense the heat. It will activate only after the fire reaches some significant stage to radiate its heat energy and sensed by the heat sensitive of heat detectors. Even though this detector has slow response to fire but it’s very suitable for application that prone to alarm false if we use smoke or flame detectors such as kitchen area, workshop, laboratory room, smoking area etc.

There are three types of heat detectors:

-          Fixed type.

-          Rate of rise type.

-          Fusible plugs.

Fixed heat detectors

A fixed heat detector is a detector that set at a fixed temperature set point. Once the ambient temperatures (caused by a fire) reach the set point, the detectors will send an alarm signal. As a standard, the fixed heat detectors set point is 77 Celsius. Fixed heat detectors are very suitable for application that has a swings temperature on it i.e. kitchen, workshop, laboratory room etc.

Rate of rise heat detectors

Rate of rise heat detectors is a detector that senses the rate of rise in the ambient temperature. It’s very sensitive heat detectors and can cause a false alarm at a room having swing temperature. Normally it will activate when the heat rate of rise is between 12 Fahrenheit or 15 Fahrenheit. It’s suitable to use at smoking room, warehouse etc. It’s also suitable to use at room that doesn’t have a high air flow that can cause a smoke detector slow to response such as storage room or warehouse.

Fusible Plugs

Fusible plugs are a metal cylinder that has a sealed metal with low melting point. It is connected to a pneumatic tubing loop. When the ambient temperatures reach the melting point of the seal metal, then the seal will broke and causing a pneumatic air leak through it. This leak will be detected by a pressure switch and it will initiate an alarm that activates a very large capacity valve to operate. This large capacity valves (called deluge valve) will spout a fire water to extinguish fires. The fusible plug is designed to protect a closed vessel from a fire. When a closed vessel exposed to a fire, it can causing a dangerous rise in internal pressure of vessels and causing a blow up. Therefore usually it put out around a closed vessel.