FIRE & BILGE ELARM SYSTEMS

BY Lt NS Wickramasinghe Bsc E & E Eng(Hons )

Heat Detectors - Principle of Operation

There are four principal methods for detecting fire Heat, Smoke,

Flame Combustion detectors

Heat Detectors

There are two methods for detecting fire from the presence of heat

Fixed temperature heat detectors - operate when the ambient temperature increases sufficiently to predetermined level heat detector will operate

A rate-of-rise heat detector - operates when the ambient temperature increases over time equal to or greater than the rate of change the detector was manufactured to operate

HEAT DETECTION TECHNOLOGIES

These technologies can be broken down into four main types.Electromechanical - As the name suggests electromechanical heat detectors operate due to a combination of mechanical movement creating an electrical circuit.

Optomechanical - This type of heat detector is a modern variation of the electromechanical line-type heat detector and could be classified as an optomechanical distributed heat.

Electropneumatic - 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.

Electronic (Thermistor) - A thermistor is a type of resistor whose resistance changes significantly according to temperature.

HEAT DETECTION TECHNOLOGIES

Smoke Detectors

Ionization Detectors - Ionization detectors have an ionization chamber and a source of ionizing radiation. The source of ionizing radiation is small quantity of americium-241 which is a source of alpha particles (helium nuclei). The ionization chamber consists of two plates separated by about a centimeter. The battery applies a voltage to the plates, charging one plate positive and the other plate negative.

Advantages of Ionization Smoke Detectors

Detects invisible products of combustion

detect other aerosol Quick acting Disadvantages of Ionization Smoke

Detectors

Detects the presence of smoke only, not toxicity Has a potential for high false alarm rate Typical locations or hazards for ionization detection

Typical locations or hazards for ionization detection

Clean rooms Computer rooms Mechanical air ducts Locations

where sensitive detection methods are needed

Smoke Detectors

Photoelectric Detectors - In one type of photoelectric device, smoke can block a light beam. In this case, the reduction in light reaching a photocell sets off the alarm.

Advantages of Photoelectric Smoke Detectors

Sensitive to visual particles of smoke Detects smothering low heat fires Provide early warning

Disadvantages of Photoelectric Smoke Detectors

Early contamination by dust causing reduced sensitivity

Detects presence of smoke, not toxicity

Must be cleaned on a regular basis Has a potential for high false alarm

rate

Typical locations or hazards for photoelectric detection

Office areas Clean rooms Raised floor spaces Computer rooms Telecommunications rooms Electrical equipment rooms Sleeping rooms

Flame Detector

Flame detectors are used to detect the light radiation component of a fire. Typical detectors of this type detect the wavelength of either IR or UV or a combination of the two. These detectors are extremely fast acting and are used in areas where rapidly occurring fires or explosions could occur.

Advantages of Flame Detection

Extremely fast acting

Disadvantages of Flame Detection

Narrow field of vision Expensive Requires unobstructed field

of view Difficult to maintain

Fire-Gas Detector

These detectors respond to the various gases produced during the combustion process.

Carbon monoxide Carbon dioxide Steam Other elements

The Fire-Gas detector employs two types of technology to predict the fire. One method uses a semiconductor material that changes the metals conducting potential in a fire situation. The other method uses a catalytic element encased in an Aluminium bead.

Advantages of Fire-Gas Detection

Detects products of combustion Sensitive enough to detect levels of gases produced between the occurrences of detectable particulate levels

and detectable heat levels Detects gases prior to reaching lethal levels

Disadvantages of Fire-Gas Detection

Can be prone to false alarms Must be mounted at a low level, leaving it susceptible to damage Can be poisoned Not suitable for areas where CO and CO2 and produced as part of

the functions within the area Cannot be considered as a universal replacement for smoke

and/or thermal detectors High cost

Air Sampling Smoke Detectors

For environments where detection of smoke is most critical, an air-sampling system provides the earliest possible detection. An air sampling or aspirating type fire detection system is a self-contained smoke detection package compromised of five primary components

Air-sampling system Aspiration system Filter assembly Detector Control system

BILGE PUMP

Bilge pumps are placed in the hold or lower levels of a ship to remove accumulated water from the lowest portions of the vessel. Ship or boat owners install bilge pumps at the lowest possible level of the ship below the sea surface in an area called the bilge. The bilge forms the joint where two sides of a ship or boat connects.

PROCEDURE OF INSTALLINGMount the pump - You must not place the pump in the bilge

unrestrained. If it falls over it will suck air and burn out.

Use smooth-bore hose - Corrugated hose reduces pump output by as much as 30%, so always connect the pump to the discharge fitting with hose that has a smooth interior surface.

Place the discharge above the waterline - If the discharge is submerged when the pump runs, the ocean siphons back

through the pump into the bilge when the pump shuts off.Connect to the battery- When you turn the power off to

leave the boat unattended, you don't want to turn off the bilge pump. Connect an automatic bilge pump directly to the battery, not through the distribution panel.

PROCEDURE OF INSTALLING