Linear Heat Detection With Optical Fibres

The Series 5500 FOF Linear Heat Detection Systems consist of:

Electro-Optic Transducer (EOT).
Fusible Optic Fiber (FOF) Sensor.
Fire Alarm Control Panel.

The EOT injects light into one end of the FOF Sensor and memorizes the intensity of light exiting the other end. When the Sensor is subjected to heat, it begins to melt or fuse, thereby reducing its optical transmittance.

The EOT detects this loss of optical transmittance and interprets it as being indicative of the fusion of the FOF Sensor due to an overheat or fire condition (U.S. Patent 4,712,096). The EOT also supervises the FOF Sensor for continuity and renders a trouble signal if the fiber is abraded or broken. The EOT is self-calibrating thereby eliminating the need for manual calibration. Through the process of Integral Automatic Initialization (IAI), the EOT automatically adjusts for:

Type and Length of FOF Sensor.
Number and Quality of Splices.

Protecting Electrical Switchgear

Electrical Switchgear present a very unique Fire Detection scenario since conventional Fire Detection Systems are subject to interference by the strong fields generated by the high voltages and current densities present within their enclosures.

Conventional Electrical Linear Heat Detection Systems may not be reliable since the electrical interference between the Detector and the Switch-gear can cause false alarms leading to downtime. Also, Switchgear often have forced air cooling systems where the air supply is derived from unfiltered production facility air thus creating a "dirty" environment within the enclosure which makes Ionization and Photoelectric type Smoke Detectors unstable and hence, unreliable. Further, while Spot type Heat Detectors can survive the dusty environment and are usually not affected by electrical interference, they do not have the fast response time that is required.

The key to preventing Switchgear enclosure fires lies in early detection. Timely detection prevents overheat conditions from developing into a fire which may consume not only the effected enclosure but also adjacent enclosures. A fast acting and reliable detection system can only be achieved with the detector mounted within and in physical contact with or in close proximity of each Switchgear component.

The Series 5500 FOF Detectors have certain unique characteristics which make them ideal for detecting overheat and fire conditions within Electrical Switchgear enclosures. Since optical fibers transmit light, the FOF Detection System is completely immune to electrical interference. The high voltages and high current densities present in Electrical Switchgear do not affect the light transmittance through the optical fiber. The low melting point, low thermal mass, and the ability to be placed in physical contact with the equipment to be protected makes the FOF Detector the most responsive of all detection means available. The FOF Sensor can be placed in physical contact with buss bars, thyristors, triacs and other components which generate heat. To achieve timely detection, it is important that the FOF Sensor be placed in physical contact or, at the very least, in the immediate proximity of the Switchgear component.

When using a FOF Detection System for protecting Switchgear Enclosures, several aspects must be taken into consideration:

Location of the EOT unit.
Looping the FOF Sensor.
Detection Zones.
Electrical Supply to the EOT.

For most applications, one (1) FOF Detector should be used for each Switchgear Enclosure.

The EOT should be mounted outside the enclosure(s) in compliance with NFPA 72 and NFPA 70. The FOF Sensor is then run through the Enclosure to be protected. Using nonmetallic cable ties or other approved fasteners, the FOF is looped around critical equipment and connected back to the EOT. The FOF Sensor should be looped in a manner such that, as far as possible, it is in direct physical contact with components which have a history of thermal failure. However, the exact positioning should be determined at site based on the internal layout of the Switchgear components.

Protecting Electrical Floating Roof Tanks

The ODTI Series 5500 Fusible Optic Fiber Linear Heat Detector provides very reliable and cost effective detection of Floating Roof Tank fires. The key to preventing Floating Roof Tank fires lies in early detection of the overheat condition which may develop into a fire and consume not only the effected Tank but also other Tanks in the area. An ideal detector is one that is in physical contact with the Tank Seal. The characteristics of the Series 5500 FOF Detectors make them ideal for detecting overheat conditions within Floating Roof Tanks. Since optical fibers do not transmit electricity, the FOF Detection System is Intrinsically Safe. The low melting point and thermal mass with the ability to be placed in physical contact with the Tank Seal makes the FOF Detector the most responsive of all detectors.

The EOT may be located either outside the Dike Area, on the Floating Roof or at the top of the Tank Shell Ladder. When the EOT is mounted outside the Dike Wall, the FOF Sensor run from the EOT to the Tank must be protected by metal conduit. When the EOT is mounted on the Roof, the wiring must be protected from damage since the Roof level also changes with the level of stored fuel. The Detection wiring must be shielded and the shield must be grounded. When the EOT is mounted on the Tank Shell, it must be protected from lightning strikes.

The FOF Sensor must be positioned relative to the tank seal to be as close to the flame as possible. In Tanks where the top seal does not flip up and down as the roof moves, the FOF Sensor can be mounted directly on the top seal, positioned on the center-line between the roof edge and the tank shell. In cases where the upper seal is mobile, the FOF Sensor must be mounted above the center-line of the seal with a bracket. The FOF Sensor must be protected from abrasion and tangling due to roof level changes. A 1 inch (2.5 cm) diameter self-coiling PVC hose may be used to route the FOF Sensor from the top of the Tank Shell to the Roof. The FOF Sensor must be routed from the EOT to the seal in such a manner that it will not be crushed or broken by people walking on the tank roof. Sufficient FOF Sensor should always be kept as spare so as to be able to restore the System at any time.

Protecting Electrical Coal Conveyor Belts

Coal Conveyor Belt fires are often caused by the overheating of conveyor idler bearings. Factors such as lack of lubricant, accumulation of grease and dirt or mechanical breakdown, cause the bearings to lock. The resultant friction between the non rotating idler and the moving belt causes the bearings to overheat.

As the bearing overheats, it has the opportunity to ignite the coal dust which has accumulated on the conveyor support structure. Within a short time period, this ignition can spread into coal being conveyed and develop into a sustained fire or explosion, ultimately destroying the conveyor belt and its support structure.

The key to preventing catastrophic fires lies in early detection which can prevent the overheat condition from developing into a fire that consumes the entire conveyor structure. A fast acting and reliable detection system can only be achieved with the detector mounted in physical contact with or in close proximity of each idler bearing.

The Series 5500 Fusible Optic Fiber (FOF) Linear Heat Detectors have been designed keeping the above in mind.

The low melting point, low thermal mass, and the ability to be placed in physical contact with the equipment to be protected makes the FOF Detector the most responsive of all detection means available. The inherent flexibility of optical fibers makes it possible to attach the FOF Sensor to the bearing support structure. As the bearing overheats, the FOF Sensor fuses and renders an alarm signal.

The FOF detector is capable of responding to overheat conditions on conveyor rollers long before a fire, it is necessary to locate and repair or replace the overheated bearing(s). Consequently, a conveyor should segmented into a number of detection zones with individual FOF detectors mounted on both sides of the conveyor. The number of detection zones and detector locations should be determined based on the conveyor length and FOF Sensor length required to accommodate crossover and termination lengths.

Protecting Electrical Cable Trays

Fires in Cables and Cable Trays are generally caused by overheating due to electrical faults, welding etc. When the Cable starts to burn, it has the opportunity to ignite adjacent Cables on the tray. Within a very short period of time, the fire can spread to the adjacent trays ultimately destroying the Cable Tray, its support structure and potentially the entire Cable vault.

The key to preventing such catastrophic fires lies in the early detection of the overheat condition. A fast acting and reliable detection system can only be achieved with the detector mounted in physical contact with or in close proximity of the Cable Tray.

To achieve timely detection, it is important that the FOF Sensor be placed in physical contact with or in the immediate proximity of the Cable/ Cable Tray support.

Even though, the FOF detector is capable of responding to overheat conditions on Cable Trays before a fire, it may be necessary to clean the Cable Vault and locate and replace the overheated Cable(s).

The Cable Vault should segmented into a number of detection zones. The number of detection zones and detector locations should be determined based on the Cable Vault length, the number of Cable Trays, the number of tiers per Cable Tray and FOF Sensor required to accommodate crossover and termination lengths.

In addition to providing audio-visual alarms, the FOF Detection System can be used to actuate localized extinguishing systems and relays to trip the conveyor belt. These factors are generally considered in zoning FOF detectors for protecting Coal Conveyor Systems. The Series 5500 FOF Linear Heat Detector may use either one of the following FOF Sensors:

The Silica Core Model 5500-50 FOF Sensor is designed for a Fusion Temperature of 300 C (572 F) with a maximum length of 1500 M (4500 ft) and is provided with a Tefzel buffer and a PVC jacket.

The Acrylic Core Model 5500-51 FOF Sensor is designed for a Fusion Temperature of 140 C (284 F) with a maximum length of 100 M (300 ft) and is provided with a Polyethylene jacket.