Fire eyes

Flame supervision devices (including fire eyes and flame rods) are primary safety techniques that are used to avoid a fire or explosion from a flammable mixture caused... 

Fire eye (ultraviolet) High speed High sensitivity Moderate cost Potential for false alarms Blinded by thick smoke Outdoors or indoors... 

Fire eye (infrared) High speed Moderate sensitivity Easy to test manually Moderate cost Affected by temperature Subject to false alarms from the many other sources of IR radiation Outdoors or indoors... 

Older types of fire eye detector, which worked in the ultraviolet range, sometimes had difficulty distinguishing between the fire radiation and other sources of radiation, such as that from a lightning bolt. Modern detectors, many of which use infrared, generally do not suffer from this defect. 

Smoke detectors are particularly useful in those situations where the fire is likely to generate a substantial amount of smoke before temperature changes are sufficient to actuate a heat detection system and before a fire eye will detect a flame. Smoke detectors use a photoelectric beam between a receiving element and light source. If smoke obscures the beam an alarm is sounded. There are... 

Fusible links are made of low melting point materials designed to vent pneumatic systems as the fire melts the link. The depressurization can open fire deluge valves. Fusible links are very reliable, but they do require that the fire be well under way before they work, whereas otho- detectors, such as fire eyes, act more quickly. Depressurization of a fusible loop is considered to be a confirmed detection of a fire, and will automatically initiate appropriate shutdowns and activate fire protection equipment. 

For example, in furnace operation it is important that there be a check for the continued presence of a flame at the burners. If the flame goes out but fuel continues to flow into the hot furnace, an explosive gas cloud may be created in the furnace. Were this cloud to ignite on, say, a hot surface in the firebox, the furnace could be destroyed by the resulting explosion. To prevent such an explosion from occurring, it is common to install a fire eye in the furnace. This is a device that detects the presence of a flame at the burners. If the fire eye does not see a flame it sends a signal that cuts off the flow of fuel. Since it is vital that this shutdown system works properly, a second fire eye is often installed. Then, if the first fire eye does not work for some reason, the second fire eye will detect the problem and shut down the system. 

Fire eyes can generate false alarms. If the instrument has internal problems, or if there is smoke or dirt in the furnace, the fire eye may (incorrectly) determine that the flame(s) have gone out, leading to a spurious or nuisance system shutdown. In such situations, it is possible to enhance both safety and reliability by installing three fire eyes, and put their signals through a voting system, in which two of the three must detect a flame-out condition before the furnace is shut down. 

The addition of safety equipment to a process may lead to an increase in the number of shutdowns. For example, adding fire eyes to a furnace will improve its safety because they will warn of situations where a burner flame has gone out. However, fire eyes may fail internally, thus leading to spurious alarms and trips. 

A fire eye or flame detector detects the radiation from a flame. It requires line-of-sight capability. There must be no blockages between the instrument and the potential fire locations. A fire eye s field-of-vision usually covers a larger area than that of a heat detector, but it will not detect a smoldering fire as quickly as some smoke detectors. 

Fusible links are very reliable but they do require that the fire be well under way before they work, whereas other detectors, such as fire eyes, act more quickly. 

Flame detectors are not affected by airflow characteristics. They are suitable for inside or outside use, but they must be shielded from external sources of ultraviolet or infrared radiation such as welding arcs, lightning, or radiating black bodies such as hot engines or manifolds. Ideally, the fire detection system should have more than one fire eye detecting a fire so that false alarms can be weeded out. Flame detectors can be... 

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