Detector voting

Detector voting philosophy during normal operation as well as the start-up and shutdown periods... 

A combination of detectors may be appropriate. They may activate an alarm only, or actuate a combined alarm/extinguishment system. With a bank of detectors a voting system may be used to increase reliability and reduce the frequency of spurious alarms. Detection/alarm systems may also be interlinked with, e.g., fire-check doors held back on electromagnetic catches such that the doors close automatically upon activation of the detection system. 

There are two types of detectors under UV/IR classifications. Both of the types respond to frequencies in the UV wavelength and IR in the CO2 wavelength. In both types simultaneous presence of the UV and IR signals must be available for alarm conditions. In the simple voting device an alarm is generated once both conditions are met. In the ratio device, satisfaction of the ratio between the level of UV signal received and IR signal received must also be achieved before an alarm condition is confirmed. 

Flame response for a ratio type is affected by attenuators, while in the voting type there is negligible effect. The detectors are limited to applications involving hydrocarbon materials. 

Secondly small quantities of airborne pollutants may poison the catalyst in the detector. This severely reduces its sensitivity. The detector becomes less reliable and often makes duplication, voting logic and frequent maintenance necessary. 

They will typically have two levels of alarm 20% LFL and 60% lower flammable limit (LFL). If multiple detectors are installed in a single location then a voting system can be installed. For example, if just one 20% alarm sounds then all hot work must stop but other work can continue as normal. If three 20% alarms or one 60% alarm sounds, then an emergency response is called for. 

Scenario coverage assessment It is defined as the fraction of the released scenario that would occur as a result of the loss of containment in a defined monitored process area that can be detected by detectors considering the frequency and magnitude of release scenarios and the defined voting arrangement [21]. 

If an instrument is prone to problems, say an oxygen detector in a gas line, this is the point where a 2 out of 3 voting (2oo3) scheme is proposed. 

Another different scenario is seen in gas detector applications where a small leak is not considered serious imtil at least two locations respond with high levels of concentration. In this case voting is not for sensor reliability but rather for the logic of the physical conditions. 

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