Read this article on Gas detection System Questions and Answers useful for your interview preparation to crack the job as Fire and Gas System Engineer.
Combustible gas mixture is the proper ration of fuel and air within the ignition limits.
Gas detection system measures the imbalance in the current loop caused by its active and in-active filaments in the presence of a combustible gas.
Presence of combustible gas Is the first stage of a possible occurrence of a fire.
A gas sensor, 3 wire system and a gas monitor.
Flame arrester is used on the sensors head. The sensors are continuously powered to burn the combustible gas for measuring. The flame arrester prevents the occurrence of fire outside the sensor.
There are two filaments. They are called ‘active’ and ‘non-active’ filaments.
From manufacturer to manufacture the sensor resistance varies. It is approximately 3 ohms on each of the filament (refer to the station drawings for the exact parameters).
Some gas monitors are provided with a self test facility to check its alarm and shutdown functions(levels).
The three-wire system is used to compensate the line resistance. The active and non-active filament loop form a whetstone bridge in the gas monitor. The amount of unbalance in the current loop during the occurrence of combustible gas is detected as the percentage of combustible gas.
The commonly use gas monitor systems are:
Methane gas is used for calibrating a gas monitor.
Generally methane 2% by volume is used as a test gas to calibrate a gas monitor.
Depending on the manufacturer, either it is the loop current or the voltage at the sensor head which has to be set precisely for better accuracy and sensitivity.
Halon (fire extinguishing agent) is not released on detection of combustible gas because the presence of gas is not a fire.
The presence of combustible gas in the air intake may result in the failure of stopping the gas turbine during a shutdown, due to the fuel and air mixture continuously available to the turbine.
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