Fire Detection System Interview Questions and Answers


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Fire Detection System Interview Questions and Answers

1.What is a ‘fire triangle’?

A fire triangle represents the three elements, which causes a fire in a combustible mixture. The three elements are fuel, air and ignition.

2.What is importance or a ‘hood’ on a gas turbine?

During a gas turbine normal running condition, a hood provides:
· prevention of turbine high dB noise to outside areas
· keeps the gas turbine clean from external dust
· provides a draft for the gas leak to the exhaust through the hood fan
During a gas turbine shutdown condition a hood provides:
· cooling the turbine body by way of the hood fan
· During a fire shutdown it facilitates to put out the fire by confining the fire extinguishers on the gas turbine.
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3.Why do we use ‘Halon’ as a fire extinguisher in a gas turbine hood?

Halon is stored in liquid form in a cylinder. When it is released in a hood during the occurrence of the fire, it discharges the halon in gas from. Halon does not act or react on electrical components.

4.What is the expansion from of B.C.F?
The expansion form of B.F.C is Bromo chloro floride.

5.In a Ruston gas turbine, why are there two Halon cylinders in each bank? How do they function?

The first bottle is the ‘first shot’ and the second bottle is ‘Extended shot’. The first bottle discharges the Galon into the hood through a 1’’ pipe in approximately 15 seconds, where as the extended bottle discharges the Halon through a ¼’’ pipe for another ½’’hour period to maintain the inert atmosphere.

6.How many UV detectors are installed in Solar, Ruston TB-5000 and Ruston TA-1750 gas turbine hoods?

Solar gas turbine hood: 4 UV detectors
Ruston TA-1750 hood: 4 UV detectors
Ruston TB 5000 hood: 12UV detectors

7.What is voting Logic of UV detectors in Soar and Ruston gas turbines?

There are two voting logics, they are: 1 out of 4 UV s and 2 out of 4 UV s

8.What happens when UV detectors detect a fire?

1out of 4 UV s: creates annunciation, audible alarm on the control panel and siren in the field (refer to the station drawings for the exact function and operations).

2 out of 4 UV s: creates annunciation, audible alarm on the control panel, a siren in the field, shutdown of the turbine and release of the fire extinguisher (refer to the station drawings for the exact function and operations).

9.How much is the time delay between fire sensing by a UV detector and confirming with an alarm?

Generally it is set for 4 secs. The UV detectors initiates a fire alarm only when the UV is detecting the fire continuously for 4 secs (refer to the station drawings for the exact settings parameters).

10.What are the Halon manual release facilities available on a Solar gas turbine?

Auto Halon cylinder can be discharged manually from the fire & gas control panel in the control room. On instiating the manual release:
· The unit shuts down.
· The auto Helon cylinder gets discharged in the hood.
· Auto halon discharge confirmation and the auto Halon cylinder pressure low alarm appears on the control panel.
· Audible alarm in the control room and siren in the field occurs.

Manual Halon cylinder can discharged manually from the field through a pall string. On initiating the manual release:
· The unit shutsdown.
· The manual Halon cylinder gets discharged in the hood.
· Manual Halon discharge confirmation and the manual Halon cylinder pressure low alarm appears on the control panel.
· Audible alarm in the control room and siren in the field occurs.

11.Why are there ‘heat switches’ in side the hood, when there are UV detectors?
Heat switches are a mechanical type and they are considered to be a positive type of fire detection system. The heat switch settings are much higher than the hood temperature.

12.What maybe the reasons if a unit shutdown on false heat detected alarm?

The reason could be:
· Hood ventilation fan has failed or stopped.
· Major hot gas leak inside the hood.

13.What happens when a heat switch actuates?

On detection of heat, the heat switch initiates the following.
Annunciation, audible alarm on the control panel, a siren in the field, shut down of the turbine and release of fire extinguisher (refer to the station drawings for the exact settings parameters).

14.Wxplain the operating principle of a gas monitor?

Gas monitor measures the imbalance in the current loop caused by its active and in-active filaments in the presence of a combustible gas.

15.Explain the calibration procedure of a gas monitor system.

· Gas sensor loop current or voltage at the sensor head is set as per the manufacturer’s recommended value.
· Gas monitor zero is adjusted to the instrument air.
· A test gas with a known quantity of combustion gas (Generally methane 2% by volume) is fed to the sensor and the span is adjusted to read 40% on the monitor scale (refer to the station drawings for the exact settings parameters).
· The calibration procedure is repeated until the zero and span reads correctly.

16.What are the alarm and shut down setting on a gas monitor?

Generally on the gas monitor, the alarm is set at 20%rising and the shut down is set at 60%rising (refer to the station drawing for the exact settings parameters)

17.What has to be done prior to entering a gas turbine hood?

Prior to entering a gas turbine, turn on the ‘ventilation defeat switch’.

18.What is a ventilation defeat switch?

On activating, the ventilation defeat switch inhibits the release of Halon in the auto mode and also the unit shutdown on ‘ventilation failure’.

19.How much is the delay between fire detection and Halon release? Why is the time delay required?

A time delay of 15 secs. Is set between the detection of fire and the initiation of Halon release. This is to achieve effective fire extinguishing by allowing the hood fan to run-down to zero speed and the bleed valves to release the compressed air.

20.What does a hood ventilation fan do when a fire is detected? And why?

On detection of fire, the fire system initiates the hood fan shutdown. This is to minimize the presence of air the hood for releasing the extinguisher.