Introduction

There are two very distinct flame detection families that are used within the Oil and Gas Industry:

1.Radiation Flame Detectors: Consisting; Ultraviolet, Single Frequency Infrared, Combination UV/IR, and Multiple Frequency Infrared

2.Visual Flame Detectors: Consisting; Visual Flame

Radiation-type flame detectors collect radiation from the area under surveillance; sum the total radiation within the field of view; analysing the total intensity of the radiation and any flicker frequency that exists. The second family; Visual Flame Detectors, are based on a near IR CCTV camera with flame detection recognition algorithms built into the detector. This type of detector is spatially aware; in that it analyses each area of interest within the field of view and determines if each area meets the criteria for fire. The visible radiation from each potential fire source is analysed individually.

Ultraviolet

Radiation-type, such as Ultraviolet (UV) detectors are good general-purpose fire detectors as virtually all fires emit UV radiation. However, UV flame detection is well known for its false alarm susceptibility to arc welding, X-raying and light- ning. Seldom discussed are the factors which cause UV flame detectors to miss a fire. Hydrocarbon films, caused by oil lube sprays from gas turbines or diesel fuel, on the windows of the device render the detector blind. Even solvents in the atmos- phere have been found to inhibit the device from responding.

As UV flame detectors are prone to severe degradation by oil and smoke they should not be used in most petrochemical applications. UV flame detectors should also not be used on sites where direct or reflected flare radiation is present. Figure 1 shows a typical UV flame detector footprint with appropriate desensitization applied. Many operators do not allow UV detection to be applied for general application due to the draw- backs of the technology.

Single Frequency Infrared

Infrared (IR) detectors were introduced to alleviate the prob- lems associated with UV detectors. They operate by detecting the heat element of a fire; analysing amplitude and flicker fre- quency of the flame. IR flame detectors solve a number of the false alarm problems associated with UV detection. They are not affected by hydrocarbon films, however, black body radia- tion does cause false alarms and water on the optical surface, attenuates the heat energy from a fire resulting in decreased sensitivity of the device. The vast majority of IR devices are designed to detect the product of combustion from a hydro- carbon fire—hot CO2 emissions. This results in some devices, only being sensitive to hydrocarbon fires.

This type of device can reject transient or periodic sources of infrared radiation while remaining responsive to genuine fires. The approach cannot, however, reject infrared radiation asso- ciated with flare reflections or turbine combustion exhausts, and can result in false alarms. This detection also only allows for relatively short viewing distances even before desensitisa- tion. Within its well-understood limitations, this is a reliable and robust technology. Figure 1 shows a typical IR flame de- tector footprint with appropriate desensitisation applied.

Ultraviolet Infrared UVIR

UV / IR combined detectors are generally not considered for duty across the industry as the use of the combined technolo- gies, not only brings together the strengths of both, but also the limitations. The UV section of the device is prone to contamination by oil mist and grime and will frequently indicate fault. In an enclosure fire, smoke is likely to ‘blind’ the UV sec- tion of the detector. The detector also features the drawbacks of a single IR flame detector (false alarm to black body, blinding due to fog/ water) and, therefore, the detector is unreliable in detecting hydrocarbon fires. Figure 1 shows a typical UV/IR flame detector footprint with appropriate desensitization applied.

Multi Frequency Infrared IR3

With the advent of Multi-Frequency detectors, guard bands were added to the 4.4uM IR sensor to reduce false alarms and increase the sensitivity. The signals from the sensors are corre- lated at either two or three optical wavelengths.

These devices may be less prone to spurious alarm from black body radiation although the sensitivity of this type of detector is also reduced, sometimes by a large amount, in the presence of blackbody radiation. This reduces the effective viewing dis- tance of the detector and even then does not show the severi- ty of desensitisation in certain cases (where the viewing dis- tance can be reduced to only a couple of meters).

On many sites this type of detector has been very prone to disruption - fault, reduced sensitivity, unwanted alarm - by water/contamination in one or more of the three independent optical path(s) and reflected flare radiation. Micropack is aware of a large number of shutdowns caused by false alarm from this detection technology and operators should be wary when installing in certain areas - particularly where flare radia- tion and/ or hot surfaces may be present. Figure 1 shows a typical Triple IR flame detector footprint with appropriate de- sensitisation applied.

Visual Flame Detectors

Flame detector family type 2; Visual Flame Detectors, employ a video imaging based technique, utilising CCTV and advanced algorithms. The advanced algorithms process the live video image from the CCTV array and interpret flame characteristics.

This is a technology that provides a control room operator with real time images of each detector’s field of view, there- fore allowing a potential incident to be assessed and con- trolled from a safe distance, which in turn reduces the risk to personnel and reduces the risk of unwanted shutdown. The device operates in the near Infrared and uses extensive signal processing to detect and annunciate fires while rejecting the common sources of false alarm found within the oil and gas industry.

The emission of exhaust gases from gas turbines emit very strongly at 4.4µm; the prime detection wavelength for IR de- tectors; causing them to false alarm. As a visual flame detector is monitoring for bright burning fires visually, false alarm im- munity is assured to hot CO2 emissions.

Black body radiation, at certain high temperatures, emits strongly at 4.4um, which we learned causes desensitisation or spurious alarms with IR flame detection. The flame detection algorithms, and the wavelength at which visual technology operates at, ensures that the detector completely ignores this source of radiation and will not false alarm.

The limitation with the visual technology is that it cannot de- tect clean burning fires. This type of fire is present when Methanol, Hydrogen and Sulphur are burnt. In 2011, an independent review on loss prevention by FM Global recommended that visual imaging flame detection systems be applied as the default technology for the fol- lowing commercial and industrial applications:

• Outdoor, open areas such as oil rigs, oil fields, mining operations, and forest products
• Indoor locations such as industrial plants, boiler or other large vessel protection, turbines, and some clean/ chemical rooms

The study also recommends to use radiant energy sensing detectors to match the radiant emissions expected from the source to be detected, as required by the applicable occupancy-specific data sheet. Since each fuel emits unique spectra, not all detectors are capable of detecting all fuels. For example, the use of an IR3 to detect a methanol fire (special hazard).

Authors
This article was produced by Fire Engineering Industry Professionals from Micropack Engineering Ltd