Flame Detection Basic Principles 262ez

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FLAME DETECTION

technical note

BASIC PRINCIPLES

1.

INTRODUCTION

The aim of this technical note is to introduce the basic technology and options currently available for flame detection. This information provides a general guide regarding the application of various detector types. However we recommend expert guidance should always be sought as many applications are covered in detail by national and international standards / codes of practice which do not form part of this technical note.

2.

OVERVIEW

Several different fire detection technologies exist all aimed at providing the earliest warning of a fire incident and offering false alarm immunity to the local environmental conditions.

This document focuses on flame detection technologies suitable for hydrocarbon gas or liquid based fuels, which operate by detecting the optical characteristics of a fire or flame.

Selecting the best detection technology for a given application is based upon the type of combustible material to be detected and the environmental conditions present.

Alternative fire detection technologies such as point heat and smoke etc. are the subject of a separate technical note.

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OPTICAL FLAME DETECTION

Standard optical flame detection operates by detecting specific wavelengths of radiation i.e. ultraviolet, visible, infrared, or a combination of these (see Figure 1). Another form of flame detection uses the visible wavelength via a CCTV system. This principle of detection is still in the early stages of development and therefore will not be discussed in this document.

Typical non fire hazard radiation sources include: solar radiation, artificial lighting, furnaces, welding and lightning. Several different types of detector exist. Each type employs one or a number of detection principles along with a range of signal processing techniques all aimed at improving false alarm discrimination.

4.

Carbon Dioxide Spike Ultraviolet

Radiation intensity

There are numerous sources of naturally occurring radiation in addition to those emitted by man made equipment which do not represent any form of fire hazard but could inadvertently activate a flame detector.

Visible Light

Infrared

Ultraviolet Detection Range Infrared Detection Range

0.2µ

0.29µ

0.4µ

0.8µ

2.7µ

4.3µ Wavelength (µm)

Figure 1 - Typical Emission Spectrum of a Hydrocarbon Fire

OUTLINE SPECIFICATION OF FLAME DETECTORS

Flame detectors are available in either stainless steel, aluminium or poly-carbonate designed to withstand external corrosive atmospheres or meet local engineering preferences. Hazardous area certification is typically Exd to both US and European standards with some devices being certified intrinsically safe. Detector outputs are either: a.

Alarm relay s for fire and fault.

b.

4 - 20mA output, where 0mA = fault, 4mA = operational and 20mA = fire. Power supply is typically 24V dc.

c.

Operating current change in the same way as a conventional smoke detector would operate.

High temperature versions designed and certified to operate in temperatures up to 125oC for turbine enclosures are also available. All detectors require the use of a swivel bracket for mounting and alignment to the target area.

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5.

TYPES OF DETECTION AND SELECTION

5.1

Ultraviolet (UV) Flame Detection

This detector uses a UV detection tube which responds to UV radiation operating within the 0.18µm 0.29µm region of the electromagnetic spectrum. This is the fastest principle of flame detection with typical response times of less than 10ms. However other sources of UV radiation will also activate the detector, these include: welding and lightening. UV radiation emitted by the sun is filtered by the ozone layer and generally causes little concern although the depletion of the ozone layer in some geographical regions means extra care must be taken. The presence of contamination or oily film on the optical surfaces of a UV detector can limit the detectors performance. To offset this effect most commercially available devices include optical test facilities. These test facilities include a UV source integral to the detector which is directed through the optical window at known time intervals. The resultant signal is compared to reference signal levels to determine the level of contamination. This in turn can be used to initiate a fault alarm if contamination is to high. Applications tend to be restricted to indoor clean environments ie: gas turbine enclosures and vehicle / engine test cells.

5.2

Infrared (IR) Flame Detection

Hydrocarbon fires emit IR radiation in the range of 4.1µm and 4.6µm of the electromagnetic spectrum. This IR range is associated with the wavelengths emitted by the hot CO2 gases in a flaming fire. IR detectors operating on this principle have a typical response time of less than three seconds. Other sources of IR radiation will also activate the detector, these include: solar radiation, ovens and furnaces. To reduce the possibility of spurious alarms various techniques are employed, the most common of which is to look for the frequency of flicker within a flame in conjunction to the intensity of IR radiation. Although this improves the level of false alarm discrimination it does not compensate for all external sources of IR radiation. This detection principle is more tolerant to the presence of contamination and unaffected by oily film on the optical surfaces of the detector, and is therefore more suitable for outdoor applications

5.3

Multi Wavelength IR Flame Detection

These detectors operate by looking at infrared radiation at two or more specific but separate bands of IR radiation. One type of detector compares the separate wavelengths of IR radiation generated by the target hydrocarbon fire. Whilst others look for the fire along with background levels of IR radiation at a different wavelength. Both types are designed to improve false alarm discrimination, and do not suffer from any of the limitations associated with UV detectors. The main limitation of this type of detector comes from the atmospheric absorption of IR radiation at the detectors target wavelength of 4,3mm. This aspect limits the detectors long range capability. Although less prone to spurious alarms generated from solar radiation this still remains a prime consideration when locating devices outside.

5.4

Ultraviolet / Infrared (UV/IR) Flame Detection

These detectors employ both the UV and IR detection principles as stated above. This configuration is designed to improve false alarm discrimination from radiation generated from sources other than hydrocarbon fires. Comparative signal processing of the detector outputs is used to add an additional level of false alarm discrimination.

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Performance limitations are concerned with the likelihood of non fire generated UV and IR sources occurring at the same time or of optical contamination which could adversely effect the signal ratio between both types of detector. This type of detector is the most widely used configuration being suitable for both indoor and outdoor applications ie offshore oil and gas platforms and onshore petrochemical plants.

5.5

Application Selection Chart

Type of Detector

Advantages

Disadvantages

Application

Ultraviolet (UV)

Fastest speed of response, Affected by external UV Highest sensitivity, sources. Automatic self test. Blinded by thick smoke or oily film.

Indoor.

Infrared (IR)

High speed. Moderate sensitivity.

Affected by temperature and sources of IR radiation. eg sunlight, furnaces.

Indoor.

Multi Wavelength Infrared (IR)

Moderate speed, Moderate sensitivity, Good false alarm discrimination.

Limited operating temperature range. Affected by some sources of IR radiation.

Indoor / outdoor.

Combined UV/IR

High speed of response, High sensitivity, Wide temperature range, Auto self test.

Dense smoke detection range.

Indoor / outdoor.

can

limit

ZELLWEGER ANALYTICS LTD. Zellweger Analytics Ltd. is a member of the Zellweger Luwa international group of companies, based in Switzerland. Backed by a worldwide network of after-sales service facilities, the company manufactures high quality safety and environmental pollution monitoring instruments under the product names of Sieger, Polymetron, Neotronics, Solomat, MDA Scientific and Astro. Each is a well known brand in its own right, but together they represent more than 150 years of experience and applications know-how in the detection and monitoring of toxic and flammable gases, fire, and the measurement of pollutants and other parameters in liquids.

Please Note: While every effort has been made to ensure accuracy in this publication, no responsibility can be accepted for errors or omissions. Data may change, as well as legislation, and you are strongly advised to obtain copies of the most recently issued regulations, standards and guidelines.

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OF THE ZELLWEGER ANALYTICS DIVISION INTERNATIONAL ENQUIRIES Zellweger Analytics Limited Hatch Pond House 4 Stinsford Road Nuffield Estate Poole Dorset BH17 0RZ, UK Tel: (44) 1202 676161 Fax: (44) 1202 678011 Email: [email protected]

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