BRHS_Chap5_Detection measures version _0_9_1
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1. The following diagram schematically presents a catharometric detector Measurement Cell Wheatstone Zero Bridge adjust Reference Cell Fig Error No text of specified style in document 3 catharometric detector Semiconductor sensors The support material of the redox reaction is no longer a metal but a n or p type semi conductor of metal oxide SnO gt ZnO etc Its conductivity is caused by shortages of oxygen oxide not exactly stoechiometric These redox reactions or simply adsorption reactions on the surface change material resistance by modifying the number of oxygen shortages The material is heated similar to the catalytic pearls but the measurement is different The resistance variation of the material itself is measured and not that of the heating element This resistance variation is connected to hydrogen concentration by a non linear correlation The following figure schematically presents a semi conductor detector co co O H CH H O NO O H O C H co Selector membrane 5nm Detection Layer Wheatstone 500nm Bridge Fig Error No text of specified style in document 4 semi conductor detector FED Field effect Gas Sensor This sensor type is based on a metal oxide field effect transistor Hydrogen diffuses into the transistor bulk and changes dependent on hydrogen concentration its electrical properties Hydrogen presence induces an increase of the threshold voltage and a decrease of2. BiennalBeportOn HudrogenSafety V NP AP f HYDROGEN SAFETY BARRIERS a et ae BiennalReportOnHydrogenSafety Table of content L2 Deteccion ie ACS io isch ich i i r a a a i i E E A aes seaessbaeasiaats iineoa 2 1 2 1 Detection of explosive ae ji i i ji o kilis tad Aiks base 2 1 2 1 1 References and SOUTCES sssssssnssssisssrtssstsssrtssnesssrtssnisssrtsenesssetssntsssrtssnissnrtssnissnrtssntsssrtssntnsneesanensa AEN 1 2 2 1 Expected performance of hydrogen fire detectors lasas Ii das io ais sa 13 1 2 1 2 Possible means to detect a hydrogen fire ai abia 1 3 Conclusion Ei ass E A AE a AE AAA eer ee rere E 16 tesa 67 kdk kA 1 2 DETECTION MEASURES Contributing author Main contributions Organisation e mail gt A k t hofer d Armin Kessler Chapter coordinator T pete essen 1Ge Pao bo ed S bastien Bouchet Detection techniques INERIS Sebastien bouchet ineris fr I Paolo Castello Detection of explosive atmosphere FRE Lionel Perrette INERIS Lionel perrette ineris fr Flame detectors Lois Boon Brett C TNO Boon Brett pml tno nl Maintenance detection layout Contributing reviewer Information reviewed Organisation e mail Peter de Brujin TNO Peter debruijn tno nl Stuart Hawksworth HSE HSL Stuart Hawksworth hsl gov uk Thomas Hiibert BAM Thomas huebert bam de 1 2 1 Detection of explosive atmosphere
3. Electrochemical 10 000 ppm Slectivity 10 lt 1 min 05 h L mW low Hydro carbons Combustible Catalytic bead 100 LEL gases amp Vapors 10 o lt 0 5 min 5 min 1W medium low selectivity i CH CO He Heat conduction avn ae 05 P lt 0 5 min iin 10W medium Catharometer high selectivity Heated i 100 LEL low selectivity 5 lt 0 5 min 5 min 25 mW Very low Semiconductor PAK KAS 9 EA 5 r Kk rre ia 67 y 2 Field Effect 30 000 ppm High selectivity 0 lt 10s 1 min medium Transistor Ultrasonic 100 Vol Low selectivity 0 lus is Pee Cae ila Very high 0 1 min 3h Very high S O Very high 04 7 10 ms 6h Very high MEMs 10 ppm 100 ow selectivity 0 his Vol Fig Error No text of specified style in document 12 typical sensor properties References and sources 1 Antoinette ACCORSI Explosim tres d tecteurs de gaz Techniques de l Ing nieur R 2380 2 Didier JAMOIS Caract risation des dysfonctionnements des d tecteurs de gaz provoqu s par la pr sence de polluants sp cifiques dans l atmosph re Rapport Minist re de l Environnement et de l Am nagement du Territoire 1997 3 Shiou Ying CHENG A hydrogen sensitive Pd GaAs Schottky diode sensor Materials Chemistry and Physics 78 2002 pp 525 528 4 Jihyun KIM B P GILA G Y CHUNG C R ABERNATHY S J PEARTON F REN Hydrogen sensitive GaN Schottky diodes Solid State Electronics 47 2003 pp 1069
4. June 2005 20 D K BENSON C E TRACY G A HISHMEH P A CISCEK Se Heeh LEE Roland PITTS D P HABERMANN Low cost fiber optic chemochromic hydrogen gas detector 1999 Proceedings of the U S DOE Hydrogen Program review 19 Michel MOLIERE Philippe COZZARIN GE Energy France S bastien BOUCHET INERIS Philippe RECH Technicrea GE sub contractor Catalytic detection of fuel leaks in gas turbines unit gaseous and volatile hydrocarbon based fuel ASME GT June 2005 1 2 1 3Detection layout As a colourless odourless and tasteless gas hydrogen cannot be detected by human senses and so means should be provided to detect the presence of hydrogen in places where leaks and or accumulations may occur The hydrogen detection system should be compatible with other systems such as those for fire detection and fire suppression Hydrogen detection devices themselves should not be a source of ignition and the response times of these devices should be as rapid as possible Some important performance factors which should be considered when selecting a hydrogen sensor for a particular application include Response time Detection range Durability lifetime Calibration maintenance Cross sensitivity specificity Area coverage The correct location of reliable sensors is crucial for timely detection and warning of hydrogen leaks before an explosive mixture is formed Recommended locations ISO TR 15916 2004 for sensors includ
5. The major hazards due to an unwanted release of hydrogen are connected with the possibility of building up explosive conditions In this respect hydrogen is potentially more hazardous than other conventional fuels methane propane or their vapors gasoline in most confined situations because of its large flammability and detonability ranges and of its low ignition energy R F Cracknell 2003 Although its high buoyancy makes the risks connected to an unwanted release likely to decrease rapidly to acceptable levels in outdoor situations and or in the presence of adequate ventilation the deployment of an adequate system for the detection of explosive atmospheres should always be taken into consideration as a possible safety measure In regulatory terms the issue is covered within the existing legislation for the safe use of flammable and explosive gases in general Alongside other protection measures the European Parliament and Council Directive 1999 92 EC on the minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospheres Directive 1999 92 EC prescribes that Where necessary workers must be given optical and or acoustic warnings and withdrawn before the explosion conditions are reached It follows that the necessity of putting a detection system in place needs to be estimated as part of a preliminary analysis of the operational hazards posed by the use of flammable gases
6. Basic considerations for the safety of hydrogen systems Technical Report First edition 2004 NASA Safety standard for hydrogen and hydrogen systems Guidelines for hydrogen system design materials selection operations storage and transportation NSS 1740 16 1997 Directive 94 9 EC of the European Parliament and of the Council of 23 March 1994 on the approximation of the laws of the Member States concerning equipment and protective systems intended for use in potentially explosive atmospheres Official Journal of the European Communities Series L 100 19 4 1994 p 1 European Standard EN 50073 Guide for the selection installation use and maintenance of apparatus for the detection and measurement of combustible gases or oxygen European Committee for Electrotechnical Standardization CENELEC April 1999 S International Standard IEC 61779 6 Guide for the selection installation use and maintenance of apparatus for the detection and measurement of flammable gases International Electrotechnical Commission IEC First edition 1999 1 2 1 4 Maintenance of detectors A detector includes two elements a sensor and a transducer The sensor is the sensitive element responsible for converting a physical measure e g gas concentration into a useful output signal The transducer turns the output signal into meaningful information displayed by the user interface Sensor or and transducer ageing may cause drift in time Maintenance is
7. 1073 5 X BEVENOT A TROUILLET C VEILLAS H GAGNAIRE M CLEMENT Hydrogen leak detection using an optical fibre sensor for aerospace application Sensors and Actuators B67 2000 pp 57 67 6 Boonsung SUTAPUN Massood TABIB AZAR Alex KAZEMI PD coated elastooptic fiber optic Bragg grating sensors for multiplexed hydrogen sensing Sensors and Actuators B60 1999 pp 27 34 7 Massood TABIB AZAR Boonsung SUTAPUN Rose PETRICK Alex KAZEMI Highly sensitive hydrogen sensors using palladium coated fiber optics with exposed cores and evanescent field interactions Sensors and Actuators B56 1999 pp 158 163 8 Petr Tobiska Olivier HUGNON Alain TROUILLET Henri GAGNAIRE An integrated optic hydrogen sensor based on SPR on palladium Sensors and Actuators B74 2001 pp 168 172 9 X BAO Jabulani DHLIWAYO Nicol HERON David J Webb D A JACKSON Experimental and therorical studies on a distributed temperature sensor based on brilloin scattering Journal of Lightwave Technology vol 13 n 7 July 1995 pp 1340 1347 10 D UTTAMCHANDAN B CULSHAW M S OVERINGTON M PARSEY M FACCHINI L THEVENAZ Distributed sensing of strain in synthetic fibre rope and cable constructions using optical fiber sensors Fiber Optic sensor Technology and applications September 1999 pp 273 275 11 Alex A KAZEMI Fiber Optic Hydrogen Detection System Fiber Optic sensor Technology and applications September 1999 pp 507 515 12 M A NAIBING A no
8. and the reproducibility of the measurement a third chemically non active electrode is added to the cell A potentiostat carried out using an operational amplifier is used to maintain the potential of the working electrode at the same value as this third electrode called the reference electrode The following figure schematically presents an electrochemical detector Aye f 2 s k kak AAA E PKT ee Membrane Diffussion barrier ze Potentiostat electrode Reference electrode Counter electrode Fig Error No text of specified style in document 1 electrochemical hydrogen detector Catalytic bead sensors The detection principle is based on combustion heat measurement of combustible gas at the surface of a metallic catalyser This means heating a pearl covered with a catalyser called pellistor or catalytic pearl or even a platinum filament by Joule effect and to measure the electric power Combustion of gas molecules at the element surface causes an increase of its temperature and therefore a modification of its resistance This resistance modification imbalances a Wheatstone bridge where the measurement element is inserted Hydrogen concentration in air is linked to the imbalance of the bridge by a linear correlation To overcome the influence of temperature and room humidity variations a second element similar to the one used for the measurement but with a non catalytic surface is inserted to
9. based sensor is most often used in research laboratories whereas the MEMS micro electro mechanic system are used in the aeronautic and spatial industries The operating principle of electrochemical catalytic catharometric heat conduction semi conductor MEMS and some sensors under development is presented succinctly below Electrochemical sensors The principle used is amperometry i e the measurement of current driven by redox reduction oxidation reactions The process is based on an electrochemical cell covered by a semi permeable selective membrane which exclusively enables the diffusion of hydrogen The diffusion rate through this membrane is on one hand proportional to partial pressure of hydrogen and therefore to its concentration in air and on the other hand to air temperature Once diffused through the membrane Hydrogen comes in contact with the boundary layer between membrane and the electrolyte which consists of sulphuric acid Here Hydrogen becomes instantly ionised at the solid liquid interface of a platinum catalytic electrode working electrode This ionisation enables a redox reaction with the second electrode auxiliary electrode consisting of platinum oxide These reactions cause a potential difference between the electrodes which enables the determination of hydrogen concentration by a non linear correlation The reaction products generate charge barriers which tend to restrict the reaction To improve the stability
10. be carried out The operating principle of the Schottky diode is the following the palladium enables the adsorption and the dissociation of the hydrogen molecule into hydrogen atoms The hydrogen atoms diffuse through the palladium up to the PdCr interface and modify the surface charge This change is detectable by measuring the voltage current pair and is dependent on the hydrogen concentration by a non linear correlation In the case of the resistive sensor the formation of palladium hydrides caused by the adsorption and the dissociation of the hydrogen molecule into hydrogen atoms increase the resistance compared to the pure palladium The following figure schematically presents a Schottky diode based sensor AAA AAS 6 oa Sate l 3 X x Rte Hoke AXX 45 a ee Wheatstone Bridge Schottky Diode Fig Error No text of specified style in document 7 PdCr Schottky diode based hydrogen sensor 1 2 1 1Some emerging technologies for detecting hydrogen In this chapter technologies for hydrogen detection under development depending on whether they are based on optical principles or not are presented Technologies not based on optics According to the bibliographical research carried out the technologies being developed are the following a semiconductor Schottky diode palladium wire network surface acoustic wave sensor on a nano structured sensitive layer The developments concerning semi conduct
11. combustion of 5 0 L min of gaseous hydrogen at NTP flowing through a 1 6 mm orifice to produce a 20 cm high flame 1 2 1 2Possible means to detect a hydrogen fire Hydrogen fire can eventually be detected by using thermal detectors as rate of temperature rise or overheat detectors to pick up radiative convective or conductive heat These reliable detectors of various types are suitable hydrogen fire detection means as long as they are located very near where the fire breaks out Other common fire detectors type like those with ionising cells are not appropriate to detect hydrogen fires KAA he 13 gt 4 1 IVS s Th te tt AA 5 2 Though hydrogen fires tend to emit radiation over a broad range and are not characterised by extreme peaks Hydrogen fire detectors can also rely on UV and IR light detection Besides the radiation itself hydrogen flames can be indirectly visible by their strong heat effect and turbulence heat ripples of the surrounding atmosphere Optical flame detectors detect specific spectral radiation emitted during the combustion process by the various chemical species ions radicals molecules that are either intermediates or final products of combustion Chemical species emit radiation at wavelengths characteristic to the particular species e The hydroxyl radical OH and water are the main emitting chemical species in the hydrogen combustion process These species emit radiation at specific s
12. hydrogen flame detection The main challenge remains the same as before that is to say to discriminate IR related to hydrogen fire from those from the sun any light sources or any hot materials IR sources powered with alternative electric currents can be filtered due to their own 100 Hz modulated signal However neither hot bodies nor sunlight display a modulated signal that can be picked up and filtered Solution consists in focussing on the 1 7 um wavelength that corresponds to a peak emission of steam having in mind that the atmosphere absorbs sun emitted IR wavelengths between 1 81 amp 1 88 um as well as between 2 55 amp 2 9 um The 1 7 um wavelength is the only one of the three IR peaks mentioned above that falls within the IR filtering spectrum of the atmosphere 100 80 Atmospheric ssion L000 ft 1 1 MM PR H 0 60 40 Transmission Percent Relative Flame Intensity 20 0 5 1 0 1 5 2 0 2 5 3 0 Microns Fig Error No text of specified style in document 13 Atmospheric IR Transmission and H2 Air Flame Emission from 2 THERMAL detectors Thermal detectors e g temperature sensors detect the heat of the flame Such detectors need to be located very close to or at the site of a fire and are not specific to hydrogen flames IMAGING systems Imaging systems mainly are available in the thermal IR region and do not provide continuous monitoring with alarm capability A trained operator is re
13. the Wheatstone bridge In the absence of combustible gas each of the two elements undergoes identical resistance variations and the bridge keeps balanced The following figure schematically presents a catalytic detector Compensator Element Passive Pellistor Wheatstone Zero Bridge adjust Sintered disk Detector Element Active Pellistor Fig Error No text of specified style in document 2 catalytic bead detector Heat Conduction Sensors Catharometers Heat conduction sensors use the significantly thermal conductivity of hydrogen gas A material heated by the Joule effect is stabilised at a temperature which depends on the electrical power provided and thermal exchanges with the gaseous environment A change in the composition of the atmosphere causes changes of the sensor temperature The derivative of this temperature change which varies the electrical resistance of the element is linked to the concentration of hydrogen gas in air by a linear correlation For the measurement a metallic wire conductor coated with chemically inert material is exposed to the gas probe A second identical wire conductor is exposed to a reference atmosphere for temperature compensation The electrical resistance variation is also measured using a Wheatstone bridge Signals caused by the varying thermal conditions are weaker than the signals of catalytic sensors FP veisialeieieie 7 4 HZ 2 ete tee AO p
14. therefore essential for keeping detector at a high performance level required for a safety use Regarding maintenance detectors should be gt sais 12 fac riSato ks Ki X Th te tt AA aa ee e regularly cleaned especially the head of the detector to allow gas to reach the sensitive element e regularly inspected for possible malfunctions visible damage or other deterioration calibrated zero and sensitivity adjusting with a standard gas in accordance with the procedure outlined in the instruction handbook Maintenance intervals depend on both context of use and kind of detectors detection technique portable or fixed detector The best mean to determine maintenance interval for a detector is based on experience learned from the use of this detector For new installations it may be wise to carry out maintenance frequently at first perhaps weekly increasing the time intervals to perhaps monthly as confidence grows on the basis of the maintenance records with experience in the installation concerned Information on maintenance protocol should be found in the user manual IEC 61508 deals also with the need for periodic maintenance 1 2 2 Detection of hydrogen flames Hydrogen burns with very pale blue flames and emits neither visible light in day time sun radiation can overpower the hydrogen flame light nor smoke it produces water when it burns in air unless e g sodium is added or dust particles are entra
15. The point is further detailed in a subsequent Communication of the European Commission Commission communication 2003 on the good practice for implementing the Directive which states that Concentrations in the vicinity of a plant can be monitored e g by means of gas alarms As major prerequisites for the use of such alarms the substances likely to be present the location of the sources maximum source strength and dispersion conditions must be adequately known and the instrument performance must be appropriate to the conditions of use especially as regards response time alarm level and cross sensitivity Failure of individual functions of gas alarm systems should not generate dangerous situations and the number and location of measuring points must be so chosen that the anticipated mixtures can be detected quickly and reliably Last but not least gas alarms for use in hazardous places must be approved and suitably marked as safe electrical equipment pursuant to the European Directive 94 9 EC Directive 94 9 EC 1994 which in turn is supported by a number of European standards prepared by CENELEC Guidelines on the application of Directive 94 9 EC 19945 Whereas ensuring safety of industrial operation in the presence of flammable gases is a well recognized issue for which a number of established technologies can be used there is a need to re KAA AA He X Ty 2 HW 4 X Pe tke AAA Pn consider the exist
16. accident escalates In the case of hydrogen fires detection meets both needs However we have underlined that it may be a challenge to discriminate hydrogen related signals from parasitic ones Therefore to prevent wrong alarms and related automatic actions it may be an option to favour human analysis and actions to automatic ones Finally rescue services or maintenance team can also use a broom to locate small fires The intent is a dry corn straw or sage grass broom easily ignites as it passes through a hydrogen flame ies 16 L r Sato ie ki X Te tt AA R F Cracknell J L Alcock J J Rowson L C Shirvill and A Ungiit Safety in retailing hydrogen in Hydrogen and its future as a transportation fuel D Holt Ed SAE publ February 2003 ISBN No 0 7680 1128 0 Directive 1999 92 EC of the European Parliament and of the Council of 16 December 1999 on minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospheres 15th individual Directive within the meaning of Article 16 1 of Directive 89 391 EEC Official Journal of the European Communities Series L 23 28 1 2000 p 57 Commission communication concerning the non binding guide of good practice for implementing Directive 1999 92 EC of the European Parliament and of the Council on minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospher
17. c with a palladium micro mirror The following figure schematically presents such a sensor Its construction is based on a multi modal fibre optic Pd asl micromirror Fig Error No text of specified style in document 8 hydrogen sensor based on a fiber optic with a palladium micro mirror Hydrogen is absorbed by the palladium micro mirror located at the end of the fibre The optical and electric properties of the palladium change Consequently the reflected wave is modified whereas the incident wave remains the same Each fiber constitutes a selective sensor Exposed fibre optic coated with a palladium layer The following figure schematically presents such a sensor Pd thin layer Fibre coating Fibre coating Fig Error No text of specified style in document 9 hydrogen sensor based on a fiber optic covered with a palladium layer The light travelling through the fibre causes evanescent waves on the fiber core surface If the core of the fibre is covered with a palladium layer the evanescent fields are altered In fact if the hydrogen is absorbed by a palladium film the refractive index of the Pd coating changes reduction This change in refractive index modifies the absorption of the guided light which can be detected by monitoring the light intensity via interferometer techniques Fabry Perot etc Each fiber constitutes a selective sensor Bragg network fibre optic A Bragg network causes periodic or ap
18. e the following Locations where hydrogen leaks or spills are possible At hydrogen connections that are routinely separated for example hydrogen refuelling ports Locations where hydrogen could accumulate Jn building air intake ducts if hydrogen could be carried into the building In building exhaust ducts if hydrogen could be released inside the building A generally accepted and commonly used concentration level for alarm activation is 1 Yo hydrogen volume fraction in air which is equivalent to 25 of the lower flammability limit This level normally should provide adequate time to respond in an appropriate manner such as system shutdown evacuation of personnel or other measures as necessary In designing a reliable hydrogen detection and monitoring system the following recommendations have been made by NASA NASA 1997 Evaluate and list all possible sources to be monitored valves flanges connections bellows etc and provide valid justification for sources not monitored R siais sais A 11 far Sato k Ki X Th te tt AA 45 1 valuate the expected response time of the leak detection system to ensure compatibility with the responding safety system Provide visual and audible alarms as necessary when the worst allowable condition red line is exceeded The allowable condition must still be in the safe range but a warning indicates a problem Provide portable detectors for field op
19. erations or isolated areas and permanently installed detectors for remote automated operations Utilize a program to maintain and periodically recalibrate detectors to ensure acceptable performance Determine the number and distribution of sampling points in the hydrogen detection system based on the possible leak rate ventilation amount and area size Consideration should be given to methods of routing hydrogen to the detector At a European level and to the knowledge of the present authors no EN standard or recommendation for detection layouts specific to hydrogen systems has been made publicly available so far However an obligation is posed under the ATEX directive Directive 94 9 EC 1994 for the necessary instructions to be provided for detection or alarm devices for monitoring the occurrence of explosive atmospheres to be provided at the appropriate places The European Standard EN50073 1999 supporting the Directive dedicates several pages to the criteria of selection installation and placement of combustible gas sensors which are essentially coherent with what laid down in the previous paragraphs European Standard EN 50073 1999 The international standard IEC 61779 6 International Standard IEC 61779 6 1999 very similar to the EN 50073 also proposes in annex a two pages document that summarizes the above points in the form of a typical environmental and application check list References and sources ISO TR 15916
20. eriodic disruption of the effective absorption ratio or of the effective refractive index of a fiber optic cable Predetermined wavelengths of the light beam are reflected by the bragg network while all other wavelengths pass The following figure schematically presents such a sensor FP eisialeleiaie 1 8 fa TiSate Z aia TONNE NnalReportOnHydrogenSafetuyu V a Bragg network Fibre coating Fig Error No text of specified style in document 10 hydrogen sensor based on a Bragg network fiber optic This sensor operating with UV light is based on the mechanical stress caused by the palladium layer when it absorbs hydrogen This stress stretches or compresses the Bragg network and therefore the wavelengths or optical lengths of reflected or transmitted light By using several Bragg networks with different lattice constants several hydrogen sensors may be multiplexed on a single fibre Wolfram Trioxide WO The Measurement principle bases on the fact that Wolfram Trioxide shows hydrogen concentration dependent changes in its refraktive Index 20 Benson et al 1999 Fiber coating Fig Error No text of specified style in document 11 hydrogen sensor based on Wolfram Trioxide Survey on typical Sensor properties Range Respons Tiume Warm up Time Power consumption Costs Standard SpecialSensor Type Hydrogen Concentration Cross sensitivity Selectivity Accuracy Long Term Stability 2 CO high k
21. es COM 2003 515 final Bruxelles B 25 08 2003 Directive 94 9 EC of the European Parliament and of the Council of 23 March 1994 on the approximation of the laws of the Member States concerning equipment and protective systems intended for use in potentially explosive atmospheres Official Journal of the European Communities Series L 100 19 4 1994 p 1 5 Guidelines on the application of Directive 94 9 EC of 23 March 1994 May 2000 annexes 5 to 7 downloadable at http europa eu int comm enterprise atex guide index htm 6 F DiMeo Jr Integrated micro machined hydrogen gas sensor final report DOE GO 10451 F prepared for the U S Department of Energy under Cooperative Agreement No DE FG36 99GO10451 7 Hydrogen Safety Codes and Standards Research Announcement of funding opportunity DE PS36 04G094000 Issued on February 2004 Draft report of the Strategic Research Agenda within the European Hydrogen and Fuel Cell Technology Platform
22. he user interface Surface acoustic wave sensor on a nano structured sensitive layer A surface acoustic wave sensor is built around two inter giddied transducers placed on the surface of a piezoelectric substrate By connecting alternating current to the metallic conductors of the entrance transducer an alternation of compressions and expansions occurs which generates a surface wave This wave moves towards the second transducer to be converted back to an electric signal During the transit between the two electrodes it is possible to influence the wave using a nano structured Sato 2 a s k Th te tt A ia p 7 sensitive net which is nothing else than a palladium wire network mentioned in the previous paragraph This nano structure with absorbed hydrogen present in the air changes on one hand the phase speed of the wave and attenuates it on the other hand The action of the hydrogen on the palladium nano wire sensitive layer has been to modify its physical characteristics density rigidity conductivity thickness The disruptions can indirectly be translated by the variations induced to the propagation of the sound waves In order to compensate room temperature influences two identical surface sound wave sensors are used One is covered with a sensitive layer enabling the comparison of the exit signals Technologies based on optics Emerging optical technologies for detecting hydrogen use fibre optics Fibre opti
23. ined and burned along with the combustible mixture Compared to hydrocarbon combustion hydrogen flames radiate significantly less heat and so human physical perception of this heat does not occur until direct contact is made with the flame Therefore a hydrogen fire may remain undetected and propagate in spite of any human direct monitoring in areas where hydrogen can leak spill or accumulate and form potentially combustible mixtures Hydrogen fire detectors can help to take immediate actions in these situations Hydrogen fire detectors can be either fixed for continuous monitoring of remote operations or portable for field operations 1 2 2 1Expected performance of hydrogen fire detectors For an efficient and a reliable use a hydrogen fire detector should fulfil the following criteria e to pick up every true alarm and to avoid false ones e to be specific and pick up hydrogen fire signals among various ones that become even more numerous when detector sensitivity is increased e to have a limited response time especially if it triggers a safety action e and if possible to have an automatic periodic check up In terms of performance its ability e to detect a hydrogen flame at a sufficient distance e and to detect small flames should be considered when installing a hydrogen flame detector For instance NASA 2 indicates that a fire detection system should at least be capable of detecting at a minimum distance of 4 6 m the flame from the
24. ing knowledge on hydrogen detection in the perspective of a future hydrogen economy A breadth of novel applications could be in sight some of which may bring this hydrogen much closer to the general public that it has even been before thus requiring hydrogen sensors as ubiquitous as computer chips in our society F DiMeo Jr Both the U S Department of Energy DoE and the European Hydrogen Fuel Cell Platform HFP have been identifying new directions for hydrogen sensors development envisaging innovation on both materials and concepts for applications ranging from large area physical sensing to in situ detection of leaks from portable devices Hydrogen Safety Codes and Standards Research 2004 Draft report of the Strategic Research Agenda Efficiency over a wide range of hydrogen and oxygen concentrations low sensitivity to gaseous contaminants and poisoning are outstanding requirements along with the possibility to efficiently integrate intelligent sensing devices in hydrogen systems so that safety or emergency measures can be actuated automatically in case of necessity 1 2 1 1 References and sources 1 2 1 2Detection techniques Commercial available technologies for hydrogen detection Several types of hydrogen sensors are in use depending on the operating conditions The electrochemical catalytic and thermal conductivity sensors are mainly used in the industries where the hydrogen risk is present The semi conductor
25. ors and the Schottky diodes mainly aim to improve the selectivity of the different layers as well as to test new metallic substrates deposits combinations Although these technologies are in market and sold research continues in order to enable a reduction of the drift and better selectivity The operation of semi conductor and Schottky diode technologies are described above This chapter succinctly presents among others the operating principles of palladium wires network based sensors and the surface sound wave sensors on a nano structured sensitive layer Palladium wire network These sensors are made of a network of palladium nano wires or mesoscopic wires from 20 to 100 wires These networks of palladium nano wires are prepared by electro deposition on a graphite surface in order to be then transferred onto a glass slide covered with a cyanacrylate film The nano wires are then connected on either side by silver contacts These palladium nano wires are in fact broken and do not conduct the current In the presence of hydrogen the palladium slightly swells and the nanoscopic spaces or breakages are repaired enabling the passage of electric current The resistance change depends on the hydrogen concentration in a concentration range from 2 to 10 In order to be operative these sensors require permanent power connection or even to be heated They require a transmitter to compute a signal and to route it to t
26. pectral bands according to their electronic structure and the typical energy translation vibration rotation of the process e OH being an active intermediate with an available free electron emits strongly in the UV spectral band at the 0 306 amp 0 282 um peak and additional weaker emission peaks at 0 180 0 240 um It also emits infrared energy in the near IR band vibration and rotation of the molecule with several peaks within the 1 3 um spectral band e H2O emits mainly in the near IR band vibration and rotation with a strong peaks at 2 7 1 9 amp 1 4 um ranging from the highest to the lowest intensity These detection techniques assume that no interfering shield is placed between the flame and the UV IR detector Though optical techniques are available to pick up these various wavelengths the main challenge consists in discriminating hydrogen flame emitted signals with other potential sources that emit similar signals in frequency and intensity UV detectors UV systems are favoured to IR because they are extremely sensitive Besides probability to encounter interfering signal is lower as long as UV detectors are shaded from sun light Drawbacks are on one hand the cost and on the other hand their reduced efficiency with liquid hydrogen flames as fog blocks UV rays The same remark applies whenever fog is present False alarms can be released by random UV sources such as lightening or arc welding The ability of the detector
27. quired to interpret whether the image being viewed is a flame UV imaging systems require special optics and are very expensive BROOMS Putting flammable objects or dust particles into a hydrogen flame will cause the flame to emit in the visible spectrum Corn straw brooms dirt and dry fire extinguishers have been used for this purpose Extreme caution needs to be taken with such practice due to the required proximity to the flame References 1 C Jungbluth amp all El ment pour un guide de s curit hydrog ne Commission des communaut s europ ennes volumes 1 et 2 EUR 9689 1985 2 NASA Safety standard for hydrogen and hydrogen systems Guidelines for hydrogen system design materials selection operations storage and transportation NSS 1740 16 1997 3 ISO TR 15916 Basic considerations for the safety of hydrogen systems Technical Report First edition 2004 2 rr hk 15 E riSato ks Ki X Th te tt AA 4 Directive 94 9 EC Directive 94 9 EC of the European Parliament and of the Council of 23 March 1994 on the approximation of the laws of the Member States concerning equipment and protective systems intended for use in potentially explosive atmospheres Official Journal of the European Communities Series L 100 19 4 1994 1 3 CONCLUSION Detection may consist in supervising an unattended site or looking at signals that are not perceptible by attending employees and to trigger an action before
28. to discriminate sunlight induced UV radiation from hydrogen flames to avoid false alarms is the main challenge Various techniques can be applied e The use of a filter to cut any wavelength above 0 29 um to keep those wavelengths attributable only to a hydrogen fire accident Indeed even on a sunny day the atmosphere filters sunray wavelengths below the proposed threshold of 0 29 um As a drawback this solution also cuts down nearly 2 3 of the UV band and therefore decreases the detector acuity e The use of two concomitant cells that watch the same zone One of the cells mostly analyses the visible spectrum where the sunlight signal is predominant in comparison with hydrogen flame emitted signal whereas the other one focuses on the UV band The UV signal from the UV cell is only taken into account if it diverges from the signal from the concomitant cell e The flickering behaviour of a flame can also be taken into account In that case the modulated part of the UV signal would be looked at This technique may not be compatible with a fast response need e Finally if parasitic signals are known to be minor a positive signal may be assumed whenever a given threshold is reached IR detectors p sais 14 SL Safe fe Ki X Th te tt AA 43 1 We have mentioned above that fog may hinder UV transmission to the sensor cell IR detectors are not sensitive to these issues Besides hydrogen flames emit significant IR to use them for
29. transconductance in an electrical connection as shown below These transconductance changes are linked to hydrogen concentration by a non linear correlation PF sinielsisinis 1 5 4 Ha si hte AI SF Source Fig Error No text of specified style in document 5 semi conductor FED detector Resistive Palladium Sensor This Sensor Type consists of a catalytic active Palladium surface Hydrogen is adsorbed dissociated to hydrogen atoms and generates palladium hydride which has a higher electrical resistance as the pure palladium This resistance change is measured and linked to hydrogen concentration by a linear correlation ca 40 Electrical Connections x6 L gt m SS tube 6 x 1 Epoxy Alumina tube 10 x 2 u Kapton Pd Fig Error No text of specified style in document 6 Resistive Palladium Sensor MEMS Micro Electro Mechanic Systems The micro electro mechanic systems combine calculators and miniscule devices such as sensors valves gears mirrors actuators loaded on a semi conductor chip The detector chip is comprised of Two hydrogen detection devices namely a Schottky palladium chrome diode PdCr for low concentrations and a resistive palladium sensor for high concentrations A temperature sensor as well as a heating element to control the temperature The electronic enables the treatment of the signals from the different devices present on the chip to
30. vel optic microbend pressure sensor Fiber Optic sensor Technology and applications September 1999 pp 516 523 13 Fr d ric FAVIER Erich C WALTER Michael P Zach Thorsten BENTER Reginald M PENNER Hydrogen sensors and switches from electrodeposited palladium mesowire array Science vol 293 September 2001 pp 2227 2231 14 R BILLINGTON Measurement methods for stimulated Raman and Brilloin scattering in optical fibres NPL Repot COEM 31 June 1999 34 p 15 Masahico MATSUMIYA Woosuck SHIN Noriya IZU Norimitsu MURAYAMA Nano structured thin film Pt catalyst for thermoelectric hydrogen gas sensor Sensors and Actuators BXX 2003 16 O K TAN W ZHU M S TSE X YAO Hydrogen sensitive I V charactristics of metal ferroelectric gas sensor device fabricated by sol gel technique Materials Science and Engineering B58 1999 pp 221 228 i 2 10 fa cr Sato ie L X Th te tt AA 45 I 17 R GLENN SELLAR Robert C YOUNGQUIST Fred FIRNSCHILD Novel technique for detection and imaging of hydrogen leaks 2003 3 presentation collected in Internet 18 Huey Ing CHEN Yen I CHOU Chien Kang HSIOU Study on hydrogen detection by Schottky diode sensors Material Chemistry and Physics 19 Michel MOLIERE Philippe COZZARIN GE Energy France S bastien BOUCHET INERIS Philippe RECH Technicrea GE sub contractor Catalytic detection of fuel leaks in gas turbines unit gaseous and volatile hydrocarbon based fuel ASME GT
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