Determination of maximum explosion pressure and the maximum rate of pressure rise of gases and vapours - Part 2: Determination of the maximum rate of explosion pressure rise

The standard test method is designed to produce measurements of the rate of explosion pressure rise and the maximum rate of explosion pressure rise of a flammable gas/air/inert mixture in a closed volume at ambient temperature and pressure. In this European Standard, the term "gas" includes vapours but not mists. Detonation and decomposition phenomena are not considered in this European Standard.
The rates of pressure rise measured by the procedures specified in this European Standard are not applicable to flameproof enclosures, that is enclosures intended to withstand an internal explosion and not to transmit it to an external explosive atmosphere, or any other closed volume where the internal geometry can result in pressure piling.  Even in an enclosure of relatively simple geometry the disposition of the internal components can lead to rates of pressure rise significantly higher than those measured using this European Standard. Flameproof enclosures shall be constructed and tested in accordance with the requirements contained in EN 60079-1 for electrical equipment and EN 13463-3 for non-electrical equipment.

Verfahren zur Bestimmung des maximalen Explosionsdruckes und des maximalen zeitlichen Druckanstieges für Gase und Dämpfe - Teil 2: Bestimmungsverfahren für den maximalen zeitlichen Druckanstieg

Diese Europäische Norm legt ein Verfahren fest zum Bestimmen des zeitlichen Druckanstieges und des
maximalen zeitlichen Druckanstieges von Gemischen aus brennbaren Gasen, Luft und Inertgas in einem
geschlossenen Behälter bei Umgebungsbedingungen. In dieser Europäischen Norm schließt ?Gas? auch
Dämpfe ein, jedoch nicht Nebel. Dieses Dokument gilt nicht für Bedingungen, unter denen Detonations- bzw.
Zersetzungsphänomene auftreten können.
Die nach den in dieser Europäische Norm festgelegten Verfahren ermittelten zeitlichen Druckanstiege können
nicht Grundlage für die Schutzart druckfeste Kapselung sein. Druckfeste Kapselung ist ausgelegt,
Explosionen im Inneren von Betriebsmitteln standzuhalten und so den Zünddurchschlag zu verhindern in
äußere explosionsfähige Atmosphäre oder in ein umschlossenes Volumen, dessen Einbauten zu
Drucksteigerung führen können. Selbst in relativ einfachen Geometrien können Einbauten zu deutlich höheren
als nach dieser Europäische Norm bestimmten zeitlichen Druckanstiegen führen. Druckfeste Kapselung für
elektrische Betriebsmittel muss nach EN 60079-1 und für nicht-elektrische Betriebsmittel nach EN 13463-3
ausgelegt und geprüft sein.

Détermination de la pression maximale d'explosion et de la vitesse maximale de montée en pression des gaz et des vapeurs - Partie 2: Détermination de la vitesse maximale de montée en pression

La méthode d'essai normalisée est conçue pour fournir des mesures de la vitesse de montée en pression et
de la vitesse maximale de montée en pression d'un mélange combustible/air/gaz inerte, dans un volume clos,
a température et pression ambiantes. Les phénomenes d?explosion et de décomposition ne sont pas pris en
compte dans la présente Norme européenne.
Les vitesses de montée en pression mesurées selon les modes opératoires spécifiés dans la présente norme
ne sont pas applicables aux enveloppes antidéflagrantes, c?est-a-dire aux enveloppes destinées a résister a
une explosion interne et a ne pas la transmettre a l?atmosphere explosive externe ou a tout autre volume
fermé ou la géométrie interne peut entraîner une accumulation de pression. Meme dans une enveloppe dont
la géométrie est relativement simple, la disposition des éléments internes peut conduire a des vitesses de
montée en pression nettement supérieures a celles mesurées selon le présente document. Les enveloppes
antidéflagrantes doivent etre construites et soumises a essai conformément aux exigences de l?EN 60079-1
pour le matériel électrique et a l?EN 13463-3 pour le matériel non électrique.

Ugotavljanje najvišjega tlaka eksplozije in največje hitrosti naraščanja tlaka plinov in hlapov - 2. del: Ugotavljanje največje hitrosti naraščanja tlaka eksplozije

General Information

Status
Withdrawn
Publication Date
31-Jan-2006
Withdrawal Date
29-Aug-2011
Current Stage
9900 - Withdrawal (Adopted Project)
Start Date
30-Aug-2011
Due Date
22-Sep-2011
Completion Date
30-Aug-2011

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2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Verfahren zur Bestimmung des maximalen Explosionsdruckes und des maximalen zeitlichen Druckanstieges für Gase und Dämpfe - Teil 2: Bestimmungsverfahren für den maximalen zeitlichen DruckanstiegDétermination de la pression maximale d'explosion et de la vitesse maximale de montée en pression des gaz et des vapeurs - Partie 2: Détermination de la vitesse maximale de montée en pressionDetermination of maximum explosion pressure and the maximum rate of pressure rise of gases and vapours - Part 2: Determination of the maximum rate of explosion pressure rise13.230Varstvo pred eksplozijoExplosion protectionICS:Ta slovenski standard je istoveten z:EN 13673-2:2005SIST EN 13673-2:2006en,fr,de01-februar-2006SIST EN 13673-2:2006SLOVENSKI
STANDARD



SIST EN 13673-2:2006



EUROPEAN STANDARDNORME EUROPÉENNEEUROPÄISCHE NORMEN 13673-2September 2005ICS 13.230; 75.160.30 English VersionDetermination of maximum explosion pressure and themaximum rate of pressure rise of gases and vapours - Part 2:Determination of the maximum rate of explosion pressure riseDétermination de la pression maximale d'explosion et de lavitesse maximale de montée en pression des gaz et desvapeurs - Partie 2: Détermination de la vitesse maximalede montée en pressionVerfahren zur Bestimmung des maximalenExplosionsdruckes und des maximalen zeitlichenDruckanstieges für Gase und Dämpfe - Teil 2:Bestimmungsverfahren für den maximalen zeitlichenDruckanstiegThis European Standard was approved by CEN on 1 August 2005.CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this EuropeanStandard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such nationalstandards may be obtained on application to the Central Secretariat or to any CEN member.This European Standard exists in three official versions (English, French, German). A version in any other language made by translationunder the responsibility of a CEN member into its own language and notified to the Central Secretariat has the same status as the officialversions.CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia,Slovenia, Spain, Sweden, Switzerland and United Kingdom.EUROPEAN COMMITTEE FOR STANDARDIZATIONCOMITÉ EUROPÉEN DE NORMALISATIONEUROPÄISCHES KOMITEE FÜR NORMUNGManagement Centre: rue de Stassart, 36
B-1050 Brussels© 2005 CENAll rights of exploitation in any form and by any means reservedworldwide for CEN national Members.Ref. No. EN 13673-2:2005: ESIST EN 13673-2:2006



EN 13673-2:2005 (E) 2 Contents Page Foreword.4 Introduction.5 1 Scope.6 2 Normative references.6 3 Terms and definitions.6 4 Test method.6 4.1 Principle.6 4.2 Apparatus.7 4.3 Preparation and preservation of test samples.10 4.4 Procedure.10 4.5 Expression of results.13 4.6 Test report.14 Annex A (normative)
Verification.15 Annex B (normative)
Smoothing of pressure-time curves.18 Annex C (informative)
Conversion of the values for the flammable substance content.22 Annex D (informative)
Example of an evaporator equipment for liquid flammable substances.26 Annex E (informative)
Example of a test report.27 Annex ZA (informative)
Relationship between this European Standard and the Essential Requirements of EU Directives 94/9/EC and 98/37/EC.29 Bibliography.30 Figures Figure A.A1 — Plot of the rate of explosion pressure rise (dp/dt)ex as a function of the test vessel volume V for H2-air mixtures (xH2 ≅ 35 mol %).16 Figure A.A2 — Plot of the rate of explosion pressure rise (dp/dt)ex as a function of the test vessel volume V for CH4-air mixtures (xCH4 ≅ 10 mol %).16 Figure A.A3 — Plot of the rate of explosion pressure rise (dp/dt)ex as a function of the test vessel volume V for NH3-air mixtures (xNH3 ≅ 23 mol %).17 Figure B.B1 — Example of a raw p(t) curve showing oscillations.19 Figure B.B2 — Example of a raw p(t) curve showing oscillations.20 Figure B.B3 — Schematic diagram showing the variation of (dp/dt)ex as a function of a smoothing parameter.21 Figure D.D1 — Evaporator equipment for producting test mixtures from liquid flammable substances.26 Tables Table 1 — Rules for rounding up (dp/dt)ex and (dp/dt)max values.13 SIST EN 13673-2:2006



EN 13673-2:2005 (E) 3 Table A.A1 — Values for verification of the apparatus (the (dp/dt)ex values are not rounded, according to Table 1 in 4.5).15 Table C.C1 — Equations for converting the values of the flammable substance content.25 Table ZA.1 — Correspondence between this European Standard and Directive 98/37/EC.29 Table ZA.2 — Correspondence between this European Standard and Directive 94/9/EC.29
SIST EN 13673-2:2006



EN 13673-2:2005 (E) 4 Foreword This European Standard (EN 13673-2:2005) has been prepared by Technical Committee CEN/TC 305 “Potentially explosive atmospheres - Explosion prevention and protection”, the secretariat of which is held by DIN. This European Standard shall be given the status of a national standard, either by publication of an identical text or by endorsement, at the latest by March 2006, and conflicting national standards shall be withdrawn at the latest by March 2006. This European Standard has been prepared under a mandate given to CEN by the European Commission and the European Free Trade Association, and supports essential requirements of EU Directives. For relationship with EU Directives, see informative Annex ZA, which is an integral part of this document. EN 13673 Determination of maximum explosion pressure and the maximum rate of pressure rise of gases and vapours consists of:  Part 1: Determination of the maximum explosion pressure  Part 2: Determination of the maximum rate of explosion pressure rise According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following countries are bound to implement this European Standard: Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom. SIST EN 13673-2:2006



EN 13673-2:2005 (E) 5 Introduction This European Standard describes a test method for the determination of the rate of explosion pressure rise and the maximum rate of explosion pressure rise of a quiescent flammable gas/air/inert mixture at ambient temperature and pressure. Rate of explosion pressure rise and maximum rate of explosion pressure rise are used for designing explosion protection measures, such as explosion pressure resistant or explosion pressure shock resistant apparatus, explosion venting and explosion suppression. These characteristics are particularly influenced by:  the size and shape of the vessel;  the type and energy of the ignition source;  the temperature and pressure;  the turbulence. It is therefore necessary to standardise the conditions at which the rate of explosion pressure rise and the maximum rate of explosion pressure rise are determined. SIST EN 13673-2:2006



EN 13673-2:2005 (E) 6
1 Scope The standard test method is designed to produce measurements of the rate of explosion pressure rise and the maximum rate of explosion pressure rise of a flammable gas/air/inert mixture in a closed volume at ambient temperature and pressure. In this European Standard, the term "gas" includes vapours but not mists. Detonation and decomposition phenomena are not considered in this European Standard. The rates of pressure rise measured by the procedures specified in this European Standard are not applicable to flameproof enclosures, that is enclosures intended to withstand an internal explosion and not to transmit it to an external explosive atmosphere, or any other closed volume where the internal geometry can result in pressure piling.
Even in an enclosure of relatively simple geometry the disposition of the internal components can lead to rates of pressure rise significantly higher than those measured using this European Standard. Flameproof enclosures shall be constructed and tested in accordance with the requirements contained in EN 60079-1 for electrical equipment and EN 13463-3 for non-electrical equipment. 2 Normative references Not applicable. 3 Terms and definitions For the purpose of this European Standard, the following terms and definitions apply. 3.1 rate of explosion pressure rise (dp/dt)ex highest value of the slope (first derivative) of the pressure-time curve (smoothed if necessary), measured in a closed vessel during the explosion of a specific mixture of flammable gases with air or air and inert gases determined under specified test conditions 3.2 maximum rate of explosion pressure rise (dp/dt)max maximum value of the rate of explosion pressure rise, when varying the content of flammable gas in the mixture NOTE For the purpose of this document, all pressures are expressed in bar absolute and rate of explosion pressure rises are expressed in bar/s. 4 Test method 4.1 Principle An explosive test mixture is ignited by a defined ignition source which is positioned in the centre of the test vessel. By means of a pressure measuring system, the pressure-time curve developed following ignition of the test mixture is recorded and the highest rate of explosion pressure rise (dp/dt)ex is calculated. The maximum rate of explosion pressure rise (dp/dt)max is determined from measurements of the highest rate of explosion pressure rise (dp/dt)max by varying stepwise the content of flammable gas in the mixture, until the maximum (dp/dt)ex is found. SIST EN 13673-2:2006



EN 13673-2:2005 (E) 7 4.2 Apparatus 4.2.1 General The test apparatus consists of:  a test vessel;  equipment for preparing the test mixture;  an ignition system;  a pressure measuring system;  a temperature measuring device;  safety equipment. 4.2.2 Test vessel The test vessel shall be spherical or cylindrical. The internal volume of the test vessel shall be equal to or greater than 0,005 m3. If a cylindrical vessel is used, the length to diameter ratio shall be equal to 1. The test vessel and any equipment (valves, igniter, transducer etc.) fitted on the vessel shall be designed to withstand a maximum pressure of at least 20 bar. NOTE Guidance on the design of the test vessel can be found in prEN 14460 [1]. The vessel shall be made of stainless steel or any material free of any catalytic effects and resistant to corrosion from the initial gas mixture and the products of combustion. The test vessel shall be fitted with sufficient ports to allow filling, evacuating and purging of the vessel. 4.2.3 Equipment for preparing the test mixture The test mixture can be prepared by the method of partial pressures or by the method of mixing flows, either inside or outside the test vessel. If the test mixture is prepared by partial pressures, the vessel used for the preparation of the mixture shall be fitted with:  a vacuum pump and a vacuum gauge;  pressure gauges or manometers;  a means of achieving a uniform test mixture (e.g. a stirrer). If the test mixture is prepared by mixing flows, the necessary components are:  flow meters (mass or volume flow meters);  a means of achieving a uniform test mixture (e.g. mixing chamber);  an evaporator if liquid samples are used (see Annex D for an example). The equipment for preparing the test mixture has to be designed in such a way that the flammable gas content in the test mixture is measured with at least an uncertainty of measurement of ± 10 % relative for a flammable gas content up to 2 % mol or ± 0,2 % absolute for a flammable gas content above 2 % mol. SIST EN 13673-2:2006



EN 13673-2:2005 (E) 8 4.2.4 Ignition system 4.2.4.1 General The igniter shall be positioned in the centre of the test vessel. Recommended ignition systems are the induction spark and the fusing wire. In the test report it has to be stated which ignition source was used. For some special mixtures it may be necessary to use a different ignition system in order to achieve ignition of the mixture. If an alternative ignition source is used it shall be fully described in the test report. It is also recommended that specialist advice is sought on the interpretation of the results. 4.2.4.2 Induction spark A series of induction sparks between two electrodes is used as the ignition source. Stainless steel is a suitable material for the electrodes. The electrodes shall be positioned at the centre of the vessel. They shall be pointed rods with a maximum diameter of 4 mm. The angle of the tips shall be 60°. The distance between the tips shall be (5 ± 0,1) mm. The electrodes shall be mounted in the vessel so they are gas tight at the highest pressures generated during the test. The mounting shall be resistant to heat and the test mixture, and provide adequate electrical resistance from the test vessel body. A high voltage transformer, with a root mean square of 13 kV to 16 kV and a short circuit current of 20 mA to 30 mA, shall be used for producing the ignition spark. The primary winding of the high voltage transformer shall be connected to the mains via a timer set to the required discharge time. The spark discharge time shall be adjusted to 0,2 s. If a spark discharge time of 0,2 s does not result in ignition of the test mixture, the test may be repeated with a spark discharge time of up to 0,5 s. The power of the spark depends on the gas mixture and its pressure. In air at atmospheric conditions according to calorimetric and electric measurements such a source gives a spark with a power of approximately 10 W. 4.2.4.3 Fusing wire This ignition device generates an electric arc by passing an electrical current along a length of straight fusing wire connected between two metal rods. The electrical power for melting the wire and generating the arc is supplied from an isolating transformer. The ignition energy delivered by the arc depends on the duration of the arc and the power rating of the isolating transformer. The energy delivered shall be in the range 10 J to 20 J, as over this range of energies there is no significant effect on the rate of explosion pressure rise. This is achieved by limiting the power rating of the isolating transformer to between 0,7 kVA and 3,5 kVA and by the use of a phase control technique. The latter is a chopping technique that allows only part of the AC waveform from the transformer secondary windings to energise the wire. Brass or stainless steel are suitable materials for the rods. The rods shall be parallel to each other with a separation distance of (5 ± 1) mm. For the fusing wire a straight length of NiCr wire (diameter 0,05 mm to 0,2 mm) shall be soldered to the tips of the metal rods. The rods shall be positioned in the test vessel so the fusing wire is at the centre of the vessel. The electrodes shall be mounted in the vessel so they are gas tight at the highest pressures generated during the test. The mounting shall be resistant to heat and the test mixture, and provide adequate electrical resistance from the test vessel body. To reduce the time required for replacing the fusing wire after a test, the rods may be mounted in a plug that can be screwed into the test vessel wall. The cross-section of the wires connecting the transformer to the rods shall be between 2,5 mm2 and 7 mm2. The length of the wires shall be less than 5 m. The diameter of the rods shall be between 1,5 mm and 5 mm. If for practical reasons the diameter of the rods has to be less than 3 mm additional mechanical support may be necessary. SIST EN 13673-2:2006



EN 13673-2:2005 (E) 9 4.2.5 Pressure measuring system The pressure measuring system consists of:  a pressure transducer;  an amplifier;  a data recording system. To insure reliability, two pressure measuring systems may be used. The pressure measuring system shall have an accuracy such that the initial pressure and explosion pressure are measured to ± 0,05 bar or better. The pressure measuring system shall have a time resolution of at least 1 ms. Pressure transducer: The pressure transducer shall be fitted in the test vessel, with the head flush with the internal wall. It shall have a resonance frequency greater than 10 kHz. It shall be able to measure pressures up to 20 bar. Pressure transducers of lower range may be used if lower explosion pressures are expected. Data recording system: The data recording system shall have a resolution of 12 bit or 8 bit and its time resolution shall be equal to or shorter thanext5001, the time from ignition to the maximum explosion pressure (see Figure B.1 and B.2). 4.2.6 Temperature measuring system Any suitable thermocouple with appropriate recording equipment may be used. 4.2.7 Safety aspects Precautions shall be taken to safeguard the health of personnel conducting the tests against the different hazards that can occur during the test:  to prevent a leak of the mixture or waste gases outside the vessel, the gas tightness of the vessel shall be checked;  to prevent rupture of the test vessel, it shall be designed to withstand a maximum pressure of at least 20 bar (see 4.2.2), as this can be assumed to be higher than the maximum explosion pressure likely to be generated during the test;  if the test mixture is prepared in a separate vessel, this vessel and the connecting line shall be designed to withstand the maximum explosion pressure;  to prevent injuries to the operator from flying fragments, all parts of the apparatus that may contain an explosive mixture shall be adequately shielded;  adequate ventilation shall be provided to prevent the build up of an explosive atmosphere in the laboratory as a result of purging of the apparatus, exhaust from the vacuum pump or leaks from the apparatus;  all electrical connections shall be adequately shielded to prevent electrocution or shock to personnel,  measures shall be taken prior to preparing the mixture to ensure that the substances can be mixed without risk; SIST EN 13673-2:2006



EN 13673-2:2005 (E) 10  measures shall be taken to prevent hazards arising from the handling of toxic flammables gases or combustion products;  the handling of flammable liquids shall be carried out in such a manner that the risk of a fire is minimised;  the handling of gas cylinders shall be carried out in such a manner that the risk of an explosion is minimised. 4.3 Preparation and preservation of test samples The components of the test mixture shall fulfil the following requirements:  Air: the air shall be free of water and oil. If synthetic air is used, it has to be stated in the report;  Inert: the purity of the inert, or the mixture of inerts, shall be 99,8 % mol or better. If a mixture of inerts is used, the composition of the mixture shall be stated in the test report.  Flammable gas: the flammable gas may be derived from:  a single substance or a mixture of substances;  a process sample (of known or unknown composition). When a single substance or a mixture of substances is used, the purity of each substance shall be 99,8 % mol or better. In the case of a mixture of substances or a process sample of known composition, the precision of the composition shall be stated in the test report. In the case of a process sample of unknown composition, the sample shall be defined as well as possible (e.g. process conditions, lower explosion limit). If the flammable gas is derived from a liquid containing more than one component, the gas phase composition can differ from the composition of the liquid phase and when large quantities of the gas are drawn off, the composition of both the liquid and gas phases can change with time. For these reasons, the test sample shall be taken from the liquid phase. 4.4 Procedure 4.4.1 Preparation of the test mixture 4.4.1.1 General If liquefied gases or liquids are used, it is necessary to ensure that there is no condensation. NOTE Condensation can be prevented by checking the vapour pressure of the substances and by local heating to prevent cooling at certain parts of the apparatus (e.g. valves). The test mixture can be prepared by the method of partial pressures or by the method of mixing flows, either inside or outside the test vessel. 4.4.1.2 Preparation of the test mixture by partial pressures If the preparation of the test mixture includes evacuating the vessel, the amount of air remaining has to be taken into account when calculating the pressures of combustible and air required. In preparing the test mixture, precautions may be necessary to prevent condensation. The mixture components are sequentially introduced into the vessel to give the required partial pressure. The partial pressure measuring system shall have an accuracy of ± 0,005 bar or better. It is necessary to ensure that the mixture in the vessel is thoroughly mixed during the introduction of each component. If the volume of the feed lines is not negligible compared to the volume of the vessel, they also need to be evacuated or purged. NOTE For practical reasons, air is often introduced as the last component, especially if atmospheric air is used. SIST EN 13673-2:2006



EN 13673-2:2005 (E) 11 4.4.1.3 Preparation of the test mixture by mixing flows The test mixture is prepared by thoroughly mixing metered flows of the gaseous components. If liquid components are used they shall be vaporised totally before mixing. NOTE It is recommended that if possible the composition of the test mixture is also measured, to check the metering devices are operating correctly and that there are no leaks in the mixing system. 4.4.2 Determination of the rate of explosion pressure rise (dp/dt)ex and the maximum rate of explosion pressure rise (dp/dt)max
4.4.2.1 Test procedure If the test mixture is not prepared in the test vessel, the vessel and all the feed lines shall be evacuated and filled with test mixture to the initial pressure. Alternatively, the test vessel may be filled by purging with the test mixture. Evacuation of the test vessel and the feeding lines shall be done to a pressure of 5 mbar or less. Before filling, purging shall be done in such a way that the test vessel atmosphere is totally replaced. This is achieved by purging with a volume that is at least ten times the vessel volume. Once the test mixture has been introduced into the test vessel, the inlet and outlet valves shall be closed. The test mixture shall be left for a period of at least two minutes to ensure it is quiescent before proceeding with the test. The pressure and temperature of the test mixture are recorded. The test mixture is then ignited and the pressure-time curve of the explosion recorded. After the test, the vessel shall be purged with air to remove the combustion products. The combustion products and purge air shall be discharged safely. Before purging begins, any residual overpressure shall be released. The humidity of the gas mixture can influence the rate of pressure rise, so it is important to ensure that the test vessel and feed lines have been purged of all moisture before starting the next test. If soot is formed during the test, the test vessel and the igniter shall be cleaned. 4.4.2.2 Determination of (dp/dt)ex The whole test procedure shall be carried out five times for the given composition of the test mixture. The highest (dp/dt)ex on each of the five pressure-time curves is determined by the following method. The pressure-time plot from each test is differentiated to obtain the highest value of the slope (first derivative) for each test. In many cases it may be first necessary to smooth the raw pressure-time data, otherwise erroneous values of the slope may be calculated. Advice on smoothing techniques can be found in Annex B. The rate of explosion pressure rise (dp/dt)ex is the highest value of the slope derived from the five tests. 4.4.2.3 Determination of (dp/dt)max. The maximum rate of explosion pressure rise (dp/dt)max is determined from measurements of the rate of explosion pressure rise (dp/dt)ex by varying step wise the content of flammable gas in the mixture until the maximum (dp/dt)ex is found. For the determination of (dp/dt)max, the number of determinations of (dp/dt)ex for one given composition can be reduced to three, provided the relative standard deviation is not greater than 10 %. (dp/dt)max is normally found for mixture compositions near the stoichiometric mixture. In order to determine (dp/dt)max with sufficient accuracy and with the minimum number of measurements, the following iterative procedure shall be used.
SIST EN 13673-2:2006



EN 13673-2:2005 (E) 12 Step 1 Choose from existing knowledge, calculation or estimation, the flammable gas content at which (dp/dt)max is expected to occur. This chosen value of flammable gas content is taken as the reference value. If the stoichiometric mixture for the reaction of the flammable gas can be calculated, then 1,1 times the stoichiometric mixture could be used as the reference value. Otherwise, the reference value has to be estimated, for example by analogy to other flammable gases of similar composition or in the same homologous series. In the case of process samples of unknown composition, it is recommended that twice the measured value of the lower explosion limit, if known, be used as an estimate of the stoichiometric mixture. Step 2 Follow the procedure given in 4.4.2.1 and 4.4.2.2 for the four mixtures with a flammable gas content of 0,8; 1,0; 1,2 and 1,4 times the reference value. In cases where there is a high degree of confidence that (dp/dt)max occurs near the reference value, the number of mixtures can be reduced to 3 and the incremental value can be reduced to less than 0,2 times the reference value. Step 3.1 Calculate the mean of the three or five (dp/dt)ex values obtained for each flammable gas content. Find the highest value of these means determined so far. Step 3.2.1 If the highest value found in step 3.1 is at the highest or lowest value of flammable gas content used, then extend the range of flammable gas content used. Choose two additional values of flammable gas content, either at the lower or higher end of the range as appropriate, at incremental values of 0,2 times the reference value. Otherwise proceed to step 3.2.2. Step 3.2.2 If the highest value found in step 3.1 lies within the range of flammable gas content used, choose two additional values of flammable gas content at the midpoints of the intervals to the left and right of the point giving the highest mean. Step 3.2.3 If there is more than one highest mean value, e
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