SIST EN 14994:2007

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Gas explosion venting protective systemsSistemi za razbremenitev tlaka plinskih eksplozijSystemes de protection par évent contre les explosions de gazSchutzsysteme zur Druckentlastung von GasexplosionenTa slovenski standard je istoveten z:EN 14994:2007SIST EN 14994:2007en;fr;de13.240Varstvo pred previsokim tlakomProtection against excessive pressure13.230Varstvo pred eksplozijoExplosion protectionICS:SLOVENSKI
STANDARDSIST EN 14994:200701-julij-2007







EUROPEAN STANDARDNORME EUROPÉENNEEUROPÄISCHE NORMEN 14994February 2007ICS 13.240 English VersionGas explosion venting protective systemsSystèmes de protection par évent contre les explosions degazSchutzsysteme zur Druckentlastung von GasexplosionenThis European Standard was approved by CEN on 15 December 2006.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 CEN Management Centre 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 CEN Management Centre has the same status as theofficial versions.CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland,France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal,Romania, 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© 2007 CENAll rights of exploitation in any form and by any means reservedworldwide for CEN national Members.Ref. No. EN 14994:2007: E



EN 14994:2007 (E) 2 Contents Page Foreword.4 1 Scope.5 2 Normative references.5 3 Terms and definitions.6 4 Venting of enclosures.7 5 Venting of isolated compact enclosures.8 5.1 General.8 5.2 Venting of isolated compact enclosures.8 5.3 Situations outside the constraints of the basic method (turbulence inducing elements, partially filled enclosures).9 5.3.1 General.9 5.3.2 Elevated initial pressure.9 5.3.3 Effect of initial turbulence.10 5.3.4 Effect of partial filling.11 5.3.5 Venting of enclosures containing turbulence inducing elements.11 5.4 Elongated enclosures.11 5.4.1 General.11 5.4.2 Venting of elongated enclosures vented at each end.12 5.4.3 Venting of elongated enclosures vented along the enclosure.13 5.5 Pipes.14 5.6 Interconnected enclosures.16 5.7 Vent ducts.16 6 Supplementary design aspects.17 6.1 General.17 6.2 Positioning and shape of explosion vents.17 6.3 Choice of venting device.17 6.4 External effects.18 6.4.1 General.18 6.4.2 Flame effects.18 6.4.3 Pressure effects.18 6.4.4 Deflectors.19 6.5 Recoil forces.20 7 Information for use.21 7.1 Marking.21 7.2 Accompanying documents.22 Annex A (informative)
Assessment of the level of congestion in rooms containing turbulence including elements.23 Annex ZA (informative)
Relationship between this European Standard and the Essential Requirements of EU Directive 94/9/EC.26 Bibliography.27



EN 14994:2007 (E) 3 Figures Figure 1
— Value of exponent
as a function of AV/V2/3.10 Figure 2 — Pressure reduction of partially filled enclosures as a function of filling ratio.11 Figure 3 — Maximum pressure developed during deflagration of propane-air mixtures flowing at 2 m/s or less in a smooth, straight pipe closed at one end .14 Figure 4 — Vent spacing needed to keep pred from exceeding 0,2 bar for propane in pipes flowing at an initial velocity of between 2 m/s and 20 m/s.15 Figure 5 — Design of a flame deflector plate (basic principles).20 Tables Table A.1 — Values for the complexity factor c.24 Table ZA.1 — Correspondence between this European Standard and Directive 94/9/EC.26



EN 14994:2007 (E) 4 Foreword This document (EN 14994:2007) 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 August 2007, and conflicting national standards shall be withdrawn at the latest by August 2007. This document 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 Directive 94/9/EC. For relationship with EU Directive 94/9/EC, see informative Annex ZA, which is an integral part of this document. According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.



EN 14994:2007 (E) 5 1 Scope This European Standard specifies the basic design requirements for the selection of a gas explosion venting protective system. This European Standard, EN 14797 and EN 14460 form a series of three standards which are used together. NOTE 1 These three standards together represent the concept of gas explosion venting. NOTE 2 To avoid transfer of explosions to other communicating equipment one should also consider applying prEN 15089. This European Standard is applicable to:  vent sizing to protect against the internal pressure effects of a gas explosion;  flame and pressure effects outside the enclosure;  recoil forces;  influence of vent ducts;  influence of initial temperature and pressure. This European Standard does not provide design and application rules against effects generated by detonation reactions or runaway exothermic reactions including decomposition in the gas phase. This European Standard is not applicable to:  fire risks arising either from materials processed, used or released by the equipment or from materials that make up equipment and buildings;  design, construction and testing of explosion venting devices, which are used to achieve explosion venting1);  protection against overpressures caused by events such as overfilling, overpressurisation, fire engulfment, overheating etc. NOTE 3 Protection by venting against dust and hybrid explosions is specified in EN 14491. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. EN 1127-1:1997, Explosive atmospheres — Explosion prevention and protection — Part 1: Basic concepts and methodology EN 13237:2003, Potentially explosive atmospheres — Terms and definitions for equipment and protective systems intended for use in potentially explosive atmospheres
1) This is covered by EN 14797.



EN 14994:2007 (E) 6 EN 13673-1, Determination of the maximum explosion pressure and the maximum rate of pressure rise of gases and vapours — Part 1: Determination of the maximum explosion pressure EN 13673-2, 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 EN 14797:2006, Explosion venting devices 3 Terms and definitions For the purposes of this document, the terms and definitions given in EN 1127-1:1997 and EN 13237:2003 and the following apply. 3.1 atmospheric conditions conditions of the surrounding medium where the atmospheric pressure can vary between 80 kPa and 110 kPa and the temperature between − 20 °C and 60 °C 3.2 gas for the purpose of this European Standard, gas, vapour or any mixture thereof at atmospheric conditions 3.3 compact enclosures enclosures having a length (height) to diameter ratio of less than 2 [EN 14373:2005, 3.14.1] NOTE The length L is calculated along the axis parallel to the main flow during the explosion, with D being the diameter measured perpendicular to this axis. For non-circular cross-sections, D is the diameter of a circle with the same area as the actual cross-sectional area. 3.4 elongated enclosures enclosures with length (height) to diameter ratio of 2 to 10 [EN 14373:2005, 3.14.2] NOTE The length L is calculated along the axis parallel to the main flow during the explosion, with D being the diameter measured perpendicular to this axis. For non-circular cross-sections, D is the diameter of a circle with the same area as the actual cross-sectional area. 3.5 pipe construction with a ratio length (height) to diameter greater than 10 [EN 14373:2005, 3.14.3] NOTE The length L is calculated along the axis parallel to the main flow during the explosion, with D being the diameter measured perpendicular to this axis. For non-circular cross-sections, D is the diameter of a circle with the same area as the actual cross-sectional area. 3.6 explosion venting device device which protects a vessel or other closed volume by explosion venting [EN 14797:2006, 3.4] NOTE Examples of such devices are: bursting discs, vent panels and explosion doors.



EN 14994:2007 (E) 7 3.7 effective vent area AE product of the geometric vent area Ad and the venting efficiency Ef for the venting device.
NOTE It is the effective vent area that should be used in making up the vent area for explosion venting [EN 14797:2006, 3.2] 3.8 gas explosion constant KG maximum value of the pressure rise per unit time (dp/dt)max during the explosion of a specific explosive atmosphere in a closed vessel under specified test conditions normalised to a vessel volume of 1 m3 multiplied by V1/3 3.9 static activation pressure pstat differential pressure at which the retaining element activates such that the venting element is able to open [EN 14797:2006, 3.11] 3.10 turbulence motion of a fluid having local velocities and pressures that fluctuate randomly NOTE Turbulence is a very effective transporter and mixer, and generally causing an overall increase of combustion rates. 3.11 turbulence inducing elements obstructions inside protected enclosures at which during an explosion turbulence is generated increasing the combustion rate 3.12 venting efficiency Ef dimensionless number used to define the efficiency of the explosion venting device [EN 14797:2006, 3.14] 4 Venting of enclosures Explosion venting is a protective measure preventing unacceptable high explosion pressure build-up inside enclosures. Weak areas in the walls of the enclosure open at an early stage of the explosion, releasing un-burnt gas/vapour and combustion products from the opening so reducing the overpressure inside the enclosure. Normally the explosion venting is applied such that the maximum reduced explosion pressure shall not exceed the known design pressure of the enclosure. All parts of the enclosure e.g. valves, sight-glasses, man-holes and ducts, which are exposed to the explosion pressure shall be taken into account when estimating the design pressure of the enclosure. The vent area is the most important factor in determining the maximum reduced explosion pressure. Information required for calculation of the vent area includes the design pressure of the enclosure, the explosion characteristics of the gas, the shape and size of the enclosure, presence of turbulence inducing elements (including congestion) inside the enclosure, the static activation pressure and other characteristics of the venting device, and the condition of the explosive atmosphere inside the enclosure.



EN 14994:2007 (E) 8 Venting does not prevent an explosion, it limits the explosion pressure. Flame and pressure effects outside the enclosure and flying debris shall be expected and in practice accounted for. In a system consisting of two connected enclosures, a gas explosion ignited in one can propagate into the second. The propagation of this explosion generates turbulence, can cause pre-compression and can act as a large ignition source in the second enclosure. This combination can enhance the violence of the secondary explosion (see 5.6). Turbulence inducing elements such as shelves in a drying oven may cause considerably more violent gas explosions. This will increase the venting requirements. As this mechanism is not covered by the general method presented in this standard, more intricate methods may need to be applied. In the informative Annex A rules are given when to apply the general method of the present standard and when one shall use more intricate methods if turbulence inducing elements are present. A general description of intricate methods is given in the informative Annex A together with requirements for the experimental validation of these methods. 5 Venting of isolated compact enclosures 5.1 General Venting devices shall comply with the requirements of EN 14797. Two principle venting device parameters are pstat and Ef, which is affected by values of vent cover inertia and enclosure volume. Accurate sizing of vents is the most important aspect of vent design. Venting requirements depend in practice on the combustion characteristics of the gas, the state of the flammable mixture (concentration, turbulence, distribution), and the geometry of the enclosure (including the presence of turbulence inducing elements). Combustion characteristics of flammable gases shall be measured according to appropriate methodologies. In this European Standard the combustion characteristics gas explosion constant KG and maximum explosion pressure pmax are used. The gas explosion constant is derived from the maximum rate of pressure rise (dp/dt)max. The latter characteristic and the maximum explosion pressure pmax shall be determined according to EN 13673-2 and EN 13673-1 respectively. 5.2 Venting of isolated compact enclosures A method to size vent openings of compact enclosures is presented. The method applies to isolated enclosures essentially free from turbulence inducing elements (see 5.3.5). Appropriate measures (explosion isolation) shall have been taken to prevent explosion propagation to/and from other enclosures. The method assumes that the explosive atmosphere inside the enclosure is essentially quiescent at the time of ignition. According to this method the vent area shall be calculated using the following equation: ()()[]()[]{}325722,05817,01,01754,00567,0lg1265,0VbarpppKAstatredredG−+−=−− (1) fVEAA= (2) where A
is the geometrical vent area (Ef = 1), in m²; Av
is the vent area of an explosion venting device with efficiency Ed < 1, in m²;



EN 14994:2007 (E) 9 KG
is the gas explosion constant, in bar·m·s-1; pred
is the reduced explosion overpressure, in bar; pstat
is the static activation pressure of explosion venting device, in bar; Ef
is the venting efficiency of explosion venting device; V
is the volume, in m³. Equations (1) and (2) are valid for:  isolated enclosures essentially free from turbulence inducing elements;  KG ≤ 550 bar·m/s;  0,1 bar ≤ pstat ≤ 0,5 bar;  pred ≤ 2 bar;  pred > pstat + 0,05 bar;  V ≤ 1 000 m3;  L/D ≤ 2;  initial conditions: atmospheric;  Ef = 1 for explosion venting devices with an area specific mass of less than 0,5 kg/m2;  Ef = 1 for explosion venting devices with an area specific mass greater than 0,5 kg/m2 and smaller or equal to 10 kg/m2 provided Av/V0,753 < 0,07, where AV is the vent area and V the vessel volume. This is valid for pstat ≤ 0,1 bar and 0,1 bar < pred < 2 bar;  for all other conditions and for explosion venting devices with an area specific mass greater than 10 kg/m2 the efficiency Ef has to be determined by tests (see EN 14797). 5.3 Situations outside the constraints of the basic method (turbulence inducing elements, partially filled enclosures) 5.3.1 General The methods proposed in 5.3.2 to 5.3.4 apply to isolated compact enclosures essentially free from turbulence inducing elements. 5.3.2 Elevated initial pressure The following equation shall be used for estimating reduced explosion pressures when the initial pressure is above atmospheric pressure: ()γ1212+=pppredred (3) where p2
is the elevated initial gauge pressure, in bar;



EN 14994:2007 (E) 10 pred1
is the reduced explosion pressure calculated by the method given in 5.2 for atmospheric
conditions, in bar: pred2
is the actual reduced explosion pressure for elevated initial pressure p2, in bar;
is the exponent, a function of vent area and vessel volume. The value of the exponent
varies inversely with AV/V2/3, where AV is the vent area and V the vessel volume. Plots of
versus AV/V2/3 are given in Figure 1 for propane, ethylene and hydrogen.
Key 1 propane 2 ethylene 3 hydrogen
exponent Figure 12)
— Value of exponent
as a function of AV/V2/3 Figure 1 is valid for initial pressures of up to an overpressure of 3 bar. The solid lines for propane and hydrogen were developed from experimental data. The line for propane shall be used for gases that have KG-values no higher than 1,3 times that for propane. The line for ethylene represents an untested interpolation. The extension of broken lines represents extrapolation. In applying Equation (3) the value used for p2 shall be chosen to represent the maximum pressure at which the protected installation can be operating at the time of the ignition. 5.3.3 Effect of initial turbulence Experimental evidence shows that initial turbulence is important and its effect on the reduced explosion pressure cannot be ignored. However, at present there is insufficient information available to be able to quantify its effects for the type of practical applications for which explosion relief is used.
2) Reprinted with permission from NFPA 68 Guide for Venting of Deflagrations. Copyright 1998 National Fire Protection Association, Quincy, MA 02269. This reprinted material is not the complete and official position of the National Fire Protection Association, on the referenced subject, which is represented only by the standard in its entirety.



EN 14994:2007 (E) 11 5.3.4 Effect of partial filling An estimate of the explosion pressures in compact enclosures provided with a vent and partially filled with an explosive mixture shall be made using Figure 2. Having calculated the vent size for an enclosure completely filled with an explosive gas atmosphere the reduced overpressure for a partially filled enclosure can be estimated. The figure shall only be applied for reduced overpressures below 1 bar for a 100 % filled enclosure. The figure is only valid for a static activation pressure of the venting device pstat = 0,1 bar.
Key X degree of filling (%) Y ratio of maximum reduced overpressure to maximum reduced overpressure for 100 % filling Figure 2 — Pressure reduction of partially filled enclosures as a function of filling ratio 5.3.5 Venting of enclosures containing turbulence inducing elements The necessary venting requirements for enclosures containing turbulence inducing elements such as trays in drying ovens can normally not be calculated with the relationships mentioned above. The turbulence inducing elements may result in strong combustion rate increases during the explosion causing considerably higher pressures than anticipated on the basis of the relationship given in 5.2. To judge whether another more intricate method shall be applied than the one described in 5.2 one can use the guidance given in Annex A. 5.4 Elongated enclosures 5.4.1 General The equations given in this subclause apply to isolated elongated enclosures (with length to diameter/width ratios of between 2 and 10), containing quiescent gas mixtures that are essentially free of turbulence inducing elements (see 5.3.5) and do not contain any bends or changes of cross-section. The equations concern only gases or gas mixtures with a burning velocity close to that of methane or propane. The aim in designing explosion relief for elongated enclosures is to minimise the reduced explosion pressure. This is done by ensuring that explosion vent areas are as large as is practicable and the static activation pressure is as low as is practicable in the prevailing process conditions. Vents shall be positioned, as far as is practicable, to minimise the distance between any potential ignition sources and the nearest vent; this may mean that vents are evenly distributed along the enclosure. Where multiple vents are fitted, each vent shall have the same area and open at the same static activation pressure. The following equations for calculating the reduced explosion pressures are based on test results.



EN 14994:2007 (E) 12 5.4.2 Venting of elongated enclosures vented at each end When the elongated enclosure is vented at each end, either through end vents or side vents close to the end, then the reduced explosion pressure shall be taken to be the maximum value produced by application of Equations (4) to (6). These equations also apply when additional vents are spaced along the enclosure. ()31312/023,0VDLWKSppuistatred+= (4) where pred
is the reduced explosion overpressure, in bar; pstat
is the static activation pressure, in bar; Sui
is the gas burning velocity, in m/s; K
is the vent coefficient (Acs/A) Acs
is the vessel cross sectional area, in m2; A
is the total area of all vents, in m2; W
is the weight per unit area of vent panel, in kg/m2; V
is the enclosure volume, in m3; L
is the enclosure length, in m; D
is the enclosure diameter, in m. bar06,0for;015,0≤=statredpKdp (5) bar06,0for;15,0015,0>+=statredpKdp (6) where pred
is the reduced explosion overpressure, in bar; d = x/D,
where x is the maximum possible distance that can exist between a potential ignition source
and the nearest vent, and D is the diameter of the enclosure; K
is the vent coefficient (Acs/A); Acs
is the vessel cross sectional area, in m2; A
is the total area of all vents, in m2. Equations (4) to (6) shall be used to estimate the vent areas and spacing necessary to limit the reduced explosion overpressure to given value. Equations (4) to (6) are only valid for:  atmospheric conditions;  burning velocity Su ≤ 0,46 m/s (i.e. burning velocity equal to or smaller than that of propane);



EN 14994:2007 (E) 13  volume V ≤ 200 m3;  weight per unit area W: 0,5 kg/m2 ≤ W < 5 kg/m2;  pstat ≤ 0,1 bar;  pred ≤ 1 bar;  2 < L/D ≤ 10. 5.4.3 Venting of elongated enclosures vented along the enclosure When the elongated enclosure is not vented at each end but only by vents along the enclosure, then Equations (5) and (6) may significantly underestimate reduced explosion overpressures. In these circumstances, Equations (4), (7) and (8) shall be applied with the highest value that is calculated being used for the estimate of reduced explosion overpressure. For methane: Kdppstatred070,0+= (7) For propane: Kdppstatred085,0+= (8) where pred
is the reduced explosion overpressure, in bar; d = x/D,
where x is the maximum possible distance that can exist between a potential ignition source
and the nearest vent, and D is the diameter of the enclosure; K
is the vent coefficient (Acs/A); Acs
is the vessel cross sectional area, in m2; A
is the total geometrical vent area, in m2. Equations (7) and (8) are only valid for:  atmospheric conditions;  burning velocity Su ≤ 0,46 m/s (i.e. burning velocity equal to or smaller than that of propane);  volume V ≤ 200 m3;  weight per unit area W: 0,5 kg/m2 ≤ W < 5 kg/m2;  pstat ≤ 0,1 bar;  pred ≤ 1 bar;  2 < L/D ≤ 10. For gases more reactive than propane, enclosures with bends or changes of cross-section, or situations where the gas mixture is flowing no methods for estimating the explosion relief required are available.



EN 14994:2007 (E) 14 5.5 Pipes The method described in this subclause apply to pipes, enclosures with length to diameter ratios of greater than 10. In sizing explosion relief the gas flow through the pipe, which may be significant in industrial plant, needs to be taken into account. The long distances over which the explosion can propagate in pipe work systems can also lead to a detonation developing. The methods presented in this subclause apply to pipes essentially free from turbulence inducing elements (see 5.3.5) unless otherwise mentioned. Figure 3 shall be used to estimate the maximum pressure developed in a smooth straight pipe, closed at one end and vented at the other (the size of the vent is equal to the cross-section of the pipe). It is valid for gases with a fundamental burning velocity of less than 0,6 m/s and for flow velocities of 2 m/s or less. The distance between the ignition source location and vent loca
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