SIST EN 14491:2012
(Main)Dust explosion venting protective systems
Dust explosion venting protective systems
This European Standard specifies the basic requirements of design for the selection of a dust explosion venting protective system. The standard is one of a series including EN 14797 Explosion venting devices and EN 14460 Explosion resistant equipment. The three standards together represent the concept of dust explosion venting. To avoid transfer of explosions to other communicating equipment one should also consider applying EN 15089 Explosion Isolation Systems. This European Standard covers: vent sizing to protect an enclosure against the internal pressure effects of a dust explosion; flame and pressure effects outside the enclosure; recoil forces; influence of vent ducts. This European Standard is not intended to provide design and application rules against effects generated by detonation reactions or runaway exothermic reactions. This European Standard does not cover fire risks arising from either materials processed, used or released by the equipment or materials that make up equipment and buildings. This European Standard does not cover the design, construction, testing and certification of explosion venting devices that are used to achieve explosion venting).
Schutzsysteme zur Druckentlastung von Staubexplosionen
Diese Europäische Norm legt die grundlegenden Anforderungen an die konstruktive Gestaltung und für die
Auswahl eines Schutzsystems zur Druckentlastung von Staubexplosionen fest. Diese Norm wurde im
Rahmen der Reihe erstellt, zu der EN 14797 Einrichtungen zur Explosionsdruckentlastung und EN 14460
Explosionsfeste Geräte gehören. Die drei Normen zusammen bilden das Konzept der Druckentlastung von
Staubexplosionen. Zur Vermeidung der Übertragung von Explosionen auf andere in Verbindung stehende
Einrichtungen sollte auch EN 15089 Explosions-Entkopplungssysteme beachtet und angewendet werden.
Diese Europäische Norm deckt Folgendes ab:
⎯ Größe der Entlastungsöffnung, um gegen die inneren Druckauswirkungen einer Staubexplosion zu
schützen;
⎯ Auswirkungen von Flammen und von Druck außerhalb des Behälters;
⎯ Rückstoßkräfte;
⎯ Einfluss der Abblasrohre.
Diese Europäische Norm gibt keine Gestaltungs- und Anwendungsvorschriften, durch die Auswirkungen von
Detonationsreaktionen oder unkontrollierten exothermen Reaktionen vermieden werden können. Brandgefahren,
die entweder von den verarbeiteten Materialien, von den vom Gerät benutzten oder freigesetzten
Materialien ausgehen oder die durch Materialien hervorgerufen werden, aus denen Geräte und Gebäude
bestehen, sind nicht Gegenstand dieser Europäischen Norm. Konstruktive Gestaltung, Fertigung, Prüfung und
Zertifizierung von Einrichtungen, die im Fall von Explosionen für die Druckentlastung verwendet werden, sind
ebenfalls nicht Gegenstand dieser Europäischen Norm.1)
Systèmes de protection par évent contre les explosions de poussières
La présente Norme européenne décrit les exigences fondamentales se rapportant à la conception et au choix d’un système de protection par évent contre les explosions de poussières. Cette norme fait partie d'une série à laquelle appartiennent également l'EN 14797 Dispositifs de décharge d'explosion et l'EN 14460 Appareil résistant à l'explosion. Ensemble, ces trois normes illustrent le concept de protection par évent contre les explosions. Pour éviter la propagation de l'explosion à d'autres équipements communicants, il convient également de tenir compte de l'EN 15089 Systèmes d'isolement d'une explosion.
La présente Norme européenne traite des aspects suivants :
- le dimensionnement des évents pour la protection contre les effets de la pression interne d'une explosion de poussières ;
- les effets de la flamme et de la pression à l'extérieur de l'enceinte ;
- les forces de recul ;
- l'influence des canalisations de décharge.
La présente Norme européenne n'est pas destinée à fournir des règles de conception et d'application contre les effets engendrés par les réactions de détonation ou l'emballement des réactions exothermiques. Elle ne traite pas des risques d'incendie provenant soit des matériaux traités, utilisés ou émis par les appareils, soit des matériaux qui constituent les appareils et les bâtiments. Elle ne traite pas non plus de la conception, de la construction, des essais et de la certification des dispositifs de décharge d'explosion ).
Zaščitni sistemi za razbremenitev tlaka eksplozije prahu
Ta evropski standard določa osnovne zahteve glede oblike za izbor zaščitnega sistema za razbremenitev tlaka eksplozije prahu. Standard je eden iz serije standardov, ki vključujejo naprave za razbremenitev tlaka eksplozije iz standarda EN 14797 in opremo, odporno na eksplozije, iz standarda EN 14460. Ti trije standardi skupaj predstavljajo koncept razbremenitve tlaka eksplozije prahu. Da bi se preprečil prenos eksplozij na drugo komunikacijsko opremo, je treba preučiti tudi uporabo ločevalnih sistemov eksplozij iz standarda EN 15089. Ta evropski standard zajema: določitev velikosti oddušnika za zaščito okrova pred učinki notranjega tlaka eksplozije prahu; učinke plamena in tlaka zunaj okrova; sile odsunka; vpliv kanalov oddušnika. Namen tega evropskega standarda ni določiti pravil o obliki in uporabi za zaščito pred učinki reakcij detonacije ali eksotermnih reakcij brez nadzora. Ta evropski standard ne zajema nevarnosti požara zaradi materialov, ki jih oprema obdeluje, uporablja ali sprošča, ali materialov, iz katerih so oprema in zgradbe narejeni. Ta evropski standard ne zajema oblike, gradnje, preskušanja in certificiranja naprav za razbremenitev tlaka eksplozij, ki se uporabljajo za doseganje razbremenitve tlaka eksplozij.
General Information
Relations
Standards Content (Sample)
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Schutzsysteme zur Druckentlastung von StaubexplosionenSystèmes de protection par évent contre les explosions de poussièresDust explosion venting protective systems13.230Varstvo pred eksplozijoExplosion protectionICS:Ta slovenski standard je istoveten z:EN 14491:2012SIST EN 14491:2012en,fr,de01-oktober-2012SIST EN 14491:2012SLOVENSKI
STANDARDSIST EN 14491:2006/AC:2009SIST EN 14491:20061DGRPHãþD
SIST EN 14491:2012
EUROPEAN STANDARD NORME EUROPÉENNE EUROPÄISCHE NORM
EN 14491
August 2012 ICS 13.230 Supersedes EN 14491:2006English Version
Dust explosion venting protective systems
Systèmes de protection par évent contre les explosions de poussières
Schutzsysteme zur Druckentlastung von StaubexplosionenThis European Standard was approved by CEN on 30 June 2012.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC 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 translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management Centre has the same status as the official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre:
Avenue Marnix 17,
B-1000 Brussels © 2012 CEN All rights of exploitation in any form and by any means reserved worldwide for CEN national Members. Ref. No. EN 14491:2012: ESIST EN 14491:2012
EN 14491:2012 (E) 2 Contents Page Foreword .31 Scope .42 Normative references .43 Terms and definitions .44 Venting of enclosures .65 Sizing of vent areas .75.1 General .75.2 Venting of isolated enclosures .85.3 Special dust cloud conditions .95.3.1 General .95.3.2 Pneumatic conveying of product with axial introduction into vessels and silos .95.3.3 Pneumatic conveying of the product with tangential introduction into vessels and silos. 105.3.4 Free fall filling . 115.4 Protection of interconnected enclosures . 115.5 Protection of pipes . 125.6 Influences of vent ducts . 135.7 Design of vent ducts . 145.8 Hybrid mixtures . 156 Supplementary design considerations . 166.1 General . 166.2 Explosion effects external to the vent . 166.2.1 General . 166.2.2 Flame effects . 166.2.3 Pressure effects . 176.2.4 Effects of flameless explosion venting devices . 186.2.5 Recoil forces . 186.2.6 Vacuum breakers . 197 Marking . 208 Information for use . 20Annex A (informative)
Explosion venting of dust filters . 21Annex B (informative)
Explosion venting of cyclones . 23Annex C (informative)
Estimating the L/D ratio when calculating vent areas for elongated enclosures . 24Annex D (informative)
Protection of buildings . 31D.1 General . 31D.2 Calculating the vent area . 31D.3 Calculation of internal surface area . 32Annex E (informative)
Deflectors . 33Annex F (informative)
Significant changes between this European Standard and EN 14491:2006 . 35Annex ZA (informative)
Relationship between this European
Standard and the Essential Requirements of EU Directive 94/9/EC . 38Bibliography . 39 SIST EN 14491:2012
EN 14491:2012 (E) 3 Foreword This document (EN 14491:2012) 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 February 2013, and conflicting national standards shall be withdrawn at the latest by February 2013. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights. This document supersedes EN 14491:2006. Annex F provides details of significant technical changes between this European Standard and the previous edition. 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(s), see informative Annex ZA, which is an integral part of this document. According to the CEN/CENELEC Internal Regulations, the national standards organisations of the following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom. SIST EN 14491:2012
EN 14491:2012 (E) 4 1 Scope This European Standard specifies the basic requirements of design for the selection of a dust explosion venting protective system. This European Standard is one of a series including EN 14797, Explosion venting devices and EN 14460, Explosion resistant equipment. The three standards together represent the concept of dust explosion venting. To avoid transfer of explosions to other communicating equipment, one should also consider applying EN 15089 Explosion Isolation Systems. This European Standard covers: vent sizing to protect an enclosure against the internal pressure effects of a dust explosion; flame and pressure effects outside the enclosure; recoil forces; influence of vent ducts; hybrid mixtures. This European Standard is not intended to provide design and application rules against effects generated by detonation reactions or runaway exothermic reactions. This European Standard does not cover fire risks arising from materials either processed, used or released by the equipment or from materials that make up equipment and buildings. This European Standard does not cover the design, construction, testing and certification of explosion venting devices that are used to achieve explosion venting1). 2 Normative references The following documents, in whole or in part, are normatively referenced in this document and are indispensable for its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. EN 13237:2003, Potentially explosive atmospheres — Terms and definitions for equipment and protective systems intended for use in potentially explosive atmospheres EN 14460:2006, Explosion resistant equipment EN 14797:2006, Explosion venting devices EN 15089, Explosion isolation systems 3 Terms and definitions For the purposes of this document, the terms and definitions given in EN 13237:2003, EN 14460:2006 and the following apply. 3.1 building enclosed, roofed space that contains a working environment that may include process plant, offices and personnel, either separately or together, but is not, in itself, an item of process plant
1) This is covered in EN 14797. SIST EN 14491:2012
EN 14491:2012 (E) 5 3.2 enclosure vessel that forms a distinct and identifiable part of a process plant and to which explosion protection by explosion venting can be applied as described in this European Standard 3.3 hybrid mixture mixture of flammable substances with air in different physical states [SOURCE: EN 13237:2003] 3.4 dust explosion constant KSt maximum value of the pressure rise per unit time (dp/dt)max during the explosion of a specific explosive atmosphere involving dust in a closed vessel under specified test conditions normalised to a vessel volume of 1 m3 multiplied by V1/3 Note 1 to entry: See EN 14034-2. 3.5 gas explosion constant KG maximum value of the pressure rise per unit time (dp/dt)max during the explosion of a specific explosive atmosphere involving gas or vapour in a closed vessel under specified test conditions normalised to a vessel volume of 1 m3 multiplied by V1/3 [SOURCE: EN 14994:2007, 3.8, modified] 3.6 geometric vent area Av ratio of required vent area A and venting efficiency Ef for the venting device Note 1 to entry: The geometric vent area is the minimum cross-sectional flow area of the vent opening, taking into consideration the possible reduction of the cross section, e.g. by back pressure supports, retaining devices and parts of the explosion venting device which remain after bursting or venting. 3.7 required vent area A vent area required for the explosion venting assuming optimum venting efficiency 3.8 overpressure required vent area pressure above ambient pressure 3.9 maximum explosion overpressure pmax maximum overpressure occurring in a closed vessel during the explosion of an explosive atmosphere and determined under specified test conditions Note 1 to entry: See EN 14034-1. SIST EN 14491:2012
EN 14491:2012 (E) 6 3.10 pipe connection between two or more enclosures Note 1 to entry: A pipe cannot be explosion protected by the explosion venting methods for enclosures described in this European Standard. 3.11 explosive atmosphere mixture with air, under atmospheric conditions, of flammable (combustible) substances in the form of gases, vapours, mists or dusts, in which, after ignition has occurred, combustion spreads to the entire unburned mixture 3.12 maximum reduced explosion overpressure pred,max resulting maximum overpressure generated by an explosion of an explosive atmosphere in a vessel at optimum fuel concentration, after effective explosion venting or explosion suppression 3.13 maximum rate of explosion pressure rise (dp/dt)max maximum value of the pressure rise per unit time during explosions of all explosive atmospheres in the explosion range of a combustible substance in a closed vessel determined under specified test conditions Note 1 to entry: This parameter is numerically identical with the parameter KSt, if the test vessel is 1 m3 in volume, but the unit of the latter is bar·m·s–1 whereas the unit of the (dp/dt)max is bar·s–1. Note 2 to entry: See EN 14034-2. 3.14 maximum external overpressure pext,max external maximum value of the overpressure generated by vented dust explosion 3.15 static activation overpressure pstat differential pressure at which the retaining element activates such that the venting element is able to open [SOURCE: EN 14797:2006, 3.11] 3.16 vacuum breaker device which prevents damage to a vessel when the internal pressure falls below atmospheric pressure 4 Venting of enclosures Explosion venting is a protective measure for enclosures by which unacceptably high internal explosion overpressures are prevented. Weak areas in the walls of the enclosure open at an early stage of the explosion, burning and/or un-burnt material and combustion products are released and the overpressure inside the enclosure is reduced. Information required for calculation of the vent area includes the explosion resistance of the enclosure, the explosion characteristics of the dust, the shape and size of the enclosure, the static activation overpressure and other characteristics of the vent closure, and the condition of the dust cloud inside the enclosure. Explosion venting shall not be performed if unacceptable amounts of materials that are classified as poisonous, corrosive, irritant, carcinogenic, teratogenic or mutagenic can be released. Either the dust or the SIST EN 14491:2012
EN 14491:2012 (E) 7 combustion products can present a hazard to the immediate environment. If there is no alternative to explosion venting, an endangered area shall be specified. NOTE 1 There is no direct guidance for estimating an endangered area for toxic or other harmful emissions, but the safe discharge area for external flames calculated according to the formulae in 6.2 gives some indication of the area required in direct line from the vent. Harmful emissions will be dispersed by air movements, however, and an extensive area in lateral directions can be required. This European Standard shall be used together with EN 14797 and EN 14460. Venting neither prevents or extinguishes an explosion; it only limits the explosion overpressure. Flame and pressure effects outside the enclosure and flying debris are to be expected and suitable precautions shall be taken. Fires inside the enclosure can also occur. NOTE 2 If burning continues inside the vented vessel after the explosion, it can cause damage to the vessel, even though it has been protected from damage caused by overpressure. The increase of the length-to-diameter ratio of an enclosure results in an increase of the rate of flame propagation. This is taken into account in the formula for vent sizing (see Clause 5). Enclosures in this European Standard are limited to L/D ≤ 20. In a system consisting of connected enclosures, a dust explosion ignited in one enclosure can propagate through the connection, generating increased turbulence, perhaps causing some pre-compression and then act as a large ignition source in a connected enclosure. This combination of effects can enhance the violence of the secondary explosion. The venting requirements of the system thus need to be increased, or the enclosures isolated (see 5.4). Internal dust explosions can endanger buildings or parts of buildings and explosion venting can be applied to protect the integrity of the building. A separate method for calculating the venting requirements for buildings is given in Annex D. The effects of internal or external obstructions on venting effectiveness shall be taken into account. Recoil forces shall be taken into account when considering the location and distribution of the vent area. Explosion venting devices shall be positioned so that the effectiveness of the venting process is not impeded. Positioning shall be such that personnel and the nearby plant will not be at risk from the venting action. If the enclosure is small and relatively symmetrical, one large vent can be as effective as several small vents of equal combined area. For large enclosures, the location of multiple vents to achieve uniform coverage of the enclosure surface to the greatest extent practicable is recommended. NOTE 3 In the formulae presented in this standard, it is important to use the correct units, which are not always SI-units. The units are indicated for every parameter used in the limits of application. Where log is used in the formulae, log10 is meant. 5 Sizing of vent areas 5.1 General Accurate sizing of vents is the most important aspect of vent design. The size of the vent depends on the explosion characteristics of the dust, the state of the dust cloud (concentration, turbulence and distribution), the geometry of the enclosure and the design of the venting device. Two explosion characteristics of the dust are the maximum overpressure pmax and the dust explosion constant KSt. For cubical enclosures, pmax and KSt are essentially independent of enclosure volume. The volume of the enclosure and the length-to-diameter ratio L/D relevant to the shape of the enclosure and the position of the explosion vent are required for sizing vents. The explosion resistance of the enclosure pred,max is also required for vent sizing. All parts of the enclosure, e.g. valves, sight-glasses, man-holes and ducts, that are exposed to the explosion pressure shall be taken into account and the explosion resistance of the weakest part shall be taken as the explosion resistance for the enclosure. SIST EN 14491:2012
EN 14491:2012 (E) 8 The two principal vent device parameters are the static activation overpressure pstat and the venting efficiency of the venting device. When sizing vents, the nominal value of the static activation pressure pstat can be used when the tolerance range of the static activation overpressure does not exceed ± 25 %. Otherwise, the maximum value of the tolerance range of the static activation overpressure shall be used. A is the required venting area that shall be fitted to the enclosure assuming the venting efficiency factor of the venting device is 1 and that therefore the effective venting area is equal to the geometric venting area. Some venting devices have a venting efficiency factor less than 1, and the effective venting area is thus less than the geometric venting area. To compensate for the lower efficiency of the venting device, the geometric venting area Av shall be larger than the required vent area A. fv/EAA= (Ef: venting efficiency) (1) NOTE See EN 14797 for details. 5.2 Venting of isolated enclosures The following formulae are designed to calculate vent areas for most practical applications: an enclosure completely full of a turbulent dust cloud of optimum dust concentration. The formulae shall apply to single enclosures where appropriate measures (explosion isolation) have been taken to prevent flame propagation between enclosures. For enclosures, the following formulae allow the calculation of the required vent area A. The required vent area can, in practical applications, be divided into several smaller areas as long as the total area equals the required vent area: a) 0,1 bar overpressure ≤ pred,max < 1,5 bar overpressure ()DLCBA/log1×+= in m2 (2) with ()[]753,05,0maxred,stat569,0maxred,Stmax51,027,010264,3VpppKpB××−×+×××⋅=−−− (3) ()758,0log305,4maxred,+×−=pC (4) b) 1,5 bar overpressure ≤ pred,max ≤ 2,0 bar overpressure BA= (5) The formulae are valid for: enclosures volume 0,1 m3 ≤ V ≤ 10 000 m3 ; static activation overpressure of the venting device 0,1 bar ≤ pstat ≤ 1 bar; for pstat < 0,1 bar, use pstat = 0,1 bar; maximum reduced explosion overpressure 0,1 bar < pred,max ≤ 2 bar; and pred,max shall be at least pstat + 2 times the tolerance range of pstat KSt and maximum explosion overpressure 5 bar ≤ pmax ≤ 10 bar for a dust specific parameter of 10 bar·m·s–1 ≤ KSt ≤ 300 bar·m·s–1; SIST EN 14491:2012
EN 14491:2012 (E) 9
5 bar ≤ pmax ≤ 12 bar for a dust specific parameter of 300 bar·m·s-1 < KSt ≤ 800 bar·m·s–1; initial process conditions conditions prevailing inside the protected enclosure at the moment of ignition: absolute pressure ≤ 110 kPa; oxygen concentration ≤ 21 %; temperature between -20 °C and +60 °C;
NOTE 1 The formulae can be applied outside this temperature range if the explosion characteristics are corrected to the actual process conditions. length-to-diameter ratio of the vessel 1 ≤ L/D ≤ 20
NOTE 2 Examples for calculating L/D are given in Annex C. If one or more of the above conditions are not fulfilled the applicability of the above formula shall be verified. 5.3 Special dust cloud conditions 5.3.1 General Subclause 5.3 outlines vent area calculations for specific situations verified by testing. Vent areas, which have been sized in accordance with 5.3, can be used for these specific situations provided the parameters stay within the range of validity given for the formulae. 5.3.2 Pneumatic conveying of product with axial introduction into vessels and silos The following empirical formulae may be used to calculate the required vent area A for pneumatic filling of vessels where the filling line is axial near the centre of the roof. NOTE 1 A typical example is a silo filled from a pipe in the centre of the roof. For vessels with a height L ≤ 10 m: ))/log(1(DLYXA×+= in m2 (6) For vessels with a height L > 10 m: ))/log(1(1,0DLYXLA×+××=in m2 (7) with FStmax red,max red,Z011,0)7,3log5,5)6log6,8(/1(DKppDX×××+×−−××= (8) 27,1max red,0715,1−×=pY (9) where
L/D is the length-to-diameter ratio of the vessel;
NOTE 2 Examples for calculating L/D are given in Annex C. SIST EN 14491:2012
EN 14491:2012 (E) 10
DF is the diameter of conveying pipe;
DZ is the effective diameter of the vessel and is calculated as follows: 34ZπVD= (10) The formulae are valid for: axially filling near the centre from above through one pipe with a diameter DF (in m) into a vessel/silo without obstructions (measurement devices are not taken into account); vessel volumes
10 m3 ≤ V ≤ 250 m3; maximum volume flow rate
2 500 m3/h; air conveying velocities
vL ≤ 30 m · s-1; diameter of the pipe
DF ≤ 0,3 m; static activation overpressure of pressure venting device
pstat ≤ 0,1 bar; maximum reduced explosion overpressure
0,1 bar < pred,max ≤ 2 bar; and pred,max shall be at least pstat + 2 times the tolerance range of pstat; maximum explosion overpressure
pmax ≤ 9 bar; dust specific characteristic
50 bar·m·s-1 ≤ KSt ≤ 300 bar·m·s-1. NOTE 3 The formulae can be used for vessels with integrated filters as long as the enveloping volume of the filter elements is less than 5 % of the overall vessel volume. The pressure resistance of these integrated filters needs to be at least equal to that of the vessel. Separate filters on top of the vessel with a chute into the vessel require explosion isolation and explosion venting of these filters. 5.3.3 Pneumatic conveying of the product with tangential introduction into vessels and silos The following empirical formulae may be used to calculate the required vent area A for pneumatic filling of vessels where the filling line is mounted tangential at the perimeter near the top of the silo. ))/log(1(DLYXA⋅+= in m2 (11) with +−−−=max red,Stmax red,Zlog)5,5()513,0)44(log)6,8(()1((pkKpkDX FStSt011,0)191,0)69(DKK×××+ (12) kpKeY27,1max red,129st166,0−××= (13) with k = 1 for 0,1 bar ≤ pred,max ≤ 1 bar; k = 2 for 1 bar < pred,max ≤ 1,7 bar. SIST EN 14491:2012
EN 14491:2012 (E) 11 The formulae are valid independent from the product load of the conveying stream in case of a tangential pneumatic filling for: tangential product introduction through one pipe with a diameter of DF ≤ 0,2 m; round vessels/silos without obstructions (measuring devices are not to be taken into account); vessel volume
10 m3 ≤ V ≤ 120 m3; length/diameter ratio
L/D with 1 ≤ L/D ≤ 5; NOTE 1 Examples for calculating L/D are given in Annex C. maximum volume flow rate
2 500 m3/h; air conveying velocities of
vL ≤ 30·m · s–1; static activation overpressure of pressure venting device:
pstat ≤ 0,1 bar; maximum reduced explosion overpressure:
0,1 bar < pred,max ≤ 1,7 bar and pred,max shall be at least pstat + 2 times the tolerance range of pstat; maximum explosion overpressure:
pmax ≤ 9 bar; dust specific characteristic:
100 bar·m·s–1 ≤ KSt ≤ 220 bar·m·s–1; DZ is calculated according to Formula (10). Alternatively the calculation according to 5.3.2 may be used, taking into account the stated boundary conditions. NOTE 2 The formulae can be used for vessels with integrated filters as long as the enveloping volume of the filter elements is less than 5 % of the overall vessel volume. The pressure resistance of these integrated filters needs to be at least equal to that of the vessel. Separate filters on top of the vessel with a chute into the vessel require explosion isolation and explosion venting of these filters. 5.3.4 Free fall filling Formulae (6) to (10) may be used to calculate the required vent area in case a product enters the vessel by free fall (gravity) from, e.g. a rotary valve or screw feeder. The feed rate shall be limited to smaller or equal 8 000 kg·h–1 and the (equivalent) diameter of the feed opening has to be substituted for DF in the formulae. Apart from these requirements, the conditions remain the same as for the numerical formulae given in 5.3.2. NOTE The formulae can be used for vessels with integrated filters as long as the enveloping volume of the filter elements is less than 5 % of the overall vessel volume. The pressure resistance of these integrated filters needs to be at least equal to that of the vessel. Separate filters on top of the vessel with a chute into the vessel require explosion isolation and explosion venting of these filters. 5.4 Protection of interconnected enclosures 5.4.1 Vent areas determined by the Formulae (1) to (5) are too small if a dust explosion propagates from one vessel into another through a pipe. Increased turbulence, pressure piling and broad flame jet ignition may result in an increased explosion violence, especially with duct length > 6 m. This results in an elevated maximum reduced explosion overpressure. Measures for explosion isolation in the connecting pipe are therefore needed in most situations. SIST EN 14491:2012
EN 14491:2012 (E) 12 In the following sub-clauses, two alternative methods are presented for specific situations without explosion isolation. 5.4.2 Explosion venting may be used without explosion isolation for vessels interconnected with pipes having a nominal diameter up to 300 mm and a connecting length up to 6 m and for dusts with KSt values not exceeding 200 bar·m·s-1, in accordance with the following criteria: a) Both vessels of the same size (size differences not greater than 10 %) are to be vented according to Formulae
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