Alternatively mathematical models may be used for the design of isolation systems. There are two types of mathematical models:
a) phenomenological models and
b) computational fluid dynamics (CFD)-based prediction tools.
Phenomenological models rely on experimental data as well but performed in the absence of isolation systems/devices. The experimental data cover a large number of enclosure volumes, pipe diameters and explosive atmosphere types and reactivity. The data in combination with simple theoretical models allow for sound extrapolation of flame propagation in ducts to larger ignition enclosure volumes, higher explosive atmosphere reactivity, etc.
Phenomenological methods heavily rely on measured delay times of detectors and CIE’s, closing times of isolation valves, deployment times of extinguishing barriers, etc. The model also contains a model for prediction of the amount of suppressant needed in extinguishing barriers depending on the local flame speed also supported by experimental data.
Based on these times, predictions can be made for minimum and maximum installation distances of isolation systems/devices, the number of HRD-Suppressors used for extinguishing barriers, etc. To this end several scenarios (variation of explosive atmosphere reactivity and ignition location) should be considered.
Figure A.2 shows an example of the output of a phenomenological design method. The Figure shows the predicted position of the isolation device in the duct (installation distance D measured from the duct mouth) as a function of the ignition source position in the vessel connected to the duct. The isolation device is part of an isolation system consisting of a detector (flame or pressure), a CIE and the device itself. The position of the isolation device is predicted for two types of detection (flame detector or pressure detector) and for two explosive atmosphere reactivities (using pressure detection).
Curve 2 shows the calculated minimum installation distance in the duct measured from the duct mouth as a function of ignition location using pressure detection. Inspection of the Figure shows that, for this configuration, locating the isolation device close to the duct mouth is sufficient provided that ignition occurs towards the centre of the vessel or even further away. The pressure rise due to the explosion in the vessel at these ignition locations reaches the activation pressure of the isolation system sufficiently early to assure that the isolation device is fully deployed before the flame reaches the mouth of the duct. At a distance from the mouth of the duct smaller than 0,25 times the diameter of the vessel the isolation device is not deployed before the flame reaches the mouth of the duct and as a result the isolation device must be installed further down into the duct. This distance increases when the ignition occurs closer to the mouth of the duct.
Curve 3 shows a similar development but for a mixture with a lower reactivity. The curve shows that even when igniting closer to the mouth of the duct the isolation device can still be positioned at the mouth of the duct since the explosion needs more time to reach the duct due to the lower reactivity. Moving even closer, however, the moment of detection starts playing a role. Due to the low reactivity the pressure rise is slow, allowing the flame to propagate deeper into the duct before being detected by the pressure detector. As a result the isolation device needs to be positioned further down into the duct for lower reactivity explosive atmosphere.
Curve 1 shows the optical flame detection case. Since the flame will be detected essentially at the instant it enters the duct (there is a general requirement to install the detector within a couple of diameters from the duct mouth),
is then a longer time available for the explosion pressure to develop and, consequently, turbulence is generated at the duct mouth. When the flame arrives at the flame detector position it accelerates into the duct leaving less time for the isolation device to respond. Hence it must be installed further into the duct at a longer distance. This effect increases with increasing distance of the ignition source from the duct mouth into the vessel.
Since in reality neither the ignition source position nor the reactivity of the explosive atmosphere is known beforehand, the worst case installation distance needs to be chosen. When both flame detection and pressure detection are used the minimum installation distance is determined by the cross point of the curves.
(V, pmax, pa and pipe diameter are kept constant)
Key
1, 2, 3 Z1, Z2, Z3 Location of ignition source 6 curve 3, Pressure detection, Kreduced
4 curve 1, Flame detection D Installation distance 5 curve 2, Pressure detection, Kmax,
Figure A.2 — Effect of ignition location, detection system and K-value on minimum installation distance
Alternatively dedicated CFD-based tools can be used. These dedicated CFD-tools describe explosion processes in detail. They allow for describing the effects of geometrical details, explosive atmosphere reactivity and ignition source location. The tools predict the position of the flame in the geometry at any moment in time as well as the pressure development at any location in the geometry.
As for the phenomenological models a combination of detailed knowledge of the isolation system components: delay in detector and CIE, deployment time of a cloud of suppressant and closing time of an isolation valve, and predicted flame behaviour for various scenarios, the minimum installation distance can be calculated. The pressure resistance of the device allows in combination with the pressures predicted by the model to predict the maximum installation distance.
Also mathematical models should be validated by performing predictions for tests to be performed afterwards. It should be demonstrated that the installation distances are well reproduced by the model. The minimum installation distance calculated by the model should be equal to or larger than the measured value. The maximum installation distance calculated by the model should be equal to or smaller than the measured value. In addition to that the calculated pressure value in front of the isolation device should be equal to the measured one by ± 20 %. The extent
of the validation process should reflect the number of parameters the model can predict as well as the number of isolation devices/systems the mathematical model can handle.
Each dependent parameter used in the mathematical model should be tested to validate applicability. Dependent factors should be tested at the extremes of their range. The extent of the validation programme should be determined by the notified body but a minimum of three different tests should be performed for each parameter the method can predict for each set of isolation system components the method can handle. In total a minimum of 100 tests should be performed in at least 4 different test volumes.
Annex ZA
(informative)
Relationship between this European Standard and the Essential Requirements of
EU Directive 94/9/EC
This European Standard has been prepared under a mandate given to CEN by the European Commission and the European Free Trade Association to provide a means of conforming to Essential Requirements of the New Approach Directive 94/9/EC of 23. March 1993.
Once this standard is cited in the Official Journal of the European Communities under that Directive and has been implemented as a national standard in at least one Member State, compliance with the clauses of this standard given in Table ZA confers, within the limits of the scope of this standard, a presumption of conformity with the corresponding Essential Requirements of that Directive and associated EFTA regulations.
Table ZA.1 — Correspondence between this European Standard and Directive 94/9/EC Clause(s)/sub-clause(s) of this EN Essential Requirements (ERs) of Directive 94/9/EC Qualifying remarks/Notes whole document 1.0.1 Principles of integrated
explosion safety
6 1.0.2 Design consideration
8 1.0.3 Special checking and
maintenance conditions
9 1.0.5 Marking
8 1.0. 6 a) Instructions
6.2, 7 1.1.2 Limits of operating
whole document 1.2.1 Technological knowledge of explosion protection for safe
7.2.3 1.2.9 Flameproof enclosure
systems
5.4 1.5.1 Independent function of
safety devices of measurement and control. Fail safe principles for electric circuits. Safety
related switches independent of software
and command 5.3, 5.4 1.5.2 Safety device failure 5.3, 5.4 1.5.3 Emergency stop controls
5.4 1.5.4 Control and display units
5.4 1.6.2 Emergency shutdown
system
5.4 1.6.3 Hazards arising from
Table ZA.1— Correspondence between this European Standard and Directive 94/9/EC (continued) Clause(s)/sub-clause(s) of this EN Essential Requirements (ERs) of Directive 94/9/EC Remarks
5.4 1.6.4 Hazards arising from
connections
whole document 3.0 General requirements 7.2.1, 7.2.3 3.1.2 Withstanding shock wave
effects of explosions
7.2.1 3.1.3 Accessories to withstand
maximum pressure of explosions
Whole document 3.1.4 Planning protective systems to take account of pressures on pipework, etc.
6.2.2 3.1.5 Pressure relief systems
6.2.3 3.1.6 Explosion suppression
systems
whole document 3.1.7 Explosion decoupling systems
whole document 3.1.8 Protective systems integrated into a circuit with an alarm
WARNING — Other requirements and other EU Directives may be applicable to the product(s) falling within the scope of this standard.
Bibliography
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[15] EN ISO 13850, Safety of machinery – Emergency stop – Principles for design (ISO 13850:2006)
[16] ISO 4225, Air quality – General aspects – Vocabulary
[17] VDI 2263-1, Dust fires and dust explosions, hazards, assessment, protective measures; Test methods for the determination of the safety characteristic of dusts
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[19] Eckhoff, R. K.: Dust explosions in the process industries; Chapter 6. Oxford: Butterworth-Heinemann (1991) [20] Faber, M.: Explosionstechnische Entkopplung VDI-Berichte 701 (1988), S. 659 - 680
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[23] Schuber, G.: Zünddurchschlagverhalten von Staub/Luft-Gemischen und hybriden Gemischen, Schriftenreihe Humanisierung des Arbeitslebens, Bd. 72, VDI Verlag Düsseldorf (1988)
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USA (1997)
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