CC BY-NC-ND
0IChemE
0957-5820/98/S10.00+0.00 © Institution of Chemical Engineers Trans IChemE, Vol 76, Part B, November 1998
AN EVALUATION OF NEW PROCEDURES FOR TESTING
EXPLOSION ARRESTERS
G. O. THOMAS and A. TEODORCZYK*
Centre for Explosion Studies, The University of Wales, Aberystwyth, UK *Warsaw University of Technology, Warszawa, Poland
The paper first outlines the practical need for explosion arresters, including pressures arising from recent legislation. The nature of pipeline explosions is then considered, including the problem of potential transition to detonation together with the implications for arrester testing. Current testing procedures for flame and detonation arresters are then reviewed and potential problems identified. Finally, recent results from potentially more rigorous and reproducible testing approaches are discussed.
Keywords: explosion arresters; testing procedures; detonation; flame acceleration.
INTRODUCTION
Explosion arresters provide an attractive and widely used method for the suppression of explosions in pipelines that transport potentially combustible gases or vapours. They are also used on the vent pipes of storage tanks containing flammable liquids and many other situations where, for safety reasons, it is necessary to halt the propagation of an accidentally initiated combustion wave.
An even wider use of arresters is being stimulated by international protocols aimed at reducing the emission of volatile organic compounds (VOCs) to the atmosphere. In November 1991, the UK (together with 20 other parties) signed a new protocol to the United National Economic Commission for Europe (UNECE) Convention on LongRange Transboundary Air Pollution. Two European Commission Directives were issued to control VOC emissions: one to reduce vapour emissions during gasoline distribution and the other to limit releases during the refuelling of automobiles. There are therefore increasing pressures on the process industries to improve their procedures for limiting the release of waste products to the atmosphere. This requires the development of vapour recovery systems, combined with thermal oxidizers, that can render the waste materials harmless. During such procedures potentially hazardous mixtures can be formed accidentally.
Deflagration and detonation arresters are thus required as part of any new installations constructed to implement vapour control. In Europe a further Directive lays down requirements for equipment and protective systems, including flame and detonation arresters, for use in potentially explosive atmospheres. These directives now result in increased probabilities of forming explosive mixtures, often with multi-component fuels and waste gases.
Given their importance as safety and environmental protection devices, it is disturbing to find that little detailed attention has been given to the fundamental mechanisms by which arresters are effective. Similarly, it is somewhat surprising, given their widespread use, that existing procedures for evaluating arresters do not address fundamental aspects of arrester performance. This is often
compounded by the lack of precise definition of the terminology used to describe the different explosion events that are possible.
Further, there are at present no universally agreed guidelines for such evaluations and there are concerns as to reproducibility in the application of existing standards. These concerns may be summarized as: uncertainty and variability in flame acceleration rates between tests and between different facilities; an uncertainty in what constitutes the most severe test of a detonation arrester; uncertainty as to how to repeatably generate test conditions for over-driven detonations and uncertainty in the application and interpretation of endurance burning tests.
In the present paper, the factors that can lead to pipeline explosions are outlined and current testing procedures and their perceived deficiencies are discussed. Finally, the results from tests with new or modified procedures that give rise to more reproducible test conditions are summarized and their potential for application in practical testing procedures are discussed.
FLAME ACCELERATION AND TRANSITION TO DETONATION
Pipelines form a crucial part of process plant, conveying chemical materials from one section to another. In many instances the gases and vapours they contain are potentially combustible. If the contents mix accidentally with air or other oxidant then an explosive atmosphere can be formed. Once ignited the confining nature of the pipe, often coupled with their long lengths, can readily lead to the development of an explosion.
After ignition by a source, the hot combustion products first expand at essentially constant pressure, and this, together with wall confinement, leads to a bulk displacement of the burnt and unburnt gases in the tube. Initially the flame front, the interface between burnt and unburnt gases, will be smooth and the reaction front will propagate relative to the unburnt mixture at the laminar burning velocity Su, the laminar phase. This laminar burning velocity is an
Figure 1. Sketches showing transition from laminar to turbulent conditions (a) in pipe flow and (b) for a propagating flame front.
essentially fundamental property of the mixture and can be derived from basic physical and thermodynamic parameters. For more discussion of laminar flames see, for example, Kuo1. If the combustion wave persists then the flow can rapidly become turbulent as the mean velocity of the unburnt gas ahead of the flame increases, see Figure 1.
Turbulence is significant in explosion development as the fluctuations in the flow field cause the flame front to become distorted and the area increase associated with this causes increased rates of combustion of the fuel on a mass basis. The apparent propagation rate of the combustion front increases and can be represented as a turbulent burning velocity, St. The energy release from this increased mass burning rate is then communicated to the bulk flow further increasing the turbulence intensity. The result is a positive feed-back system that can lead to rapid flame acceleration. This can occur relatively easily in a confined pipe, but can also arise in larger more unconfined situations if sufficient congestion is present, i.e. pipe racks and vessels. More detailed reviews of the development of turbulent explosions have been given by Lee . In pipelines the degree of flame acceleration will be controlled by a number of factors. One of the most important is the surface roughness, whose influence is already well known to process engineers through its influence on pressure losses. The presence of bends, junctions and area changes can also lead to explosion enhancement. If the flame acceleration is fast enough a shock wave can form. Beyond this point a new regime of shock initiated combustion can develop and there is a possibility of a transition to detonation.
In a detonation the chemical reactions are initiated in a different way to those in deflagration. In deflagration the combustion reactions are strongly dependent on heat and mass diffusion in the region of the energy release zone, and are more complex in turbulent combustion fronts. In detonations, however, the reactions are initiated by the pressures and temperatures associated with the shock. The initial pressure rise is called the von Neumann peak and the time before exothermic reaction starts is called the
induction time. As the shocked gas is continually receding from the shock front with time, this gives rise to an induction zone, the zone between the shock front and the onset of exothermic reaction. This is shown schematically in Figure 2.
Stable detonations propagate at a unique velocity, equal to the sum of the sound speed and local gas velocity at a location slightly downstream of the shock, at the end reaction of the zone, the Chapman-Jouguet or CJ state. Simple theory shows that important macroscopic detonation properties, such as velocity and peak pressure, can be computed accurately using only the thermodynamic properties of the initial gas mixture. The pressure and velocity of most fuel-air detonations are close to 18 bara peak pressure and 1800ms1. The pressure histories also exhibit characteristic self similar features. Detonation theory and the calculation of detonation properties are discussed in detail by Nettleton3.
The initiation of a detonation often involves a discontinuous transition from a more loosely coupled shock and reaction wave. This point can be reached by direct generation of the lead shock, as in the case of direct initiation4'5'6, or as the result of flame acceleration in a confined region7. If a sufficiently strong shock front is
Figure 2. Sketch showing the evolution of a detonation behind an initiating shock.
Distance
Figure 3. Schematic of the final stages of deflagration to detonation transition. (a) Initial shock-flame complex. The incident shock (S) and exothermic reaction front (R) propagate together. (b) Hot spot (HS) formation. Energy release here may also lead to a slight acceleration of the leading shock (A). (c) Transition leading to an overdriven detonation O. (d) Steady state detonation D.
formed ahead of an accelerating flame, the shock induced reactions (often but not always correctly called auto-ignition) can lead to the formation of a so called 'hot spot'. Furthermore, if the conditions of temperature and gradients of extent of chemical reaction are appropriate, this reaction centre can increase coherently leading to a rapid localized explosion in the shocked gas, as described by Khokhlov et al.8. This leads to a second shock wave that rapidly manifests itself as an overdriven detonation.
The eventual transition in the flame acceleration case is often termed deflagration to detonation transition, or DDT. In this case, a local and rapid change in the combustion mechanism occurs giving rise to the potential of extremely large overpressure, potentially of the order of 100 bar for gases at initially atmospheric pressure. The mechanism is shown schematically in Figure 3 and a typical pressure trace from a pressure gauge close to a transition is shown in Figure 4. In either case, after transition, the peak overpressure decays rapidly to the C-J equilibrium value. Notwithstanding this, the magnitude of the pressures generated during a transition to detonation means that it is still seen as potentially the most severe explosion that an arrester must withstand by many engineers, although this
3Q. 30
3 4 5 6 7 8 9
Time (ms)
Figure 4. Typical experimental pressure history observed in the vicinity of an actual deflagration to detonation transition event.
may not be the case in practice, an issue addressed by Thomas9. The practical problem in investigating DDT and of explosion arrester response is how such transitions can be generated with sufficient reproducibility to allow the process to be studied and understood in a quantitative manner.
BASIS FOR ARRESTER OPERATION
Explosion arresters are basically heat and momentum dissipation devices. To prevent passage of an explosion from one section of a pipe to another an explosion arrester must restrict the passage of high velocity and high temperature reaction products. This is normally done by different arrangements that give a large number of narrow channels. The width of the channels is such that there is rapid heat transfer to the walls cooling the gas and hopefully quenching combustion. It is assumed, therefore, that the channels slow and cool the gas flow such that any high pressure gases emerging from the arrester on the downstream side are below the ignition temperature of the downstream gas. This restriction in flow is crucial to operation but it also limits the normal performance characteristics of the arrester, where it restricts the desired process flows. Any explosion arrester must thus strike a compromise between these two conflicting requirements.
The aim of standard explosion arrester testing is to provide information on the relative explosion mitigation performance characteristics of different arresters, in providing these desired losses under explosion conditions . A number of such testing protocols currently exist or are under development. Some existing and proposed standards are reviewed briefly in the next section.
EXISTING STANDARDS FOR ARRESTER TESTING
A German standard has been in use for many years. This requires that deflagration arresters be placed no more than 20 pipe diameters from any potential ignition sources, thus reducing the possibility of flame acceleration and any significant overpressures at the arrester when the flame impinges on it. Other existing requirements are closer to the current United Kingdom procedures for testing flame and detonation arrester performance, BS724410. There is also a requirement for endurance burning tests. To qualify, the standards require that the arrester withstand a set number of flame or detonation impacts without combustion propagating beyond the arrester.
Other standards currently set world-wide include that published by International Maritime Organisation (IMO)11 and the more recent US Coast Guard standard1 2 for marine vapour recovery systems. A new Canadian Standard is now in force, which is again similar in many respects to other existing standards1 3 .
At present no common European standard exists and there are only a few countries that provide guidelines for their arrester testing or installation. To provide a European standard a new CEN committee was convened and a new European CEN standard for flame and detonation arresters for general use was released for general comment in 19971 4 . Testing has been recognized as an important part of this specific standard for flame and detonation arresters15.
The working group's initial work has been based on existing national standards and drafts (BS 7244, DIN). It has been found that problems of repeatability and of efficiency (in terms of the total number of tests required) still exist in the testing of both defl agration and overdriven detonations. Stable detonation tests are easily reproduced and are well understood.
Appropriate testing will be necessary due to the new EU directive adopted on March 10th 1994: 'Directive on the approximation of the laws of the member states concerning equipment and protective systems intended for use in potentially explosive atmospheres'. Definitions and practical loads to be tested are given in prEN 1127 'Safety of machinery; fire and explosions: Part 1: Explosion prevention and protection' where resistance to deflagration, detonation and long time burning is required for fl ame arresters.
Other CEN standards for explosion protected equipment are near completion and will have to refer to the fl ame arrester standard (CEN/TC 150/WG 7; 'Fork-lift trucks for use in potentially explosive atmosphere' and CEN/TC 270/ WG 2; 'Safety requirements for the design and construction of internal combustion engines for use in potentially explosive atmosphere'). Detailed requirements concerning the installation of fl ame arresters will be based on a draft proposal for a Council Directive 'Directive concerning minimum requirements for the safety and health protection of workers potentially at risk from explosive atmospheres' which is in preparation. More special directives aim at a reduction of the emission of Volatile Organic Compounds (VOC Stage 1 and Stage II) and will require widespread application of flame arresters in Vapour Recovery Units (VRU) when enforced. In compliance with existing ADR and RID regulations, road and rail tankers for flammable liquids have had to be equipped with flame arresters for a long time; a specification that equipment in accordance with European standards is still lacking.
CURRENT PROCEDURES
For flame arresters, the basis of an evaluation is often a determination of the maximum length of flammable gas mixture upstream of the arrester for which a flame does not pass through the arrester. Once this critical condition for successful arrester operationhave been identified, the ability of an arrester to prevent flame transmission must then be shown to be repeatable over ten tests. This approach is based on the principle that longer lengths of pipe result in higher flame velocities and thus present a more severe test of the arrester.
There is also a requirement that the arrester must be able to withstand a continuous flame established due to the combustion of a flammable mixture flowing through the arrester, without allowing a flame to propagate through the arrester and ignite the gas downstream.
For detonation arresters, tests are also required where overdriven detonations are incident on the arrester. Overdriven detonations, which are associated with deflagration to detonation transition (DDT), can exhibit transient velocities up to 50% in excess of the CJ value. The corresponding peak pressures can vary between 70 and 100 bara.
The current standard implicitly assumes that the worst case conditions correspond to the point at which DDT
occurs. Some have argued that there is an increasing risk as the length of detonating gas before the arrester increases. Recent studies would appear to have proved these fears unfounded and the point at which transition to detonation occurs is now widely considered to be the worst case. Even so, the problem exists of establishing a means of generating such conditions repeatably at a wide range of facilities.
DEFICIENCIES IN CURRENT PROCEDURES
Several studies have been undertaken recently to investigate deficiencies and requirements during existing procedures for arrester testing. In a wide ranging study of the US Coast Guard test procedures several major problem areas were identified12. These involved:
(i) the uncertainty and variability in flame acceleration rates between tests, and also between different facilities,
(ii) the uncertainty in what constituted the most severe test of a detonation arrester,
iii) uncertainty in the repeatability of generating test condition for over-driven detonations.
More recently, in Europe, whilst developing a small scale arrester test facility problems were reported similar to those listed above1 7 . The study also concluded that there were a number of deficiencies in the existing procedures. The main deficiencies were:
(i) the reliance placed on pipe length before the arrester;
(ii) the use of flame velocity data, but not of pressure data;
(iii) the stochastic nature of the flame accelerations obtained;
(iv) testing against steady detonations and not transient overdriven detonations.
The stochastic nature of the results can be seen in Figure 5. Figure 5(a) shows the variation in flame velocity at an arrester for essentially identical initial conditions. There is some degree of correlation between the flame velocity and pressure, Figure 5(b), but it would still require far more than ten tests to obtain ten results at the specific conditions of interest.
The present position regarding the test procedures for endurance burning for flame and detonation arresters is also unsatisfactory for regulatory authorities, manufacturers and users. The current test procedures in various standards have been summarized recently9 .
ORIGINS OF PRESENT DEFICIENCIES
Under existing guidelines, problems arise due to the stochastic nature of the flame acceleration process. This is due to the positive influence of gas turbulence on combustion2,7 . Turbulence arises due to viscous forces that distort the velocity flow field. In explosions this is particularly severe when obstacles are present in the flame's path. The initial flow generates turbulence which increases combustion rates which in turn increases the flow ahead of the flame, giving increased turbulence and even faster combustion velocities.
If the degree of flame acceleration is sufficient, a shock wave can form ahead of the combustion front and conditions are attained where transition to detonation occurs. The time
Figure 5. Graphs showing variations in (a) measured flame velocities and (b) correlation between pressure and velocity, recorded at an explosion arrester for essentially identical initial conditions.
and position along the tube at which this occurs is very strongly dependent on the early stages of flame acceleration. This leads to a severe problem when testing detonation arresters against transition to detonation as, at a fixed location, significant variations in measured peak overpressures result from otherwise identical initial conditions. It may thus take many tests to achieve the number of overdriven cases required under some testing protocols.
In addition, the initial flame acceleration phase has a factor determined by the resistance to flow of the arrester itself, depending on its construction, thus introducing a further non-quantifiable and irreproducible element into the test.
RECENT INVESTIGATIONS OF REVISED TESTING PROCEDURES
In the light of the deficiencies identified above, a programme of experimental work has been undertaken to investigate further the practicality of the suggestions made by Thomas and Oakley1 7 on possible new approaches to arrester testing methods. A key concern in this work was the need to establish procedures that could be implemented by any testing agency, and give reproducible test conditions. The initial work on 50 mm diameter pipes was reported recently by Teodorczyk and Thomas19 and the results are summarized in the remainder of this section. The primary diagnostic used in this work was that of pressure measurement. A number of gauges were used, with gauges 0.1m immediately before and after any attenuating device used. Any additional gauges upstream or downstream of the attenuating sections were located 0.2 m apart.
Deflagration Arrester Test Configuration
Flame acceleration occurs in smooth pipes due to turbulence induced by shear arising from viscous interactions of gas with the tube wall. In this case the process is highly dependent on the initial stages after ignition. If this initial stage develops in a slightly different way each time, then subsequent flame development will always be different in different tests due to the high inter-dependency between the combustion and flow-wall interaction and turbulence generation. One means of overcoming this is to introduce repeated obstacles into the tube so that the obstacle generated turbulence dominates that from the pipe alone and the acceleration is more repeatable. By varying obstacle size and position, variations in flame speeds can be obtained. The ratio of the tube cross section obstructed by the obstacle to the unobstructed tube cross section, often termed the blockage ratio, was 37% in all tests.
Such an experimental arrangement was developed recently, and is shown schematically in Figure 6. It comprises the primary flame tube, diameter 50 mm, whose length can be varied up to 9 m, fitted with ports for the placement of pressure gauges to measure shock development, and photodiodes and ionization probes to monitor the combustion. Pressure and flame detectors were also placed just prior to and after the arrester element, within the arrested casing. The accelerating sections and ignition location can be varied at will along the pipe. The accelerating section, of length M, is formed from a repeated ring obstacle configuration, as shown again in Figure 6.
Figure 6. Schematic of flame acceleration apparatus and accelerating section.
Figure 7. Schematic of the apparatus for generating overdriven detonation test: (a) Basic structure of the test apparatus, (b) anticipated velocity profile and (c) sketch of various phases (i) steady incident detonation, (ii) decaying decoupled phase and (c) overdriven phase after transition.
Detonation Arrester Test Configuration
A number of the studies have been concerned with the conditions for an eventual transition to detonation. A review of all stages of DDT, from the development of initial flame front instabilities to complex shock-flame interaction and eventual transition is available2'7'8. All show that once a critical shock velocity has been achieved then transition to detonation becomes inevitable. Thus if these shock conditions could be established during detonation arrester testing, repeatable testing against transition and overdriven detonations would be possible. A method for achieving this has been tested using a detonation to generate an initial shock sufficiently strong to lead to incipient DDT.
The arrangement, shown schematically in Figure 7 (a), consists of a initial detonation booster section formed from a small volume of a reactive and highly detonable gas mixture that is readily ignited by an electric spark and ensures that a detonation is rapidly established in a first donor section. The next section is an attenuator section, capable of decoupling the shock and reaction associated with the detonation front. The requirement is sufficient attenuation of the shock front so that a sufficient volume of gas can undergo transition to detonation occurs in the acceptor section. Once conditions are optimized, the arrester can be located at the appropriate transition point. For certain gases, the transition process is more reproducible if some partial obstruction is placed at the start of the acceptor section. Figure 7 (b) shows the expected velocity profiles along the tube and Figure 7 (c) gives details of the development of the various phases of initial steady detonation, quenching region and transition to detonation.
Previous studies have shown that an inert air gap can be used successfully as an attenuation device5 and that on a small scale this can be achieved easily using slide valves. This is not practicable on a larger scale and the use of sections with acoustically absorbing walls has also been investigated.
Three fuels were used, to represent the three classes of reactivity noted in the British standard BS 7244. These are, in increasing reactivity, propane, ethylene and hydrogen, all stoichiometric with respect to air. Combustion and pressure wave motions were monitored using photodiodes and pressure gauges.
Flame Acceleration Test Results
Typical pressure and light emission histories obtained in these tests are shown in Figure 8, for each mixture. Strong flame acceleration was obtained in all cases, and the emission records for ethylene- and hydrogen-air indicate that the arrester failed in these cases. In this case, the obstacles were spaced one tube diameter apart (S/D =1) and the overall length of the accelerating section was equal to 1 m (MID = 20).
Velocities and peak overpressure were measured upstream of the arrester as a function of a range of accelerating device length (M/D) and obstacle spacing (S/D). Peak velocity and overpressure measured as a function of accelerator length for an S/D of 1 are shown in Figure 9. The error bars indicate the spread obtained for repeat tests.
In assessing the performance of the arrester, use was made of the pressure time history of the gauge immediately prior to the arrester element. From these records, the time varying impulsive load on the arrester could be calculated. Plots showing the impulse time histories for different mixtures for an accelerator section with an obstacle spacing S/D = 6 and overall length M/D = 60 are shown in Figure 10. In addition, it is possible to relate these to the effectiveness of the arrester as the solid symbols indicate the time at which arrester failure was first observed.
Detonation Test Results
The experiments were again performed in a circular tube, internal diameter 50 mm. The overall length L was 3 m. In these experiments, an initial booster section containing fuel-oxygen was used to initiate the donor detonation. Pressure gauges were again located prior to the arrester as well as in the arrester housing itself. Three fuels were again used, to represent the three classes of reactivity noted in the British standard BS7244.
The attenuating section was formed from an 80 mm internal diameter section of an air gap. The attenuating schemes used were as follows:
1. Enlarged cross-section tube: 0.5m long, 80mm in internal diameter;
Figure 8. Pressure evolution upstream and light emission downstream of the arrester: SID — 1 and two lengths of accelerating device length (MID) for (a) propane, (b) ethylene and (c) hydrogen.
C/l 1400
a o 800
QJ > 600
50mm tube m M
- S/D = 1
* Ï :©= -
o propane
A ethylene
[i □ hydrogen _1_
50mm tube S/D-1
propone ethylene
hydrogen
Figure 9. Measured (a) flame velocity and (b) peak overpressure just upstream of the arrester as a function of accelerating section length (M/D) for an obstacle spacing S/D = 1.
line the damping section and (b) a transition to detonation when the perforated mesh is replaced by a 25 cm long air gap.
DISCUSSION AND CONCLUSION
The initial results summarized above show that more reproducible means of generating fast flames are possible, for use in testing flame arresters. In addition, it seems possible to characterize arrester performance based on the integrated impulse incident on the arrester up to failure.
2. Perforated tube: 0.5 m long, 54mm ID, 3 mm hole diameter, 21% porosity;
3. Perforated tube as in 2 with wire mesh or steel wool inlay;
4. Air gap, 0.1 m or 0.2 m long.
Pressure histories obtained for a propane-air detonation incident on a 10 cm long air gap are shown in Figure 11 and illustrate a case where the detonation failed to re-initiate. Figure 12 shows pressure histories for an ethylene-air detonation and these illustrate (a) failure of the transmitted detonation and (b) a transition to detonation when a series of disks (as used in the accelerating section described earlier) were introduced downstream of the attenuator. Pressure histories for a hydrogen-air detonation are show in Figure 13. These illustrate a) continuous transmission of a detonation when an acoustic absorbing material is used to
0.0 0.5 1.0 1.5 2.0 2.5
Time (ms)
Figure 10. Plot showing impulse time histories for different mixtures for an accelerating device with an obstacle spacing S/D = 6 and overall length M/D = 60. Solid symbols indicate the time at which arrester failure is first observed.
Figure 11. Pressure histories obtained for a steady propane-air detonation incident on a 0.1 m air gap, showing the failure to re-initiate.
Figure 12. Pressure histories for a stoichiometric ethylene-air detonation showing (a) failure and (b) transition to detonation when a series of disks are introduced downstream of the attenuator.
The results of attempts to generate deliberate controlled transition to detonation are equally encouraging, via a combination of attenuating sections and obstacles. Not surprisingly, different configurations are required for each
Figure 13. Pressure histories for a hydrogen-air detonation showing (a) transmission and (b) transition to detonation when the perforated mesh is replaced by a 0.25 m long air gap.
fuel type, but the overall saving in length of pipe and operational costs might be significant. Further support for this approach has been provided by Knystautus and Lee20 who have used accelerating obstacles to generate overdriven DDT test conditions.
The developments described above have already been implemented successfully on a 150 mm diameter facility21. Accelerating obstacles of similar MID were again used and a combined diaphragm and slide valve used for the DDT tests. Flame acceleration by obstacles has also been demonstrated as being readily implemented in a 250 mm diameter pipe. The development of the DDT test is slightly more problematic as the size of slide valves start to become impractical. The use of plastic diaphragms is a likely solution but care is required as initial experience at this laboratory with 150 mm pipes has shown that arrester elements can become fouled by plastic residue from the diaphragms. Studies of the transmission characteristics with thin plastic diaphragms forming the air gap were reported recently by Bjerkedvedt et al.22.
An alternative test for the overdriven case has been proposed in the new draft CEN standard, by testing with a steady detonation at elevated initial pressure. The validity of this test has yet to be fully considered. In this case, the magnitude of the peak pressures will only be 1.25 times that of a steady CJ detonation whereas the pressures during a
transient overdriven phase are significantly greater than this. No evidence exists at present to indicate whether the short duration of this overdriven phase justifies the use of a lower maximum peak loading pressure. A further untried idea proposed for testing against an event representative of a DDT event is to generate an overdriven detonation by means of a converging nozzle. By this means the detonation energy and momentum are presented in a more directed way onto the arrester.
The overall conclusions that may be drawn from the above is that new procedures for the testing of explosion arresters are available that will produce more reproducible conditions for the independent evaluation of individual arrester performance. These tests can be implemented with relatively simple modification to existing testing equipment.
REFERENCES
1. Kuo, K. K., 1986, Principles of Combustion, (John Wiley).
2. Lee, J. H. S. and Moen, I. O., 1986, The mechanisms of transition from deflagration to detonation in vapour cloud explosions, Progress in Energy and Combustion Science, 6: 359-389.
3. Nettleton, M. A., 1987, Gaseous Detonations—Their Nature, Effects and Control, (Chapman and Hall).
4. Edwards, D. H., Thomas, G. O. and Williams, T. L., 1981, Initiation of detonation by steady planar incident shock waves, Combustion and Flame, 43: 187-198.
5. Thomas, G. O., Sutton, P. and Edwards, D. H., 1991, The behaviour of detonation waves at concentration gradients, Combustion and Flame 84: 312-322.
6. Edwards, D. H. ,Hooper, G., Morgan, J. M. and Thomas, G. O., 1978, The quasi-steady regime in critically initiated detonation waves, J Phys D: ApplPhys, 11: 2103-17.
7. Moen, I. O., 1993, Transition to detonation in fuel-air explosive clouds, J Hazardous Materials, 33: 159-192.
8. Khokhlov, A. M., Oran, E. S. and Wheeler, J. C., 1996, A theory of DDT in unconfined detonations, Combustion and Flame 108: 503-517.
9. Thomas. G. O., 1998, Characterisation of pressure histories observed during flame acceleration and transition to detonation in pipeline explosions, Internal report UWA/det50598, (Centre of Explosion
Studies, University of Wales Aberystwyth, Draft paper for submission to Trans IChemE).
10. British Standards Institution, 1990, Flame arresters for general use, BS7244, (British Standards Institution).
11. International Maritime Organisation, 1988, Revised standards for the design, testing and location of devises to prevent the passage of flame into cargo tanks in tankers, MS/CIRC 373 Rev 1, 1988.
12. United States Coast Guard, 1990, Marine vapour control systems, US Federal Register 25396-25451.
13. Roussakis, N. and Lapp, K., 1991, A comprehensive test method for inline flame arresters, Plant/Operations Progress, 10: 85-92.
14. CEN draft standard, 1997, prEN 12874, (European Commission for Standardization, rue de Stassart 36 B-1050 Brussels).
15. Wingerden, K. V., 1994, Standardisation of devices and systems for explosion prevention and protection—a status report, Paper presented Explorisk94, Gent, Belgium23-25 March 1994.
16. White, R. E., 1982, Southwest Research Institute Report, Project No. 06-4116.
17. Thomas, G. O. and Oakley, G. L., 1993, On practical difficulties encountered when testing flame and detonation arresters to BS7244, Trans IChemE, Part B—Proc Safety andEnv Prot, 71: 187-193.
18. Capp, B., 1994, Flame arresters: Endurance burning, Proc Hazards XII, Manchester, IChemE Symp Series No 134: 405-414.
19. Teodorczyk, A. and Thomas, G. O., 1995, Archivium Combustionis, 15: 59-80.
20. Knystautus, R., Lee, J. H. and Goroshin, S., 1996, The testing of detonation arresters, Proc Second Specialised Meeting on Fuel-Air Explosions,,Christian Michelsen Reserch, Bergen, Norway, pp 7.17.14.
21. Teodorczyk, A. and Thomas, G. O., 1997, Poster presentation, Twenty Fifth Int Symp on Combustion, Naples.
22. Bjerkedvedt, D., 1996, Some preliminary results from experiments with re-initiation of gaseous detonations after a thin solid membrane, Proc Second Specialised Meeting on Fuel-Air Explosions,Christian Michelsen Reserch, Bergen, Norway, pp 6.33-6.44.
ADDRESS
Correspondence concerning this paper should be addressed to Dr G. O. Thomas, Centre for Explosion Studies, The University of Wales, Aberystwyth SY23 3BZ, UK.
The manuscript was received 29 December 1997 and accepted for publication after revision 16 September 1998.
