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The 2013 edition of NFPA 68 Standard on Explosion Protection by Deflagration Venting will replace vent area sizing equations containing the gas Deflagration Index, KG, for high-pressure applications with equations containing the laminar burning velocity. The 2013 equations are essentially a more transparent casting of the Swift-Epstein-Fauske derivations to provide a framework for including approaches to model flame accelerations and flame wrinkling/stretching in large enclosures with and without internal obstacles. These equations compare favorably with the reference data sets for unobstructed and minimally obstructed enclosures. Calculated vent areas using the new method also prevent under-sized vent areas for large enclosures with large equipment representative of many petrochemical process structures. However, the new methodology does not yet reproduce large-scale test data with repeated small obstacles simulating highly congested piping arrays and more work on this issue is necessary. This is an abstract of a paper presented at the 2013 AIChE Spring Meeting and 9th Global Congress on Process Safety (San Antonio, TX 4/28/2013-5/2/2013).

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... Afterwards, the updated version (NFPA-68, 2007) were achieved by using the NFPA-68 2013 edition's new correlations (Rodgers and Zalosh, 2013). ...

... The latest version of NFPA-68 standard uses the laminar burning velocity, scale dependent c value in sub-sonic flow correlations, and scale dependent flame enhancement factor in sonic-flow correlations to consider the flame instabilities inside the enclosure and through the vent. Therefore, the calculation accuracy is improved comparing to earlier editions (Rodgers and Zalosh, 2013). However, in order to predict the peak overpressure of vented gas explosion, the calculation procedure by using the new equations becomes complicated, the backward induction has to be used to derive the peak overpressure prediction equations of NFPA-68 2013 edition in this study. ...

This paper presents new correlations developed through numerical simulations to estimate peak overpressures for vented methane-air explosions in cylindrical enclosures. A series of experimental tests are carried out first and the results are used to validate the numerical models developed with the commercial CFD software FLACS. More than 350 simulations consisting of 16 enclosure scales, 12 vent area to enclosure roof area ratios, 8 gas equivalence ratios and 9 vent activation pressures are then carried out to develop the Vented Methane-air Explosion Overpressure Calculation (VMEOC) correlations. Parameters associated with burning velocity and turbulence generation, oscillatory combustion and flame instabilities in vented gas explosion are taken into account in the development of new correlations. Comparing to CFD simulations, the VMEOC correlations provide a faster way to estimate the peak overpressure of a vented explosion. Additionally, it is proved in this study that the VMEOC correlations are easier to use and more accurate than the equations given in the up-to-date industrial standard- NFPA-68 2013 edition.

... The flame wrinkling factor λ calculation is not trivial, it depends on various parameters such as the presence of obstacles and the Reynolds number at the vent. Rodgers et Zalosh (2013) showed that NFPA68:2013 model gave conservative estimations of the vent are in a vast majority of industrial cases. Fakandu et al. (2013) compared both NFPA68:2013 and EN14994:2007 models and demonstrated a better fit on experimental data with the former, despite it seem insufficient in cases with turbulent hydrogen, or when many small obstacles are present near the vent. ...

The case of a gas explosion occurring in a geometrically simple enclosure, equipped with a vent is considered. It is well known in the gas explosion scientific community that the calculation of the reduced explosion overpressure, determinant in safety studies, is not trivial. Not only there is a strong dependency on the chemical kinetics of the combustible but also on the enclosure geometry, the fluid flow, the vent mechanical behaviour, shape, etc … As a result, the modelling of the physics at stake is challenging, a wide range of models are proposed in the scientific literature and this reference situation is still the object of extensive research. A new simulation approach ignoring a large part of the underlying physics is investigated. It is based on the use of an artificial neural network (ANN). The focus is given on the method of use and results obtained with the ANN rather than on the neural network itself. Our observations are discussed within the scope of industrial safety problems. Calculations performed with the relatively simple ANN proposed in the official TensorFlow tutorial, on a vented explosion database containing 268 tests, led to surprisingly good results considering the ANN implementation efforts. The tool might look promising but is also far from being as trivial as it seems at a first glance: not only the results of simulations obtained with this type of model must be examined with the greatest care but also the initial data base must be very well controlled. Routes are proposed to enhance the initial database and perform relevant analyses of the neural network predictions.

This paper presents an extension of previous study on estimating the internal and external pressure and impulse from vented gas explosion in large cylindrical tanks. Unlike the small-medium scale explosion from cylindrical tanks in previous work, explosion pressure and impulse from large-scale explosions are numerically and analytical investigated in this study. CFD simulations are conducted and validated by using experimental data. Sensitivity study on grid selection for large-scale explosion simulations is performed. By adopting two scale-up factors for the analytical correlations, the internal pressure and external pressure on adjacent tanks from vented gas explosion in a tank are accurately predicted. Parametric study of the effectiveness of separation gap between adjacent tanks on pressure and impulse mitigation is also carried out. Influences of the venting size, tank diameter and tank height on the effectiveness of separation distance for pressure and impulse mitigation from large-scale vented gas explosion in an adjacent tank are studied.

In the problem of the protection by the consequences of an explosion is actual for many industrial application involving storage of gas like methane or hydrogen, refuelling stations and so on. A simple and economic way to reduce the peak pressure associated to a deflagration is to supply to the confined environment an opportune surface substantially less resistant then the protected structure, typically in stoichiometric conditions, the peak pressure reduction is around the 8 bars for a generic hydrocarbon combustion in an adiabatic system lacking of whichever mitigation system. In general the problem is the forecast of the peak pressure value (P MAX) of the explosion. This problem is faced using CFD codes modelling the structure in which the explosion is located and setting the main parameters like concentration of the gas in the mixture, the volume available, the size of vent area and obstacles (if included) and so on. In this work the idea is to start from empirical data to train a Neural Network (NN) in order to find the correlation among the parameters regulating the phenomenon. Associated to this prediction a fuzzy model will provide to quantify the uncertainty of the predicted value.

R.G. Zalosh

From this investigation of scaling laws for methane explosions in roomlike enclosures it is concluded that (second) peak pressure data for enclosure volume less than about 12 m**3 are well correlated by the parameter A//C/A//v and satisfy Equations (2) and (3) in the text. Enclosures with volumes greater than about 12 m**3 require a different peak pressure correlation, because of the suddenly accelerated combustion or flame instability at the later stages of the explosion, when the flame reaches the most remote wall. Second peak pressures in the large-scale enclosures are an order of magnitude larger than second peak pressures in the smaller enclosures with comparable values of A//c/A//v or A//v/V**2**/**3. In both the small-scale and large-scale low-pressure enclosures, the (second) peak pressure due to insufficient vent area was independent of the (first) peak pressure due to vent deployment.

A set of large-scale vented deflagration tests involving methane- and propane-air mixtures in a congested enclosure was conducted. The enclosure utilized had a 24 ft by 24 ft footprint and 6 ft high. Steel plates were attached to the roof and all four sides served as vents. The tubes were arranged with a pitch/diameter ratio of 7.6 and provided area and volume blockage ratios of 13% and 1.5%, respectively. The average peak pressures achieved with methane and propane mixtures were 4 and 5 psig, respectively. These pressures were 20 to 40 times the values predicted by the NFPA 68 weak and strong enclosure correlations, respectively, when the constraint imposed by the vent panel release pressure was not explicitly considered. The NFPA 68 duration expression yielded reasonable results (i.e., within a factor of 2) when a reasonable estimate of the peak pressure was utilized. This is an abstract of paper presented at the AIChE Spring National Meeting (Orlando, FL 4/24-26/2006).

Starting from a detailed mathematical model previously developed, which accounts for each physicochemical phenomenon involved in a vented deflagration process, a simplified (or short-cut) model has been derived based on suitable assumptions and algebraic manipulations. The simplified model reduces to a system of three ordinary differential equations, the solution of which can be readily achieved on a small personal computer. The reliability of such a simple model has been tested by comparison with a collection of published experimental data (the same one considered when developing the original detailed model), covering a wide range of values for vessel volume (0.001–199 m3), initial pressure (0.1–0.4 MPa), and bursting pressure (0.1–2.96 MPa) and including various vassel shapes and fuel-air compositions. The error in the maximum absolute pressure values, averaged over 91 experimental data relative to vented explosions, is equal to 28%. The proposed model can be regarded as a simple tool for predicting with reasonable accuracy the maximum pressure value during a vented deflagration in a wide range of operating conditions, thus providing a significant contribution to the venting area design.

M. Epstein
Ian Swift
Hans K. Fauske

Two closed-form approximate solutions are presented for the final pressure produced by a hydrocarbon explosion in a spherical vessel with sonic venting. A constant factor which multiplies the ideal spherical flame velocity is used to describe the effect of flame acceleration. One of the solutions is a simple, easy-to-use equation which may appeal to vent designers; it agrees well with reported results from a comprehensive computer model and correlates available experimental data as well as previous models involving several variable turbulence factors.

M. G. Cooper
M. Fairweather
J. P. Tite

Experiments conducted in near-cubic enclosures with low failure pressure explosion relief panels have been used to identify the physical mechanisms responsible for the generation of significant pressure peaks during vented explosions. For ignition at the center of a vessel the combustion process was found to give rise to four major pressure peaks, and these peaks have been quantified in terms of those parameters most frequently considered in the design of reliefs for explosion venting. Increasing the failure pressure of relief panels was found to result in two particular peaks becoming the dominant features of observed pressure-time profiles. The implications of all these findings for the design of explosion reliefs in practical situations are discussed.

B.J. Lowesmith
Chris Mumby
G. Hankinson
J. S. Puttock

The EC funded Naturalhy project is assessing the potential for using the existing gas infrastructure for conveying hydrogen as a mixture with natural gas (methane). The hydrogen could then be removed at a point of use or the natural gas/hydrogen mixture could be burned in gas-fired appliances thereby providing reduced carbon emissions compared to natural gas. As part of the project, the impact on the safety of the gas system resulting from the addition of hydrogen is being assessed. A release of a natural gas/hydrogen mixture within a vented enclosure (such as an industrial housing of plant and equipment) could result in a flammable mixture being formed and ignited. Due to the different properties of hydrogen, the resulting explosion may be more severe for natural gas/hydrogen mixtures compared to natural gas. Therefore, a series of large scale explosion experiments involving methane/hydrogen mixtures has been conducted in a 69.3m3 enclosure in order to assess the effect of different hydrogen concentrations on the resulting explosion overpressures. The results showed that adding up to 20% by volume of hydrogen to the methane resulted in a small increase in explosion flame speeds and overpressures. However, a significant increase was observed when 50% hydrogen was added. For the vented confined explosions studied, it was also observed that the addition of obstacles within the enclosure, representing congestion caused by equipment and pipework, etc., increased flame speeds and overpressures above the levels measured in an empty enclosure. Predictions of the explosion overpressure and flame speed were also made using a modified version of the Shell Global Solutions model, SCOPE. The modifications included changes to the burning velocity and other physical properties of methane/hydrogen mixtures. Comparisons with the experimental data showed generally good agreement.

Robert DeGood
Kris Chatrathi

A test programme was developed to study factors that may influence the maximum pressure during venting developed during deflagration. The factors considered were vent burst pressure, vent mass, ignition source location, discharge ducting, downstream obstructions and water covers.

Venting is a widely applied method to protect process equipment from being destroyed by internal explosions. The key problem in venting is the appropriate design of the vent area necessary for an effective release of the material. For gas explosions different calculation methods exist, but there are no clear recommendations which one should be preferred for the practical cases under consideration. The present paper gives a review of different calculation methods, their ranges of validity, their physical background and applicability. The presented examples include a comparison of computed reduced explosion pressures for methane-air, propane-air and hydrogen-air mixtures with experimental data and two fictitious test cases. The results of different methods show a wide range of scatter, however some recommendations for their applicability can be given.

Ian Swift
Mike Epstein

The internal pressure developed during deflagrations in low-pressure structures depends on the dynamic performance of the explosion vents used for protection. Large scale tests were performed to evaluate the effectiveness of different types of commercially available vents.

C. Regis Liu Bauwens
J. Chaffee
S.B. Dorofeev

Experimental data obtained for hydrogen mixtures in a room-size enclosure are presented and compared with data for propane and methane mixtures. This set of data was also used to develop a three-dimensional gasdynamic model for the simulation of gaseous combustion in vented enclosures. The experiments were performed in a 64 m3 chamber with dimensions of 4.6 × 4.6 × 3.0 m and a vent opening on one side and vent areas of either 2.7 or 5.4 m2 were used. Tests were performed for three ignition locations, at the wall opposite the vent, at the center of the chamber or at the center of the wall containing the vent. Hydrogen–air mixtures with concentrations close 18% vol. were compared with stoichiometric propane–air and methane–air mixtures. Pressure data, as function of time, and flame time-of-arrival data were obtained both inside and outside the chamber near the vent. Modeling was based on a Large Eddy Simulation (LES) solver created using the OpenFOAM CFD toolbox using sub-grid turbulence and flame wrinkling models. A comparison of these simulations with experimental data is discussed.

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W Bartknecht

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W Bartknecht

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Z W Rogowski
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An Experimental Study of the Physics of Gaseous Deflagrations in a Very Large Vented Enclosure

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C J M Van Wingerden
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van Wingerden C. J. M., Zeeuwen J. P.-Venting of Gas Explosions in Large Rooms-4th International Symposium on Loss Prevention and Safety Promotion in the Process Industries, EFCE Publication Series, No. 33, Vol. 3, F38-F47, 12-16 September (1983)

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