On flammability hazards from pressurised high-flashpoint liquid releases

Hazardous area classification is well established for dust and vapours, however this is not the case for high flashpoint liquid fuels. This study highlights the limitations of current guidance in relation to flammable mists, through demonstration of flammability of a representative high flashpoint fuel for releases in the range of representative industrial operating pressure, complemented by a phenomenological analysis and semi-quantification of the results observed. Flammability results are presented from low-pressure practical releases ( 61 °C), through a plain orifice, at temperatures well below its flashpoint. Based on a proposed two-phase flow-regime diagram, a semi-quantitate analysis of the results observed is offered via a simple 1-D phenomenological model, accommodating jet breakup length, spray quality, air entrainment and droplet dynamics. The complex scenario of liquid releases impinging onto an unheated flat surface is also considered. An impingement model is utilised to show the relative increase in volume of fine secondary spray induced post-impingement relative to the unobstructed case, resulting in a significant volume of flammable mist. This is demonstrated experimentally by showing flammability of a 5 barg release post impingement whereas the unobstructed 10 barg case would not ignite

Area classification of flammable mists: Summary of joint-industry project findings

This paper presents the findings from a programme of research on mists produced from flammable or combustible fluids at temperatures below their flash points that was undertaken at the Health and Safety Laboratory and Cardiff University over the last four years. The purpose of the research was to improve our understanding of flammable mists and, in particular, to identify when leaks of high-flashpoint fluids may produce a flammable atmosphere and to help define the extent of the flammable cloud. The work was funded by a consortium of industry and regulatory sponsors. The research programme consisted of five main elements: 1.) A detailed literature review (presented previously at the Hazards XXIII conference) 2.) Development of a classification system for releases of high-flashpoint fluids 3.) Experiments to determine the ignitable range of droplet size and concentration for different classes of spray release 4.) Computational Fluid Dynamics (CFD) modelling of the experiments and other releases relevant for area classification 5.) A final stage of analysis and comparison to area classification guidelines This paper concentrates on the final four elements. The experiments and CFD modelling considered three fluids: Jet A1 (kerosene, flashpoint = 38 °C), a hydraulic oil (flashpoint = 223 °C) and a light fuel oil (flashpoint = 81 °C). These were chosen as representative of the range of high-flashpoint fluids used across industry. A single release geometry was studied that involved an orifice diameter of 1 mm and a downwards-directed spray. For pressures where the spray was fully atomised, it was found that the CFD model using the 'DNV Phase III JIP RR Primary Breakup Model' provided reasonably good predictions of the droplet size and concentration. Predictions from the validated CFD model were compared to hazard distances presented in the EI15 Model code of safe practice. The paper concludes with amendments to some of the existing guidelines for area classification of mists