Using CHETAH to estimate lower flammable limit, minimum ignition energy, and other flammability parameters

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When hot vapor condenses the nucleation particles initially formed coalesce rapidly and exothermically. Most of the heat of coalescence is released in less than a millisecond while droplets are less than 100 nm diameter. Consequently, the heat is released close to the source of the vapor and has no effect on dispersion. Coagulation rate is governed by an inverse square law with respect to number density; calculations showed that a vapor cloud initially within its flammable range should produce droplets smaller than 10 µm after a minute of coalescence. Such micronized droplets completely vaporize in flame preheat zones and behave as premixed vapors rather than as individual droplets; this characteristic is not due to the increased surface energy accompanying subdivision into micronized droplets but to short evaporation times in the thick preheat zones of limit flames. Droplet growth time is a nonlinear function of cloud mass concentration and if the latter exceeds about 10 times the minimum explosible concentration (MEC), droplets quickly grow to above 20–40 µm. These larger droplets tend to burn individually and the estimation of MEC becomes complicated, although rain‐out of large droplets might hasten dispersal to below the MEC. With the assumption that micronized crystalline organic materials behave similarly to liquid droplets, we derive “intrinsic” MEC values for both hydrocarbons and a series of CHOx fuels having increasing O:C ratios. In each case, the MEC attains a constant “terminal value” as carbon number is increased. Terminal MEC values are 40 g m−3 for CH (hydrocarbons), 50 g m−3 for CHO (aldehydes, ethers, etc.), 60 g m−3 for CHOO (esters), 67 g m−3 (polyethylene glycols), and 103 g m−3 (carbohydrates). MEC values for CHO and CHOO decrease rapidly with increased carbon number since additional hydrocarbon units dilute the functional oxygen group. For polyethylene glycols and carbohydrates, the MEC is less dependent on carbon number since oxygen is contained in a repeating group. Although scission and/or decomposition replace simple evaporation as carbon number increases, flash pyrolysis data for polyethylene in air show that the products of decomposition can have little effect on MEC. Nevertheless, owing to experimental deficiencies published MEC values for polyethylene are as low as 10–30 g m−3. Of the many factors affecting MEC tests, we focus on the effects of gravity and flame radiation. Radiation not only affects MEC but also the burning rates of metal dusts such as titanium; such tests should be carried out on an adequately large scale. © 2017 American Institute of Chemical Engineers Process Process Saf Prog 37:4–17, 2018

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Estimating Mec Of Vapors and Mistsmentioning

confidence: 99%

Section: Estimating Mec Of Vapors and Mistsmentioning

confidence: 99%

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Minimum explosible concentrations of mist and dust clouds

Britton¹,

Harrison

2018

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Static hazards of container filling

Britton

Walmsley

2021

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The maximum filling rates recommended by IEC and NFPA for conductive intermediate bulk containers (IBC) conflict with each other and take no account of the filling pipe diameter. A model previously developed to predict maximum liquid surface potentials in medium tanks was re-derived using analytical approximation rather than curve fitting techniques and modified for application to conductive containers. Physically consistent flow rate limits are given for all sizes of container. Relative to the 25 kV hazardous potential threshold, IEC's flow velocity limit of 2 m/s theoretically provides a large safety factor for IBC filling that increases linearly from 2 to 5 as the inside diameter of the filling pipe decreases from 78 to 27 mm. However, because potential varies with the inverse cube of pipe diameter, the safety factor provided by NFPA's flow rate limit of 225 L/min diminishes from 8 to 1/2 over the same range and surface potentials exceed 25 kV for filling pipe diameters below about 35 mm. The model is extended to address elevated dielectric constants and practical guidance is provided for different filling pipe options. We also warn of specific practices that could increase the likelihood of a fire or explosion.

Risks and limitations of steam curtains protection in case of flammable gases leakage

Kulich¹,

Herink

2022

Refinery and petrochemical production facilities process and produce gaseous and liquid hydrocarbons. In case of any failure, improper design, or human error, flammable or toxic substances can be released into the environment. In such cases, vapor barriers of steam curtains are used in the chemical industry to primarily dilute the released gases as well as to divert the gases to the safe area away from a potential source of ignition. When steam is ejected from a nozzle in the pipe wall, the medium rubs against the edge of the nozzle, liquid water impinges on the wall of the nozzle, and an electrostatic charge can therefore be expected. From the above description, it can be deduced that if the operation of the steam curtains is associated with the generation of electrostatic charge and the possibility of its discharge and the generation of electrostatic sparks, the activation of the steam curtains during the discharge of flammable gases paradoxically represents a certain risk. From the point of view of