Flashover Potential Research Articles

Assessing thermal hazards and toxicity of raw biomass particles from prevalent agricultural crops in China

Fire and explosion hazards pose significant safety concerns in the processing and storage of biomass particles, warranting the safe utilization of these particles. This study employed scanning electron microscopy, thermogravimetric analysis, and cone calorimetry to investigate the thermal hazards and toxicity of raw biomass particles from four prevalent agricultural crops in China: rice, sorghum, corn, and reed. Among the samples, corn exhibited the highest heat output of 8006.82 J/g throughout the thermal decomposition process. The quantitative evaluation of critical heat flux, heat release rate intensity, fire growth rate index (FIGRA), post-ignition fire acceleration (PIFA) and flashover potential (X) revealed a substantial fire risk inherent to all the examined straw samples. Notably, corn displayed the lowest FIGRA value of 8.30 kW/m2 s, while rice demonstrated the minimum PIFA value of 16.11 kW/m2 s. Moreover, the X values for all four biomass particle types exceeded 10 under varying external heat flux levels, indicating their high propensity for fire hazards. Analysis of CO and CO2 emissions during combustion showed all four biomass samples exhibited high concentrations throughout, from the initial stages to the end. The present study offers crucial insights for formulating comprehensive fire safety guidelines tailored to the storage and processing of biomass particles.

Fire parameters, behaviour, and comparative thermal hazard of food grains based on the cone calorimeter tests

Agricultural food grains have been involved in various large fires, but little is known about the fire parameters for many varieties required to evaluate their fire hazard. This research experimentally studied various food grains, with an emphasis on African food grains to obtain their important fire parameters. Oven-dried cowpea, lentils, millet, soybean, flax (linseed), unshelled peanuts, sunflower, shelled peanuts, and sesame were subjected to heat fluxes of 25 kW/m2, 35 kW/m2, and 50 kW/m2 in a cone calorimeter. Heat release rate, effective heat of combustion, mass loss, critical heat flux, time to ignition, peak heat release rate, total heat release, and time to peak heat release rate were obtained. Thermal fire hazard indices derived from these primary parameters, such as flashover potential, fire growth rate, and percentage mass loss were determined and compared with wood pellets. Under a 50 kW/m2 heat flux, the peak heat release rate ranged from 230 to 608 kW/m2 for millet and sunflower respectively. The correlation of materials characteristics and fire parameters shows that the oil content is the dominant determinant of the thermal fire hazard level in the food grains tested. These grains were also found to have a higher fire hazard compared to wood pellets.

Heat release and burning behaviour of foam and foam/Basofil fabric combination

A comprehensive characterization of the heat release rate and burning behaviour of foam and foam/Basofil fabric combination has been carried out at different levels of heat flux varying from 10 to 70 kW/m 2 using cone calorimetry. Peak heat release rate (PHRR) was found to increase for foam and foam/fabric combination with the increase in the level of heat flux. However, considerable reduction in PHRR was noted for foam/fabric combination vis-à-vis that of foam alone. Foam/fabric combination was found to exhibit two-step decomposition behaviour at heat flux levels of 40 kW/m 2 and above due to the cracking of the char formed from the cover fabric. Flashover potential of the foam was considerably reduced by the fabric covering the foam. Carbon dioxide and carbon monoxide yields were found to be lower for foam/fabric combination. Smoke toxicity, as indicated by the index of combustion completeness, was found to be lower for the foam/fabric combination.

Evaluation Of The Hazard Of Fire Resistant Materials Using Measurements From Laboratory And Parallel Panel Tests

This paper describes new concepts for evaluating the fire propagation hazard of exposed polymer materials exhibiting low heat release rates (typically less than 200 kW/m). While the intent of most fire propagation models is to predict details such as the position of flame or pyrolysis fronts as a function of time in a room-corner scenario, engineers often simply need to know whether a material will support sustained fire propagation or not. To accomplish this objective, a 2.4-m high parallel panel configuration is studied as a reference scenario for determining fire propagation behavior apart from other complicating factors, such as flashover potential or excess pyrolyzate production. Coupled to this test configuration is laboratory analysis of the polymer that provides quantitative information on two key parameters for a simplified predictive model of fire behavior: 1) the ratio of the heat of combustion to the heat of gasification and 2) particulate yield of the polymer flames. Procedures for obtaining these parameters are described for three materials, two nominally identical polyvinychloride polymers of different thickness and a fire-retarded polypropylene. The behavior of these three materials in the parallel panel configuration when exposed to gas burner flames of varying intensity is analyzed and compared to model predictions. Based on the analysis, it is concluded that accurate determination of the heat of gasification in an inert, nitrogen environment is important for understanding the hazard of fire resistant materials.

Dielectric surface flashover in a simulated low Earth orbit environment

Surface flashover on insulators under UV irradiation or with a plasma background was investigated with high-speed electrical and optical sensors in order to clarify differences in the breakdown development compared to the pure vacuum case. Results with a plasma background show a more rapid development in the breakdown initiation compared to measurements in vacuum with no plasma. With a magnetic shielding technique using permanent magnets, the duration of an applied voltage pulse can be increased by a factor of 2-3 without causing flashover. UV illumination on the electrodes decreases the flashover voltage (for the dc case) or the voltage pulse duration without breakdown (for the pulsed case), whereas UV illumination on the dielectric surface increases the flashover potential.

Vacuum flashover on diamond-like carbon-coated insulators

Thin coatings of diamond-like carbon (DLC) were applied to cylindrical alumina and high temperature polyimide insulator samples. The coated insulators and uncoated controls were tested for resistance to surface flashover in vacuum under a dc voltage gradient. The DLC-coated insulators had significantly higher flashover potential than the controls and showed no visible tracking on their surface, even after multiple discharges. DLC coatings, applied at temperatures <350/spl deg/C, show promise for improving the performance of HV vacuum feedthroughs, switches, and in other applications.

Methods of increasing the surface flashover potential in vacuum

The effects of surface coating and magnetic fields on dielectric surface breakdown in vacuum are described. Coatings of metals and meta oxides increase the pulsed surface hold-off voltage by factors of up to three, depending on the material. Measurements of the secondary emission coefficient show substantial differences between coated and uncoated samples at high energies. The influence of magnetic fields on dielectric surface breakdown for uncoated samples is investigated in the pressure range of 10/sup -5/ to 1 Pa, for both DC and pulsed electric and magnetic fields. Insulation effects (increase in breakdown voltage of up to 2* at magnetic field amplitudes of 0.5 T) are observed when the magnetic field is parallel to the surface of the dielectric and perpendicular to the applied electric field. Magnetic insulation shows a strong material dependence and decreases with increasing pressure and surface roughness.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A treatment which improves surface withstand voltage in vacuum

A treatment which significantly increases the pulsed surface flashover potential of some insulator materials in vacuum is described. The flashover potential is measured by placing a short cylindrical sample between brass electrodes in a vacuum chamber at a pressure of 10/sup -4/ Pa and applying a high-voltage (HV) double exponential pulse with 1.5/135- mu s time constants. The treatment consists of exposing a sample to 80000 sparks from a spark plug in vacuum. For each material, an untreated and a treated sample are measured under identical conditions, and the results compared. Results on Lexan show an increase in flashover potential from 100 to 170 kV. On the machinable ceramic Macor, the results were 60 kV untreated and 140 kV treated. Surface analysis using electron spectroscopy for chemical analysis shows that the treatment alters the surface composition of the material. Also, the secondary electron emission coefficient (SEEC) has been measured for treated and untreated samples over a range of incident electron energies. The SEEC changes dramatically after treatment.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Pulse Flashover of Solid Dielectrics in Vacuum

In an effort to understand the mechanisms which lead to changes in the surface flashover potential of insulators used in high power spark gaps a systematic study of the flashover potential of various organic insulators was undertaken. The flashover potential of these insulators was measured both before and after they had been exposed to the by-products of an arc from a surface discharge ignitor plug. These insulators were analyzed using Electron Spectroscopy for Chemical Analysis (ESCA) to determine what, if any, structural changes occurred on the surface of the insulator as a result of the flashover events and/or irradiation. A Scanning Electron Microscope (SEM) was also used to investigate the tracks formed on the insulators as the result of a surface flashover.

Estimating room flashover potential

Flashover is the ultimate event in a room fire signaling the final untenability for room occupants and greatly increased hazard to other building spaces. Despite this importance, hazard evaluations of furnishings and other common fuel loads have normally not been based on estimates of flashover potential. This paper considers a simple combustion model and examines available experimental data.

The Effect of Humidity on the Dry Flashover Potential of Pin-Type Insulators

This paper shows that humidity affects considerably the flashover voltages of porcelain and Pyrex insulators. The flashover potential rises as the humidity is increased. The tests were made on pin-type insulators and on rods. The results show that this rising characteristic is a surface effect which varies with the absolute water content of the surrounding atmosphere.