A quantitative approach for optimal alarm identification
Abstracts:Alarm identification refers to selecting a set of measurements to be configured to the alarm system. Contrary to prior literature which uses qualitative cause-effect based techniques, the present work incorporates quantitative aspects such as the time taken by measurements for deviation, to make alarm identification more reliable.
Risk assessment of process system considering dependencies
Abstracts:Risk assessment is conducted in process systems to identify potential accident scenarios and estimate their likelihood and associated consequences. The bow-tie (BT) technique is most frequently used to conduct the risk assessment. It is a simple, comprehensive and straightforward technique; however, it considers independence among the causation factors (initiating events) of an accident scenario and the safety barriers in place to minimize the impact of the accident scenario. This is a serious limitation and can lead to erroneous results. This paper presents a simple yet robust approach to revise the Bow-tie technique considering interdependence. It employs copula functions to model the joint probability distributions of causations in the BT model of the accident scenario. This paper also analyzes the impact of dependence on two common logic gates used to represent the potential accident scenario. The probability of a potential accident scenario in a hexane distillation unit using both the traditional BT technique and the revised approach is compared. Results confirm that the revised approach is reliable and robust.
Extraction of the remnant coal pillar in regular and irregular shapes: A case study
Abstracts:With the increasing coal mining rate, many coal mines in China are facing the coal resource exhaustion problem. This paper presents a case study on extracting a coal pillar which was left in a longwall retreat mine, Wangzhuang mine, in China. The left coal pillars usually have boundaries of regular and irregular shapes, which result in complex stress distributions within the coal pillar. It is of great importance to study the stress distributions and to determine the roadway layout prior to extracting the left coal pillar. This study numerically analyzes the stress distributions in the left coal pillar, based on which the roadway layout and support techniques are proposed. The roadway stability were well maintained throughout the mining process and the mining safety and productivity were ensured. The left coal pillar in Wangzhuang mine was successfully extracted. This case study would be helpful for extracting the left coal pillar in the longwall retreat mines.
Investigation of dust dispersion in a modified Hartmann tube using positron emission particle tracking and simulations
Abstracts:An important research tool that is used for assessing fundamental dust explosion characteristics is the Hartmann apparatus, where dust is dispersed by a pressure wave. Nevertheless, it is questionable as to whether the formed dust cloud is uniformly dispersed as well as how the solid particles behave as they flow. In this study we used two research tools. The first one is the novel experimental technique Positron Emission Particle Tracking (PEPT). It derives from Positron Emission Tomography (PET) technique that is normally used in the medical environment. PEPT is a technique of tracking individual particles and can be used for studying multiphase flows. Thus the main objective of this paper is to demonstrate how this method can be used for studying such systems. The second tool we used in this research is numerical simulations in which the Eulerian-Lagrangian approach was adopted. Therefore the second main objective of the paper is to investigate the flow of a single particle in the Hartmann apparatus and show the complexity of the problem. According to the experimental results the process is highly stochastic and influenced not only by the injection pressure but also by the initial conditions and geometry. High injection pressures lead to frequent particle collisions with the walls resulting in a potential loss of kinetic energy. The simulations showed that the vertical velocity profile of the fluid flow distribution was non-symmetric even though the previous numerical studies showed the opposite. This may have influenced the dust dispersion process. The influence of the gas injection velocity is only important in the beginning of the dispersion process but not further into the process. During this period we observed frequent collisions that counteracted the acceleration of the particle, and the value of the coefficient of restitution influenced the rate of deceleration.
Long-term emission measurements at a floating roof tank for gasoline storage
Abstracts:In our paper we present the results of a long-term emission measurement of volatile organic compounds emitted by a single-hull floating-roof tank. The tank has a capacity of and is used to stock commercially available gasoline. Due to imperfect seals and the formation of lubricating films during withdrawal, subsequent emissions of volatile compounds have to be expected. Therefore, a non-zero probability has to be assumed for the formation of ignitable concentrations of flammable gases, resulting in according an explosive gas area classification. Here a representative single-hull floating-roof tank is monitored using an appropriate network of infrared (IR) detectors, accompanied with measurements using a high resolution photoionization detector (PID). Measured values have been compared to the expectations estimated applying the API 2517/2519 and the VDI 3479 standards.
Non-destructive testing and temperature distribution of coal mine roadway lining structure under exogenous fire
Abstracts:In order to study the thermal damage of concrete lining under exogenous mine fire conditions, the temperature and P-wave velocity distribution of naturally ventilated concrete roadway lining are investigated through an experiment using a 1/10 reduced-scale roadway model. In this experiment, the lining is composed of concrete cubes, the temperature is monitored with thermocouples installed inside and outside the specimens, and the velocity of concrete specimens is measured before and after the fire. Through the analysis of the results, it is found that the temperature is almost symmetrically distributed on the inner surface of sidewall lining, which corresponds to the stratification of smoke. Moreover, the temperature reaches its maximum value on the inner surface of the lining structure and gradually decreases outward from the inner surface. In addition, the decrease of the P-wave velocity of the concrete is closely related to the fire temperature, while the variation of the P-wave velocity can reflect the damage degree of the concrete lining and thus indicate structural damage and changes in mineral content. Finally, the damage characteristics obtained can provide a basis for the design of roadway linings.
Ethylene/polyethylene hybrid explosions: Part 2. effects of polyethylene particle size distributions on flame propagations
Abstracts:To reveal clearly the effects of particle size distributions on flame propagations during hybrid explosions, ethylene/polyethylene hybrid explosions with different polyethylene particle size distributions were experimentally studied. Experimental results showed that flame propagation velocities and maximum flame temperatures increased with the decrease of particle size distribution as a whole. Due to the serious agglomeration of polyethylene when the concentration of ethylene was 0.5%, flame propagation velocities and maximum flame temperatures of the <75 μm polyethylene hybrid mixture were lower than those of the 75–100 μm polyethylene hybrid mixture when dust concentration was less than a certain range. The maximum flame temperature of the 100–212 μm polyethylene hybrid mixture was higher than those of the <75 μm and 75–100 μm polyethylene hybrid mixtures when dust concentration >500 g/m3, which was attributed to the combustion behavior of the molten polyethylene particles. The maximum flame temperatures of 2.3% ethylene hybrid mixtures increased slightly as particle size distribution decreased. The maximum flame temperatures of the 2.3% ethylene hybrid mixtures of particle size distributions <75 μm, 75–100 μm, and 100–212 μm were 1776 °C, 1759 °C, and 1758 °C respectively. Ethylene/polyethylene hybrid flame consisted of the premixed gas flame followed by the diffusing dust flame. With the increase of particle size distribution, the large particle pyrolysis zone without visible flame would exist between the premixed flame zone and the dust flame zone.
Construction of a 36 L dust explosion apparatus and turbulence flow field comparison with a standard 20 L dust explosion vessel
Abstracts:By modifying the dispersion system and the ignition delay time, and hence the flow field and turbulence intensity during the combustion process, various 20 L dust explosion vessels have been calibrated to give results comparable to the 1 m3 vessel as prescribed in the former ISO standard (ISO-6184, 1985). However, the results obtained from experiments conducted in the two vessels do not always agree for the same dust. There can be several reasons for this discrepancy: turbulence decays faster in the 20 L vessel compared with the 1 m3 vessel, the energetic ignition sources used in standardized tests may overdrive the combustion process in the 20 L vessel, and the interaction between the flame front, including radiation emitted from the flame, and the vessel walls is more pronounced for the smaller vessel. This paper details an approach for calibrating a new 36 L dust explosion vessel by utilizing principles for factorial design and analyzing the decay of turbulence following the transient dispersion of the dust clouds by means of computation fluid dynamics (CFD). In the present work, the CFD simulations were used to examine transient injection of air into the 20 L and 36 L vessels, in conjunction with experimental data reported for the 20 liter spherical vessel. Although the vessels considered here had slightly different shapes, sizes, dispersion systems and operating conditions, the simulated turbulence levels were similar at the time of explosion. In addition, the estimates for the laminar burning velocity (S L ) obtained using experimental results from the 20 L and 36 L vessels, and assuming the validity of a correlation for the turbulent burning velocity (S T ), were also in good agreement.
Predicting triggering and consequence of delayed LNG RPT
Abstracts:We develop a model for delayed rapid phase transition (RPT) in LNG spills based on thermodynamics and nucleation theory which includes predictions of both triggering and vapor explosion consequence. We discover that the model predictions can be accurately characterized by two independent parameters alone: The initial fraction of methane and the molar mass of the remaining non-methane part. Based on this we develop correlations for risk assessment which may be used without access to the underlying advanced algorithms, and we give practical advice for risk mitigation. The model is consistent with an often reported empirical triggering criterion for cryogen RPT. We show that spilled LNG must typically boil down to about 10–20% of the original amount before RPT may occur, and after triggering one may expect energy yields of 10–20 g TNT per kg of triggered LNG. Explosive pressures in the range 20–60 bar can be expected.
Effect of metal mesh on the flame propagation characteristics of wheat starch dust
Abstracts:A self-designed vertical dust combustion pipeline platform was used to study the explosion of wheat starch dust along with the different characteristic parameters of metal meshes and to highlight the influence of the metal mesh on the flame propagation of wheat starch dust. The temperature and pressure of flames were measured using a micro-thermocouple and pressure sensors, and spontaneous images of flames under different working conditions were captured by a high-speed photography system. It was observed that the metal meshes damaged the structural characteristics of flame propagation and the flame front surfaces, which led to the distortion and prevention of flame spread. This blocking effect increased with an increase in the number of metal meshes and layers. When the metal mesh had a smaller aperture, there was an increased suppression of the flame propagation. The presence of metal meshes in flame propagation also quenched a large portion of the flame, with the decrease in flame temperature being large and inversely proportional to the number of meshes and layers. The metal mesh also moderately inhibited the propagation of the combustion pressure wave. The inhibitory effect on pressure wave propagation was more evident and the combustion pressure decreased with an increase in the number of metal meshes and layers.