CHAIRMAN'S ADDRESS - Coke Oven Managers Association

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APPROACH TO THE HAZARDOUS AREA 81CLASSIFICATION - STOIKOUThe equivalent dust Zones are 20, 21 and 22. Fig. 1 shows the symbols for allgas zone types.Fig 1Gas Zone TypesComputer ModelingThe Process Hazard Analysis Software Tool (PHAST Prof. Version 6.0) isused for the calculation of the zonal extent of a hazardous area. It uses built-inchemical and parameter data, along with scenario and meteorological datasupplied by the user to predict the flammable effects from atmospheric releasesof hazardous materials.The first stage in PHAST's calculations is the discharge model whichpredicts the flow rate and state of the material as it is released into theatmosphere. The results from the discharge calculations are then used byPHAST, along with the meteorological information, in the dispersion model.The dispersing cloud can follow a range of paths, all of which are modelled byPHAST as well as all possible flammable outcomes.A side view example of a dispersion cloud displayed dynamically is shownin Fig. 2. This case is time dependent and represents a Coke Oven Gas,outdoor, horizontal release from a valve flange. The scenario type is a 'leak',with a hole diameter of 9.4mm, at 10 metres height above ground level. Threedifferent concentration levels are shown in Fig. 2; the one of most interest isthe same value as the Lower Explosive Limit of the gas, which is 4.45% vol.
82 THE COKE OVEN MANAGERS’ASSOCIATIONAll weather inputs (wind speed, stability class, atmosphere and surfacetemperature, surface roughness, and humidity) can be varied. However, thestability class and wind speeds are linked. The weather category chosen forPHAST calculations at Dawes Lane Coke Ovens is 1.5D which stands for awindy day with little sun (wind speed: 1.5m/s) or overcast/windy night. Theimported process conditions in the example at Fig. 1, are 30C and a pressure of56mbar.Fig. 2PHAST ModelThe base averaging time used in PHAST for all the cases considered is thatfor Flammables (=18.75 sec). Averaging time comes into effect when thecloud becomes passive. When the cloud is active, the concentration profile willbe constant. When the cloud becomes passive, the wind will cause the cloud tomeander. If there is a monitor where the cloud is passive, sometimes it mightmeasure the edge of the cloud. Other times it might be measuring the centre or
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CHAIRMAN'S ADDRESS - Coke Oven Managers Association

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