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Sign up for free Log in. EMBED for wordpress. Dillution is achieved by ventilation and ventilation can be expressed as the air velocity around the source of release. The model is based on the assumption that, the higher the air velocity near the source of release is, the higher the dillution is. Based on the values, the intersection of the corresponding lines, results in the dillution class.
For indoor situations the resulting dillution class from figure C. The dillutin class for indoor situations depends also on the average background concentration in the area. The average background concentration in the area under consideration, must be lower than the critical concentration. The critical concentration is a certain percentage of the LEL of the substance. The background concentration Xb can be calculated by the following equation:.
The f factor is a safety factor, for the efficiency of the ventilation under consideration. When the background concentration is lower than the critical concentration, than the check is "OK" and the dillution class remains the same as the prelimenary dillution class, based on figure C.
When the calculated background concentration is higher than the critical concentration, than the check is "Not OK" and the dillution class becomes low. The availability of the ventilation is classified in three levels of availability:. Based on all the assessed steps the zones can be classified based on the the following table D. The extent of the hazardous area or region where flammable gas may occur depends on the release rate and several other factors such as gas properties and release geometry and surrounding geometry.
Figure D. Which line to choose depends on the form of the release and the substance properties. There is a cut-off in the graph for heavy gases at a 1,5 mtr radius and for jet and diffuse releases the cut-off is 1 mtr radius.
In order to simplify all the calculations and assessments, we developed Hazcalc. This is an online calculation model, which is based on scenarios, and fully compliant with the current European standard EN-IEC All figures and graphs are implemented in the software, even as a substance database.
Assessment reports can be printed as pdf and be downloaded in Excel format. For more information visit hazcalconline. Principles of Hazardous Area Classification. IEC : Edition 3. Download IEC pdf file. IEC Edition 3. It is an approach which is defided into the following steps: Identify sources of release Calculate the grade of the release; Classification of the dillution of flammable substances in air; Classification of the availability of the ventilation; Determination of the zone classification see table below ; Determination of the extent of the zone.
What are we actually assessing? Approach IEC 1. Sources of release The first step is to identify the sources of release from an installation of work process. Source Description Example Continuous grade of release Release which is continuous or is expected to occur frequently or for long periods The surface of a flammable liquid in a fixed roof tank, with a permanent vent to the atmosphere.
Primary grade of release Release which can be expected to occur periodically or occasionally during normal operation Seals of pumps, compressors or valves if release of flammable substance during normal operation is expected. Secondary grade of release Release which is not expected to occur in normal operation and, if it does occur, is likely to do so only infrequently and for short periods Flanges, connections and pipe fittings, where release of flammable substance is not expected during normal operation.
Grade of the release The grade of the release in, mass per second, can be calculated using the equations from the standard. Kind of release Equation from standard Example Pressurized non-choked gas release e.
Dillution When the grade of release is known, it is important to evaluate, how the concentration of flammable gases is dilluted. This can be evaluate with figure C. Background Concentration for indoor situations For indoor situations the resulting dillution class from figure C.
Availability of the ventilation The availability of the ventilation is classified in three levels of availability: good: ventilation is present virtually continuously; fair: ventilation is expected to be present during normal operation. Discontinuities are permitted provided they occur infrequently and for short periods; poor: ventilation which does not meet the standard of fair or good, but discontinuities are not expected to occur for long periods.
Zone classification Based on all the assessed steps the zones can be classified based on the the following table D. Extent of the zone The extent of the hazardous area or region where flammable gas may occur depends on the release rate and several other factors such as gas properties and release geometry and surrounding geometry.