RR630 Area classification for secondary releases from low pressure natural gas systems
A review has been carried out on the ventilation of enclosures focusing on measures of ventilation effectiveness and how ventilation rates can be measured for input into an area classification methodology. The most accurate approach to calculating air change rates for naturally ventilated enclosures is to make measurements of the decay rate of a tracer gas within the enclosure. However, the time and expense required to do this means that it is not an approach suitable for area classification. BS 5925 describes a method for calculating air change rates that is simple to apply and should provide data of sufficient accuracy to be appropriate for area classification. The approach has been applied to two enclosures where the air change rate was measured experimentally. In the first of the two cases considered, the calculated air change rate was in good agreement with the measurements, whereas in the other case it under-predicted the ventilation rate. An appropriate conservative choice of the wind speed, say 0.5 m/s, should provide corresponding conservative estimates of the ventilation rate.
As part of the ventilation review, BS EN 60079:10 has been reviewed in detail and in this report we have made a clear distinction between the two definitions given for the gas cloud volume Vz. They are:
- a hypothetical volume that can be calculated using the formula in BS EN 60079:10 and is proportional to the mass release rate of a leak divided by the air change rate of the enclosure. Vz in this context is therefore simply a measure of ventilation effectiveness and the criterion Vz less than 0.1 m3 is used to define zone 2 NE.
- a gas cloud that has an average gas concentration of ½ LEL.
The presence of a heat source in an enclosure has been shown to lead to reduced mixing and therefore greater gas cloud build up in some cases. In particular, large gas clouds can result from cases where a strong thermal stratification exists coupled with a confined leak location. However, in the absence of a confined leak location it would appear that the cout < 10% LEL criterion is still appropriate.
The CFD model for the above work has been validated against 29 experimental tests carried out in a purpose built enclosure. The experimental tests consisted of releases of simulated methane gas for a range of leak rates and ventilation rates. Three different configurations of the release location and direction were tested and measurements of the point gas concentration measurements were used as the basis for the model validation. The results of the CFD simulations showed good agreement with the experimental data.
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