Geometry implementation

The overpressure in a gas explosion is very dependent on the degree of congestion made up by piping, equipment, structure, cable trays etc. In general, the higher the degree of congestion, the higher the overpressure. It is therefore extremely important to take into account also smaller geometry elements in an analysis. This also applies to natural ventilation calculations, where a congested area offers higher resistance to the flow than does an uncongested region. Gas dispersion is also strongly affected by the degree of congestion, as gas mixing with air is improved in a more congested region.

In ventilation, dispersion and explosion calculations it is hence recommended that the geometry is modelled in sufficient detail such that flame propagation and acceleration will be accurately represented. Typically details and piping down to 2-5 cm size need to be included.

The figure below illustrates how equipment density affects ventilation and explosion overpressure in FLACS simulations. The geometry on the right is the realistic one. For identical wind conditions the number of air changes in the geometry on the left was twice that of the one on the right. For identical gas clouds and ignition locations the explosion overpressures in the geometry on the right were approx. 5 times that calculated in the one on the left.

 

Geometry implementation may either be done by electronic transfer from a CAD system (PDMS or PDS/Intergraph), or by manual implementation, based on drawings.
Smaller details will be implemented either by using GexCon's Anticipated Congestion Method (ACM), which is a formalized procedure for generating smaller geometrical details based on similar existing installations, or based on detailed drawings if such exist. The ACM is faster and cheaper than basing the implementation on actual drawings of the installation (which may or may not exist at the present time), but gives for the purpose of performing explosion analyses, a robust representation of the geometry of the installation. By using the ACM, detailed geometry representations can be generated even in a concept phase where very little geometry information exists. Hence reliable explosion simulations can be performed and applied even in the concept evaluation phase.

The figure below shows an example FPSO geometry implemented in FLACS.

The ACM may also be applied to verify geometries intended to be used for explosion simulations.

Contact
Lars Rogstadkjernet
Phone no: +47 55 57 43 22
Email: larsr(at)gexcon.com