Review of COMEX and NVBANG Explosion Models
COMEX and NVBANG are two explosion models developed by DNV. These codes are used in safety case submissions.
The information about the models is scarce and HSE Specialist Inspectors in the Offshore Safety Division 3.2 asked that HSL would critically assess the strengths and weaknesses of the models.
Information was sought from DNV, who kindly provided documents giving descriptions of the models. A set of further queries was sent to DNV, who again kindly provided answers. The developer was then sent relevant parts of the report for comments and corrections of any factual inaccuracies.
COMEX, a spreadsheet program, is a suite of five empirical models. The models in COMEX fall into two categories that are suitable either for confined geometries or for geometries with vents on all walls. COMEX must be run by an authorised person at DNV and is not available to customers on a license basis. The model constants have been calibrated against a series of experiments in enclosures, whose volumes ranged from 0.0036 m3 to 425 m3 – these experiments were performed between 1978 and 1983. Obstacles of different sizes, pitches and orientations can be accounted for – the obstacles are grouped into grids. The user can specify up to 20 grids that have to be identical, in terms of blockage ratio, obstacle size, etc., and the grids must be equally spaced. The constants were re-calibrated when the results from the largescale experiments performed as part of Phase 2 of the Blast and Fire Engineering for Topside Structures (BFETS) Joint Industry Project became available. COMEX was also used to calculate the explosion-generated over-pressure for one of the tests conducted in the BFETS project.
NVBANG is a phenomenological model. The model is not normally used as a stand-alone tool; rather it is used in conjunction with COMEX or other tools that can provide an estimate of the over-pressure. NVBANG has a feedback factor that has to be tuned so that NVBANG gives the same answer as, say, COMEX. Other input parameters will have to be adjusted to account for external explosions, non-stoichiometric fuel-air mixtures, and fuels other than propane or methane, etc. NVBANG code was used to calculate the pressure time history, with COMEX providing maximum over-pressure, of one of the tests involving a propane cloud performed during Phase 2 of the BFETS Joint Industry Project.
This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the author(s) alone and do not necessarily reflect HSE policy