Modelling of Plant Congestion with TNT Equivalent Explosion Method

The TNT equivalent explosion method is a versatile tool for hydrocarbon processing plants explosion modelling. It is not however a rigorous calculation tool as it is only applicable to open unconfined areas and not taking into account the effects of process plant congestions, confinements and obstacles which would affect flammable vapour cloud explosion loads. If obstacles are present then expansion-generated flow, created by the combustion of the unburnt gas passing through the obstacles will generate turbulence. This will increase the burning velocity and further enhances the turbulence. The turbulent mixing of vapour and air, and ignition of the cloud at a point remote from the release, increases the peak overpressures and explosion impact.

This paper presents a straightforward methodology to introduce plant congestions into TNT equivalent modelling which can be used as a preliminary explosion overpressure impact assessment for process plant piping, equipment layout and structural designs.

The modelling assumes that the explosion occurs from a hydrocarbon release source as its centre and a specific distant location as radius, generating a hemispheric shaped hypothetical blast load zone from the ground level with a finite volume. This location could include the target or the congestion. The first step is to calculate the TNT equivalent peak overpressure from a release source to the target area. Then by using the ideal gas and explosion cubic laws calculate the pressure rise due to congestions.  The congestion (or obstruction) is represented by the volume occupied by the buildings or plant items such as piping, piperack, structures, vessels, tanks and rotating equipment. This technique models the energy released at explosion and the rise in overpressure on the basis of the reduction of volumetric hemispheric space due to presence of the congestions. As expected by the ideal gas law the pressure will rise if the volume decreases. The resulting overpressure accounts for the congestion at remote locations.