Deflagrations By Design

The consequences of large VCEs and the potential for deflagration to detonation transition (DDT) are current topics of considerable interest in the petroleum refining and chemical processing industries. The potential for a DDT is particularly relevant, since a detonation can significantly increase explosion energy and decrease standoff distance, both of which act to increase blast load. A number of potential mitigation options to limit VCE severity have recently been explored and tested (e.g., multiple simultaneous ignition sources to limit flame travel distance, suppressants to limit flame speed). Another mitigation approach is to control the congested volume geometry in order to limit the flame travel distance (i.e., minimize flame acceleration within a congested volume) and incorporate open areas between congested volumes (e.g., “safety gaps”). An additional congested volume geometry mitigation approach is to minimize the flame travel distance at which lateral venting becomes effective (i.e., the minimum distance to free vent). Several simplified VCE blast load prediction methods consider flame travel distance within a congested volume as an input parameter, but do not explicitly consider the distance to free vent.

Decreasing the distance to free vent will limit flame acceleration and decrease the potential for a DDT for a given congestion/confinement level and fuel mixture. High fuel reactivities and elevated congestion/confinement levels would require smaller distances to free vent to preclude a DDT, as well as limit the maximum flame speed to a prescribed value. This paper describes an internal research program executed by BakerRisk that takes a first step towards demonstrating this mitigation option. Tests were performed using near-stoichiometric ethylene-air mixtures in an elongated test rig configured with a medium level of congestion. The free vent distance required to prevent a DDT in this configuration was determined. This paper presents the results of the test program, the associated FLACS predictions, and potential areas for future research.