Venting Strategies in Elevator Legs

Facilities that manufacture, process or handle combustible dusts can be subjected to fire and explosion hazards presented by these materials. Fire and/or explosion incidents involving combustible dusts can result in very significant facility damage and potential injury or loss of life.  The NFPA has developed several standards to aid these industries in mitigating the hazards associated with combustible dusts.  One of these strategies is deflagration venting in accordance with the NFPA 68 standard.  However, in some cases, practical constraints such as available space, configuration of existing facilities, or operations requirements, can prevent a company from bringing their operations and equipment into compliance with the prescriptive requirements of this standard.  Alternate venting geometries that do not meet the prescriptive requirements, can be used as a performance based approach to NFPA 68, but this requires detailed analysis of a potential deflagration inside the equipment.

This paper focusses on the review of the NFPA 61 and 68 requirements associated with deflagration venting in a bucket elevator leg in agricultural service.  In particular, the present paper investigates a scenario where the elevator vent sizes and locations are restricted for practical or operational reasons.  Computational Fluid Dynamics (CFD) calculations of vented deflagration inside bucket elevators are performed for a number of geometries and conditions.  These models can be used to ensure that the life and safety goals of the NFPA standards are met for alternate venting strategies.  The present work highlights the differences between single sided venting and double sided (symmetric) venting.  The results are discussed both in terms of overpressures in the elevator leg, but also in terms of forces and impulses imparted on the elevator structure.