This document describes the recommendations for the design and use of screw conveyors and product receivers which can in addition be used as a means for explosion isolation to prevent a dust explosion transmission into connected plant items by using the bulk material which is inside.
The recommendations given in this document are procedural measures since the properties of the bulk material affect the efficacy of this measure essentially (e.g. flow and explosion characteristics). Product receivers and screw conveyors cannot be considered as protective systems under the scope of the ATEX Directive.
As far as screw conveyors are concerned, the scope of this document is limited to rigid, tubular, singular screw conveyors which consist of a spiral blade coiled around a shaft held by external bearings (the rotating part of the conveyor is sometimes called “auger”).
NOTE Additional internal bearings can be necessary if the tubular screw conveyor exceeds a certain length.
This document includes limits of application where a plug of bulk material in a screw conveyor is not possible/sufficient to achieve explosion isolation and also application ranges where a plug of bulk material is not necessary to achieve explosion isolation.
This document does not address the mandatory risk analysis and ignition hazard assessment, which are performed for the application of the screw conveyors and product receivers. The mandatory risk assessment includes start-up and shut-down conditions, when potentially no plug of material is present to prevent explosion propagation. To mitigate this residual risk, it is possible to use as an extra measure, e.g. a traditional gate valve which prevents flame transmission and is able to withstand the expected maximum explosion pressure.