The interaction between rotary valves and pneumatic conveying pipelines
The object of this work was to investigate the interaction between rotary valves and pneumatic conveying pipelines and the effect which this can have on overall system performance. A review of previous work and current industrial practice revealed that very little work has been published on this subject, although it was evident that some manufacturers of pneumatic conveying systems do have preferred entrainment configurations. Consequently, a preliminary experimental investigation was undertaken with a transparent model of a rotary valve and drop-out box in order to observe the air and solid flow patterns inside the drop-out box. This provided the most important outcome of this work, that is, the discovery that two distinctly different modes of flow can exist in the chamber of a conventional drop-out box. The first of these is a turbulent swirling motion caused by the conveying airstream and is the most desirable operating condition because it results in the most effective entrainment of material into the conveying line. The second is a situation where the drop-out box is effectively 'choked' with product. Models to explain these two conditions have been developed and subsequently tested against data obtained from a full size positive pressure conveying system specifically constructed for this purpose. An extensive experimental programme has been carried out in which the performance of this system was examined with a selection of different entrainment configurations and different test materials. The principal variables investigated were the height and volume of the drop-out box, the orientation of the rotary valve with respect to the pipeline and the conveying air velocity. As a result of this work guidelines have been produced for interfacing rotary valves with pneumatic conveying pipelines. These have been presented as a simple list of eight points and it is anticipated that they will enable systems to be designed with more confidence than has been possible previously.