Understanding Bends In Pneumatic Conveying Systems

solids has been successfully practiced — in industries as diverse as chemical, agricultural, pharmaceutical, plastics, food, mineral processing, cement and power generation — for more than a century. Pneumatic conveying provides advantages over mechanical conveying systems in many applications, including those that require complex routing, multiple source-destination combinations and product containment. Pneumatic conveying transfer lines are often routed over pipe racks and around large process equipment, giving process operators great layout flexibility. Such design flexibility is made possible by the use of bends (such as elbows and sweeps, discussed below) between straight sections (both horizontal or vertical), which enable convenient change of direction in the flow of the conveyed solids. However, among all the components of a pneumatic conveying system, bends — despite their apparent simplicity — are probably the least understood and most potentially problematic for process operators. Findings from various research studies are often not consistent, and often times public findings do not match field experience. The importance of bends in any pneumatic conveying assembly cannot be overstated since — if not properly selected and designed — they can contribute significantly to overall pressure drop, product attrition (degradation) and system maintenance (due to erosive wear). Historically, a basic long-radius bend (shown in Figures 1 and 2, and discussed below) has been the bend of choice for designers of pneumatic conveying systems, for a variety of reasons: • Long-radius bends provide the most gradual change in direction for solids, and hence are most similar to a straight section of piping • The angle of impact on the pipe wall is relatively small, which helps to minimize the risk of attrition or erosion • For lack of other experience, to maintain the status quo Years of field experience and a variety of studies conducted to troubleshoot common problems — such as line plugging, excessive product attrition (degradation), unacceptably high bend wear and higherthan-expected pressure drop — clearly indicate that the flow through bends in pneumatic piping is very complex. One should refrain from generalizing the findings until the underlying physics are well understood. This complexity is exacerbated when innovative designs are introduced to address existing issues with common-radius bends (also discussed below). Today, most of the data still resides with vendors and there is a need for fair, unbiased and technically sound comparative evaluation. The purpose of this article is to summarize the key concepts, outline key metrics used to evaluate bend performance, and provide guidance for their selection. We will limit our discussion to dilute-phase conveying. (Issues related to pipe bends for dense-phase conveying systems will be addressed at a future date.)