Spouts, chutes, and transitions: Gravity flow design is critical for efficient operation of many dry bulk processing facilities
By Jeremy Cychosz
Many commodities do not lend themselves to transfer by pumps or blowers. For these types of dry bulk solids, gravity flow is the primary method of material transfer. Facilities that process or handle these materials have special design parameters.
The flow characteristics of a commodity (e.g., flour doesn’t flow in the same manner as soybeans) is a major design parameter to consider. The key property in bulk solids design is the angle of repose, or the slope that a stable pile of the material makes relative to a horizontal plane. Another design parameter to consider is the flow rate of the commodity. Moving 10,000 bushels per hour of wheat requires a much larger facility than moving it at 1,000 bushels per hour. Other material properties and characteristics, such as moisture content, abrasiveness, and bulk density, are elements that are considered in a gravity flow design. Special coatings or processes that have changed the commodity’s flow characteristics also need to be considered.
Facilities that rely on gravity flow to move their commodity will employ a network of spouts, chutes, and transitions. Spouts and chutes typically have a circular or square cross section; are constructed of steel or perhaps aluminum; are joined by flanges, clamps, couplings, or welds; and can be lined with materials to resist abrasive wear or improve the flow of the commodity.
Since the flow of the material is only propelled by gravity, the routing of spouts and chutes is a critical design element. Often, the placement of the spouting and process elements governs the design; support structures and systems are designed around their locations. Close design coordination of systems is a key feature of facility design. Some commodities require more gentle handling than others – aggregates and fertilizers can tolerate high velocities, trajectory and impact, but specialty grains, seeds and food products will degrade if dropped or handled in excess velocity. Consequently, an analysis of spouting slope, length, and material characteristics may be necessary to ensure material is not needlessly damaged. Flow retarders, cushion boxes, or letdown ladders are among the items used to promote gentle handling of fragile commodities.
Even with properly arranged spouting or chutes, a gravity flow facility will always have a need to translate the commodity horizontally or vertically. An entire industry of handling equipment for dry bulk solids has been developed for this requirement. Belt conveyors, vibratory conveyors, bucket elevators, cup elevators, screw conveyors, and more are all employed to move dry goods from one point to another. This specialized equipment is integral to the design of a gravity flow facility.
Through careful attention to material flow characteristics, spouting routes, and equipment arrangement, a well-designed facility can improve the efficiency of operations by reducing the quantity of equipment or arranging bins to match equipment flow rates and improve product quality by designing spouting and transitions to reduce material damage and spillage. In addition, well-planned maintenance access points can make preventive maintenance easier and reduce time to make repairs.
Consideration of all these gravity flow elements – along with coordination of all disciplines involved – can result in a safe, efficient, and high-functioning facility.
Case Study: University of Arkansas Seed Facility
The University of Arkansas selected IMEG to design a new foundation seed cleaning and storage facility at the Rice Research and Extension Center in Stuttgart, AR. IMEG worked with the university to establish the new facility’s operational requirements, storage capacity, production rates, and quality specifications. The facility needed to clean three crop types: rice, soybean, and wheat. Variety purity and seed quality were the top design considerations, so the ability for operators to inspect and clean seed handling equipment was paramount in the design.
The bulk storage needs of the facility required the use of twenty 1,200-bushel cone-bottom, round storage bins. For the bin filling operations, IMEG proposed a unique bin fill conveyor that had not been previously used in the rice seed industry: an enclosed head-house structure above the storage bins with a belt conveyor outfitted with a tripper and shuttle cart. The tripper and shuttle cart can be positioned directly above any bin opening in the headhouse floor to fill the bins. The unique characteristics of this design allowed for all material handling equipment to be located indoors, eliminating the need for an exterior tower or ladder up to a bucket elevator head, long spouting runs down to the bins, or augers or drag conveyors to distribute the seed. The use of a belt conveyor allows for complete cleanout for variety changes, full inspection, and ease of cleaning operations. The enclosed headhouse allows operators to safely pull product samples directly from the bins and provides full access to the equipment for any maintenance activities.
To reduce capital and operational costs, one bucket elevator serves a three-fold purpose. The main receiving elevator performs the functions of three separate legs: it receives the raw grain and elevates it to the pre-cleaner; it is part of the bulk storage bin unloading system where it elevates grain to the primary cleaner; and it is part of a recycle system that transfers cleaned seed from the packaging bin back to the primary cleaner or bulk storage. IMEG carefully reviewed the operational requirements of the system with the university to establish the sequence of operations and protocols to allow the single elevator to perform the three functions without hampering the operations of the facility.
IMEG also designed a semi-horizontal flow system for the equipment arrangement of the facility. While larger commercial seed facilities frequently utilize tall, vertical flow arrangements, the cost and accessibility constraints of this project directed the design to a semi-horizontal flow, with vertical gravity flow utilized wherever possible within the elevation constraints of the facility. Grain is re-elevated to the top of the facility by use of gentle handling cup elevators during the cleaning process. The facility uses two additional “lifts” when compared to a typical vertical flow facility. The semi-horizontal arrangement allows all the equipment to be located on three service levels indoors and above the ground floor. This arrangement allows operators quick access to all areas of the facility during operation, inspection, cleaning, and maintenance activities.