Bin design also involves making decisions about other system components, including: • outlet gate or shut-off valve. In general, a slide gate at the bin outlet should only be used for maintenance purposes, not to modulate the solids flowrate. Therefore, it should be operated only in the full-open or full-closed position. • number of outlets. Although a multi-outlet hopper may be attractive for production flexibility, its design can have negative consequences structurally and from a flow perspective. This also applies to hoppers that have asymmetric geometries (e.g., one sloping wall, one vertical wall). • bin vent or dust collector. Depending on the method of vessel filling, an air-solids separator may be required. • conservation vent. A dual-acting conservation vent is typically installed on thin-shelled steel tanks to avoid excessive pressure and vacuum conditions and prevent damage. • level detectors. There are two different types of level detection: point level detection and continuous level detection. Point level detectors are typically attached to the side wall or roof of a bin, and measure solids by direct contact using capacitance sensors, pressure diaphragms, or other means of proximity sensing. Common point level detectors include rotary paddles, tilt switches, and vibrating rods. Continuous level detectors usually consist of a roof-mounted device that emits radar or ultrasonic signals. The signals project down to the surface of the bulk material, and then rebound to a receiver for processing. Ultrasonic, guided-wave radar, or plumb-bob (i.e., yo-yo) detectors are commonly used to provide continuous detection of the top surface of the bulk material. • explosion protection. Depending on the explosivity characteristics of the powder, explosion vents, isolation, suppression, or inerting may be required. Numerous explosionprotection equipment and consulting firms are available to
provide testing and expertise. Reference 9 explains how to limit dust explosion hazards and Ref. 10 outlines test procedures for determining explosivity. • access doors, manways, and poke holes. Poke holes (nozzle ports on the hopper walls) are not recommended in mass-flow hoppers, as they have a tendency to prevent flow along the walls — creating a problem that mass-flow bins are intended to solve. Access doors are also a frequent cause of problems; if they are essential, it is better to locate them in the cylinder rather than in the hopper. • ladders, railings, and platforms. Although these seem like minor details, depending on the need to access the vessel roof for maintenance, they may become important.