Five Tips for Ensuring Bulk Solids Storage Success
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As appeared inNovember 2017 PBE powderbulk.com Copyright CSC Publishing Five tips for ensuring bulk solids storage success Eric Maynard Jenike & Johanson This article provides tips for selecting an but this approach can be risky. Two material samples effective bulk solids storage system and with the same name may have very little in common. avoiding material flow problems resulting Coal, for example, comes in many forms, including anthracite, bituminous, sub-bituminous, and lignite. from poorly designed storage vessels. The ash production and heating values from burning each of these coals can vary significantly. Also, the average moisture content for each coal type is different, ranging from 5 percent moisture in anthracite up to 40 percent moisture in lignite. If you aving been active in engineering projects at use an assumed coal moisture content of 10 percent to plants handling a myriad of powders and bulk design your storage system but are actually handling Hsolids for more than two decades, I’ve heard lignite at 40 percent moisture, you’ll likely experience the phrase, “I wish I’d thought of that!” many times. flow problems. Though some bulk material handling processes get it right the first time, in many cases I’m called in to Such physical differences between materials that are correct basic engineering design mistakes that have nominally the same can dramatically affect handling resulted in increased costs from reduced operating efficiency, processability, and mass balance, so it’s vital efficiency, increased equipment maintenance, and lost that you test your specific material’s properties when or out-of-spec final products. designing a storage system. Even fly ash no longer has “standard” properties since the material’s flowability If you’re in charge of implementing a new bulk solids can be dramatically altered by the dry sorbents used in storage and handling system, don’t assume that the EPA-mandated air pollution control and scrubbing equipment will meet your needs; instead, ask systems. This point is more critical than ever with questions, do the necessary engineering, and follow traditional fossil fuel power plants co-firing with through with a design that’s engineered to succeed. biomass. The flow properties of the biomass — even a Most problems are caused by upfront design mistakes material as simple as wood chips — can vary greatly rather than operating errors. Spending 10 to 20 percent depending on whether the chips are from a hardwood, more upfront to ensure that your system is engineered such as oak or maple, or from a softwood, such as pine correctly can save you 10 times that amount in long- or spruce. Softwood chips are often stringy, prone to term operating costs and costs to fix poorly designed interlocking, and contain a high percentage of pitch (or equipment. tar), making them stick in diverter gates, feeders, and transfer chutes. Also, defining the particle size for anisotropic (or irregularly shaped) wood chips can be The following tips will help you select or design the best tricky because the particle size must be defined in three bulk solids storage equipment for your application. dimensions rather than by the screen size the material can pass through, which is commonly done for other materials. 1Avoid using assumed bulk material flow properties. You must also carefully consider your material’s bulk A common mistake when designing or selecting density since material-induced structural loads are storage equipment is to use assumed material directly proportional to the material’s bulk density. properties. For many common materials, such as coal, Don’t just measure the material’s loose or “as poured” limestone, sugar, or wood chips, bulk density, bulk density; also measure the material’s moisture content, or particle size data is readily compressibility to determine the bulk density over the available either online or in industry-specific reference full range of pressures you expect the material to be books. It may be tempting to use this published data, subjected to in storage. Copyright CSC Publishing • Flowrate limitation, which is an insufficient material flowrate from the vessel, typically caused by counter- 2 Focus on discharge rather than storage. flowing air slowing the gravity discharge of a fine Storing a material is relatively easy. Anyone with a powder basic understanding of geometry and bulk density can • Segregation, as shown in Figure 2, which is the design a bin or silo to “store” a powder or bulk solid at separation of particles by size, shape, or density a certain tonnage or volumetric capacity. The difficult part is ensuring that the vessel will reliably discharge Flow problems in a storage vessel can have many the material at the required flowrate. In the late 1950s, adverse consequences for production, product quality, 1 Andrew Jenike developed unique scientific methods and worker safety. Ratholing will limit a vessel’s live for designing bins, hoppers, and silos. (or usable) capacity so much so that the vessel’s active volume may only be 10 to 20 percent of its rated While material flow can be a problem with a variety of storage capacity. Depending on the material, bulk material handling equipment, such as feeders, stagnation in a poorly designed storage vessel can lead transfer chutes, and dust collectors, flow problems to material caking, spoilage, or other forms of quality most commonly occur in storage vessels. Common degradation. Also, the non-uniform material loading flow problems and their consequences include: caused by arches and ratholes and their tendency to • Arching or bridging, as shown in Figure 1a, in which occasionally collapse can cause localized, or even 2 material forms a stable arch-shaped obstruction over catastrophic, silo failure. The segregation of a material the hopper outlet, stopping material flow by particle size may prevent a desired chemical reaction, cause the final product to be out-of-spec, or • Ratholing, as shown in Figure 1b, in which material require costly rework to desegregate the material. forms a stable open channel in the bin, stopping or causing erratic material flow Accurately measuring your material’s flow properties • Flooding or flushing, where an aerated bulk solid allows you to predict and control how the material will behaves like a fluid and flows uncontrollably through behave in your storage vessel and during discharge. A an outlet or feeder bulk solids testing lab can measure these flow properties1, 3 under conditions that accurately simulate Figure 1 how you handle or process the material in your plant. If your material’s properties change rapidly over time or if special precautions are required, you should have a. Arching the tests conducted onsite. Flow problems are often the result of a hopper discharging material in an undesirable flow pattern. The flow pattern you choose for your bin can directly Material influence your material’s flow performance. Unfortunately, most standard-design storage Stable arch Figure 2 Segregation Coarse particles b. Ratholing Stagnant material Flow channel Fine particles Copyright CSC Publishing equipment can yield undesirable performance with Even if the equipment supplier guarantees your vessel’s difficult-to-handle materials, such as fine powders, performance, the supplier likely won’t replace the sticky materials, or materials whose particles tend to equipment for free or reimburse you for production interlock or cake. downtime if you experience flow problems. You can prevent these flow problems by installing a Carefully evaluate whether you should use a storage vessel specifically designed to discharge your 3standard design. material in a mass-flow pattern. With mass flow, all material moves whenever any is withdrawn in a first-in Most standard storage silos and bins discharge first-out flow sequence, as shown in Figure 3b. material in a funnel flow pattern. With funnel flow, Material flow is uniform and reliable with no stagnant some of the material moves while the rest remains regions, so material won’t cake, level indicators work stationary, as shown in Figure 3a. This first-in last-out reliably, and segregation of the discharge stream is sequence is acceptable if the material is coarse, free- minimized. Also, with mass flow the material’s bulk flowing, and non-degradable and if segregation during density at discharge is independent of the amount of discharge isn’t an issue. A funnel-flow silo typically material (or head) in the vessel. uses a 60-degree (from horizontal) hopper angle. This standard hopper angle can be an economical choice from a capital expense perspective because man - To achieve mass flow in a storage vessel, the hopper ufacturing units with this geometry is easy and wall must be steep enough and sufficiently low in inexpensive and generates minimal waste. friction to allow the material to flow along the wall surface despite the hopper’s converging geometry. Also, with abrasive materials, such as sand or bottom A 60-degree hopper angle will promote funnel flow for ash, the hopper walls must be lined with an abrasion- most bulk solid materials. If you’re storing a difficult- resistant liner to minimize wear to the hopper wall over to-handle material, selecting such a hopper can cause time. Achieving mass flow often requires a flow problems and greatly increase operating costs. nonstandard vessel design, which may cost more upfront but will save money in the long run in reduced Figure 3 downtime and repair costs. a. Funnel flow 4Make sure the feeder will maintain reliable flow. In many cases, poor flow from a storage vessel can be caused by the feeder drawing material from the vessel and discharging it into the downstream process. Effective material discharge from storage requires that Flowing Stationary the feeder withdraws material uniformly through the material material entire cross-section of the hopper’s discharge outlet.4 Regardless of the hopper design, an obstructed outlet due to a poorly designed feeder or partially closed slide gate will result in funnel flow.