Having been active in engineering projects at plants handling a myriad of powders and bulk solids for more than two decades, I’ve heard the phrase, “I wish I’d thought of that!” many times. Though some bulk material handling processes get it right the first time, in many cases I’m called in to correct basic engineering design mistakes that have resulted in increased costs from reduced operating efficiency, increased equipment maintenance, and lost or out-of-spec final products.
If you’re in charge of implementing a new bulk solids storage and handling system, don’t assume that the equipment will meet your needs; instead, ask questions, do the necessary engineering, and follow through with a design that’s engineered to succeed. Most problems are caused by upfront design mistakes rather than operating errors. Spending 10 to 20 percent more upfront to ensure that your system is engineered correctly can save you 10 times that amount in long-term operating costs and costs to fix poorly designed equipment. The following tips will help you select or design the best bulk solids storage equipment for your application.
Avoid using assumed bulk material flow properties.
A common mistake when designing or selecting storage equipment is to use assumed material properties. For many common materials, such as coal, limestone, sugar, or wood chips, bulk density, moisture content, or particle size data is readily available either online or in industry-specific reference books. It may be tempting to use this published data, but this approach can be risky. Two material samples with the same name may have very little in common. Coal, for example, comes in many forms, including anthracite, bituminous, sub-bituminous, and lignite. 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 use an assumed coal moisture content of 10 percent to design your storage system but are actually handling lignite at 40 percent moisture, you’ll likely experience flow problems.
Such physical differences between materials that are nominally the same can dramatically affect handling efficiency, processability, and mass balance, so it’s vital that you test your specific material’s properties when designing a storage system. Even fly ash no longer has “standard” properties since the material’s flowability can be dramatically altered by the dry sorbents used in EPA-mandated air pollution control and scrubbing systems. This point is more critical than ever with traditional fossil fuel power plants co-firing with biomass. The flow properties of the biomass — even a material as simple as wood chips — can vary greatly depending on whether the chips are from a hardwood, such as oak or maple, or from a softwood, such as pine or spruce. Softwood chips are often stringy, prone to interlocking, and contain a high percentage of pitch (or 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 tricky because the particle size must be defined in three dimensions rather than by the screen size the material can pass through, which is commonly done for other materials.
You must also carefully consider your material’s bulk density since material-induced structural loads are directly proportional to the material’s bulk density. Don’t just measure the material’s loose or “as poured” bulk density; also measure the material’s compressibility to determine the bulk density over the full range of pressures you expect the material to be subjected to in storage.
Carefully evaluate whether you should use a standard design.
Most standard storage silos and bins discharge material in a funnel flow pattern. With funnel flow, some of the material moves while the rest remains stationary. This first-in last-out sequence is acceptable if the material is coarse, free-flowing, and non-degradable and if segregation during discharge isn’t an issue. A funnel-flow silo typically uses a 60-degree (from horizontal) hopper angle. This standard hopper angle can be an economical choice from a capital expense perspective because manufacturing units with this geometry is easy and inexpensive and generates minimal waste.
A 60-degree hopper angle will promote funnel flow for most bulk solid materials. If you’re storing a difficult-to-handle material, selecting such a hopper can cause flow problems and greatly increase operating costs.
Even if the equipment supplier guarantees your vessel’s performance, the supplier likely won’t replace the equipment for free or reimburse you for production downtime if you experience flow problems.
You can prevent these flow problems by installing a storage vessel specifically designed to discharge your material in a mass-flow pattern. With mass flow, all material moves whenever any is withdrawn in a first-in first-out flow sequence. Material flow is uniform and reliable with no stagnant regions, so material won’t cake, level indicators work reliably, and segregation of the discharge stream is minimized. Also, with mass flow the material’s bulk density at discharge is independent of the amount of material (or head) in the vessel.
To achieve mass flow in a storage vessel, the hopper wall must be steep enough and sufficiently low in 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 ash, the hopper walls must be lined with an abrasion-resistant liner to minimize wear to the hopper wall over time. Achieving mass flow often requires a nonstandard vessel design, which may cost more upfront but will save money in the long run in reduced downtime and repair costs.
Be sure to consider combustible dust hazards.
Most dusts generated by bulk solids manufacturing operations are combustible, which means they can burn rapidly, causing either a flash fire or an explosion. Though most people know about the hazards of flammable gases and liquids, many are unaware of the combustible dust hazards associated with bulk solids storage and dust collection equipment. According to the US Chemical Safety and Hazard Investigation Board, 281 combustible dust incidents occurred during the 25-year period from 1980 to 2005, killing 119 workers, injuring 718, and causing extensive damage to plants and equipment.
A dust explosion requires five ingredients:
- Combustible dust such as sugar, plastic, wood, and most carbon-containing dusts
- An oxidant such as the oxygen present in the air surrounding most process areas
- An ignition source such as a static discharge, hot surface, or spark
- Dispersion of the dust into the air
- Confinement of the dispersed dust in an enclosed space such as a silo, dust collector, dryer, mill, or building envelope
Several National Fire Protection Association (NFPA) standards provide excellent guidance for preventing and protecting against combustible dust hazards. Prior to reviewing industry- or design-specific standards, I recommend first reviewing the more general NFPA 652: Standard on the Fundamentals of Combustible Dust. The standards detail many devices and methods for protecting your storage equipment from combustible dust hazards, including venting, containment, isolation, and suppression. These OSHA-consensus standards may be mandated by an appropriate authority having jurisdiction (AHJ), such as a plant owner, insurance provider, fire chief, or building inspector. Ignoring your plant’s combustible dust hazards or the requirements of your local AHJ can result in fines at best and catastrophic consequences at worst.