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28 / March 2020 powderbulk.com properties and the conveying velocity. A highly abra- sive material moving at a low velocity will cause less equipment abrasion than a somewhat abrasive material moving at a high velocity. Therefore, in nearly all cases, reducing the system's velocity will minimize the risk of abrasion. Conversely, a lack of control over the system's velocity will unnecessarily increase the risk of abrasion. There are many examples of seemingly harmless materi- als, which appear soft and nonabrasive, wearing holes in pipes and elbows; usually, the culprit can be traced back to excessive air velocity. Degradation. Degradation isn't necessarily a material property, but materials can be considered fragile or friable and, as a result, degrade when they're handled roughly. In pneumatic conveying, degradation happens when the air-entrained materials interact with hard surfaces such as pipe walls. The particle momentum and resulting impact breaks larger material pieces into mul- tiple smaller pieces. Again, there's no exact definition for what constitutes degradation because how you measure a material's degradation level varies depending on the material being handled. Materials such as coffee beans, sugar granules, or microspheres would have differing measurements for what constitutes as degradation. It can be said, though, that the impact velocity from the material's momentum has the strongest correlation with the resulting particle size reduction. In nearly all dilute- phase conveying cases studied, an increase or decrease in conveying velocity produced a proportional increase or decrease in degradation. Buildup. The ability of particles to attach themselves to pipe walls is a difficult material attribute to study or quantify. However, certain materials over others have been known to demonstrate adhesive and cohesive qualities by sticking to a pipe's internal surface while additional particles attach to those stuck particles until a layer of material is formed. This material layer can continue to build, eventually partially blocking the line and affecting the conveying line's ability to pro- cess material. While the underlying cause of buildup is often not fully understood, one reason could be because fatty materials release encapsulated oils that can attach themselves to hard surfaces. Another reason could be because the water in high-moisture materials can make particles cohesive, which cements them in place when they dry. Extremely small particles some- times attach to surfaces due to static force. Whatever the reason, the material's ability to build up is often noted after the process already happens, so this infor- mation isn't well-defined. When studied, the material's treatment, including exposure to air velocities, cor- relates with the degree of buildup. In other words, or worst-case duty (most difficult-to-move material, greatest transfer rate, longest conveying distance) is considered and equipment is selected with this factor in mind. The parameters at which to run the equipment and components in your pneumatic conveying system, such as the base operating speed of a blower, is deter- mined with the worst-case conditions in mind, and without an operator adjustment after installation, the equipment may operate this way for its entire service life. Therefore, nearly every dilute-phase system can operate at a speed greater than what's required to per- form the duty, and for many systems, the extra speed is excessive. This is the cumulative effect of the effective air velocity and the worst-case considerations. The classic dilute-phase system would normally be designed and operated at full speed with across- the-line power of 50 or 60 hertz. Until recently, adding a variable-frequency drive (VFD) for blower speed control came with a significant cost. Unless there was a specific process need for changing blower speed, many projects didn't integrate speed controls due to the cost. At full speed, the blower would operate the system reli- ably across a range of material feedrates and conveying distances if required. However, running the blower at full speed when a lesser speed will do results in great- er-than-planned air velocities, potentially from 5 to 20 percent greater, in the conveying line. Generally, the system would continue working as expected and the elevated velocities wouldn't be evident to the operator. Reduce velocity, reduce risk As we've established, elevated velocities can occur as part of the dilute-phase conveying system's initial design or by operating the system below the maximum design conditions. Air velocity matters in a dilute- phase flow system because the effective air velocity is a strong contributing factor in equipment abrasion, mate- rial degradation, material buildup in conveying lines, and overall process power consumption. Each of these potential system issues can pose problems in a pneu- matic conveying system. Abrasion. Normally, abrasiveness is considered a material property, and materials can be abrasive, somewhat abrasive, or nonabrasive. There's no exact definition but rather an aggregate of experience regard- ing how quickly equipment wears out around specific materials. To estimate abrasiveness, a material can be assigned to the hardness scale and compared to another material. Other custom or standard tests can be con- ducted to put the material on another relative scale, but what matters most is how the material interacts with the process equipment. In pneumatic conveying, equipment abrasion occurs because of a combination of the material