Rock Products

JUL 2019

Rock Products is the aggregates industry's leading source for market analysis and technology solutions, delivering critical content focusing on aggregates-processing equipment; operational efficiencies; management best practices; comprehensive market

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24 • ROCKproducts • July 2019 Weighing W eighing continuously is the eyes it takes to satisfy the mind on whether process control is delivering to our expectation or not. Without the data, we are blindly shooting in blue sky. When deciding on the accuracy require- ments of the weighing system, consider what accuracy range is acceptable, from overkill to what you can get by with. When continuous weigh measurement is important, determining where in the process weight measurement is possi- ble is the next step. Some continuous weigh solutions are considerably less expensive than batch weighing into hoppers, trucks and rail- cars. Consider the expense of a railcar scale compared to a $10,000 continu- ous weigh scale. Shop around for manufacturers and engineers of measuring devices and control systems. Pick their brain on what the market can supply and what support they will provide. Support is just as important as the potential performance the scale system delivers. Continuous Weighing Is Difficult Once the current weight is known, that information is soon obsolete. The information is useless unless data is monitored and the process control is adjusting itself to the information delivered. Over time, the industry has developed more options from which to choose. Some of those choices are dictated by the application. Retrofit applications are usually more difficult to master over new applications. The new process design can usually be worked around standard model scales. And when a scale does not exist for an application, custom design engineers may come up with a design with an acceptable outcome. We have seen designs that include an adaptive weighing system that corrects a volumetric blending signal to be cor- rected by the continuous weigh device. The device adjusts the volumetric signal to the volumetric blending control. This system was added without any hardware/software changes to the plant volumetric blending control. In another instance, we have seen a system that required both volumetric metering and gravimetric weighing to accomplish the job that could not be done alone, by either. Both Systems Have Pros and Cons The combination of both systems' pos- itive features was enough to get the job done that neither could have done alone. Thinking outside the box sometimes is a plus. Applications for "weighing on the fly" include batching, blending, inven- tory and truck/railcar/ barge load out. Continuous weigh scale indicators dis- play both rate and accumulated weight much like a car has a speedometer and odometer displaying miles/hour and accumulated miles. The speedometer info can be used for cruise control much like the rate signal (tph, lb./min., etc.) can be used for used blending control. The rate signal is elec- tronically smoothed (dampened) more so in blending systems than in other applications. The process is looking for a steady rate signal to help keep the process control system stable and minimize hunting. Batching control takes advantage of the accumulated weight and will be com- pared to a batch set point. When both the weight and set point match, the system feed may slow the feed or shut the feed altogether. Slowing the feed first, may help to reduce over shooting the set point. Continuous Weighing: The Big Picture When Continuous Weigh Measurement Is Important, Determining Where in the Process Weight Measurement Is Possible Is the Next Step. By Clarence Richard

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