Methods of Measuring Solids Flow

Explore ways to measure solids flow in industrial processes with some advanced methods for more accurate results.

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For many professionals in the field, the journey into process measurement often begins with the monitoring of liquid and gas flows, temperatures, and pressures. Expertise is honed in designing systems with the right instrumentation for these tasks. However, the realm of solids measurement often receives less attention initially. Yet, as projects evolve, a significant portion now involves dealing with solids control. It becomes evident that the skills acquired in liquid control don't always seamlessly apply to solids measurement.

There are five primary methods for measuring solids flow:

  1. Belt Speed Measurement
  2. Conveyor Motor Amp Draw
  3. Belt Scale Conveyor
  4. Impact Mass Flow
  5. Radiometric Absorption Conveyor

Belt Speed Measurement

Methods of Measuring Solids Flow: Belt Speed Measurement

Belt speed measurement offers simplicity but lacks precision. It merely gauges the linear speed of conveyors or screws, estimating the mass conveyed per minute. While suitable for some scenarios, it assumes a steady feed rate and product density, rendering it unsuitable for control or inventory tracking purposes.

Conveyor Motor Amp Draw

Conveyor motor amp draw measurement operates on the assumption that increased amps correlate with increased conveyed mass. Although more robust than speed monitoring alone, it suffers from significant drawbacks. It assumes mechanical steadiness in the conveyor system, which may not hold true over time due to factors like friction variations, mechanical wear, and non-linear amp-mass relationships. Hence, it may offer a false sense of measurement accuracy.

Belt Scale Conveyor

Methods of Measuring Solids Flow: Belt Scale Conveyor

Belt scale conveyors represent the pinnacle of mass flow accuracy. These systems incorporate load cells mounted on independently supported rollers beneath a conveyor belt. As the solid product passes over these load cells, its weight is measured. Combined with output from a speed sensor, this yields an extremely accurate mass flow reading, with potential accuracy levels reaching up to +/-0.25%. While not exorbitantly expensive, these systems necessitate ample space along the belt for installation and operation. They also require maintenance and periodic recalibration due to exposure to dust and moving parts.

Impact Mass Flow

Methods of Measuring Solids Flow: Impact Mass Flow

Impact mass flowmeters rank as the next most precise solids flow measurement technology. They involve lifting the product above a designated "box" and letting it fall through to a bin or secondary conveyor. This box comprises a top entry chute, a bottom outlet chute, and an angled plate attached to a load cell in the middle. As solids strike the plate, the load cell tracks the rate and mass of the passing solids, providing a reliable accuracy of around +/-0.75 to 1.00 percent. While requiring less maintenance than belt scales, they still need periodic cleaning due to dust exposure and potential wear from abrasive solids.

Radiometric Absorption Conveyor

Lastly, the radiometric absorption method offers a lesser-known but effective approach. Utilizing a radioisotope to emit gamma rays at a consistent rate, this method relies on the conveyor and conveyed material to absorb these gamma rays. As the conveyor's density remains constant, any change in absorption over time directly corresponds to a change in product mass flow. With minimal gamma emission levels, these systems are safe for field installation and generally do not require special certificates or government regulations. Positioned outside the conveyor and making no contact with the product, they are immune to dust and abrasion, resulting in a respectable accuracy rating of around +/-1-2% in most applications.

Now that you're familiar with the primary methods for measuring solids mass flow, it's crucial to integrate this information into your plant's mass balance calculations and inventory controls if you haven't already done so. Should you have any inquiries regarding the implementation of these methods into your processes or if you need assistance with rectifying installations that aren't meeting your expectations, please don't hesitate to reach out

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