Advanced automation technology from Emerson is helping to solve a range of solids level measurement challenges at a large lime processing plant in the US.

The need to reduce costs and increase operational efficiency is driving greater adoption of automation technology. For producers of lime, limestone and clay products, automation is key to continuous production. However, due to the processes involved and the nature of the products, conditions in lime processing plants are particularly harsh, which creates a challenging environment for instrumentation. In a Powder & Bulk Solids magazine article, ‘Meeting the level measurement challenges at lime processing plants’, Kurt Gieselman, a senior field sales specialist for Rosemount™ measurement products at Emerson, explains that:

 …In general, the biggest instrument challenge producers face is obtaining reliable and accurate level measurements of solid materials. That includes measurement of the crushed and powdered limestone, coal in silos that feed kilns, and hydrated lime as it is produced. All these applications have challenging operating conditions due to the weight of the material, dust, and low reflectivity of the surface.

The article describes several challenging solids level measurement applications at a large lime processing plant in the US. The first involved a hydrated lime surge bin, through which finished product passes before being conveyed to large storage silos. Achieving a stable level measurement was challenging because the material tended to compress in some areas and create gaps and bridges in others. Kurt explained that:

…To prevent this, pneumatic vibrators are used to shake and re-distribute the material. Previously, a capacitance probe had been used to perform the level measurement, but its measurements were erratic and slow to respond to level changes. In addition, the high vibration shortened the lifespan of the capacitance unit to only a few months. As the surge bin is only four feet tall, this made guided wave radar the ideal choice.

A Rosemount 5303 guided wave radar (GWR) level transmitter from Emerson was selected to replace the capacitance probes. The device is available with a remote housing extension for mounting the transmitter head away from the probe, which protects the electronics from the vibration, helping to extend the lifespan of the device. A critical advantage of GWR devices is their ability to provide accurate measurements in small tanks with rapidly changing levels, which is essential for this application. The Rosemount 5303 is proving to be reliable and maintenance-free, and the improvement in measurement accuracy has allowed the process to become more stable.

Another challenge involved two 112ft silos that store coal used to heat the kilns. Accurate level measurement is required for inventory purposes, as coal is pulled from the two silos simultaneously and the measurements are used to determine when to order more coal. The silo must be ready to receive the full amount of coal from the barge when it arrives, as receiving only a partial load is costly and inefficient, so good inventory management is essential. The article explains that:

…Previously, an ultrasonic level sensor had been used to perform the measurements in the silos. However, while the device functioned correctly most of the time, occasionally it would “lock up” and despite several work orders to investigate and eradicate the error, the cause was never determined. Because of the unreliability, there was a reluctance to empty the silo to less than 40%.

 

The production team replaced the device with a Rosemount 5408 non-contacting radar level transmitter from Emerson. Due to the height of the silo, a parabolic antenna was installed to direct the radar signals. When installed, some initial adjustments were required to the threshold settings, after which the device worked well. But as the weather warmed, the radar began to act in a similar way to the ultrasonic device, with short periods where it would lock up. The article states:

…One of the key features of the Rosemount 5408 is its standard built in data historian, which automatically collects and stores data for up to seven days, making troubleshooting much easier. Using the data historian and its accompanying tank radar echo spectrum, large signal peaks were found close to the antenna area that corresponded to the times it locked on high readings. Inspection of the device during one of these peaks revealed that moisture was condensing on the antenna and causing the high-level readings. Air lines were installed to blow off the moisture and this eliminated the condensation issue. The radar has subsequently worked well, providing accurate and reliable measurements.

A further challenge involved hydrated lime storage silos. The hydrated lime accumulates in the silos and is then removed for sale, so accurate inventory measurements are important. When the silo is filled, the hydrated lime powder creates an internal dust cloud. If the lime comes into contact with any moisture, it tends to form a hard, crusty layer that can be difficult to remove. To perform level measurements, the company had been using a non-contacting radar level transmitter. Kurt noted:

…The existing radar used a Teflon cap attached to the antenna to help reduce buildup. However, buildup still occurred and the resulting readings became unstable, requiring the units to be cleaned. This required a maintenance engineer with the appropriate tools to climb over 200 steps to the top of the silo, remove the unit to clean the antenna. This occurred every two or three days and was not only time-consuming, but also hazardous, especially during inclement weather. Each occurrence cost about $50 per trip. While this cost seems small, over time it added up. Plus, each trip was a safety concern and took personnel away from other maintenance tasks.

The article describes how the production team, working with Emerson, replaced the existing radar with a Rosemount 5408. The access point for the device was a 6in nozzle, which was too narrow for a parabolic antenna. The combination of silo height and low dielectric properties of the hydrated lime exceeded the range limits of a process seal antenna. Thus, a standard cone antenna with a flushing connection for an air purge was recommended. The new radar has provided robust measurements without the need for cleaning for more than a year now.

 

The final challenge involved measuring a specialty hydrated lime product on a railcar load-out system surge bin. Previously, the product was loaded into trucks and transferred to railcars. In the new system, the hoppers feed directly into the railcars, but the product must be measured accurately for billing purposes. Measurement is difficult because it is taken while moving through a 14ft narrow hopper. Not only is the environment very dusty and the product light and aerated, but the hopper also fills very quickly. The article explains:

…Emerson’s Rosemount 5408 level transmitter was chosen due to the success on the same product in larger storage silos. The device accurately tracks the measurements ensuring that railcars can be loaded directly for shipment to customers. The new system has eliminated the step with the truck trailers, making it more streamlined and responsive to order fulfillment.

 

By having more reliable level measurements, the plant is not only able to efficiently schedule the delivery of raw materials, but can also track movement of lime products throughout the plant. Reducing maintenance requirements and manual measurements, and eliminating downtime due to problematic devices, has increased personnel safety. With more accurate measurements, there is greater assurance of the amount of final product being delivered to the railcars, ensuring accurate billing and satisfied customers.

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