Continuous level measurement solutions during fast running process

Simone Hadraschek

Every application and every process is different. In order to define the correct technology and the right instrument it is essential to analyze all different process parameters

Every application and every process is different. In order to define the correct technology and the right instrument it is essential to analyze all different process parameters such as material properties, process temperatures, process pressure, process connections as well as the material of the silo or vessel. The efficiency of an industrial plant is not only influenced by its mechanical and electrical installations, the appropriate planning, selection and engineering of the required overall infrastructure also play a critical role. Viewed as an integrated entity, the plant’s infrastructure also includes items like optimized measurement technology. The selection of the right technologies, their correct configuration and smart positioning will save substantial costs and add up to a significant improvement in the plants’ economic efficiency over its lifecycle.

Working title:

For big plants level measurement technology mostly is just a small wheel – but that can cause a big effect. Many a “simple” type of component can soon become a problem if it’s not working reliably. In situations where level monitoring is required, costly events such as over filling of the silo can be avoided. Level measurement sensors have to be designed with a high degree of compatibility for processes, so that they can be seamlessly integrated into installations ensuring the optimal solution. During cement manufacturing processes the right level measurement for continuous and level limit detection has to be deal with dust, dirt and different material. It needs to realize a quick response during fast running processes to support the production flow. UWT shows how they installed appropriate sensors within different processes and vessels related to fulfill specific demands.

A major cement producing company near Bogotá that manufactures and sells cement and clinker to the domestic and many global export markets was looking for measurement solutions for different process steps to support the manufacturing cement production flow. Based on the following three applications it is described how the plant operator integrated measurement technology into single silos and material manufacturing processes to make the vessels´ content more transparent and improve a smooth working flow.

Continuous level monitoring for conveying processes

Processors understand the importance of reliable material movement. Conveyors transport material from unloading and storage, to processing, packaging and shipping. The cement production process of one of Colombia’s most important cement provider starts at the quarry with the study and evaluation of the raw materials (limestone and clay). Once the raw materials are evaluated and found suitable for cement production and after additional geological studies are undertaken the extraction process starts: drilling, blasting, loading and transport of the raw materials to the crusher.

The crushing process reduces the size of the extracted rock down to a size that is suitable for conveying and feeding to the grinding process. First the control of the crusher speed and material transport of the connected conveyor belts was requested. Here the measurement device needed to be a non-contacting technology. Installed was the radar sensor NivoRadar® 3000 for continuous level measurement. A high frequency signal is transmitted with a very small beam angle using two-wire technology. It is reflected by the bulk material and received back to the sensor. The frequency difference, which is directly proportional to the distance, is then further processed and output as the level signal. The unit operates at a high frequency of 78 GHz, which delivers a very small 4° beam angle enabling the radar sensor to measure very tall or very narrow silos/containers, all the way down to the discharge bottom. The high frequency ensures optimum reflection of the bulk solid material. In many cases measurement is even possible if internal struts are fitted within the silo/container. The small beam angle thus also made the use with little surface possible and facilitates the installation and alignment of the sensor. The small beam angle allows for an easy alignment of the sensor during commissioning. Measurement distances up to 100 meters can be managed by the radar sensor so that considering the installation within the plant high flexibility was offered. The solution was for the conveying application the aiming flange version of the radar. Using the adjustable aiming flange, the sensor was perfectly aligned since the probe could be fixed to the desired point for reliable material flow control.

Non-contact radar detection with short reaction time

Another conveying process that needed to be supported with continuous level monitoring was the stone conveyor to get an equal filling of the blending bed. The crushed raw material is transported from the quarry by a conveyor belt, analyzed on its way with on line analysis equipment and finally stored in the pre-homogenization beds. Raw materials are built up in two horizontal piles and extracted by cutting vertical slices. This method of storage and recovery greatly reduces the variability of the incoming raw materials. The extracted pre-homogenized material is transported to a silo from which the raw meal mill is fed. At the raw mill site, corrective materials such as iron minerals corrective limestone or sand are added to the mix before it enters the raw mill.

For the even filling of the storage area of the blending bed the radar sensor controlled the discharge height of the belt stacker. With the exactly positioned aiming flange of the NivoRadar® the smooth process flow of the filling was efficiently supported. The device used a two-wire technology and was made simple to operate via a local programmer with display and a Quick Start Wizard. The radar sensor was designed for the application with a completely dustproof construction and provides reliable measurement results in process temperatures up to 200°C. Due to the high sensitivity with dielectricity constant DC ≥ 1.6 the sensor was also appropriate to detect the fine solids. The lens antenna was equipped with air purge function for the sticky solids using an air flush connection. Even for the case if condensation occurs, the flushing connection provided a functionally reliable measurement.

Limit detection to avoid overfilling

For the shutdown of the conveyor belts the point level sensor of the rotary paddle switch Rotonivo® was installed on top of the roof of the blending bed. Limit detection was required to avoid overfilling or unnecessary downtime. The chosen solution with the Rotonivo® RN 3002 had a rope extension of 10 meters length. A motor driven shaft causes a vane to rotate. Once the material level reaches the vane, thereby preventing further rotation, switches are activated which result in an output signal and the motor stops. When the vane is free again from material, the output signal is reset and the motor driven shaft rotates again. Due to the dusty process environment the unit was suggested to be ATEX certificated and it was integrated a dustproof housing and an encapsulated ball bearing with shaft sealing. Thus, the rotating paddle switch is able to cope easily with the high dust development during filling, giving a rapid response and protecting against overfill. The rotation principle is unaffected by caking of the material and didn’t influence the switching reaction. The PNP electronics of the paddle switch RN 3000 series uses 3-wire electronics technology with a non-contact output, open collector and PNP. These electronics could be connected to any standard PLC with 24V DC power supply and PNP input card.

More efficient silo management by visualization of stock level

Within the Colombian plant the clinker grinding mills, which are vertical types or ball mills are equipped with pre-grinders or crushers. On entry to the mill the clinker is mixed with gypsum and other final cement recipes related materials. The proportions of these materials, composition of the clinker and the degree of fineness to which the resultant cement is crushed determine the cement type and characteristics. Mills are normally equipped with classifiers which return oversize product back to the mill for further grinding. The components for the grinding mill are stored in several intermediate silos where a continuous content level detection should be installed for better control of the single material stocks. Crucial for these storage application was a short reaction time to the overfill protection and rapid detection of low-level control for the small transfer containers.

For the varying types of bulk materials the sensitivity had to be high enough to detect also very light material. At the same time there were different silo heights and diameters of container so that the sensor needed to be flexible in its mounting possibilities. The installed radar sensor with adjustable aiming flange and lens antenna with 4° small beam angle could be exactly placed at the silo roof and adjusted for continuous level detection of the material without delay and spurious echo even within limited space as in some multiple chamber silos. By the non-contact technology even steep sided cones didn’t influence the measuring results. The plug in display allows programming and diagnostics on-site, making the installation and operation of the unit very comfortable.

The next consideration of the plant operator was at this point of production to connect the radar sensor to a complete system for level monitoring for an improved transparency concerning silo management. The measurement system devices communicate directly with PLC control systems via an analog 4…20ma signal or even a MODBUS RTU or Profibus digital protocol. The level signals of the installed radar sensors on each silo could be bundled by the visualization software Nivotec® combined with a Wago WebController. The plant could securely access this information (password-protected), at any time of the day via any internet broswer over a predefined IP address using the visualisation software Nivotec®. It is possible to include any number of other silos in the visualization system - without additional hardware costs. By these level monitoring and visualization the plant got a complete system for fill level and trend display, data storage and remote fill level information.

Conclusion: The gear mechanism within a plant

Measurement technology is just a small wheel – that can cause big effects

Level measurement technology is multifaceted and rarely stands still. Exciting and challenging developments in different applications mean that the product range continuously has to grow and develop to meet these requirements. For continuous and point level measurement technology plant operators often need individually tailored solutions for their material manufacturing processes. Each process has a specific requirement or demand for the sensor.

With particular attention to single steps during the manufacturing of the cement producing company in Colombia, the customized level measurement was a crucial component to support the smooth running process flow – avoid overfill protection, quick response when it comes to silo emptying, transparency and control over content level within the vessels at any time.

With application-oriented product design of the installed radar sensor and paddle switch - supported by the level control visualization system - a high degree of compatibility for the silos and vessels was given, so that they could be seamlessly integrated into the installations ensuring the optimal silo management.

UWT solution NivoRadar®
UWT solution NivoRadar®

Plant in Bogota

Precise Radar Measurement

Paddle switch

Associated projects