Measurement sensor answers to demands of grey gold manufacturing
Lot solutions as quick and easy measurement device, rotating paddles for extreme temperatures & capacitance RF probes for specialities
Plant operators are busy keeping pace of the wide range of challenges presented by raw materials of the building sector, the changing operating conditions in silos or pipelines or the need for ex-certificates. Such requirements call for measurement solutions that are versatile, whilst representing a cost-effective investment and offering versions fully equipped to meet a wide variety of these requirements. Due to the fast development of the building and quarry industry in the MENA countries during the past years, the demand for more solution approaches for applications of cement plants there has increased. Solutions of appropriate sensor technology for content and point level measurement have to be found in order to support the process flow of the cement production in the most reliable way.
In one of Saudi Arabia’s largest cement plants UWT was confronted with especially harsh working conditions. For his cement silos the plant operator was looking after reliable contents measurement technology as well as after a flexible sensor system for level limit detection. The combination of a very high dust generation and an extremely high process temperature was a challenge for the right choice of instruments.
Thus the selection of measurement technology included first of all for the storage silos an electromechanical plumb bob that fits also perfect with the connection to a visualization system. Further for limit detection during several process steps rotating paddle switch and capacitive rod probe were installed. Here respectively the high temperature version was demanded.
Lot solutions as quick and easy measurement device
The electromechanical measurement device is an invaluable instrument in the field of inventory management due to its advanced and still so simple electronics technology. The sensor weight, made of a suitable material, attached to either a tape or rope is electromechanically lowered from the top into the vessel. Once the sensor weight rests on the material, the winding direction of the motor changes and the sensor weight is rewound to the upper stop position. Used as the demand indicator during the filling stage it plays an important role. Thereby it has to cope with several requirements such as large measuring distances in high silos, rapid response to overfill protection or measurement of material within steep gradient cones. Besides during filling there are extremely dusty, polluted environments.
The reliable electromechanical lot system of UWT NivoBob® with its robust mechanical design has for that reason become the choice of the plant operator in Saudi Arabia faced with these demanding conditions. For continuous level measurement within the cement storage silos the NivoBob® NB3200 tape version was chosen, especially due to its integrated tape cleaner which reliably prevents contamination of the mechanical chamber. The cement manufacturer automated his level measurement in 50 of his silo cells containing raw cement by the lot measurement system. It was important for him, that the device was easy to set up and could cope with the exceptionally dusty environment. The user friendly software with its quick set-up option in different languages made the programming of the required parameters an easy job. Trough an aiming flange it was possible to install the instrument directly in the silo roof from where it can cover ranges up to 50 m. For the powdery materials of cement dust with a steep angle of repose the use of an optional pin to secure the sensor weight and prevent it from slipping or falling off was recommended. As an additional safety feature a maximal travel distance parameter of the sensor weight was set to avoid it from getting caught in the bottom of the bin when the silo is emptied.
The most convincing advantages of the series of plumb bob sensors were as followed:
1. Not affected by stiff products or sticky, dirty, vapor, dusty or steamy environment
2. Not affected by condensations, caking and change of humidity.
3. Not affected by materials properties, sizes, shapes, dielectric and conductivity
4. Effective self-cleaning mechanism ( tape cleaner )
5. Easy to install and commission, very user friendly, less than 5 minutes at-site setup during commissioning – can be set up prior to installation
6. Very suitable for huge tall and especially wide dimension silos
7. Due to the modular construction individual customer wishes could be easily included into the product design
One of the most commonly asked question regarding this technology was how it can be prevented that the cables of the lot system get tangled, i.e. as a result of constant winding and unwinding or by material being drawn up causing interference. UWT has addressed this problem and has introduced a design which utilizes only one or two reels. It is also equipped with a brush cleaning system or a spring-tensioned tape fitted with a scraping mechanism which removes material from the cable or tape when is retracted into the housing. The lot features a robust design, such as heavy-duty cables which prevent the sensor weights from being disconnected and causing possible damage.
More transparency by level visualization
The cement producer additionally requested an economical and practical solution in the form of a central level-remote system. Background was to save unnecessary logistics costs caused by multiple deliveries and make the storage management more transparent and efficient through continuous level control, improved truck routing and detailed planning.
Therefore a level visualization system was connected:
The electromechanical lot system devices communicate directly with PLC control systems via an analog 4…20ma signal or even a MODBUS RTU or Profibus digital protocol. Thus the lot systems were directly compatible with all conventional control systems. Because of the wide range of different level control solutions within the UWT NivoTec® series, it could be found a proper economical model for the cement manufacturing plant. The level signals of the installed NivoBob® on each silo were bundled by the visualization software combined with a Wago WebController. The information received was passed to the Internet using an Ethernet connection via a routed IP address. The plant could securely access this information (password-protected) via any internet browser at any time of the day over a pre-defined IP address). It is possible to include any number of other silos in the visualization system - without additional hardware or costs.
Optional: If the priority is to keep installation at the plant silos to a minimum a GSM modem can be used to remotely access the data. In this case for the data transmission no Ethernet connection is required, but only a SIM card in the WAGO to pass the modem. This modem collects all level signals and sends them in an encrypted log via mobile phone over the Internet to the appropriate controller.
The benefits of the whole system for the cement producer were besides the simplification of the material disposition processes leading to a reduction of costs:
~ The installation of the system was straightforward and could be carried out by the plant own service engineers
~ Control cabinets only had to be set-up once; afterwards no additional IT support was necessary
~ All silo levels can be now visualized at the same time which leads to more material planning security
~ The material flow finally follows a standard process with much greater planning security
~ The system can be dismantled at one location and installed again at another without new costs
Measurement solutions for very high temperatures up to 1100°C
There are a great many specialist processes within the cement production industry, where the use of the right measurement technology makes the difference to support a smooth process flow. During the volume or dosage process e.g. the installed sensor needs to be a flexible and adjustable limit switch with high sensor sensitivity for precise volume dosing; whereas the device within a crusher has to deal with heavy mechanical load environment.
For the mixing process of the cement plant there was also a mixture of particular demands for the sensor. First of all the production included a variety of different bulk materials that were powdery, aggressive and moisty. Than within the blender container the bulk was continuously moving within an agitator. Characteristics of the application were especially high temperatures, dust, condensation and concomitant with that, strong caking. So the measurement system needed to have high resistance to aggressive bulk goods as well as a quick response time through precise adjusted electronics. During filling processes there occurred vibrations within the process system and the environment from time to time was statically-charged.
For point level measurement UWT project management suggested their tried and tested rotary paddle switch Rotonivo® of the RN 3000 series to the cement producer. This level switch was suited for the demanding cement applications as full and empty detector. The paddle limit switch provided very precise level limit measuring as the sensor is unaffected by dust, humidity or build-up of adhesion. The high temperature models could handle process temperatures of up to +1100°C (2012°F) and - depending on the given conditions - the units were equipped with an appropriate sealing (Viton, Teflon or Ceramics). Further it was prepared with an integrated rotation control which recognizes a fault of the unit and immediately initiates an alarm. The PNP electronics of the paddle switch 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. Besides stainless steel sliding bearing with conical shaped contact, a reinforced stainless steel shaft with increased diameter was constructed. The shorter shaft at the end of the stainless steel protection cap decreased the torque at this point. The fixing bolts and paddle made from 1.4404 steel were strengthened and the stronger spring increased the switching torque of the limit switch.
In several blending tanks there were intense steam saturation and the addition of liquid components. Due to the predominating strong adhesion there was demanded sensor equipment including particular anti-caking technology. Installed was a capacitive device of the RFnivo® series designed as single rod probe and including the “Active Shield” technology whereby the adhesive bulk could be detected tout court. In the case of the Arabian cement producer the probe with ceramic insulators was equipped to cope with process temperature of up to +500°C (932°F). Due to its construction the device does not require temperature limiting components for bulk detection in the application process.
Appropriate measurement solution for in between
Throughout the whole process of cement production you come across a range of intermediate processes, such as additional storage processes, subsequent transport processes and development processes in between. In intermediate containers the material is ready for its further processing involving varying types of pre-hoppers, tempering cells or surge hoppers where ingredients for adding and mixing are stored. There the measurement sensor had to work with varying types of bulk materials in dusty environments. Indispensable was a short reaction time to the overfill protection and rapid detection of low-level control for small transfer containers. For the level limit detection in the smaller hoppers the plant could be equipped one more time with the level limit switch RFnivo® 3000. The device offers in its series varying extension lengths for the different sized containers and works through an output signal with timing control for precise punctual switching.
During production, the free-flowing bulk goods are transported either mechanically using chain conveyors, elevators, screw or pneumatic conveyors by one manufacturing process to another. Here, controlled material flow is essential to ensure an effective production process. The demand was about detection and monitoring of moving solids in these varying bulk transport systems. Faults in conveyor systems for raw material and other bulk materials should be detected early through the use of appropriate measurement principles. Besides there was little space within the processes for the measurement sensor. The chosen RFnivo® 3200 heavy duty version had a design that could withstand high container pressures up to 25bar (363 psi) and further offered a high torque setting of 525Nm. Besides he coped with tension forces of up to 40kN. The installation could be vertical, horizontal and oblique and supported the smooth process flow of the cement manufacturing especially within the smaller containers and hoppers. For the compact sensor there was no calibration needed because the probe started to calibrate itself after the set-up and the wiring in the container has been completed and the power supply connected. The operator had to do nothing further than set the signal output in the Quickstart menu according to its application.
This was also important for the plant operator, in case that he wants to change the intermediate container. If the RF is used in every other application just the supply voltage has to be previously connected and the calibration button has to be pressed. The probe automatically adjusts to the new application and is ready for use.