Interface level detection in stone reclaim slurry measurement

High solids content & moving surface


Concerning precise interface measurement of different solids in liquids there are popular project planning options for continuous level monitoring with lot systems and point level detection with vibrating forks. A company in England who manufactures stone recovery plants for the quarrying industry implements the electromechanical plumb bob NivoBob® for the detection of interface levels with the right feeling for the separating layer. The operator was looking after a principle that is sensitive and simple.


High solids content & moving surface

There are various industries that require detection of sludge levels or detection of solids in liquids, e.g. in sediment containers or basins, filters or inclined clarifiers in the metal industry, chemical plants, lime or gravel works and the wastewater industry. In this case the measurement accuracy and durability of the sensors is of particular importance as pollution, chemicals and gases can often affect measurement accuracy. UWT offers different configuration options for interface detection of solids in liquids as well as liquid layers for continuous measurement and for point level detection within these industries.

A manufacturer of stone recovery plants focusing processing solutions to the quarrying, aggregates recycling and handling industries was looking after consistent measurement equipment to improve his stone reclaim processes. He provide washing systems and aggregate washing devices in England and Wales. The stones are placed in a container filled with water, which is stirred slowly. All the dirt is removed from the stones. The stones are then sold again as building material. The dirt in the water settles as mud at the bottom of the tank where the application requires the measurement of the sludge level. From a certain height, the mud is pumped out. The plant operator had already tried a radar and an ultrasonic system, but both were unsuccessful due to the high solids content in the water and the movement of the water surface. Several tests carried out by Graeme Hughes, Managing Director of UWT (UK) Ltd with various sensors quickly made it clear that the interface measurement using the electromechanical measuring principle of the compact and prooven NivoBob® lot can be optimally realized. The electromechanical devices are robust and very reliable also due to the simple measuring principle. The multi-functional technologies found in the NivoBob® NB 3300 (rope version) and NB 3400 (tape version) plumb line systems have been specially modified for interface measurement. They are ideal partners for use as continuous measurement of sludge or salt levels in contaminated water containers or asbestos silos. The resistance to corrosive materials by the devices is of vital importance in order to achieve reliable results. All process-contacting sensor parts are therefore especially coated or made of corrosion-resistant materials. The stainless steel wire rope version can be offered with a protective PA plastic covering and is highly resistant to aggressive media in waste water which can be found in settling ponds. The calibration of the sensor is straightforward and the weight adjustment can be implemented mechanically.  


For the installations in England the NivoBob® NB 3300 with rope extension was used and proved to be a suitable measurement system for this interface application. The sensor weight, made of a suitable material, attached to a 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. For interface measurements, the sensor weight can be adjusted according to the density of the sediment. The distance is electronically measured and together with the programmed container geometry information the microprocessors convert this distance into a volumetric output signal. The lot generates an analog 4-20mA signal and a Modbus RTU or Profibus DP interface for the evaluation systems. As the system is immune to dielectricity, conductivity, humidity and caking, the bob continuously delivers a reliable level signal. The sensing weight is not affected by the water movement. It moves through the water surface to the mud layer and detects quick and precise the separating layer. The sensing weight of the sensor can additionally be adjusted to the viscosity of the slurry. The plant was convinced by the reliable and easy-to-use system for measuring the sludge level in the wash tank.


Plumb bob measurement without tangle
One of the most commonly asked question regarding this technology was how it can be prevented that the cables or ropes 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 has been also equipped with a brush cleaning system and a spring-tensioned tape fitted with a scraping mechanism that removes material from the rope when it´s retracted into the housing. Additionally the NivoBob® lot systems feature heavy-duty ropes which prevent the sensor weights from being disconnected and causing possible damage. 


More transparency by level visualization

The plant operator furthermore requested an economical and practical solution in the form of a central level-remote system. Background was to make the process management more transparent and efficient through continuous level control 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 plant. The level signals of the installed NivoBob® on each container 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 containers in the visualization system - without additional hardware or costs.

Optional: If the priority is to keep installation at the plant 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 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. The system can be dismantled at one location and installed again at another without new costs.


Interface limits measurement by vibrating forks

To measure limit levels of sludge and solids in liquids, vibrating forks of the Vibranivo® series VN 1000 or VN 5000 can be installed very uncomplicated. The parts which come in contact with the medium are made of durable stainless steel. International requested approvals for use in hazardous locations (gas and dust) are standard. Electronically stimulated piezos cause the fork to vibrate. As soon as the sensor is covered with material, the vibration is dampened and the resulting electrical current change causes the output signal to switch. Once the material level falls below the sensor, it is free to vibrate again and the output signal is reset. The vibration probe puts the switching point where the application needs it.

The types have a flexible configurable range and are available with extension tubes or as a very short version. Using series VN 1030 or VN 5030 even depths of approx. 4 meters (13 feet) in a tank or a basin are possible as long as the installation position is from the top. The fork Vibranivo® VN 1020 and VN 5020 with a short probe arm ensure high robustness for applications with heavy mechanical loads. The VN 5000 series offers more space for wiring due to the larger housing and additionally a flameproof design. The high sensitivity required for the respective solids detection can be easily adjusted on the electronics. The point level switch fork is robust, easy to operate and can, via its series range, be precisely configured for each application.