WesTech Engineering, Inc.

Bed Level Measurement


Courtesy of Courtesy of WesTech Engineering, Inc.

Numerous studies and papers have been devoted to the subject of thickener design and operation to achieve specified throughput and discharge densities. Mathematical models have been developed that present methods for sizing thickeners and predicting performance under varying process conditions. The compression effect of deeper beds on dewatering performance is a recognized phenomenon. This is one variable of many that are inter-related and influence thickener performance. For example in the steady-state, continuous operation of a thickener, if the solids feed rate increases and the bed level remains the same, the underflow discharge rate must increase with the result of solids concentration being lower. The interactions of these parameters are at the heart of thickener models. Knowing the bed depth is critical to maintaining stable thickener operation.

Numerous methods have been employed to achieve reliable bed level measurement including bed pressure, ultrasonic, capacitance, nuclear, vibrating probes, divers and floats to name a few. They are typically limited to measuring the level at one location in the thickener and they range from being very expensive to reasonably affordable. Each type can point to processes where they have successful operating instruments. However, without universal success of any type of instrument, many plant operators feel making this measurement accurately and reliably is difficult.

The instrument started development as a way to detect the bed level interface in a thickener. In the process of testing, both in the laboratory and in an actual operating thickener, it was found to provide data that goes well beyond just bed level detection. This instrument is capable of profiling density gradients within the bed. Some bed level instruments on the market can provide a vertical density profile in a thickener bed at one location. However, this instrument has the ability to profile the entire bed in the vertical, horizontal and angular dimensions. Data collected during tests of the instrument are presented and discussed. The application to thickener control and potentially for troubleshooting and design purposes will also be discussed.

The operating principle of the instrument is that resistance to movement of any object, through a settled thickener bed, increases with increasing solids concentration and that this resistance can be measured. A sensor was developed for proof of concept tests that were conducted on different solids concentrations to determine the relationship to sensor output. The sensor was a paddle shaped probe of stainless steel instrumented with a strain gauge. Drag created by moving the sensor through settled solids of different concentrations created deflection of the paddle which was measured by the strain gauge. The property of the settled solids which was chosen for comparison was yield stress. Other prototype configurations used load cells instead of strain gauges to measure the drag on the probes resulting from movement through the settled solids.

In a thickener the instrument would consist of at least one sensor (but more likely multiple sensors) located inside the thickener and moving through the medium inside the thickener. This medium ranges from mostly liquid in upper regions to settling solids to concentrated solids having a yield stress in the lower regions. As a sensor moves through the thickener it is exposed to drag forces or pressures caused by resistance to movement through the medium.

Signals from sensors can be managed, transmitted and collected by wireless electronics to a PC, PLC or other signal conditioning equipment where the data may be analyzed and a determination made as to the types of output to send to plant displays and controls. The instrument system uses a battery powered transceiver that controls data acquisition from the sensors and data transmission to the receiver. This eliminates the need for slip rings to transmit data and power the system. To extend battery life, sensors are only powered when a data sample is collected. The time between data samples can be set or reset as necessary while in operation.

Laboratory Testing
In the lab, two different solids were prepared for testing, copper tails and clay. Different concentrations of each material were prepared by successive dilutions of the same sample. A Haake VT550 viscometer was used to determine the yield stress of each concentration. The sensor was rotated through each sample and the strain readings from the strain gauge were recorded.

The relationship between sensor output and yield stress of test samples was established. The measured strain magnitude gave a linear relationship with the yield stress over the tested range. The type of material was also a factor. The clay sample has a different linear relationship with a different slope. The clay sample was tested at lower yield stresses for the purpose of observing sensor sensitivity. The sensor produced a measureable strain value below 10 Pa yield stress. Another objective of the testing was to gain experience using the wireless data acquisition electronics.

The laboratory sensor accomplished the goal of proving the concept. Movement of the sensor through the samples provided measurable response over a range of solids concentrations that would be expected in a thickener bed. The wireless electronics and data acquisition were found to work well with the test sensor. With these positive results, development of a prototype instrument to test in a full scale thickener proceeded.

Prototype Testing
Prototype sensors were developed that could be installed and tested in an operating thickener. This design used load cells to measure drag force caused by sensor movement. Three sensors were mounted on a structure that was placed over one of the outer dewatering pickets on top of the rake arm. The three sensors were spaced 300 mm apart vertically on the support with the lowest sensor located just ahead of the rake arm at an elevation approximately 600 mm above the bottom of the tank wall. A circular disk on a tubular arm transmits the drag force to the load cell where the output signal is generated. Wiring from the sensors was routed up the support structure to the transceiver, located in a protective enclosure on top of the support and above the liquid level of the thickener.

The thickener is a 22.86 m diameter, high density thickener with a 3.66 m tank sidewall, producing 40-60 Pa yield stress underflow. The process dewaters insoluble material from soda ash production for either underground or surface deposition. The thickener has a peripheral walkway that allowed convenient access to the instrument and provided the ability to make manual bed level readings at any angular position around the tank.

Current thickener control is based on the operator conducting manual bed level reading every four hours. These bed readings were used to control the underflow discharge rate. Additional manual readings were taken at other times as needed to correlate with instrument data. The plant procedure for making manual measurements was to lower a long PVC pipe into the tank at a shallow angle until the operator could feel the bed. Then maintaining the feeling of the bed, the pipe would be transitioned to vertical along the tank wall. Markings on the pipe allowed operators to measure the depth of the bed below the liquid level.

Testing of the prototype instrument started with the focus of detecting bed level. The instrument consisted of three sensors spaced 300 mm apart vertically. The output signals from the sensors as the mud bed level varied, during normal operation, demonstrated that the bed level interface could be clearly identified. Sensor signals were seen to increase from zero to maximum output before the bed level was even detected by the next higher sensor, 300 mm above it. This indicates a very distinct bed level interface and that the instrument was able to detect it. Correlation was demonstrated to exist between manual bed level measurements and instrument signals.

Sensor signals are proportional to bed yield stress. Test data indicates that as bed level rises above the level of a sensor, the yield stress at that sensor also rises. This indicates that a relationship exists between yield stress at a position in the settled bed and the depth of the bed above that position. Potentially a single sensor may be able to detect not only when the bed level reaches the elevation of the sensor but also estimate the bed level as it rises above the elevation of the sensor. However, this will need to be confirmed through site specific operating experience. Accurate bed level sensing will enhance the ability to control the thickener.

Data from testing the prototype instrument detected significant angular variability around the thickener. The cyclic nature of this data correlated to the period of rotation of the mechanism. Plotting the data of one sensor (one elevation) showed the settled bed had areas of high yield stress while other areas had low yield stress, indicating a significant variation in solids concentration and particle size distribution in the bed as a function of angular position. Further investigation revealed the cause was due to poor distribution from the feedwell. A current was carrying coarse solids to one side of the tank. This segregation within the tank was happening even though the manually measured bed level showed very little variation at different locations around the tank. The observed segregation within a thickener is known as an island and its presence was previously unknown. Having this amount of information about the state of a thickener bed has important implications for thickener design, operation and control.

Developing a bed profile, as was demonstrated above, has the potential of detecting other operating problems within thickeners that are often difficult to diagnose. This may be done by using both vertical and horizontal sensor arrays. The ailments that may be possible to observe include rat-holes, shortcircuiting flows, doughnuts and rotating beds. In addition, this instrument can show how the bed becomes denser with depth and how that changes with varying operating conditions. A vertical bed profile that is out of the ordinary may indicate changes to the flocculating characteristics of the feed or changes in particle size distribution. The potential of this instrument as a diagnostic tool for thickeners is significant.

Testing clearly shows the potential of this instrument goes far beyond measurement of the mud bed level. The ability to measure and observe the dynamics of the thickening process opens up possibilities in the fields of thickener control, process optimization, thickener design and theoretical modelling of thickeners. The common thickener theory describing a bed as radially homogeneous is already in jeopardy. The data suggest non-homogeneous distributions and variable angular, horizontal and vertical density gradients are more likely.

The magnitude of sensor signals was found to have a linear relationship to the yield stress of the material. The instrument described provided data from which the bed level interface inside a thickener tank can be determined. It also detected increasing yield stress as the bed level increased above the elevation of the sensor. This provides unprecedented information from within the thickener bed that could be used for early warning of reduced thickener performance. The potential exists to develop control methods and strategies to optimize thickener performance using this real time feedback. The ability of sensors to measure density gradients, as they move through the thickener bed, demonstrated that profiling of the bed can be used to diagnose thickener operating problems; in this case poor feedwell distribution.

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