Slurries are found in many process industries, and measuring flow rates of slurries is often challenging due to a variety of reasons, including constant changes in the size and volume of entrained solids, varying density, laminar flow, and the high velocity often necessary to maintain the entrained solids in solution.
Common slurry measurement applications, challenges
Some common slurry measurement applications and challenges include:
Pulp and paper, with a typical consistency of 3.6% solids at the entrance of the paper machine in the sulphate process, which is the most common for production of chemical pulp as it also produces a stronger pulp than the mechanical pulp process.
Metals and mining with 5% to 8% solids in many streams, including high solids content greater than 50% in copper concentrator plant tailings lines.
Chemical injection for oil and gas and petrochemical, where reactions add to the noisy flow signal.
Fracking applications require accuracy of 0.25% of the blends of water and sand pumped downhole to reduce well challenges.
Water and wastewater treatment brines, where the high conductivity of the brine, combined with about 10% solids, generates a high variability in flow measurement.
Fortunately, most slurries are water-based and hence a conductive fluid, making them well suited to non-intrusive flow measurement using magnetic flow meters or magmeters. This measurement technology is one of the few able to measure flow in turbulent and laminar flow applications.
Magmeters are comprised of a transmitter and sensor that together measure flow. The magmeter’s sensor is placed inline and measures an induced voltage generated by the fluid as it flows through a pipe. The transmitter takes the voltage generated by the sensor, converts the voltage into a flow measurement and transmits that flow measurement to a host system.
Whether measuring flow from pulp stock, brines, or mining ore using magmeters—including those with an impingement plate to both protect the lining and electrodes of the meter—particulates in slurry flow streams can result in unstable readings. This and other issues can be addressed by careful selection of the right magmeter for each application.
Managing unstable signals in slurry flow measurement
Unstable and inaccurate magmeter readings often occur due to entrained debris in the fluid impacting the electrode sensors and causing millivolt spikes, which are then interpreted as flow spikes. Fiberglass pipe is often used in brine or chemical services, and it tends to create a lot of static current in the application, which like the particle impingement problem, affects measurement integrity. When using a standard magmeter, these noise signals measured by the electrodes struggle to separate from the flow signal consistently and accurately.
The traditional method of compensation for this type of noise is extending dampening time of the flow signal, which is done in the transmitter. Due to the nature of these slurry signals, it is not unusual to see damping times of 30 to 60 seconds. This technique produces a stable flow rate value, but it is not good for real-time control. For many flow processes, the process deadtime is often less than one second, so damping to this large an extent means control is responding to a change that occurred multiple cycles earlier, which can lead to unstable operation of the control system.
This type of unstable operation often results in oscillations of the control valve, reduced productivity and downtime. Even with damping applied in both the transmitter and the control system, in many cases there is still too much noise to regulate the process effectively.
In one pulp mill example, when the problem was at its worst the facility was forced to turn off the automatic control completely and stroke the control valve manually, leading not only to poor performance, but ineffective use of operators’ time.
A better approach to measuring slurry flow rate
One way to reduce the noise effect is to increase the available power of the signal generated by the sensor, such as increasing power from 0.5 amps with legacy designs to 2 amps in newer designs. However, increasing the signal power solves only one aspect of the measurement problem, the noisy signal, without fully addressing the challenge of debris or other process-induced spikes.
By analyzing over 200 real-world harsh environment noise samples during the development, and taking advantage of improved microprocessor capabilities, a magmeter development team was able to identify requirements for more sophisticated digital processing in the transmitter, including active processing of signals to identify and ignore outliers caused by impingement of particles.
Inside technology: Magnetic flow meter improvements
This advanced transmitter is supplied with new magmeter purchases, and it can be retrofitted to existing installations. It includes three process noise profiles, two coil frequencies, zero trim, and five preconfigured signal processing modes based on averaging time, process noise level, process noise factor/tolerance level, scan time, and time limit of the running average. There is also a fully customizable sixth “custom” signal processing mode that can be user specific based on the application. Technical support is available to help with fine tuning and development of custom configurations.
The meter’s sensor is an obstruction-free design without moving parts, making it ideal for measuring conductive slurries, where it minimizes maintenance and repair. No moving parts or obstructions also means no mechanical failure or material build up, providing a high level of reliability.
Embedded diagnostics are gaining increasing importance across industries. Implementing measurement devices that can provide insight into installation conditions, process conditions and device health are key enablers for predictive maintenance. Smart meter verification capabilities provide these diagnostics with real-time alerts, notifying maintenance of issues before they result in any process related problems. Diagnostics include indications for empty pipe, reverse flow, and electrode saturation—as well as grounding and wiring faults, along with other issues.
Transmitting diagnostic variables, user selectable secondary variables (such as electronics temperature, totalized flow, or any of the other 16 available variables), and other information to a host—such as a control or asset management system—requires a digital communication protocol. HART (from FieldComm Group) is one option, and it is superimposed on the 4-20mA flow measurement signal.
It has the advantage of being the world’s most widely used field device protocol in the process industries, so many host systems support the protocol. For those that do not, protocol converters are available to convert the HART signal to multiple discrete and 4-20mA signals, ensuring compatibility with all host systems.
End users should look for a line of magmeters with a wide range of sizes, such as 3 – 36 inches (80 – 900 mm), with accuracies of ±0.25% standard and ±0.15% high accuracy option configurations, to handle most applications.
Advanced measurement technology in action: packaging material application
Before installing the slurry magmeter, the flow signal from the meter was varying between values as high as 150L/min to values as low as 10L/minute. After installation of the new slurry magmeter, damping reduction was reduced from 15 to only three seconds. When combined with the signal processing capabilities in the transmitter, this produced a much more stable signal, with the observed measurements more representative of the actual slurry flow.
The resultant improved process control, enabled by an improved slurry flow reading, not only supported the detection of important changes in the operation, but also helped the company avoid rework due to incorrect material feeds, which affected quality of the packaging material.
A second pulp industry application, this one in Sweden, was experiencing excessive process oscillation. In addition to excessive wear on the control valve due to frequent stroking, it was necessary to revert from closed-loop to manual control in many instances. With a new slurry magnetic flow meter installed at this Swedish mill, plant personnel were able to consistently operate in an automatic closed-loop control mode, resulting in increased productivity, reduced raw material usage and fewer process disruptions.
Advanced measurement technology in action: mining application
Similar improvements can be found in other industries, including a South American gold mine where the slurry magmeter was installed on the mineral pulp distribution line. Before installation, automatic load balancing of the circulating load (ratio of the coarse material returned to the mill compared to the fine material) was not possible and had to be done manually to account for the adjusting factors. These manual adjustments were often incorrect or not made in a timely manner, leading to significant rework. The wide variability of slurry flow also made it challenging to manage the pH of the process, resulting in requiring decreased of production for safety reasons. After installation of the slurry magnetic flow meter, real-time response to flow rate changes was now available. This minimized required rework and increased throughput, largely due to automatic mass balancing now being performed in real time.
Advanced magmeter improves accuracy, controllability, throughput
Similar success stories—with significant improvements in accuracy, controllability and throughput—can be found in any industry where noisy magmeter signals are observed, especially with slurries. Though slurry flow measurement continues to be a challenge, particularly in highly conductive or abrasive environments when the flow reading is used for closed-loop control, most of these applications can be addressed with modern magmeter technology.
Using this type of cutting-edge technology in a configurable and customizable manner provides low variability of flow readings, empowering personnel to run their plants closer to operating limits. It also provides improved automatic closed-loop control, better process stability and increased throughput—along with less equipment wear and tear across a wide range of flow regimes, profiles and ranges.
