Tough slurry processes require quality measurements

In industries such as metals and mining, pulp and paper, and oil and gas, advanced mag meters offer solutions for challenging slurry applications.

AlfioManciagli/iStock
AlfioManciagli/iStock

Many different industries use slurry applications as part of their processes. In most instances, that means a pumped, pulped or mined material is mixed with water before it is moved along through a pipe to the next step in the process, which can involve adding chemicals or solvents for the next stage of the product. 

Knowing which additive to add — and when — are important parts of the process, as well as knowing how much is flowing through the pipes at any given time. This flow information will determine the amount of additive required. An inaccurate measurement means extra expenses, wasted additive, wasted product and possible process upsets. 

It is not easy to get a correct measurement without the right specialized equipment because the very nature of slurry means there will be large chunks of debris that are part of the flow, which can cause noise interference and skew the measurement data. 

Generally, the meter of choice for these kinds of applications is the magnetic flowmeter — or mag meter — and specifically, a slurry mag meter. A mag meter is obstruction-less and has no moving parts that can be damaged by the particles and chunks that flow through the pipes. Because slurry is a water suspension, it is easier to measure with a magnetic flowmeter, which operates on Faraday’s law of electromagnetism. The less pure the water is, the more conductive it becomes, making it possible to gain accurate readings of the flow of materials. However, most traditional mag meters are subject to high noise interference from the material flowing through the pipe impinging on the electrodes, which can interfere with signal processing. 

Multiple industries have slurry applications that may benefit from the use of a slurry mag meter.

Before discussing a solution to this problem, the following section takes a closer look at which industries use slurry processes — and why — to illustrate specific challenges in measuring slurry flow in each application. While multiple industries have slurry applications that may benefit from the use of a slurry mag meter, there are three main industries where slurry processes are a primary part of production: metals and mining, pulp and paper, and oil and gas.

Metals and mining

In the metals and mining industry, a producer will, in simple terms, take rock out of the ground, smash it up and mix it with water, primarily to cut down on dust exposure, which is difficult to handle and hazardous for both machines and humans. 

Mined ore is mixed with water as a transport fluid to carry the ore throughout the mill for processing, as it provides a safer and more efficient mode of transportation than mechanical conveyors. By using this approach, the mill can better measure the amount of slurry going to the various processes, helping to optimize the operation. 

Raw ore is fed to the ball crusher using mechanical transport methods, but from the ball crusher stage onward, water transportation is used to carry the ore throughout the rest of the processing steps. The crushed ore then goes to the flocculation (or froth flotation) application, where the ore that the producer wants is concentrated before it is sent to the leaching and dewatering processes. 

The desired material is harvested from the bottom and transferred to leaching pads where a chemical spray is applied to help dissolve the material (being mined) out of the remaining ore. The leach is then transferred for further processing. At each stage where material flows in a liquid slurry of any kind, accurate measurement is needed. However, at each stage, there is the possibility for process upsets due to decisions made based on poor measurement readings. A meter that is affected by debris hitting the sensors, causing signal noise in the transmitter, will take longer to readjust to what it is reading, which can result in adding the wrong amount of additives. 

Pulp and paper

In the pulp and paper industry, water is used — similar to the metals and mining industry — as a primary transport media for the pulp stock used to create various paper and wood fiber products such as cardboard and tissue. Like mining, the pulping process starts with a mechanical operation to take harvested lumber and chip it into small wood chips, which are then fed to a chemical digester using mechanical means such as conveyors.  

Because the pulping process to turn the wood chips into pulp fiber requires harsh chemicals and high temperatures, using water as a transport medium helps keep the hazardous chemicals contained in the process piping and keeps operators and technicians safe not only from exposure risks, but also from burns associated with the process temperatures. During the pulping process, the wood chips are broken down into fibers of various lengths depending on the paper product being produced. 

As the fiber flows through the line, the effect of the fibers rubbing across the electrode can interrupt the measurement signal, creating process noise. With advanced signal processing, slurry mag meters are ideally suited to measure these pulp stock slurries, which can range in concentration from as low as 3% pulp stock to 16% and higher, as they move from the pulping operation through the bleaching process to the preparation area before ultimately being fed to the headbox of the paper machine. Accurate measurements of stock flow through the process ensures that bleaching, chemical additives and paper weight ensure the desired targets.

Finally, slurry mag meters play an important role in the pulping mill when it comes to the liquor recovery process. Liquor is the term used to describe the chemical mixture that is fed to the pulping process to break down the wood chips. White liquor (virgin chemicals) is often mixed with green liquor (recovered chemicals) to help manage costs. 

While white liquor typically does not have much process noise, green liquor can have some components that create process noise, thus requiring a slurry mag meter. 

Once the processing is complete and while the pulp stock continues on its path throughout the rest of the mill, the black liquor — spent chemicals from the pulping process that also contain some lignins and other organic byproducts of the pulping process — are concentrated and sent to the recovery boiler to burn off the organics and create green liquor. These organic compounds coupled with the concentration processes create measurement challenges, and they are another ideal application for slurry mag meters.

Black liquor is the primary fuel source for the recovery boilers, which not only help to eliminate the organics from the black liquor and create green liquor in the process, but also provide much, if not all, of the power required to operate the mill. Therefore, accurate measurement of the black liquor is critical to optimizing the recovery boiler performance.

Oil and gas

In the oil and gas industry, slurry mag meters are used in hydraulic fracturing — the process of extracting oil and gas by drilling down into the earth and injecting a high-pressure water mixture directed at the rock to release the oil and gas inside. Water, sand and chemicals are injected into the rock at high pressures, which fractures the formations and allows the gas and oil to flow out to the head of the well. 

Slurry mag meters are installed on the blender trucks where water is pumped into the truck from one end and blended with sand and fracking chemicals to create the fracking fluid, which is then pumped downhole into the well. Accurate measurement of what is pumped downhole into the ground is essential for process optimization and, more importantly, for environmental compliance. 

Although a slurry mag meter might be overly sufficient on the clean water side of the blending truck, it is required for the "dirty" or blended side. Thus, many operators, for financial reasons, use the same technology and avoid the need to have different spare parts on hand to replace meters as they wear out. 

Specific slurry problems 

As previously mentioned, slurry, by definition, will have debris in the flow. This debris can have an impact on the electrodes that measure the induced voltage in the flow and send the signal back to the meter, which yields a reading on the transmitter. 

Debris can have an impact on the electrodes that measure the induced voltage in the flow and send the signal back to the meter, which yields a reading on the transmitter.

These kinds of impingements on the transmitter cause process noise, which alters the signal sent to the transmitter, creating an unstable flow measurement. Most meters on the market today employ some form of signal dampening to filter out the noise caused by debris; however, traditional dampening techniques typically require excessive averaging times. This means the transmitter flow measurement reaction to real changes in the flow rate will be delayed, affecting the ability to effectively control the process loop. This ultimately leads to inaccuracies further down the process and in the final product. 

When debris impinges on electrodes, the induced voltage generated by the fluid moving through the magnetic field (Faraday’s law) is interrupted, which results in changing signal amplitude being sent to the transmitter. Magnetic flowmeters use this induced voltage to calculate the flow rate of the fluid, thus any variation in the amplitude of that voltage signal will appear as variability in the flow output, causing inaccurate and "noisy" readings. This leads to process control and optimization challenges.

Solution

Although slurry applications pose problems for accurate measurement, solutions are available that can overcome these issues and provide what many producers are looking for in slurry mag meters: low signal noise and onboard diagnostics. 

When selecting a magnetic flowmeter for a slurry application, important considerations include how the meter addresses noise issues and how quickly the meter reacts to changes in the process flow rate.

Advanced mag meters offer those capabilities and allow producers to optimize their processes and reduce waste and process upsets. Onboard diagnostics can alert an operator to potential issues with the flow before they become critical and upset the process. When selecting a magnetic flowmeter for a slurry application, important considerations include how the meter addresses these noise issues — either through signal processing or more traditional techniques — to provide a stable flow signal, what type of onboard diagnostics are available to help alert various problems with the meter and how quickly will the meter react to real changes in the process flow rate.

Additionally, advanced diagnostic capabilities and the ability to verify the meter in-situ are key features to look for. All these capabilities will enable accurate and stable flow measurements while providing confidence that the meter is working correctly.  

 

Laura Chemler works in product management as a senior product engineer at Emerson, specializing in electromagnetic flowmeters. Laura has six years of engineering experience and holds a Bachelor of Science in mechanical engineering from Purdue University.

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