Electrical capacitance tomography can provide real-time imagery of any pipeline.
by Todd Loudin

Process scaling affects numerous industries, causing varying degrees of unwanted costs and downtime. The oil industry, for instance, spends up to 0.25 percent of the global gross domestic product (GDP) per year eradicating and treating paraffin wax deposits in piping systems. The mining industry can be negatively affected by scale in process water, cyanide leaching and tailings. Alumina and bauxite refining operations face scale in digesters and transfer piping, which often creates a significant bottleneck in production. The pulp and paper industry experiences scaling in pulp digester screens and white, green and black liquor processing. Other industries that are affected by scale include steelmaking, wastewater treatment, chemical production and power generation.

In most cases, the scaling is calcium carbonate or similar culprits. A type of nontraditional scaling is zebra mussels in the Great Lakes region. While not generally classified as scale, they can completely clog large-diameter pipeline in a short amount of time. Struvite scaling is a culprit often seen on exit lines of digesters in wastewater treatment facilities.

Image 1. Figure 1. Scaling ECT pipeline instrumentImage 1. Figure 1. Scaling ECT pipeline instrument (Images and graphics courtesy of Flowrox)

Some remedies for scaling include mechanical cleaning, chemical cleaning or pigging pipelines. One of the keys to proper treatment of scaling is knowing when to add chemicals, when to run the pig or when to shut the process down to perform mechanical cleaning. While the oil and gas industry, for example, spends large amounts of money on instrumentation to monitor and control its processes, many operations monitor scale deposits primarily by watching for pressure drops in the piping system. This method, however, is imprecise because many variables can affect pressure drop. For instance, as a pump begins to wear, the pressure output of the pump may decrease. This does not always indicate the presence of a scale deposit; it could simply be a worn impeller.

Detection Tools

New scale-detecting instrumentation uses electrical capacitance tomography (ECT) to create a visual display of pipeline scaling without having to physically open the pipeline. This instrument can monitor hard and soft scaling in numerous industries and enables plants to reduce anti-scaling chemical usage and associated costs.

Typically, a plant adds more chemicals than necessary to prevent a production shutdown or slowdown. With this instrumentation, plants can know when scale is present and in what thickness so they can use chemicals only when necessary. They can also monitor the effectiveness of the chemicals they are using, and anti-scaling chemical manufacturers can fine-tune their anti-scaling chemicals and validate the success of new formulations.

ECT can provide a new level of intelligence regarding what type of scaling is occurring as well as multiphase flow visualization. In a piping system where a vortex effect is present, the instrument can provide clear imagery of this phenomenon.

A pipeline that has two or three different chemicals or solids can be imaged to show a video of the flowing process. Even if there is a significant amount of scaling on the inside of the instrument, it will not affect the multiphase flow capabilities.

Figure 2. Imagery of scale inside of a pipeline Figure 2. Imagery of scale inside of a pipeline

The U.S. Department of Energy at the Morgantown Energy Technology Center in West Virginia and the University of Manchester Institute of Science and Technology in the U.K. both performed extensive early research and pioneering on the potential uses and applications of ECT technology.

Some types of instrumentation can monitor scaling conditions but only on a basic level. For instance, ultrasonic instruments can detect scaling conditions, but many of these instruments lose the ability to see additional accumulation of scale after only a few millimeters of scale buildup. New ECT instruments can provide real-time imagery of any pipeline—from one that has less than 1 millimeter of scaling to one that is 95 or even 100 percent built up with scale.

How ECT Works

ECT instruments do not require radiation and use very low levels of energy to generate pipeline visualizations. They are targeted at a point of interest and then measure the results of the material properties within the target. ECT applies excitation signals to the target point of interest and measures the output signals that depend on the electrical properties of the medium and scale present. The instrument measures the conductivity as well as permittivity distribution.

ECT instruments use a grouping of electrodes mounted around the target of interest. As an electrode is excited, the other electrodes are grounded and measure the capacitance. The permittivity of the distribution of the medium is measured due to resistance of permittivity to the electrical field. The higher the permittivity found within the target of interest, the greater the resistance to electrical stimulus. The electrodes create an electrical field, and grounded capacitance-measuring electrodes measure this resistance. Very low levels of electrical current are supplied to the electrodes—as little as 3 volts and sometimes up to only 12 volts alternating current (AC). Each electrode is excited in turn, and algorithms create a 3-D image of the pipeline's interior.