Electrical capacitance tomography keeps eye on scale

ECT gives real-time images of pipe interiors to monitor and manage process scaling.

By Jim Montague

While routine build-up and clogging in process applications is annoying, process scaling is more persistent, causes unplanned downtime and increases costs in many industries. It's typically composed of calcium carbonate, wax, grease or similar compound combinations, but it can also include non-traditional chemical and biological substances that can quickly clog pipelines and equipment. For example, struvite, or magnesium ammonium phosphate, often occurs on digester exit lines in wastewater treatment applications.

"Remedies for scaling include mechanical or chemical cleaning or pigging pipelines," says Todd Loudin, president, North American operations at Flowrox Inc. "Keys to proper scaling treatment include knowing when to add chemicals, when to run pigs, or when to shut down processes for mechanical cleaning. While the oil and gas industry spends lots of money on instrumentation to monitor and control its processes, many operations monitor scale deposits mostly by watching for pressure drops in their pipelines. However, this method isn't precise because many factors can influence pressure drops. For instance, when pumps begin to wear out, their pressure outputs may decrease, but this doesn't always indicate scale deposits. It could just be a worn-out impeller."

Loudin adds that newer scale-detecting instrumentation uses electrical capacitance tomography (ECT) to visualize pipeline scaling without opening them up. ECT instrumentation monitors hard and soft scaling in many process applications, and lets users determine where and how thick scale is so they can deploy and monitor their chemicals more effectively. Likewise, manufacturers of anti-scaling chemicals can tweak their products, and evaluate the success of new formulas.

"ECT can deliver a new level of intelligence about what scaling is happening, as well as multiphase flow visualization," explains Loudin. "In piping with a vortex effect, this instrumentation can provide clear imagery of it. A pipeline with different chemicals or solids can show a video of the flow process. Even if there's scaling inside the instrument, it won't affect multiphase flow capabilities."

ECT can deliver a new level of intelligence about scaling, as well as multiphase flow visualization.

Research on possible ECT applications has been conducted by the U.S. Dept. of Energy's Morgantown Energy Technology Center in West Virginia, and the U.K.-based University of Manchester's Institute of Science and Technology. "Some types of instrumentation can monitor scaling conditions, but only on a basic level," Loudin says. "For instance, ultrasonic instruments can detect scaling conditions, but many of them can't see added scale accumulation after only a few millimeters of scale buildup. ECT can provide real-time images of any pipeline from less than 1 mm of scaling up to 95-100% built up with scale."

ABCs of ECT

ECT instruments don't require radiation and use very low energy levels to visualize the pipeline. They measure conductivity and permittivity distribution at a point of interest to measure material properties within the pipe. The instrument applies excitation signals to the target, and measures output signals based on the electrical properties of the medium and scale.

"ECT instruments use electrodes mounted around the target," explains Loudin. "When an electrode is excited, the others 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 in the target, the greater its resistance to electrical stimulus. The electrodes create an electrical field, and grounded capacitance-measuring electrodes measure this resistance. Very low electrical current is supplied to the electrodes—as little as 3 V and sometimes up to only 12 VAC. Each electrode is excited, and algorithms create a 3-D image of the pipe’s interior."

Download the 2016 State of Technology Report: Flow Measurement 

Loudin adds that ECT instruments are similar to a spool piece inserted between two flanges of a pipe. The process flow passes through the device as if it was just another pipe section. In most cases, the instrument is manufactured from the same material as the mating piping, so it mimics scaling in the rest of the pipe.

"ECT for piping and other industrial systems is relatively new, and only a few companies use it this way," says Loudin. "The instrument is connected to an electrical control box, which delivers low voltage to the electrodes and collects capacitance measurements. The box also contains an industrial computer that performs algorithms to create images. The box delivers images with a 4-20 mA signal to plant monitoring systems and control panels. The box can be located close to the instrument or up to 300 feet away in non-explosion-proof environments. In explosion-proof environments, the distance to the box can be cut in half."

Long-term investment

 Scale is present in many industries, but it may not always be enough of a nuisance or capital drain to invest in detection. However, paper mills, mining operations, chemical plants, steel mills and other users that spend $1 million or more per year on anti-scaling chemicals may find it especially useful in reducing chemical consumption and operating costs. Most importantly, ECT can prevent and alarm if scale conditions reach critical levels.

"One solution using ECT that will be available in 2017 is a plug that can be inserted into a large-diameter pipeline or vessel to detect and monitor scaling," concludes Loudin. "The idea is to reduce the size and cost of the instrument for insertion in a large-diameter pipeline or vessel. At the same time, the market may also see a probe that can be inserted into a vessel to measure the phase boundary of various substances as well as the emulsion layer. This will allow chemical manufacturers involved in decantation and similar processes to reap the benefits of increased end-product recovery, decreased wastewater treatment costs and improved product quality."

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