Most process plants use substantial amounts of water. The water comes from a range of sources such as rivers, lakes, wells and oceans and contains dissolved and suspended solids. To reduce consumption, most plants reuse water; this, in turn, causes it to be highly concentrated in silicate, iron and calcium carbonate that prompt limescale build-up and rust.
Limescale deposition results in the need for regular shutdown to remove the build-up. Sometimes, this removal can take place as part of a planned maintenance program but, on other occasions, severe or even complete blockage by limescale can force unscheduled equipment outages until the blockage is cleared. In addition, scale deposits can increase corrosion due to entrapped oxygen and the scale itself sometimes can corrode the surface it contacts. Also, scale will interfere with the action of inhibitors in the system, keeping them from reacting with the surface below the scale. If the scale is patchy, the differential aeration between the clean surface and the scale surface may cause a corrosion cell to set up.
Iron reacts with oxygen to form red rust (Fe2O3). Dissolved oxygen in water causes growth of such rust inside a ferrous pipe that accelerates scale deposition (see photo). Electronic water treatment (EWT) devices such as Scalewatcher change that red rust into black rust (Fe3O4) called magnetite. This conversion makes it possible to extend the lifespan of pipe material by forming a film on the inside of the pipe wall that prevents rust growth. The transformation into black rust by EWT not only removes the red rust but also allows the ongoing flow of water to remove scale.
To explain the effect of electronic scale removal, it is important to first understand the major factors that cause scale. While scale can be a complex of many minerals, calcium carbonate is the most prevalent in industrial processes.
Aqueous solutions can become supersaturated, which means they contain higher concentrations of dissolved solute than their equilibrium concentration. Such solutions are not stable and are easily triggered into dropping back to saturation level, forcing the dissolved compound to precipitate. Even when a bulk solution is less than fully saturated, scale formation can occur spontaneously due to localized supersaturation — for example, a drop in calcium carbonate solubility on a surface can lead to the formation of deposits.
EWT is a non-invasive system that involves wrapping a solenoid coil or coils around the pipework to be treated. A signal generator that produces a continuously changing frequency within a specified range supplies current to the coils. The pulse-shaped current creates an induced electric field, concentric around the axis inside the pipe. As a consequence of this arrangement, any charged particle or ion moving within the field experiences a so-called Lorentz force generated by the interaction between charged particles and magnetic and electric fields.
The treatment influences the initial nucleation, resulting in idiomorphic, scattered crystals that do not stick together or form matted structures — in contrast to the matted structures that continuously grow in untreated water. The crystals’ large volume relative to their surface area makes them sensitive to water currents; they are easily flushed out of the pipeline. Because no new scale layers are formed, the shear force of the water flow gradually will remove existing layers of scale. The ability to adjust power, frequency and coil configurations on site enables optimizing performance without downtime and pipe replacement.
The treatment requires flowing water to be effective. In addition, to ensure the best possible results, the coils should be located 39 ft from the equipment to be protected — and not installed immediately before or after a pump.
Payback on EWT normally is less than two years. In some cases, the return on investment can be as little as three months.
EWT has proven effective in treating rust problems in a variety of applications. Here are two examples:
River water used in a compressor cooling-water line. The pipe work was clogged with scale, resulting in a safety device automatically stopping the compressor when the temperature of cooling water exceeded 194°F. The main components of the scale were calcium carbonate and iron.
The river water, gathered in a water pond, passes through a strainer, cools the compressor, and then goes to a sump from which it is returned to the water pond by a circulation pump. It was difficult to pull up the screen of the strainer because 0.2 inch of rust adhered to the screen case. A Scalewatcher was installed on the discharge side of the pump. After about three weeks, red rust inside the screen case of the strainer disappeared and remaining rust had changed into a black color. Then it became easy to pull up the screen. In the compressor, the temperature of the cooling water did not surpass 176°F, making it possible to operate the compressor safely without interruption.
Iron scale caused by chlorine at a power plant. Seawater was used in a pipeline supplying analyzing equipment. The line was clogged with micro-organisms and iron scale. The strainer had to be cleaned every week. A Scalewatcher was installed on the line between the water pump and the strainer to see if it could prevent corrosion by dissolved chlorine. After a month, the rust and hard scale inside the pipeline had softened. After a further three months, the red rust had changed to black rust, the strainer was not clogged and the microorganisms had disappeared.
JAN DE BAAT DOELMAN is president of Scalewatcher North America, Oxford, Pa. E-mail him at [email protected].