Scaling can sap energy, maintenance time

If some method can be found to control scaling, there is the potential to save energy and prevent equipment failure and reduce maintenance

By Jan de Baat Doelman April 22, 2013

Boil a kettle of tap water in a hard water area and two undesirable observations are made: A fine but harmless scum appears on the surface of the water and a hard white encrustation develops on the heating element. The latter is calcium carbonate and is usually referred to as limescale. The limescale that deposits on the heating element will, if untreated, reduce the efficiency of the kettle, induce corrosion of the element, and ultimately lead to appliance failure. However, it is not just domestic appliances that are affected; boilers and steam generators also develop limescale deposits, which will cause similar problems if left untreated.

Limescale can form wherever water is heated. The reason for the problem is that source waters (potable or ground water) contain dissolved mineral salts, sometimes described as hardness, which have low solubility in the water. When heated, the water can no longer hold these salts in solution and deposition must then occur. 

What can go wrong?

Deposits are an insulting layer on heat transfer surfaces. This leads to more power being consumed or to the need to install heavier duty equipment to compensate. It is estimated that 40% more energy is needed to heat water in a system fouled with ¼-in. of limescale.

Scale in water lines reduces the available cross-section area and the throughput. Eventually the line will become completely blocked. Equipment then needs to be shut down for cleaning, and this costs money.

Safety valves or emergency process sensors (e.g., that operate deluge systems) may not operate in an emergency. Overheated boilers can be dangerous.

Stagnant conditions can develop in void spaces beneath deposits, and this encourages corrosion of steels and other metallic surfaces. The results can be fluid leaks and equipment failure, which is potentially dangerous. Scale surfaces are also excellent growth sites for bacteria, which can crate conditions hazardous to health (e.g., production of legionella pneumophila).

In order to establish if you have a problem, try answering these questions:

  • Do appliances such as water-fed equipment contain white scale?
  • Are there signs of unexpected deposit formation around valves or at the pipe outlets?
  • Are boilers/heat exchangers performing below design?
  • Is corrosion a problem in the plant?
  • Is the water throughput less than expected? 

The more times that the answer is "yes," then the more likely it is that you have scale. 

Solving the problem

If some means can be found to control scaling, there is potential to save energy, prevent equipment failure, reduce maintenance, and save money.

A range of physical methods can be used to remove fouling deposits. Water jetting, or sand or plastic-bead blasting can be used in accessible locations. However, such methods can be expensive and abrade surfaces.

Magnetic and electronic descaling does not stop precipitation but alters the shape of the crystals to reduce the adherence and buildup of deposits on the pipewall. Perhaps the most remarkable observation is that devices can affect descaling downstream of the point of installation; a softening and loosening of existing scale several weeks after installation is commonly reported.

To understand the mechanism, some knowledge of mineral scale precipitation is necessary. We know that in order to form a scale deposit, three conditions must be met:

  1. The solution must be supersaturated.
  2. Nucleation sites must be available at the pipe surface.
  3. Contact/residence time must be adequate.  

To prevent scale it is necessary to remove at least one of these preconditions. Clearly, contact time is not an alterable factor. To be effective, any device must therefore affect either the supersaturation value or the nucleation process.

The direct effect on electronic devices is on the nucleation process and in particular to enhance initial nucleation through the creation of new nucleation sites within the bulk fluid flow. Crystal growth then occurs at these points of nucleation and not at the pipe wall. Suspended solids increase with a corresponding drop in the level of supersaturation, and these effects have been observed in the field. The localized pH increase near the pipewall caused by hydroxyl radicals formed by electromechanical interactions is one mechanism that drives the changed nucleation characteristics.

Electronic devices are not flow-rate dependent and can be built to fit pipe diameter up to at least 60 in. The units are lightweight and easy to install, can be retrofitted, and produce no significant magnetic field. They are usually effective on calcium carbonate, are claimed to reduce iron fouling, and appear to prevent fouling by various other substances. 

Jan de Baat Doelman is president of Scalewather North America Inc.