It costs U.S. industries billions of dollars a year to control and remove the limescale that builds up in industrial equipment such as heat exchangers, evaporative coolers, boilers, chillers and other water-fed equipment. Oil wells, for example, face significant scaling problems from the highly mineralized water that is extracted with the oil.
Limescale not only increases downtime and maintenance costs and causes the early renewal of capital equipment, it also increases energy usage. Scale prevention can benefit industrial water users by minimizing or eliminating unexpected production shutdowns and offering substantial savings to end users through water conservation.
Scale Formation
Scale usually refers to an intimate mixture of sparingly soluble mineral salts. Mineral scale deposition occurs as a result of heat transfer or pressure changes. Calcium carbonate scaling from hard water, and calcium phosphate and oxalate formation in sugar refineries are examples.
There are many scenarios to consider when assessing scale formation. Calcium carbonate is the predominant component of the hard and tenacious scale deposit from water and is particularly apparent in processes involving heat transfer. A concentration of dissolved solids by repeated partial evaporation of the water is the main factor causing calcium carbonate scale. Even soft water will eventually become scale forming when concentrated numerous times.
Process, maintenance and facility managers should be concerned about scale deposition. Deposits are an insulating layer on heat transfer surfaces. It is estimated that 40% more energy is needed to heat water in a system fouled with ¼ in. of limescale. This leads to a jump in power consumption or to the installation of heavier-duty, more expensive heat exchangers to compensate. Scaled boiler tubes experience mechanical failure as a result of overheating, and cooling tower plates can collapse due to the weight of scale deposits. Erosion damage can occur as a result of scale particles breaking loose and subsequently impinging on other surfaces.
Pipework scale reduces available cross-section area, and the increased pipe wall friction affects fluids. A larger, more power-consuming pump will be required to maintain throughput volumes, but this may only be a temporary solution to the problem. A plant that needs to be shut down for cleaning costs money.
The formation of a thin uniform layer of scale or wax can temporarily reduce steel corrosion; eventually, stagnant conditions will develop under the deposit and electrochemical reactions will corrode the steel surfaces, which could lead to leaks and equipment failure—both potentially dangerous. In the food industry, the incorporation of even undesirable trace particulates can lead to off flavors or colors, reduce shelf life or even make the product unsaleable.
Not only are plant and product integrity at stake, but personnel health and safety also may be compromised. Fouled safety valves or emergency process sensors may not operate in an emergency. Overheated boilers have been known to explode. Failure to control bacterial growth in cooling water can create conditions hazardous to health (e.g., production of Legionella pneumophila) or, in anaerobic conditions, may allow the production of toxic hydrogen sulphide from sulphate-reducing bacteria.
Recognizing Fouling
Because scale and other deposits generally form inside closed systems, it is not always evident that deposition is occurring. Some clues, however, may be present. It is useful to try to answer the following questions:
- Are energy/heating bills reduced immediately after cleaning the plant?
- Is it necessary to arrange significant planned and/or unplanned downtime?
- Are heat exchangers performing below design?
- Is corrosion a problem in the plant?
- Are there signs of unexpected deposit formation within the system?
The more times the answer is “yes,” the more likely it is that fouling has occurred. When it is controlled, industries can save energy, prevent equipment failure and reduce maintenance. Furthermore, a successful treatment strategy will maintain fluid flow, reduce corrosion and provide a safer environment—in addition to saving money.
Solving the Problem
A process audit can identify the extent of the problem, the point in the system corresponding to initial fouling and (most usefully) why there is a problem. From the evidence collated, it may be possible to suggest a solution without the need for expensive external control measures. Minor changes in the process temperature, pressure, pH or fluids composition could significantly reduce the fouling potential at practically no cost.
Treatment options include inhibitor chemicals, descalers, ion exchange, physical cleaning such as pipeline pigging or the installation of permanent magnets, or electronic devices such as the patented Scalewatcher computerized electronic water conditioner.
Although usually it is possible to find a chemical solution to a fouling problem, ever-increasing environmental and safety pressures demand that chemical consumption be reduced wherever possible. Increasingly, restrictions are being applied regarding the use of chemicals due to their environmental impact.
A range of physical methods can be used to remove fouling deposits. Water jetting, sand or plastic bead blasting can be used in accessible locations. Such methods are expensive and can cause abrasion of surfaces.
Electronic Devices
Unlike other preventative techniques, electronic devices do not stop precipitation, but alter the shape of the crystals to reduce the adherence and buildup of deposits on the pipe wall. The devices can affect descaling downstream of the point of installation—a softening and loosening of existing scale several weeks after installation commonly is reported.
To understand the mechanism, some knowledge of mineral scale precipitation is necessary. In order to form a scale deposit, three conditions must be met:
- The solution must be supersaturated.
- Nucleation sites must be available at the pipe surface.
- 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. Therefore, to be effective, any device must affect either the supersaturation value or the nucleation process.
The direct effect of electronic devices is on the nucleation process, particularly on enhancing initial nucleation through the creation of new nucleation sites within the bulk fluid flow. Crystal growth then occurs at the 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 pipe wall caused by hydroxyl radicals formed by electromechanical interactions is one mechanism that drives the changed nucleation characteristics.
A Lorenz force (F) is experienced by charged particles that flow through a field, F = qE + q (V x B), in which q is the charge on the particle, E is the electric field vector, V is the particle velocity and B is the magnetic field vector. Electronic devices operate at small residual magnetic fields, whereas magnets need high field strength (greater than 1,000 gauss) for optimum performance. Electronic devices are not flow-rate dependent and can be built to fit pipe diameter up to 60 in. The flow dependency of magnetic devices is explained by the velocity parameter, V, and E = 0. The flow non-dependency of electronic devices is explained by the fact that the magnetic component approaches zero, but the electric component is essentially constant. This suggests that the key performance parameter is the total value of the Lorenz force acting on the charged particles, rather than the individual magnetic and electric field vectors.
By nature, all particles in water have a negative charge and are surrounded by the so-called “electric double layer,” layers of positive and negative ions around the particle. These layers may be seen as “protective” layers preventing more ions from sticking to the surface of the particle. The Lorenz force, if strong enough, will distort these layers, and ions in the bulk of the liquid may now stick to the surface, forming crystals. These crystals will not adhere to pipe walls and will go down the drain or remain suspended in a circulating system. As a result, fewer mineral ions will be present in the liquid. Pipe walls will corrode less, as a lower amount of positive ions will be present.
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