U.S. industry spends billions of dollars each year to control and remove limescale build-up in industrial heat exchangers, evaporative coolers, boilers, chillers and other water-fed equipment. Oil wells, too, face significant scaling problems from the highly mineralized water that's extracted with the oil. Limescale increases downtime, maintenance costs, and energy consumption, and leads to the early renewal of capital equipment. Scale-prevention can benefit industrial water users by minimizing or eliminating unexpected production shutdowns and generating substantial savings through water conservation.
Types of fouling
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. Other types of fouling include growth of algae and bacteria (bio-fouling); consolidation of loose particles (particulate fouling, such as corrosion byproducts); and accumulation of coke-like deposits (chemical-reaction fouling).
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 that causes calcium carbonate scale. Even soft water will eventually form scales when concentrated numerous times.
Fig. 2. Scale causes increased pipewall friction.
Reasons for concern
Deposits create an insulating layer on heat-transfer surfaces. It is estimated that 40% or more energy is needed to heat water in a system fouled with ¼” of limescale. This leads to more power consumption or the installation of heavier-duty, more expensive heat exchangers to compensate. Scaled boiler tubes mechanically fail as a result of overheating, and cooling tower plates (Fig. 1) can collapse due to the weight of scale deposits. Erosion damage can occur as a result of scale particles breaking loose and subsequently impinging upon other surfaces.Pipework scale (Fig. 2) reduces the available cross-section area, and fluids are affected by increased pipewall friction. A larger, more power-consuming pump will be required to maintain throughput volumes, which may only be a temporary solution. 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, but stagnant conditions eventually develop under the deposit, and electrochemical reactions will corrode the steel surfaces. The result can be fluid leaks and equipment failure, which are potentially very dangerous. In the food industry, incorporation of even trace amounts of undesirable particulates can lead to off-flavors or off-colors, reducing shelf life or making products unsaleable.
Personnel are also at risk. Safety valves or emergency process sensors that are fouled 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.
Because scales and other deposits generally form inside closed systems, it is not always evident that deposition is occurring. But certain clues can be detected. It is useful to try to answer the following questions:
The more times the answer is "yes," the more likely it is that there is fouling. If fouling can be controlled, the potential exists to save energy, prevent equipment failure and reduce maintenance. Furthermore, a successful treatment strategy will maintain fluid flow, reduce corrosion effects and provide a safer environment, in addition to saving money.
Solving the problem
A process audit can identify the extent of an existing problem, the point in the system that corresponds with initial fouling, and, most useful, why there is a problem. The evidence may indicate a solution without the need for expensive external control measures. These might include minor changes in process temperature, pressure, pH or fluids composition that could significantly reduce the fouling potential at practically no cost.
Fig. 3. How Scalewatcher technology works
Treatment options include inhibitor chemicals, descalers, ion exchange, physical cleaning such as pipeline pigging or the installation of permanent magnets or electronic devices like Scalewatcher computerized electronic water-conditioners (which works as shown in Fig. 3).
Although it is usually possible to find a chemical solution to a fouling problem, increasing environmental and safety pressures demand that chemical consumption be reduced wherever possible.
A range of physical methods can be used to remove fouling deposits. Water jetting, sand or plastic-bead blasting, for example, can be used in accessible locations. Such methods are expensive and can cause abrasion of surfaces.
Unlike other preventive techniques, magnetic and electronic descaling devices do not stop precipitation but alter the shape of the crystals to reduce the adherence and build-up of deposits on the pipewall. Perhaps the most remarkable observation is that these 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 magnetic and electronic mechanism, some knowledge of mineral scale precipitation is necessary. We know, for example, that three conditions are needed to form a scale deposit:
To prevent scale, it is necessary to remove at least one of these pre-conditions. 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 of the electronic device described above is on the nucleation process. Specifically, it enhances initial nucleation through the creation of new nucleation sites within the bulk fluid flow. This is known as controlled precipitation. 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.
Key to this process is manipulation of a factor known as the Lorenz Force (see Sidebar). By nature, all particles in water have a negative charge and are surrounded by the so-called “electric double layer,” which are layers of positive and negative ions. These are considered “protective” layers because they prevent more ions from sticking to the particle surface. If strong enough, however, the Lorenz Force, will distort these layers and allow ions in the bulk of the liquid to 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, less mineral ions will be present in the liquid. An important side effect is that pipe walls corrode less as a lower amount of positive ions are present. MT
A Lorenz Force F is experienced by charged particles that flow through a field. This type of force is expressed as follows:
q = charge on the particle
E = electric field vector
V = particle velocity vector
B = magnetic field vector
Electronic devices operate at very weak magnetic fields, whereas magnets need high field strength (>1000gauss) for optimum performance. 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 force of the electric field component is independent of the flow rate. 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.
Lorenz Force Key…
EWT: F=q(E + vxB) Newton
F: Lorenz Force expressed in Newton
q: Charge of particle expressed in Coulomb
E: Electric field expressed in Volt/meter
V: Velocity of particle expressed in meter/second
B: Magnetic induction expressed in Tesla