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LIMESCALE By Jan de Baat Doelman ControllingControlling ScaleScale DepositionDeposition andand IndustrialIndustrial FoulingFouling ook at the heating element of a growth of algae and bacteria (bio-fouling), washing machine or dishwasher the consolidation of loose particles (particu- Lin a hard water area and you will late fouling, [e.g., corrosion by products]) see a white encrustation containing hard- and the accumulation of “coke” like deposits ness salts. This is commonly referred to as (chemical reaction fouling). limescale and is an example of domestic fouling. The limescale (calcium carbonate) What Can Go Wrong? that deposits on the heating element will, if Process managers should be concerned untreated, reduce the efficiency of the about fouling. Deposits are an insulating layer machine, induce corrosion of the element on heat transfer surfaces. These deposits lead and ultimately lead to appliance failure. to more power being consumed or to the Industrial fouling poses a far greater installation of heavier duty, more expensive L Treatment options for limescale build-up include problem than in the domestic sector. This heat exchangers to compensate. It is estimat- inhibitor chemicals, descalers, ion exchange, is because huge volumes of fluids are han- ed that 40 percent more energy is needed to physical cleaning, magnets and electronic devices. dled and the systems that contain the fluids heat water in a system fouled with 0.25″ of can become fouled. The quality of water calcium carbonate scale. Scaled boiler tubes to maintain throughput volumes but this is streams used by industry varies widely and mechanically fail as a result of overheating, only a temporary solution to the problem. causes many fouling problems. and cooling tower plates can collapse due to A plant that needs to be shut down for the weight of scale deposits. Erosion damage cleaning costs money. Types of Fouling can occur as a result of scale particles breaking The formation of a thin uniform layer Mineral scale deposition occurs as a loose and then subsequently impinging on of scale or wax can temporarily reduce the result of heat transfer or pressure changes. other surfaces. steel corrosivity, but eventually stagnant For example, calcium carbonate scaling Pipework scale reduces the available conditions develop under the deposit and comes from hard water, while calcium phos- cross-section area, and fluids are affected by electrochemical reactions will corrode the phate and oxalate formation occurs in sugar increased pipewall friction. A larger, more steel surfaces. The result can be fluid leaks refineries. Other types of fouling include the power-consuming pump will be required and equipment failure, which is potentially very dangerous. In the food industry, the incorporation of even trace amounts of undesirable particulates can lead to off-fla- vors or off-colors, reducing shelf-life or even making the product unsaleable. Not only is plant and product integrity at risk but also personnel health and safety may be compromised. 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 haz- ardous to health (e.g., production of legionel- la pneumophilia). In anaerobic conditions, it may allow the production of toxic hydrogen sulfide from sulphate reducing bacteria. Magnetic and electronic descaling systems alter the shape of the crystals to reduce the adherence and build-up of deposits on the pipewall. L www.waterinfocenter.com WATER Engineering & Management • JULY 2001 19 LIMESCALE Recognizing Fouling Table 1: Scale Prevention with the Lorenz Force Since scales and other deposits general- ly form inside closed systems, it is not × always evident that deposition is occurring. Electronic Scale Control: F = q(E + v B) However, some clues can provide the evi- Magnet: F = q.v × B Newton dence that is necessary. It is useful to try to Alloys: F = q.E Newton answer the following questions. F: Lorenz Force on particle or dissolved mineral ion in the liquid, measured in Newtons. q: Charge of particle or dissolved ion, measured in Coulombs. • Do energy/heating bills reduce imme- E: Electric field generated by alternation magnetic field measured in V/m (Volts/meter). diately after cleaning the plant? v: Velocity of water measured in m/s (meters/second). • Is it necessary to arrange significant planned and/or unplanned downtime? • Are heat exchangers performing ting, sand or plastic-bead blasting can be is one mechanism that drives the changed below design? used in accessible locations. Such methods nucleation characteristics. • Is corrosion a problem in the plant? are expensive and can cause the abrasion of A Lorenz force (F) is experienced by • Are there signs of unexpected deposit surfaces. charged particles that flow through a field: formation within the system? F = qE + q (V × B) where q is the charge Magnetic and Electronic Descaling on the particle, E is the electric field vec- The more times that the answer is “yes” Unlike other preventative techniques, tor, V is the particle velocity and B the then the more likely it is that there is foul- these devices do not stop precipitation but magnetic field vector. Electronic devices ing. If fouling can be controlled, there is alter the shape of the crystals to reduce the operate at very small residual magnetic potential to save energy, prevent equipment adherence and build up of deposits on the fields whereas magnets need high field failure and reduce maintenance. pipewall. These devices also can affect strength (>1,000 gauss) for optimum per- Furthermore, a successful treatment strate- descaling downstream of the point of formance. The flow dependency of mag- gy will maintain fluid flow, reduce corro- installation. A softening and loosening of netic devices is explained by the velocity sion effects and provide a safer existing scale is commonly reported sever- parameter, (V) and E = 0. The flow non- environment. Finally, it will save money. al weeks after installation. dependency of electronic devices is To understand the mechanism, some explained by the fact that the magnetic Solving the Problem knowledge of mineral scale precipitation is component approaches zero, but the elec- A process audit would identify the necessary. In order for a scale deposit to tric component is essentially constant. extent of the current problem, the point in form; three conditions must be met. This suggests that the key performance the system corresponding to initial fouling parameter is the total value of the Lorenz and, most useful, why there is a problem. • The solution must be supersaturated. force acting on the charged particles, From the evidence collated, it may be pos- • Nucleation sites must be available rather than the individual magnetic and sible to suggest a solution without the need at the pipe surface. electric field vectors. The equations for for expensive external control measures. • Contact /residence time must various devices using the Lorenz Force are Minor changes in the process temperature, be adequate. shown in Table 1. pressure, pH or fluids composition could Electronic devices are not flow-rate significantly reduce the fouling potential at In order to prevent scale it is necessary dependent and can be built to fit pipe practically no cost. to remove at least one of these pre-condi- diameters up to at least 60″. The units are Treatment options include inhibitor tions. Clearly contact time is not an alter- lightweight and easy to install, can be chemicals, descalers, ion exchange, physical able factor. Therefore, any successful retrofitted and produce no significant cleaning such as pipeline pigging, or the device must affect either the supersatura- magnetic field. They usually are effective installation of permanent magnets or elec- tion value or the nucleation process. on calcium carbonate, are claimed to tronic devices. The direct effect of electronic devices is reduce iron fouling and appear to prevent Although it usually is possible to find a on the nucleation process and in particular fouling by various other substances. chemical solution to a fouling problem, to enhance initial nucleation through the About the Author: environmental and safety pressures are creation of new nucleation sites within the Jan de Baat Doelman is the president of demanding that chemical consumption be bulk fluid flow. Crystal growth then occurs Scalewatcher North America Inc., Oxford, Pa. reduced when possible. Increasingly, at these points of nucleation and not at the For more information on this subject, restrictions are being applied regarding the pipewall. It has been observed that sus- circle 864 on the reader service card. use of chemicals. pended solids increase with a correspond- ing drop in the level of supersaturation. Physical Methods The localized pH increase near the A range of physical methods can be pipewall caused by hydroxyl radicals used to remove fouling deposits. Water jet- formed by electromechanical interactions 20 WATER Engineering & Management • JULY 2001 www.waterinfocenter.com.

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