Controlling Scale Deposition
Look at the heating element of a washing machine or
dishwasher in a hard water area and you will see a white encrustation
containing hardness salts. This commonly is referred to as limescale and is an
example of domestic fouling. The limescale (calcium carbonate) that deposits on
the heating element will, if untreated, reduce the efficiency of the machine,
induce corrosion of the element and ultimately lead to appliance failure.
Industrial fouling poses a far greater problem than anything
in the domestic sector. Huge volumes of fouled fluids are handled, and the
systems that contain the fluids can become fouled as well. The quality of water
streams used by industry varies widely and gives rise to numerous fouling
Type of Fouling
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 the growth of algae and bacteria (biofouling) and the
consolidation of loose particles (e.g., particulate fouling-- corrosion
byproducts and the accumulation of "coke-like" deposits).
What Can Go Wrong?
Process managers should be concerned about fouling. Deposits
are an insulating layer on heat transfer surfaces. This leads to more power
being consumed or to the installation of heavier duty, more expensive heat
exchangers to compensate. It is estimated that 40 percent more energy is needed
to heat water in a system fouled with 1/4 inch of calcium carbonate scale.
Scaled boiler tubes mechanically fail 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 then subsequently
impinging upon other surfaces.
Pipework scale 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 but this
may allow only a temporary solution to the problem. Plants that need to be shut
down for cleaning cost money.
The formation of a thin uniform layer of scale or wax
temporarily can reduce steel corrosivity, but eventually stagnant conditions
develop under the deposit and electrochemical reactions will corrode the steel
surfaces. The result can be fluid leaks and equipment failure, which is
potentially very dangerous. In the food industry, the incorporation of even
undesirable trace particulates can lead to off-flavors or off-colors, reduced
shelf-life or even making the product not fit for selling.
Not only is plant and product integrity at risk but
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 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.
As scales and other deposits generally form inside closed
systems, it is not always evident that deposition is occurring. There are some
clues that can provide the evidence that is necessary. It is useful to try to
answer the following questions.
-Do energy/heating bills reduce immediately after cleaning
-Are heat exchangers performing below design?
-Is corrosion a problem in the plant?
-Are there signs of unexpected deposit formation within the
The more times that the answer is "yes," the more likely
it is that there is fouling. If fouling can be controlled, there is a potential
to save energy, prevent equipment failure and reduce maintenance time and
costs. Furthermore, a successful treatment strategy will maintain fluid flow,
reduce corrosion effects and provide a safer environment. In addition, it will
Solving the Problem
A process audit would identify the extent of the
current problem, the point in the system corresponding to initial fouling and,
of most use, why there is a problem. From the evidence, 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 significantly could reduce the
fouling potential for little to no cost.
Treatment options include inhibitor chemicals, descalers, ion
exchange and physical cleaning such as pipeline pigging or the installation of
permanent magnets or electronic devices.
Although it usually is possible to find a chemical solution to
a fouling problem, environmental and safety pressures demand that chemical consumption
is 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 inaccessible
locations. For other applications, such methods may be expensive and possibly
can cause the abrasion of surfaces.
Magnetic, Electronic Descaling
Unlike other preventative techniques, these devices do not stop
precipitation but rather 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 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. We know that 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. To be effective,
any device must affect either the supersaturation value or the nucleation
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 pipewall. 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 60 inches. The units are lightweight, easy to install,
can be retrofitted and produce no significant magnetic field. They usually are
effective on calcium carbonate claimed to reduce iron fouling and appear to
prevent fouling by various other substances.