Limescale and its effects on materials…
24 April 2012
Calmag Ltd - Providing impartial advice on limescale and water treatment solutions.
The build-up of limescale in domestic hot water supplies and primary systems is a major problem that all heating engineers should be aware of. Problems are caused when the scale builds on surfaces such as the inside of the copper tube or the pump, to a point where flow rates drop, valves and pumps jam, or system efficiency decreases.
The following information on available scale inhibiting devices has been complied after talking to a number of manufacturers. The science behind the devices is still under research and the complexity of the physics and chemistry involved mean it is very difficult to ascertain the true effectiveness of such devices without carrying out extensive laboratory testing.
Installation conditions and water quality vary so much that it becomes very difficult to draw conclusions from on-site experience. At present, no official standards or methods of testing have been established, so customers often have to rely on supplier guidance.
The build up of scale may also cause corrosion. It is often believed that a lining of scale on the inside of a pipe will protect the pipe. This is not true. The blue colour of the scale is a sure sign of copper corrosion.
Scale is a metal salt and is a good conductor of electricity. When it builds up on a metal surface, such as inside the cast iron casing of a pump, it will act as an anode, with the metal becoming a cathode. This anode-cathode set up allows electrons to flow freely between the scale and the metal, allowing corrosion to take place.
Also, as the scale is not uniform, the corrosion becomes localised rather than spread evenly over the surface of the metal. In such circumstances, testing the material on the inside of the pump will show the presence of both scale and rust. The quantity of scale that water can dissolve depends upon temperature, pressure and the pH level.
As the temperature of water increases, the volume of scale in solution it holds drops, resulting in the precipitation of scale modules into the water. These molecules join into crystals either on rough surfaces or on other crystals. Likewise, as the ph increases or as pressure decreases (as it does when water is flowing along a pipe or through a control), the volume of scale held in solution drops resulting in scale formation. It is also worth noting that scale may form in areas where water is turbulent.
This is because turbulence causes a drop in pressure, which in turn leads to scale forming. To prevent scale from being deposited on surfaces, the aim is to create small ’seed’ crystals of scale within the water upon which any remaining scale will grow, forming larger particles of scale suspended in the water. These suspended particles pass along pipework, through any controls, and out through the hot water outlets, without causing problems.
Scale particles in suspension will typically grow to 50 microns in size. Particles of 25 microns may be visible to the naked eye. The way to create crystals in the water is to apply an electric field. Individual molecules of scale align themselves with the electric field, and move towards each other (a similar mechanism to the way electrons move along a wire).
Once in contact the scale bonds together, forming crystals with the scale atoms in the crystal aligned to the electric field. Crystals will continue to grow in this fashion. In-line magnetic devices rely on a flow of water through a magnetic field to generate the required electrical field.
The electric field strength will vary with the flow rate of the water, and will disappear when the water stops flowing. The electric field is only present within the vicinity of the magnets, and once scale crystals have moved past the magnets, they will break down and may build up on surfaces.
Non-intrusive electronic devices produce an electric field (sometimes referred to as a radio signal) along the pipe and within the water. The devices are connected to an electrical supply and do not require a flow of water to set up the electric field. Instead, applying electrical a.c. signal to signal wires or a nonferrous core, creates the electric field.
The critical factor that determines the effectiveness of the device is the type of signal applied. Research has shown that a randomly varying ’sinusoidal’ signal gives the best results, although the mechanism is not completely understood. As yet square-wave signal has little effect. The electrical field that is created will travel along pipework and through water, however, the distance will be determined by the position where pipework is earthed.
The position where pipework is earthed is important, because electricity runs towards earth, and if such a device is to be fitted to protect specific system components, then preferably, there should be no earth between the device and the components to be protected.
One should also avoid fitting these devices on a conducting loop, such as on a loop of copper pipework, as the loop may ’short-circuit’ the electric field, reducing its effectiveness. It is important when buying these devices that the electronics are protected against spikes and other variations in the electrical supply.
If not properly protected then the electronics may soon fail. It is also worth checking that the device does not need to be reset if the electrical supply is cut off temporarily. In-line electrolytic devices are basically self-contained batteries, producing an electric field and current through the water. The electric field creates grains of scale within the water. Again, the electric field is only present within the device, and once scale crystals have moved on they may break down and may build up on surfaces. Like a battery, the device will have a set life span; the longer the life… the lower the effect on scale.
