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Corrosion in Commercial Heating and Hot water Systems – 2
Part 2 – Testing for Corrosion
If your premises are located in a soft water area then you need to be aware of the threats of corrosion to your system, and small leaks in your metal plumbing components could well be a sign of a more serious problem.
Testing water for corrosivity factors – pH, calcium concentration, hardness, dissolved solids content and temperature – as well as dissolved lead and copper due to corrosion is relatively straightforward.
The original accepted measurement of water corrosivity, the Langelier Saturation Index (LSI), was developed to determine whether a water source is potentially scale forming or scale dissolving (aggressive). LSI, however, was not developed with the intention of testing naturally soft water, so relies on the chemical characteristics of the water, such as pH and especially hardness, to estimate the corrosivity. Positive values are indicative of non-corrosive water, negative LSI values indicate water could be corrosive.
The problem with LSI is that it based upon the oft-cited belief that calcium carbonate scale when deposited in a thin, carefully controlled, uniform layer can contribute to a protective barrier against corrosion. There is little scientific evidence to support this assumption
The highly variable physical and chemical conditions within a hot water system mean it is very unlikely that a deposit of scale would ever be truly ‘uniform’. Deposition would typically be prevalent on the hottest elements of the boiler/water heater, and with the formation of crevices actually drive greater localised corrosivity in a fashion similar to pitting which would lead to the potential failure of an appliance. Additionally, as scale accumulates microscopic deposits of metal can be incorporated generating a ‘mini-galvanic’ effect.
Scale formation in a glass-lined indirect tank
Even without the issues of generating corrosion factors, it is never recommended that scale build-up be allowed in a hot water system. An increasing build-up of scale reduces the efficiency of an appliance by restricting the rate of flow and harming efficient transfer of heat. All of which drives up the operational costs.
For a more accurate prediction of calcium carbonate, and levels of corrosivity, the Ryznar Stability Index (RSI) was developed which considers pH, conductivity in TDS, calcium ions (CA²⁺), bicarbonate (HCO₃) and water temperature. An RSI of 6.0 -7.0 indicates water with little scale and the probability of some corrosion, with higher values being increasingly corrosive. Between 7.0 – 7.5, corrosion will be significant, becoming heavy between 7.5 and 9.0. Results above 9.0 are regarded as intolerable.
Both LSI and RSI are predicated on assessing the level of scale build-up, so an alternative approach is to test for metals concentrations, especially copper, which is achieved by drawing a sample to test directly from the hot water system. Detection of iron, manganese or aluminium impurities in the water can also be indicative of corrosivity as localised deposits can also cause pitting in copper.
The only caveat is that the concentrations in open systems, such as where instantaneous water heaters are deployed, will quickly flush by-products of corrosion out of the system. So regular testing is soft water areas can be advantageous when seeking to prevent serious damage to appliances and associated system pipework.
UK water suppliers will typically provide self-assessment help for establishing local water quality, and can often provide details of most recent water tests based on a business’ postcode. Independent testing is also available, though there will be a cost associated. At Adveco our engineers can help and advise with your water assessment as part of the application sizing process.
Corrosion is a complex phenomenon, and no single dissolved substance is responsible for making water corrosive. There are several factors that can increase the likelihood of corrosion, especially the natural softness of water. When water passes through limestone and chalk in the ground, such as in the South East of the UK, it will pick up calcium and magnesium carbonates, when these minerals are greater than 280ppm the water is classed as hard. However, in Scotland, the North West and South West of England, and Western Wales, where water passes through hard igneous rock it lacks dissolved calcium and magnesium. This makes the water naturally purer (less than 100 ppm). This soft water exhibits a low pH, low total dissolved solids (TDS) and negligible buffering capacity, all of which makes it more corrosive.
When differing metals are connected in a hot water system, the water in contact with both metals acts as an electrolyte conducting the current. The current flows through the water from the positively charged less noble material to the negatively charged more noble material. Where the current leaves the less noble metal, corrosion will occur. As the current is usually greater close to the contact point of the two metals, this is where corrosion will be a greater issue. The higher the metal is on the Galvanic series, the nobler the metal will be, whilst the greater the distance between the two differing metals in the series, the greater the electrical potential will be and the greater the corrosion rate for the less noble metal.
The team will take the Lemosho Route during this early trekking season, where due to the lower temperatures there usually would be a good chance of encountering snow on the summit. As Paul observes, “The weather forecast is pretty shocking at the moment, so we are expecting heavy snow!” In preparation, Paul began a series of practice treks last year, including hiking up Ben Nevis last May, a challenge in of itself. But at a mere 1,345 metres, that peak pales in comparison to Kilimanjaro. The eight-day trek length in Tanzania is critical in enabling Paul’s body to acclimatise to the altitude and the physical exertion needed for the steeper sections of the climb towards the summit. The final camp to summit push will take a good six-seven hours, which, weather dependent usually starts at 2 am, for a sunrise arrival at the summit. That should give the team a better chance navigating the loose scree which freezes overnight and provides a full day for a safer descent back down most of the mountain.
