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Holistic approach to monitoring

13 September 2018

Water quality plays a considerable role in plant efficiency but what should we be measuring, where and how? Armand Uchtmann, commercial business manager water quality EMEA of HORIBA, offers guidance on monitoring the quality of water used for heating and cooling purposes.

Also known as ‘the universal solvent’, water has many industrial uses of which dissolving solids is but one. Other uses include heating and cooling. Here, subject to its quality, the heating and cooling of the water will affect the condition and performance of pumps, valves, condensers, pipes and storage tanks etc.

Different industries have varying requirements where water quality is concerned but, for heating/cooling purposes, the parameters of most interest are as follows.

Turbidity: This is a measure of the concentration of suspended particles/solids which tend to be invisible to the naked eye, but makes the water cloudy. For instance, water sourced from rivers may contain suspended and abrasive clay particles that can wear components.

The most popular means of determining the water’s turbidity is simply to pass light (full spectrum or just IR) through it; and light-based turbidity probes are widely available. Photometers can also be used.

Salinity / conductivity: Both are measures of the presence of salts and other dissolved inorganic chemicals, and the biggest enemy is of course limescale. This forms when water – in which carbon dioxide (CO2) has reacted with calcium carbonate (CaCO3) to form soluble calcium hydrogencarbonate (Ca(HCO3)2) – is heated. Specifically, the calcium hydrogencarbonate breaks down to water, carbon dioxide and insoluble calcium carbonate (limescale). 

The presence of magnesium hydrogencarbonate (Mg(HCO3)2) in water also leads to limescale through the same mechanism. 

Limescale tends to localise around the hottest components in the system (e.g. a boiler matrix) and, because it has a very low thermal conductivity, it insulates the water from the heat source. Net result: the boiler works much harder.

For monitoring purposes, in-line, dual-purpose, flow-through sensors are proving popular in many industries. They measure conductivity and temperature, because the latter can have a bearing on the former. In addition, monitoring systems are available for the dual measurement of turbidity and conductivity. Dedicated salinity meters are available too, but they are optimised for detecting the presence of only sodium chloride (NaCL).

Dissolved Oxygen (DO): The corrosion of ferrous-based metals is largely caused through contact with water that has a high oxygen content; creating red iron oxide (2Fe2O3). In a closed system the DO content will reduce over time and only black iron oxide (Fe3O4) will be able to form; which protects against further corrosion.

DO monitoring systems are widely available with varying levels of accuracy. Generally, higher accuracy (μg/L) systems are used where even low DO levels are to be guarded against because of the value of the product and reliance on the heating/cooling system.

pH level: The non-alloyed ferrous-based components of a heating/cooling system can be particularly susceptible to acid corrosion (a.k.a. hydrogen corrosion). The presence of carbon dioxide in water, which (along with limescale) is created when calcium hydrogencarbonate breaks down, can lead to acid corrosion.

Many handheld meters are available for manually checking pH levels but, increasingly, in-line and real-time monitoring systems are being employed in water quality management scenarios.

Taking measure

Monitoring and controlling the parameters discussed above will help protect your heating/cooling system, but be mindful of how controlling one parameter may affect another. For example, using acids to combat the build-up of limescale can, if done to excess, result in acid corrosion. Accordingly, you need to take a holistic approach to water quality monitoring and it is worth working with sensor OEMs, such as HORIBA, who cover all technologies.