Almost all of our water, sourced from rivers, lakes and bore holes and delivered to our taps, is perhaps best described as a chemical soup. Gases from our atmosphere, such as carbon dioxide and oxygen, mineral-forming elements like calcium, magnesium and silicon, and metals such as iron and manganese, all swirl through every typical domestic, commercial and industrial supply.
For the boiler and steam-generating applications commonly found in industry and commerce, this chemical loading in feedwater can have a dramatic impact on performance. Poor control of water quality increases the likelihood of deposition, carryover, and corrosion.
For boiler operators, maintaining finely balanced water chemistry is, therefore, key to longevity, lower servicing costs and better thermal efficiencies for steam plant equipment. A wide range of chemicals are added to boiler feedwater to inhibit scale and corrosion, scavenge oxygen, balance the pH and mobilise sludge, for example.
THE RISKS
Water is an effective ionic solvent and it dissolves many of the materials it comes into contact with. Besides those already established, water also typically contains suspended solids and may even contain effluent from industrial, farming or water treatment processes (see also https://is.gd/pareci).
Water quality and the materials dissolved or suspended within it is also dependent on a host of environmental factors, such as the local geology, the temperature, and the pH. Furthermore, though groundwater is more consistent, other naturally-found water sources contain different materials whose proportion may change by season, and during storm events or dry spells.
The consequences of poor water quality control in boiler applications can be dramatic. Typical dissolved minerals, such as calcium carbonate from limestone, calcium sulphate from gypsum and magnesium sulphate salts, are relatively insoluble in water and tend to precipitate out during boiling. Furthermore, under higher temperatures, these minerals can precipitate out directly onto the metal of the boiler tubes as hard, insoluble, impervious scale. As the dissolved solids come out of solution, a mixture of different mineral deposits, rust and other contaminants, inevitably cause scale and sludge to build up.
As the scale deposits are often insulating materials, build-ups and blockages can cause overheating of boiler tubes and other elements. This in turn can increase the impact of thermal fatigue stresses and accelerate corrosion. Such deposits also restrict efficient heat transfer, reducing boiler efficiency and increasing fuel consumption.
Periodic blowdowns in boilers are a typical mechanism employed to reduce sludge, but because of the need to reheat incoming feedwater, it can be costly in terms of energy used. This needs to be balanced with the risk of carryover, which increases as deposits accumulate. It is extremely important to control the level of total dissolved solids within the boiler feedwater.
As Mark Bosley, technical director at SUEZ Water Purification Systems, explains: “Its non-negotiable. You really do have to get the right water chemistry otherwise your boiler is just going to fail at some point, which could be very expensive to replace.”
Higher concentrations of precipitated solids and other materials can cause carryover, in which the steam is contaminated with boiler water solids when bubbles or foam appear as these materials increase in concentration. Contaminants like oil or high loading of dissolved naturally-occuring organics, for example, can increase the propensity for foams to develop. They can be introduced into boiler feedwater by pumps or other hydraulic equipment or direct from the feedwater supply. This can be a particular problem in applications that use so-called ‘clean steam’, that needs to be sterilized or, in power generation applications where deposits on a turbine’s blades could be catastrophically damaging.
Reactive oxygen is also a significant issue for boiler operators. A glass of water at room temperature can contain around 10 ppm (parts per million) of dissolved oxygen. As the temperature increases, the solubility of oxygen decreases. In boiler conditions the availability of free oxygen is a gift to the gods of corrosion as carbonate scales can chemically degrade, leading to oxygen pitting. Oxygen micro-bubbles start to come out of solution and cause local corrosion at the surface of the metal boiler tube. The oxygen pitting eventually eats through the tube, producing holes and causes tube failure.
FEEDWATER TREATMENT
After being used in a typical boiler process, the steam is passed through a condenser to be reintroduced to the boiler feedwater, and any losses are made up with new water. This so-called make-up water is initially filtered to remove suspended solids, organisms and similar contaminants before a number of further process are undertaken to ensure it meets the required standard. Both the quantity of impurities and the nature of the impurities are key considerations in ensuring appropriate water quality standards are met, but factors such as the volume of make-up water required and the particular operating pressures and temperatures of the boiler concerned are also critical factors in determining boiler feedwater quality.
Indeed, high-pressure boilers have a different criteria for water quality compared to low-pressure boilers operating at/under 30 bar pressure. The chemistry of water is somewhat different in a boiler that’s running at pressures of hundreds of bar. In such cases, practically all impurities must be removed, typically using technologies such as reverse osmosis, deionisation and electrodeionisation. For low-pressure boilers, simple sodium-based water softeners will be sufficient in many applications.
A boiler will typically run through certain cycles of concentration. Sludge dispersant chemicals like hexametaphosphates, which keep minerals in solution, have to be added to keep the sludge moving, so that it can be expelled during blowdown operations.
The pH of unconditioned steam is around 4.4; it is quite acidic because of the amount of carbon dioxide that’s in the water. Poor pH balance can rapidly accelerate corrosion. Bosley explains: “If it’s not clean steam, it is typical to dose in pH conditioners like AME and hygrozyme, which are volatile and float around in the steam to neutralise the acidic conditions and provide a barrier of protection to the condensate pipework.”
With final polishing to remove materials like silica, it is possible to develop quite a big chemical process on boiler feedwater pre-treatment. However, in a typical small or medium enterprise that employs just a few dozen people, a large-scale and expensive water treatment facility is not a practical proposition.
Nonetheless, recognising the importance of maintaining good feedwater quality, a number of companies and trade bodies have launched training courses to ensure that boiler operators have the required skills. Earlier this year, Deep Water Blue in partnership with Human Focus International launched an online boiler water treatment course for shell boilers, coil boilers, steam generators, and hot water boilers, for example.
Many other companies also offer similar courses on industrial boiler water treatment designed for boiler plant operators, plant managers, maintenance and engineering contractors, and water treatment specialists.
Ultimately, boiler feedwater quality issues such as scaling, corrosion and deposition can affect the reliability and longevity of a boiler, pushing up the costs of servicing and maintenance. It can also impact on the quality of the steam, as well as result in higher fuel costs associated with lower thermal efficiency.
Operators of high-pressure boilers are well aware of the importance of good feedwater quality, but even in less demanding low-pressure applications, suitable feedwater treatment is critical to addressing scale, corrosion and deposition challenges in order to optimise costs and efficiency.
UPDATE: Some word and sentence changes were requested for this article after the magazine went to the printers. OE has made those changes in this digital version.