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INDUSTRY FOCUS WATER & WASTE TREATMENT THE BEST WAYS TO TREAT FEEDWATER


Feed your boiler carefully, or you just might kill it, says Mark Bosley, technical director at SUEZ Water Purification Systems. How improving the quality of the water used in your boiler can prolong its life...


S


team has been a vital industrial resource since the start of the 18th


Century. In many industries it remains as important today as it ever was, with countless uses including power generation, as a process fluid and for cleaning and sterilisation. Wherever it is used, steam starts life as liquid water. And the quality of that water can have a profound effect on the efficiency and longevity of boilers, pipework and other steam handling equipment. Water is an excellent solvent, dissolving


minerals and gases wherever it goes. Once it is heated inside a boiler, that activity accelerates. Feedwater goes to work dissolving the boiler itself through corrosion, or re-depositing solutes as scale inside the boiler chamber itself and internal heat transfer tubes. Corrosion leads to shorter equipment life and unpredictable failures, while scaling interferes with heat transfer and fluid flow, reducing efficiency. At the high pressures, temperatures and flow rates found inside a modern industrial boiler, those effects can happen very quickly. Putting tap water directly into an industrial high-pressure boiler is likely to result in failure in days, rather than years. Another side-effect of boiler operation


presents particular challenges for water quality. As some of the water inside a boiler is converted to steam, dissolved materials tend to stay in the remaining liquid water. That means solutes build up over time, and water that is purified to a certain level eventually becomes over concentrated in minerals. Once that stage is passed, operators have no choice but to “Blowdown” the boiler, i.e. discard a proportion of the boiler water and replenish with fresh feedwater. Blowdown is expensive and inconvenient: companies are essentially throwing expensive water and energy down the drain and paying for the subsequent clean-up costs. Managing the quality of boiler feedwater


requires a multi-pronged approach: removing contaminants as efficiently as possible before the water enters the boiler, then adding chemicals to control the nature of the water once it is in the system. Scale is caused by dissolved calcium and magnesium, which precipitate at high


28 SEPTEMBER 2019 | PROCESS & CONTROL


temperatures. One way to tackle it is by softening, in which both elements are replaced by sodium using a “base-ion- exchange” process. The sodium is usually provided by passing the water over a cation ion-exchange resin charged in the sodium form. Once all the sodium has been exchanged for the calcium and magnesium ions, the resin will then require regeneration to convert it back to the sodium form by using a brine solution. Softening is effective for many low- pressure boilers that only require soft water to operate but for high pressure boilers complete de-ionisation is required. For this type of application an alternative


approach is applied by the use of Reverse Osmosis (RO) to remove all dissolved solids from feedwater. Reverse osmosis utilises a semi-permeable membrane, under pressure, which rejects up to 98 percent of the incoming dissolved ions.


ELIMINATING SCALING & FOAMING This process eliminates the problems of scaling and foaming at a stroke, leading to higher cycles of concentration within the boiler which reduces the blowdown rate and improves energy efficiency. Demineralised water produced by RO can be an aggressive solvent, as it can be rich in carbon dioxide which depresses the natural pH of the water to around 4.5-5.0. RO will not reduce the dissolved oxygen content of the feedwater, therefore, the addition of chemicals to passivate the water, de-oxygenate it and protect equipment further upstream such as the steam distribution system, will be required.


Ensuring the quality of boiler feedwater requires thought, appropriate equipment and careful management


Corrosion is caused by the presence of


dissolved oxygen and by acidity. In boilers, oxygen corrosion tends to take the form of localised pitting, which is difficult to detect and can lead to catastrophic failure. Heating water and stripping it with steam in a hotwell or deaerator is a cost- effective way to reduce the oxygen content of water, but these processes leave some oxygen behind, so corrosion risks persist. Removing the remaining oxygen requires


the addition of chemicals. Sulphites do this job very well, but their use increases the undesirable level of dissolved solids in the water. Organic oxygen reducing chemicals avoid that problem, but their presence in the resulting steam precludes their use in certain applications. When managing the pH of boiler feed


SUEZ’s industrial water purification systems include reverse osmosis units


water, the objective is to create conditions that are alkaline, but only slightly so. Caustic soda (sodium hydroxide) is inexpensive, but it can dissolve iron, so excess concentrations can create their own corrosion problems. High levels of sodium hydroxide also increase the level of solids in the water and can cause sparingly soluble salts to precipitate into scale. In softened water boilers, the other salts buffer the caustic, reducing problems associated with its use. In demineralised systems, however, other chemicals such as phosphates or amines provide a safer alternative. By understanding the potential issues associated with the quality of boiler feedwater, and the available remedies and inherent trade-offs, companies can ensure they are giving their boilers the best chance of a long, healthy and efficient life.


SUEZ Water Purification Systems www.suezwatertechnologies.com


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