FEATURE BOILERS, BURNERS & CONTROLS
RETURN TO STEAM AGE? S
Steam is a vital resource in industry and as such boilers must run as reliably and efficiently as possible, explains Mark Bosley, technical manager at SUEZ Treatment Solutions UK
team is a critical utility in a wide range of industrial applications. The efficiency and reliability and thus the operating cost of any boiler depend on the quality of the water supplied to it. Modern boilers work at higher efficiencies (stress levels) so more attention must be paid to their ongoing performance. Heat transfer rates tend to be much higher irrespective of pressure so like an F1 racing car in comparison to a Model T Ford the process fluids need to be more highly specified and constantly managed and monitored. Water should be seen as a process fluid and managed with the same attention to detail as any other. It is an excellent solvent, dissolving minerals and gases in nature and when heated (as in a boiler), starts dissolving the boiler (corrosion) or re-depositing some of those solutes to cause scaling. Corrosion leads to shorter life and unpredictable failure while scaling interferes with heat transfer and fluid flow, reducing efficiency. Scaling in your kettle is a minor annoyance but in an industrial boiler it can lead to catastrophic failure. Putting tap water directly into an industrial boiler is likely to result in failure in days rather than years. The answer in each case is to remove the contaminants as efficiently and as early as possible with equipment-based technologies and add chemicals that control the nature of the water in the chosen application.
SCALE CONTROL Scale is caused by the presence of calcium and magnesium in the water which precipitate at high temperatures. No chemistry can prevent this sufficiently in a boiler so they must be removed externally. Softening is a process where these are replaced with sodium (base- exchange where brine is used to regenerate a resin which exchanges the calcium and magnesium for sodium). This is effective although no process is 100% efficient and chemicals need to be added to the boiler to ensure performance. Softening increases the total solids dissolved in the water and as evaporation occurs the levels increase further such that they can precipitate. To avoid this, water is thrown away (purge or blowdown) to limit the solids. High solids can cause a form of scaling in addition to foaming and loss of level
control resulting in boiler water carry over into the steam main (it is designed for gaseous steam) resulting in damage to the steam main pipework. An alternative pre-treatment is to
remove all of the dissolved solids by processes including demineralisation or reverse osmosis (RO). This solves scaling and foaming, reduces blowdown and thereby improves energy efficiency. Perversely, all these treatments make the water more corrosive. Demineralised water is an extremely aggressive solvent so chemicals need to be added to passivate this property. Demineralisation involves the use of cation and anion ion- exchange resins. These exchange cations for hydrogen, anions for hydroxide and the hydrogen and hydroxide combine to form H2O. Reverse osmosis utilises a semi-permeable membrane under pressure which rejects up to 96-98% of the incoming dissolved ions.
CORROSION CONTROL Corrosion is caused by oxygen and incorrect pH. The oxygen in the water can be most cost-effectively reduced by heating and stripping with steam in a hotwell or deaerator but this still leaves too much to avoid corrosion so chemicals are needed. Sulphite is widely used to scavenge oxygen but this contributes to the level of dissolved solids leading to increased blowdown. Organic oxygen
24 DECEMBER 2015/JANUARY 2016 | FACTORY EQUIPMENT
Understanding and keeping an eye on critical areas of potential failure can help to maintain boiler efficiency and reduce downtime due to corrosion and scaling
reducing chemicals can be used and do not increase solids but require careful consideration depending on the use of the steam. Oxygen tends to cause localised pitting so there is little or no advance warning of failure. Monitoring and good control of excess chemical (oxygen scavenger) is the best defence. The pH needs to be alkaline to
minimise corrosion but not with too much sodium hydroxide because this can dissolve (corrode) iron, again increasing the level of solids in the boiler water and increase the potential to precipitate sparingly soluble salts.
CONTROL MAGNESIUM AND CALCIUM In softened water boilers the other salts buffer the caustic and it is useful to control magnesium and calcium with treatment chemicals. In a demineralised water boiler caustic is not required and the alkaline conditions can be most safely provided with chemicals including phosphates and/or amines. Understanding and keeping an eye on
these critical areas of potential failure through pro-active investment, management and monitoring can help to maintain boiler efficiency and reduce downtime due to corrosion and scaling.
SUEZ Treatment Solutions UK T: 0191 491 4274
www.degremont-industry.co.uk
/ FACTORYEQUIPMENT
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