Getting the best out of boilers

There are various issues that can affect the efficiency of hot water and steam boiler systems, as Mark Bosley, business support divisional manager at Purite, explains

Boilers can be expensive to run, especially when the levels of impurities in the feedwater are not taken into consideration or equipment not specified correctly to control them in the original design. Operating and maintenance costs can be significantly reduced by pretreating boiler feed water using some form of water treatment or chemical dosing, but to make the right specifications for your system you need to understand the different issues that can affect boiler efficiency whether they be for steam or hot water generation.

Low pressure boilers are generally used for supplying hot water typically for heating and it is these systems which are generally supported by ion-exchange softeners or chemical dosing systems.

All sources of water contain impurities which are harmful to boilers.

Common failures are due to the effects of scaling, corrosion and the build up of deposits or sludge.

Essentially, a low pressure boiler system is similar to a domestic kettle, mains water is heated in tubes that pass through the shell of the boiler furnace. Any hardness in the water - the result of rainwater coming into contact with chalk - will cause scaling of these boiler tubes, which coud lead to catastrophic boiler failure and, ultimately, costly maintenance or replacement. To remove hardness, water softening is applied, typically involving the use of an ion-exchange water softener to exchange the hardness-forming calcium and magnesium compounds with soluble sodium equivalents.

The effects of corrosion can take longer to manifest themselves. For corrosion to occur, dissolved oxygen must be present, eliminating this by dosing an oxygen scavenger is a cost effective measure.

For boilers that run at high pressures and are used to generate steam for power generation, for example, a far greater water purity above that of softened water is required. Water must be 'ultra-pure' in terms of certain critical impurities in order not to form deposits on the blades of the perfectly balanced turbines. In particular, the level of silica is a major concern in applications such as these and its presence may need to be limited to as little as 10 or 20ppb. Despite these tough challenges, high pressure systems can be supported by reverse osmosis water purification technology forming the core of the treatment process.

Reverse osmosis water purification systems are increasingly being used across many industries to provide exceptionally high levels of water purity. In a reverse osmosis system, pressurised feed water is passed through specialised semi-permeable membranes to remove inorganic ions and dissolved organic contaminants. The latest membrane elements used in reverse osmosis systems provide high levels of flow at lower operating pressures and their performance not only results in close on 100% purity but cuts the cost of purification since pump speeds, and thus energy demand, can be significantly lowered. Variable speed drives for purification units can cut costs yet again, as they enable the speed of each unit to be matched exactly to the output demands of the water treatment system.

To maximise the performance and longevity of the reverse osmosis system, it is necessary to consider pre-treatment; in fact, larger or high volume reverse osmosis systems often include a pre-treatment package designed to meet the characteristics of the feed-water. In essence, the pre-treatment stage is designed to maximise the operating life of the reverse osmosis membranes by removing contaminants that might otherwise foul, degrade or scale the membranes.

The effectiveness of a reverse osmosis membrane can be severely compromised by the accumulation of scale, organic contamination or biofilm and to fully optimise equipment, minimise running costs and extend operating life, the reverse osmosis system needs to be regularly cleaned. The period between cleans will depend on factors, such as incoming feed-water quality and the pretreatment used. However, with routine cleaning and maintenance, a reverse osmosis system will deliver consistent levels of performance, efficiency and reliability, preventing unexpected downtime and component failures.