Effective water treatment
Published: 08 July, 2011
Carl Knight, Fulton Limited’s sales and marketing manager, looks at water treatment for steam raising plant, and the benefits of an effective water treatment programme.
Plant engineers use water for heat transfer because it’s abundant, cheap, easily stored and transported, has a high specific heat capacity and high latent heat of evaporation. But before water can be used for process applications, it must be treated to help prevent scale formation and corrosion in the boiler and to control the content of dissolved solids.
The water delivered by your local water board is treated to ensure that it is potable and free from harmful substances including bacteria. It typically contains 0.05 to 0.5% dissolved solids and can therefore be classified as 99.5% pure, but no guarantees are made regarding its suitability for process purposes, including the raising of steam. Preventing scale formation and corrosion in the boiler and controlling dissolved solids is therefore a crucially important consideration in a correctly designed water treatment programme.
However there are still many cases where steam boilers are becoming less efficient or even failing due to a lack of, or poorly applied, water treatment programme. Typically, scaling and corrosion can result in the mechanical failure of the boiler itself whilst foaming can contaminate the steam raised and have further consequences in the steam pipework and equipment. All of these factors have a common and predictable economic consequence, i.e. operating costs are increased.
So what should an engineer look for in a correctly designed water treatment programme?
Pre-treated (raw) mains water contains calcium bicarbonate which when heated, decomposes to form calcium carbonate (limescale). This is insoluble and forms a hard deposit on heat exchange surfaces.
Limescale’s thermal conductivity is approximately 6% of that of steel, so even small amounts of scale on a heat exchange surface will significantly reduce efficiency and result in increased temperatures on the heat source side of the exchanger. This will ultimately result in metal failure.
To control limescale formation, boiler manufacturers usually recommend one or more of the following processes:
Water softening removes hardness – in the form of calcium and magnesium –using ion exchange softening. Raw water is passed through a bed of ion exchange resin where the calcium and magnesium are replaced with sodium.
When the exchange capacity is exhausted, the resin is regenerated by passing it through a concentrated solution of brine to release the calcium and magnesium, which are then discharged to drain. The exchange sites on the resin are replenished with sodium derived from the brine.
While softening removes the scale forming tendency of water, the dissolved solids concentration remains similar to that of the raw water.
Reverse osmosis removes virtually all dissolved solids and is achieved by forcing the input water through a differentially permeable membrane at 18 to 20 bar pressure. Water molecules easily pass through the membrane, whilst salts dissolved in the water are discharged as a concentrate. This process removes 80% to 95% of the salts and typically results in 80% treated water.
In the deionisation process raw water is passed through two ion exchange resins. The first resin removes negatively charged ions (anions) including chloride, sulphate and bicarbonate and the second removes positively charged ions such as sodium, calcium and magnesium. Deionisation uses hydrochloric acid and sodium hydroxide (caustic soda) for regeneration of the resins.
Both reverse osmosis and deionisation processes produce water with a very low dissolved solids concentration, resulting in significantly reduced blowdown and therefore energy consumption.
The primary cause of corrosion in steam boilers is the presence of oxygen in the feed water which, if not removed, passes into the boiler water with predictable results. Other causes of corrosion include caustic ‘embrittlement’ and the action of dissimilar metals, both of which can result in the fatigue and consequent failure of metals.
Boiler manufacturers generally recommend that corrosion be controlled using the following techniques/processes:
Oxygen becomes less soluble in water as the temperature increases. It is therefore important that feed water is maintained at as high a temperature as possible in order to reduce oxygen concentration.
Any remaining oxygen can be removed using an oxygen scavenger (sodium sulphite, tannin or a variety of organic compounds). The choice of scavenger is determined by the type of boiler, boiler pressure, steam application and other parameters.
It is important that the scavenger is dosed to the feed water at a point that ensures complete reaction with the oxygen prior to the water entering the boiler. It is also important that the correct quantity is added to ensure that there is a reserve in the boiler at all times to maintain the treatment.
Maintenance of the correct concentration of dissolved solids in the boiler water is a factor often overlooked in boiler operation. Incorrect concentrations can have profound effects on steam quality and energy costs.
In order to minimise blowdown, the total of dissolved solids in the boiler water should be maintained as close as possible to the limit recommended by the boiler manufacturer. It is also important to recognise that, in the absence of a heat recovery system, water blown down is actually energy lost.
The use of either reverse osmosis or deionisation to treat the boiler’s make-up water will enable the blowdown to be reduced significantly, which in turn will reduce operating costs.
In summary, water treatment for steam boilers is an essential element of maintenance practice that must be monitored correctly to include appropriate pre-treatment of the make-up water. When applied properly, water treatment will prolong the life of your steam boiler, increase its energy efficiency and therefore reduce operating costs.
For further information please visit: www.fulton.com