boiler water conditioning

Water treatment and conditioning must meet the following three priority targets :

• maintaining heat exchange by preventing scaling and the formation of deposits on walls
• protection against corrosion;
• quality of the vapour produced.

Water treatment units are responsible for purifying and de-aerating make-up water or feed water. Conditioning is the essential adjunct to a treatment programme and includes the rational injection of «conditioning products» via dispensing units.

Of the most frequently used products, we can quote :

• phosphates: combined with the alkaline nature of boiler water, phosphates neutralize hardness by forming tricalcium phosphate, an insoluble compound that can be dispersed before being removed through floor blowdown or through continuous blowdown. In systems where the feed water is less than 1° TH, phosphates are used to regulate alkalinity or boiler water pH and, therefore, to avoid acid or alkaline corrosion of boiler internal walls. Phosphates are often combined with natural and/or synthetic dispersants;

• natural polymers: lignosulphonates, tannins;

• synthetic polymers: polyacrylates, copolymer acrylate/styrene-sulphonates that also act as chelating agents;

• chelating agents: such as EDTA used to maintain impurities in solution;

• organic phosphates that inhibit with a threshold effect;

• oxygen reducers: sodium sulphite, bisulphites, tannins, hydrazine, carbohydrazine, hydroquinone, hydroxylamine derivatives (DEHA) etc. These reducing agents, whether or not catalysed, are used for oxide retrogradation (e.g. Fe₂O₃ → Fe₃O₄), reducing dissolved oxygen and, in some cases, passivating metal surfaces. The product selected and the amount used will vary depending on whether or not thermal de-aeration is available and on the use made of vapour (some products such as hydrazine are banned from food industries);

• anti-foaming or anti-carry-over products, surfactant mixtures that alter the liquid’s surface tension and eliminate foam, thereby reducing the quantity of fine water droplets carried in the vapour (carry-over).