Oxygen in boiler water
Why should you worry about oxygen in boiler water? Primarily because oxygen is a significant factor in the corrosion of metals. Accordingly, if you can remove the oxygen from water you can reduce the negative effects of corrosion. Boiler water oxygen scavengers do just that; they are products that can be added to the boiler water that help to remove oxygen and so reduce the problems associated with corrosion.
There are several types of oxygen scavenger: organic and inorganic, the difference being that inorganic products add dissolved solids to the water. This means that you have to increase the amount of ‘blowdown’ given to the boiler to maintain conditions free from boiler water carryover.
Also, inorganic oxygen scavengers tend to be temperature dependent and so the lower the boiler feed water temperature the more chemical you need to add. An example of an inorganic oxygen scavenger is sulphite, whereas examples of organic oxygen scavengers are tannin, DEHA, and carbohydrazide.
One method used to reduce the amount of oxygen in boiler feed water is to maintain as high a temperature as possible. This is because increasing the water temperature can drive off the oxygen in gaseous form. It is therefore essential to ensure good feed tank design, and that as much condensate as possible is returned to the system.
The use of condensate is preferable for heating the feed tank rather than a steam sparge or external heat source because it is a recycled heat source and so helps to conserve energy and reduce operating costs.
Advantages to the use of sulphite are that the higher the feed water temperature the less sulphite is required, its low cost and it is colourless as a product. One disadvantage is that sulphite adds to the ‘dissolved solids’ and so leads to more system ‘blowdowns’.
Tannin, DEHA and carbohydrazide
Tannins, DEHA and carbohydrazide are not temperature dependent and do not add to the ‘dissolved solids’ and hence blowdown is reduced relative to a sulphite treated boiler, however, these products are slightly more expensive than sulphite.
It is important to calculate the dosage rate for all types of chemical product and to examine the overall system design before choosing the most suitable oxygen scavenger.
Accepta have a number of highly effective, technically advanced boiler products that act to absorb oxygen from boiler water and so help prevent corrosion. For more information … boiler water oxygen scavengers
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