Boiler Feedwater (bfw)

Introduction

Boiler feedwater ("BFW") may either be the same as the makeup water, or may consist of returned steam condensate, or (as in most cases) will be a mixture of both.

The relative amounts of makeup and condensate may vary. A typical figure is 5% makeup with 95% condensate, but this depends on how the steam plant is operated and how much of the steam is condensed and recovered for recirculation.

In order to be non-scaling, the BFW must be a softened or demineralized water . In order to be non-corrosive to the carbon-steel or low-alloy steel components from which boilers are usually constructed, the water must be thoroughly deaerated.

This is partially achieved by thermomechanical means (use of deaeration heating tanks), while the last traces of dissolved oxygen are removed by chemical agents ("oxygen scavengers").

See also

Steam Condensate

Makeup Water

Water Treatment

General

Oxygen scavengers include both volatile products (e.g., hydrazine, or other organic products like carbohydrazine, hydroquinone, diethylhydroxyethanol, methylethylketoxime, etc.) and non-volatile salts (normally: sodium sulfite, Na2SO3, or a derivative thereof). The latter salts often contain catalyzing compounds to increase of rate of reaction with dissolved oxygen (e.g., cobaltous chloride).

Oxygen Scavengers

While the oxygen scavenging salts tend to react rapidly with oxygen, even at lower temperatures, their reaction products result in increased levels of total dissolved solids in the boiler feedwater.

To minimize this disadvantage, the salt should be fed to the storage tank of the deaerating heater and care should be taken that no precipitates are introduced into the boiler feed.

As rule of thumb, one typically feeds 10 parts of sodium sulfite per part of oxygen (to compensate for reaction with atmospheric oxygen and for impurities in the sodium sulfate).

Volatile oxygen scavengers are normally employed for higher pressure systems (e.g., above 1000 psig). These materials may react directly with oxygen and/or directly with carbon steel boiler surfaces to form gamma iron oxide or magnetite. The formation of such "oxygen-impervious" oxide films precludes excessive corrosion.

Except for the common hydrazine (N2H4), most other volatile organic oxygen scavangers are sold under some proprietary name. The use of hydrazine in boiler feedwater is very common and very well documented. Because hydrazine typically does not react with oxygen at a rapid rate at lower temperatures, it may not be the preferred oxygen scavenger for low temperature boiler feedwater systems. In addition, hydrazine is seriously toxic and great care should be taken in its handling.