Boiler feedwater

It was general practice to install zinc plates and/or alkaline chemicals to reduce corrosion within the boiler.

Many tests had been performed to determine the cause (and possible protection) from corrosion in boilers using distilled water, various chemicals, and sacrificial metals.

Use of lime for alkalinity control was mentioned as early as 1900, and was used by the French and British Navies until about 1935.

[2] In modern boilers, treatment of feedwater is critical, as problems result from using untreated water in extreme pressure and temperature environments.

This includes lower efficiency in terms of heat transfer, overheating, damage, and costly cleaning.

Boiler water is treated to prevent scaling, corrosion, foaming, and priming.

This is a problem because scale does not transfer heat very well and causes the tubes to fail by getting too hot.

Internal treatment protects against feed water hardness by preventing precipitating of scale on the boiler tubes.

This treatment also protects against concentrations of dissolved and suspended solids in the feed water without priming or foaming.

These treatment chemicals also help with the alkalinity of the feed water making it more of a base to help protect against boiler corrosion.

In deaerating heaters, steam sprays incoming feed water and carries away the dissolved gases.

This means of mechanical deaeration is used with chemical oxygen scavenging agents to increase efficiency.

With tray type heaters the incoming water is sprayed into steam atmosphere to reach saturation temperature.

(Sendelbach, p. 130) The feedwater must be specially treated to avoid problems in the boiler and downstream systems.

Additionally, feed water is typically alkalized to a pH of 9.0 or higher, to reduce oxidation and to support the formation of a stable layer of magnetite on the water-side surface of the boiler, protecting the material underneath from further corrosion.

This is usually done by dosing alkaline agents into the feed water, such as sodium hydroxide (caustic soda) or ammonia.

Any non-volatile salts and minerals that will remain when the feedwater is evaporated must be removed, because they will become concentrated in the liquid phase and require excessive "blow-down" (draining) to prevent the formation of solid precipitates.