Deaeration of Boiler Feedwater

Pour yourself a tall glass of cold water. Place it in front of you and read on.

The water you have just poured for yourself is much like the feedwater you may be sending directly into your boiler. 

It contains among other things, dissolved gases such as oxygen and carbon dioxide that can be particularly destructive to feed lines, condensers and to your boiler.

The oxygen in this raw feedwater is released within the boiler as a result of heat and rises in the form of bubbles. These bubbles attach themselves to the boiler tubes, water legs and the sides of the boiler drum shell at the water line.

The oxygen along with the carbon dioxide attacks the iron and set up chemical musical chairs in which the steel in your system will always lose. This destructive game will continue until either all the oxygen is entirely removed from the water or the steel or iron is dissolved.

A deaerator will prevent the game from ever starting. This piece of equipment removes corrosive gases from boiler feedwater and preheats the water prior to entrance into the boiler.

A deaerator should be considered if any of the following situations occur:

Your boiler plant operates at 75 psig or greater.
Your boiler plant has limited standby capacity.
Production depends on your continuous boiler operation.
Your boiler plant uses 25% or more cold water makeup.

Now take a good look at that glass of water you poured earlier. Those little bubbles that have formed on the inside of the glass are just what we have been describing. Imagine the inside of your boiler system with high temperatures and high pressures. If you don't have a deaerator, maybe it is time to consider one.

COMMON WATER IMPURITIES

Impurity Source Effect
Algae organic growth fouling
Calcium mineral deposits scale
Carbon dioxide dissolved gases corrosion
Chloride mineral deposits corrosion
Free acids Indus. Wastes corrosion
Hardness mineral deposits scale
Magnesium mineral deposits scale
Oxygen dissolved gases corrosion
Silica mineral deposits scale
Suspended solids undissolved matter fouling/scale

There are five major problems directly associated with water quality that will effect boiler performance. These are:

Scale formation
Corrosion
Fouling
Foaming
Embrillement

SCALE is a very hard substance that adheres directly to heating surfaces forming a layer of insulation. This layer of insulation will decrease heat transfer efficiency. Scale also results in metal fatigue/failure from overheating, energy waste, high maintenance costs and unnecessary safety risks. A one-sixteenth inch thickness of scale in a firetube boiler can result in a 12.5% increase in fuel consumption.

CORROSIONis defined as the destruction of a metal by chemical or electromechanical reaction with its environment. The metal is eaten away in much the same manner as fender rusts on a car. Corrosion dramatically increases maintenance costs and can cause unnecessary safety risks. It will occur when levels of oxygen or carbon dioxide are high, where pH values are low, where contact occurs between dissimilar metals and in damp environment or corrosive atmospheres.

Corrosion is an electrochemical process in which electricity flows through a solution of ions between areas of metal. Deterioration occurs when the current leaves the negatively charged metal or anode and travels through the solution to the positively charged metal or cathode, completing an electrical circuit in much the same manner as a battery cell. The anode and the cathode can be different metals or areas of the same metal. Corrosion occurs when there is a difference in the electrical potential between them. 

FOULING occurs when a restriction develops in piping and equipment passageways and results in inefficient water flow. The fouling of boiler room equipment directly impacts energy efficiencies and cost of operations.

FOAMING is a condition in which concentrations of soluble salts, aggravated by grease, suspended solids or organic matter, create frothy bubbles or foam in the steam space of a boiler. When these bubbles collapse it creates a liquid that is carried over into the steam system. Foaming degrades steam quality and in some cases can create a water slug that is discharged into the steam lines.

CAUSTIC EMBRITTLEMENTwill occur when there is a high concentration of alkaline salts (a pH value of 11 or greater) that will liberate hydrogen absorbed by the iron in the steel. Caustic embrittlement will be more evident in high temperature areas of the boiler's waterside and manifests itself in the form of hairline cracks.

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