Make-up water to steam boilers should be treated with oxygen scavengers to avoid serious corrosion problems.
Corrosion is a major problem in steam systems with high consumption of make-up water. Fresh water contains dissolved oxygen. Oxygen together with high temperature is highly corrosive for the carbon steel piping used in steam systems.
To avoid corrosion the oxygen in the make-up water for saturated steam production should be removed by using some kind of scavenger chemicals and/or deaeration towers.
Some commonly used scavenger chemicals are
- Sodium Sulfite - common in low and medium pressure systems
- Hydrazine - common in high pressure systems
- Sodium erythorbate
Sodium Sulfite is a white crystalline or powder soluble in water. Sodium Sulfite decomposes on heating and is prepared from sulfur dioxide and sodium carbonate or caustic soda.
Sodium Sulfite is an effective scavenger for use in systems operating below 1000 psi. For pressures above 1000 psi breakdowns may form corrosive Hydrogen Sulfide and/or Sulfur Dioxide. Sodium Sulfite may also increase the amount of dissolved solids and the conductivity of the boiler water.
Hydrazine efficiently eliminates the residual oxygen by reacting with the oxygen forming water and gaseous nitrogen.
Since hydrazine is a toxic chemical it must not be used in steam plants supporting food processing systems (FDA - U.S. Food and Drug Administration).
Erythorbic acid and its sodium salt can replace Sodium sulfite and Hydrazine as oxygen scavengers. Sodium Erythorbate is a non toxic oxygen scavenger that can also be used in food processing systems.
Adding Scavenger Chemicals
For best performance the chemical should be added in the storage tank before the feed tank. The efficiency of a scavenger is increased with longer reaction time. Adding directly to the make-up water feed line is not recommended due to short reaction time.
The make-up water should always be fed to the feed tank through a deareation tower on the top of the tank. The high temperature in the tank reduces the ability of the water to keep oxygen (and gases in general) dissolved. The oxygen will be forced out of the water and vented away from the system.
Supercritical Units, Power Plants
Condensate Polishing is an important aspect to power plant chemistry to control contaminants that can lead to corrosion and deposition in downstream components. For supercritical power plants polishing is absolutely a critical component to ensure the integrity of steam turbines.
A modern power plant that employ condensate polishing has ferrous metallurgy benefits from the presence of oxygen in the feedwater. The corrosion rate of carbon steel is dramatically reduced when operating with a positive potential (dissolved oxygen > ~50 ppb) versus what can be obtained with fully dearated water.
Provided the feedwater is relatively contaminant free the oxide formed when oxygen is present (>50 ppb) is several orders of magnitude less soluble than the oxide formed under dearated, reducing conditions. Modern once-through supercritical units operates under an oxidizing treatment without the addition of an oxygen scavenger and in most cases with the addition of oxygen to the feedwater. During operation these units either operate with their deaerator vents closed or equipped with oxygen feeders at the deaerator outlets.