Gases such as O₂ and CO₂ are not removed during pretreatment. These are exactly the gases that destroy boilers.
Oxygen is the primary corrosion agent in steam generation systems, and even at concentrations on the order of ppb it is already damaging. Corrosion rate increases with oxygen content and rises significantly with temperature. Metal loss and failures will happen quickly if left uncontrolled.
CO₂ leaves the boiler with the steam, combines with condensate and forms H₂CO₃. Since the water is ultrapure, pH drops sharply, causing corrosion in steam and condensate lines. In plants with long condensate return systems, this mechanism accounts for a significant share of maintenance costs.
The simultaneous presence of O₂ and CO₂ has a synergistic effect: total corrosion will be greater than the sum of each acting alone. If removal of these gases isn't done properly, the result shows up as weld failures, pitting in piping, and unplanned shutdowns.
Mechanical Deaeration
Mechanical deaeration is based on Henry's law:
which states that the concentration C of a gas in the liquid is proportional to its partial pressure P. The constant kH depends on the gas, the liquid and the temperature.
For most gases dissolved in water (O₂, CO₂ and N₂), increasing temperature reduces kH, making the gas less soluble. In practice: higher temperature accelerates corrosion, but the effect of the reduced concentration is stronger, resulting in a lower overall corrosion rate.
To make gas removal easier, the deaerator increases surface area by breaking the body of water into droplets or thin films, which heat up faster and let the gas escape more easily. Dry steam is also injected, which, besides raising the temperature, shifts the chemical equilibrium toward removing the gases.
Only dissolved gases can be removed by mechanical deaeration. Ionized gases such as NH₄⁺, HCO₃⁻ and CO₃²⁻ are not removed. The gases released from the body of water still need to be physically carried out of the system through a constant steam vent to atmosphere.
In industrial practice, deaerators typically operate between 102°C and 110°C (pressure slightly above atmospheric), reducing dissolved O₂ to below 7 µg/L (7 ppb), as recommended by ASME for medium- and high-pressure boilers.
Chemical Deaeration
Since oxygen is quite reactive, mechanical deaeration is often followed by chemical deaeration, carried out by reducing agents (oxygen scavengers, usually amines) that remove residual oxygen after mechanical removal. Beyond that role, these scavengers can also act as passivating agents, forming a protective film on the metal surface.
Choosing a scavenger should account for: boiler pressure, whether or not dissolved solids are added, desuperheating, deaerator performance, and any required regulatory approvals (NSF, Kosher).
A critical point: the presence of a scavenger in the water doesn't necessarily mean the absence of oxygen. Measurement matters, and given the low levels expected, it needs to be done meticulously — any contamination will have a large impact on the result.
In modern high-purity systems, controlled oxygen is sometimes deliberately used to stabilize the passive layer and reduce FAC (Flow Assisted Corrosion), but that topic will be covered in another article.
The Cost of Inaction
Dissolved oxygen in feedwater, even at very low concentrations, is one of the leading causes of localized corrosion (pitting) in steam generation systems. The resulting iron deposits on heat exchange surfaces, reducing efficiency and accelerating wear.
A TÜV SÜD study assessed 99 industrial facilities with cracking and failures related to deaeration and corrosion. The average impact was USD 1.3 million per plant, while the preventive cost is only around €4,000 per plant. [TÜV SÜD, 2024]
Based on Kakama Wera's experience, the most common problems are: inadequate venting, dosing points in the wrong locations, poor sampling and measurement systems, and delta T outside the recommended range. A careful review of these and other points on the plant can bring substantial savings.
Next Steps
In Part II, we detail the types of deaerators, selection criteria, and the main operational control points.
References
- TÜV SÜD. Deaerators — Risk Management for Boiler and Machinery Engineering. 2024. Available at: tuvsud.com.
- ASME. Consensus on Operating Practices for the Control of Feedwater and Boiler Water Chemistry in Industrial and Institutional Boilers.