What is a Deaerator in Piping Engineering?

A Complete Guide to Purpose, Types, Design, and Real-World Applications

In the world of thermal power plants, boilers, and steam systems, one piece of equipment quietly plays a critical role in ensuring operational safety, efficiency, and longevity: the deaerator. While it may not always be visible on the plant tour, a deaerator is a powerhouse behind the scenes, protecting expensive assets like boilers and turbines from corrosion and damage.

This comprehensive guide dives into the function, types, components, and real-world importance of deaerators in piping engineering, especially within steam and power generation systems. Whether you're a piping engineer, plant operator, or just curious about industrial systems, this article will give you a solid understanding of how deaerator work and why they matter.

Why Is Deaeration Necessary in Steam Systems?

The Problem: Dissolved Gases in Boiler Feedwater

Water, especially untreated or naturally sourced water, contains dissolved gases like:

• Oxygen (O₂)
• Carbon dioxide (CO₂)
• Nitrogen (N₂)

Of these, oxygen and carbon dioxide are particularly harmful to steam systems. When these gases remain in the feedwater and enter the boiler, they cause internal corrosion of tubes, headers, drums, and piping. This corrosion not only weakens the system but also reduces the thermal efficiency of the boiler.

Did You Know? Even a small concentration of dissolved oxygen—just 1 part per million (ppm)—can cause pitting corrosion in boiler tubes, leading to catastrophic failure over time.

The Solution: Mechanical Deaeration

Enter the deaerator, a mechanical device that removes these dissolved gases from feedwater before it enters the boiler. This pre-treatment process significantly reduces corrosion and improves equipment life, safety, and efficiency.

What is a Deaerator? Definition and Basic Function

A deaerator is a pressure vessel or system used in thermal power plants to remove dissolved gases—mainly oxygen and carbon dioxide—from boiler feedwater.

The key principle of a deaerator lies in Henry’s Law: The solubility of a gas in a liquid decreases as the temperature of the liquid increases. By heating water close to saturation temperature, the deaerator encourages the dissolved gases to escape.

Core Functions:

• Heat the feedwater to near its saturation temperature
• Strip dissolved gases using steam
• Vent the released gases safely
• Store pre-heated feedwater for the boiler

In piping engineering, the deaerator is an integral part of the boiler feedwater system, and it plays a critical role in corrosion prevention and thermal performance.

Types of Deaerators in Piping Engineering

1. Tray-Type Deaerator (Vertical Deaerator)

This is the most common type used in large-scale power plants.

Working Principle:

• Water enters the system and is distributed through perforated trays.
• Low-pressure steam rises from the bottom and contacts the thin films of water flowing over the trays.
• The heat from the steam increases the water temperature, causing the dissolved gases to be released.
• Gases are vented out through the top, while deaerated water collects in a storage tank below.
  
  
  

Advantages:

• High efficiency in removing O₂ and CO₂
• Suitable for high-capacity applications

Real World Example:
At a 500 MW coal-fired thermal plant, a tray-type deaerator is used to handle nearly 100 tons/hour of feedwater. By maintaining water temperature close to 105°  C, the plant ensures almost complete removal of oxygen and a corrosion-free boiler life.

2. Spray-Type Deaerator (Horizontal Deaerator)

Spray-type deaerators are generally used in smaller or medium-capacity systems.

Working Principle:

• Water enters through spray nozzles which break it into fine droplets.
• These droplets come into direct contact with steam in a chamber.
• The fine mist ensures rapid heating and gas removal.
• Deaerated water settles in the tank below.
  
  
  

Advantages:

• Compact design
• Easier to install in limited spaces
• Lower initial cost

Use Case:
Spray-type deaerators are commonly found in industrial plants such as paper mills, textile processing, and chemical plants where feedwater demand is moderate but consistent.

Key Components of a Deaerator System

Understanding the design of a deaerator helps piping engineers integrate it efficiently within a plant layout. Let’s look at its key components:

1. Deaeration Chamber: The main zone where feedwater and steam interact. This chamber is often equipped with trays or spray nozzles to maximize surface area and heat transfer.
2. Storage Tank: Usually located below the chamber, this tank stores pre-heated, deaerated water which is then pumped to the boiler.
3. Vent: A critical outlet that allows the released oxygen and other non-condensable gases to escape. Proper venting ensures continuous deaeration.
4. Level Indicators and Pressure Gauges: Essential for monitoring the water level, pressure, and temperature inside the deaerator. Automation systems may include alarms for abnormal conditions.
5. Control Valves and Pumps: These regulate the feedwater flow and steam supply, maintaining optimal conditions for deaeration.

Deaerator Integration in Piping Engineering

• Elevation: Deaerators are usually placed at an elevation above the boiler feed pump to ensure positive suction head and avoid cavitation.
• Material Selection: Since the deaerator operates at high temperatures and handles corrosive gases, materials must be selected to resist thermal and chemical stresses.
• Piping Layout: Careful design of inlet and outlet piping, vent lines, and safety relief paths is crucial to system safety and efficiency.

Design Standards and Codes for Deaerators

• ASME Section VIII: For pressure vessel design
• ASME B31.1 or B31.3: For power and process piping integration
• API 614 or API 618: If integrated with rotating equipment or compressors
• NBIC: For inspection and maintenance standards

Common Issues and Troubleshooting in Deaerators

1. Poor Venting

If non-condensable gases are not vented properly, they accumulate and reduce deaeration efficiency, leading to increased oxygen levels.

Fix: Regularly check and clean vent orifices and ensure proper steam flow.

2. Temperature Drop

Lower-than-expected water temperature can indicate insufficient steam supply or heat transfer issues.

Fix: Check steam control valves, insulation, and deaerator internals for scaling or blockages.

3. Corrosion in Tank

Over time, inadequate oxygen removal can still cause minor corrosion inside the storage tank.

Fix: Combine mechanical deaeration with chemical treatment (oxygen scavengers) for complete protection.

Real-World Application: A Case Study

Project: 660 MW Supercritical Thermal Power Station
Challenge: High dissolved oxygen (DO) levels in feedwater causing pitting corrosion in boiler tubes.
Solution: Replaced undersized spray-type deaerator with a high-capacity tray-type deaerator.
Result:

• DO levels reduced from 0.3 ppm to <0.007 ppm
• Boiler uptime increased by 15%
• Feedwater pump cavitation incidents dropped by 90%

Combining Deaeration with Chemical Treatment

While mechanical deaeration removes 90–99% of dissolved gases, it is often supplemented with chemical oxygen scavengers like:

• Sodium sulfite (Na₂SO₃)
• Hydrazine (N₂H₄)
• Carbohydrazide

These chemicals react with the remaining traces of oxygen to form harmless by-products. Piping engineers must design dosing points, injection quills, and mixing systems to properly integrate this treatment into the feedwater system.

Conclusion: Why Every Piping Engineer Should Understand Deaerators

A deaerator might not look glamorous in a plant schematic, but its role is absolutely critical. By preventing corrosion, ensuring stable feedwater temperatures, and protecting capital equipment, deaerators save thousands (sometimes millions) of dollars in maintenance and downtime.

From piping layout to elevation planning and material selection, engineers must give this device the attention it deserves. Whether you're working on a greenfield project or a retrofitting job, understanding the function and design of deaerators is an essential skill for anyone in the field of piping engineering.

Final Thoughts and Industry Tips

Tip for New Engineers: Always verify the steam pressure and feedwater flowrate while selecting a deaerator.

Did You Know? A poorly vented deaerator can lead to the same level of corrosion as having no deaerator at all!

Maintenance Tip: Include the deaerator vent line in your regular inspection schedule—blockages here are silent killers.