A refinery steam system depends on more than boilers, steam drums, and steam headers. Before water ever reaches the boiler, another critical piece of equipment helps prepare it for high-temperature, high-pressure service: the deaerator.
A deaerator is a specialized feedwater vessel designed primarily to remove dissolved gases—especially oxygen and carbon dioxide—from boiler feedwater while also heating and storing the water before it is sent to the boiler.
The basic process is:
Condensate + Makeup Water → Deaerator → Deaerated Boiler Feedwater → Boiler Feedwater Pump → Boiler
Removing these gases matters because dissolved oxygen can aggressively attack boiler tubes, economizers, feedwater piping, and other components. Over time, even relatively small amounts of oxygen can contribute to serious corrosion and equipment failure.
For refinery workers, the deaerator is an important link between the condensate-return system and the steam boiler.
Figure 1. Refinery Deaerator System. Simplified flow diagram showing how condensate and makeup water are heated by steam to remove dissolved oxygen and other non-condensable gases, with hot deaerated water collected in the storage section and supplied to the boiler feedwater pumps.
What Is a Deaerator?
A deaerator is a pressure vessel used in boiler-feedwater systems to remove dissolved gases from water.
Water naturally contains dissolved gases. Oxygen is particularly undesirable in high-pressure steam systems because it can contribute to localized corrosion and pitting.
The deaerator uses two powerful tools to remove these gases:
Heat + Contact With Steam
As the incoming water is heated close to its saturation temperature, the solubility of dissolved gases decreases. Steam contacting the water helps strip those gases out.
The released gases are then discharged through a vent.
The result is hot, treated water ready for the next stage of the boiler-feedwater system.
Why Dissolved Oxygen Is a Problem
Oxygen and high-temperature metal surfaces are a bad combination.
When dissolved oxygen remains in boiler feedwater, it can attack carbon-steel surfaces and contribute to oxygen pitting.
Unlike uniform corrosion that gradually affects a broad area, pitting can create highly localized areas of metal loss.
A component may therefore appear relatively sound overall while having deep individual pits.
Potentially affected equipment includes:
- Boiler feedwater piping.
- Economizer tubes.
- Boiler tubes.
- Condensate systems.
- Feedwater pumps.
- Heat exchangers.
- Boiler drums and associated components.
Preventing oxygen from entering the high-pressure steam cycle is therefore much better than attempting to repair the damage later.
Where the Deaerator Fits in the Steam Cycle
The deaerator sits between the condensate/makeup-water system and the boiler-feedwater pumps.
A simplified refinery steam cycle looks like:
Boiler → Steam Header → Process User → Condensate → Condensate Return → Deaerator → Boiler Feedwater Pump → Economizer → Boiler
Not all steam returns as condensate.
Some steam may be consumed directly by the process, lost through vents and leaks, used for stripping, or otherwise unavailable for recovery.
That lost water must be replaced with treated makeup water.
The deaerator commonly receives both returning condensate and treated makeup water.
How a Deaerator Works
The operating principle is based heavily on temperature and gas solubility.
Cold water can contain dissolved gases.
As water temperature increases toward its saturation temperature at the deaerator’s operating pressure, its ability to retain dissolved gases decreases.
Steam then provides intimate contact with the incoming water.
A simplified sequence is:
Water Enters → Water Is Distributed → Steam Contacts Water → Water Heats → Dissolved Gases Release → Gases Vent → Hot Deaerated Water Collects
The deaerated water then flows into the storage section.
Step 1: Condensate and Makeup Water Enter
Returning condensate and treated makeup water enter the deaerator.
The exact inlet arrangement varies by design.
The incoming water must be distributed so that it can contact steam efficiently.
Simply dumping a large stream of water into a tank would not provide enough surface area for effective gas removal.
Step 2: Water Is Broken Into Smaller Streams
The deaerator spreads the incoming water into thin films, droplets, or small streams.
Depending on the design, this may be accomplished with:
- Spray nozzles.
- Trays.
- Distribution devices.
- Internal baffling.
Increasing the exposed water surface area allows steam to contact the water more effectively.
Step 3: Steam Contacts the Water
Steam enters the deaerator and contacts the incoming water.
The steam transfers heat into the water and helps strip dissolved gases from it.
As the feedwater approaches saturation temperature, oxygen and other non-condensable gases are released.
Step 4: Dissolved Gases Leave Through the Vent
The released gases travel toward the deaerator vent.
A small amount of steam normally accompanies the vented gases because maintaining proper venting is necessary for effective deaeration.
The vent must allow non-condensable gases to escape.
If they cannot leave, they can accumulate and reduce deaerator performance.
Step 5: Deaerated Water Enters Storage
After the water has been heated and stripped of dissolved gases, it collects in the storage section.
This creates a supply of hot boiler feedwater.
From there:
Deaerator Storage → Boiler Feedwater Pump → Economizer → Boiler
The boiler-feedwater pumps then raise the water pressure sufficiently to overcome downstream system pressure and deliver water into the boiler.
Major Components of a Deaerator
Deaerating Section
The upper portion contains the equipment responsible for bringing steam and water into close contact.
Depending on the design, this may contain trays, spray nozzles, or other distribution equipment.
Storage Tank
The lower vessel stores deaerated feedwater.
This provides an inventory of hot water for the boiler-feedwater pumps.
Steam Inlet
Steam enters the deaerator to heat the incoming water and strip dissolved gases.
Water Inlet
Condensate and makeup water enter through designated connections.
Vent
The vent allows oxygen, carbon dioxide, and other non-condensable gases to leave the system.
Overflow
An overflow connection protects the vessel from excessively high water level under applicable conditions.
Drain
The drain allows water to be removed from the vessel during shutdowns and maintenance according to facility procedures.
Level Instrumentation
Level transmitters, indicators, alarms, and control systems help maintain the required storage-water inventory.
Pressure and Temperature Instrumentation
Because deaeration depends strongly on pressure and temperature, these operating conditions are closely monitored.
Tray-Type Deaerators
One common industrial design uses internal trays.
Incoming water flows across or through multiple trays while steam moves through the deaerating section.
The trays break the water into thinner layers and increase the amount of surface exposed to steam.
The concept is:
Water Downward + Steam Contact → Gas Removal
Good steam-to-water contact is essential.
Damaged, displaced, or fouled trays can reduce deaerator effectiveness.
Spray-Type Deaerators
Another design uses spray nozzles.
Incoming water is sprayed into small droplets.
This dramatically increases surface area.
Steam heats the droplets, allowing dissolved gases to escape before the water enters the storage section.
Some systems combine spray and tray arrangements.
Why the Water Is Kept Hot
The deaerator doesn’t remove oxygen and then intentionally cool the water back down.
Keeping the feedwater hot provides several benefits.
Hot feedwater requires less additional energy inside the boiler to reach boiling conditions.
Maintaining appropriate temperature also helps prevent gases from readily dissolving back into the water.
The deaerator therefore serves as both a gas-removal device and feedwater heater.
Deaerator Pressure and Temperature
Pressure and temperature are directly connected.
A pressurized deaerator operates at a temperature corresponding closely to the saturation temperature at its operating pressure.
If the temperature is too low for the operating pressure, effective deaeration may not occur.
Operators therefore evaluate pressure and temperature together rather than treating them as unrelated readings.
The Importance of the Vent
The vent may appear to be a small part of a very large vessel, but it performs an essential function.
The deaerator is intentionally removing gases from water.
Those gases need an escape path.
If the vent is restricted or incorrectly operated, non-condensable gases can accumulate.
However, excessive venting can also waste steam and energy.
Proper vent operation therefore balances effective gas removal with controlled steam loss.
Boiler Feedwater Pumps
Large boiler-feedwater pumps are commonly located downstream of the deaerator.
These pumps must take suction from a supply of hot water.
This introduces another important consideration: Net Positive Suction Head, commonly called NPSH.
Hot water is closer to its boiling condition than cold water.
If pressure at the pump suction falls too low, some of the water can vaporize locally.
That can contribute to cavitation.
For this reason, deaerators are frequently installed at an elevated position so gravity provides additional static head at the boiler-feedwater pump suction.
The arrangement may look like:
Elevated Deaerator
↓
Vertical Suction Piping
↓
Boiler Feedwater Pump
That elevation is functional, not simply architectural.
Why Deaerators Are Often Installed High
Workers frequently notice deaerators mounted high on structural steel.
The elevation helps provide sufficient pressure at the suction of the boiler-feedwater pumps.
More vertical water column means greater static head.
This helps maintain adequate pump suction conditions while handling hot water near saturation.
Understanding this relationship explains why the deaerator and feedwater pumps may be physically separated by considerable elevation.
Common Deaerator Problems
Poor Oxygen Removal
If oxygen levels remain too high, possible causes may include insufficient steam, incorrect temperature, poor water distribution, damaged internals, venting problems, excessive flow, or abnormal operating conditions.
Vent Problems
A restricted vent can prevent gases from escaping.
Excessive venting can waste steam.
Spray-Nozzle Problems
Plugged or damaged spray nozzles can reduce water distribution and steam-to-water contact.
Damaged Trays
Tray-type deaerators depend on properly positioned internals.
Damaged trays can reduce effective gas removal.
Level-Control Problems
Incorrect water level can affect storage capacity and feedwater-pump operation.
Extremely low level can threaten boiler-feedwater pump suction.
Excessively high level can create other operating problems.
Corrosion
Ironically, the equipment designed to protect the steam system from corrosion can itself experience corrosion if chemistry or operating conditions are incorrect.
Vibration and Water Hammer
Steam and water interaction can create unstable conditions when systems are improperly operated or equipment is malfunctioning.
Unusual hammering or vibration should never simply be accepted as normal.
Deaerator Cracking and Mechanical Integrity
Deaerators operate under pressure and experience repeated thermal and mechanical stresses.
Certain designs and service histories have experienced cracking around welds and highly stressed areas.
Inspection programs may therefore include examination of:
- Vessel welds.
- Nozzle connections.
- Support attachments.
- Internal components.
- Storage-tank areas.
- Areas identified by engineering or inspection history.
Repairs must follow the facility’s applicable engineering, inspection, and pressure-vessel requirements.
Field Knowledge for Pipefitters
A deaerator system can involve several important piping services.
Pipefitters may encounter:
- Condensate-return piping.
- Makeup-water piping.
- Steam-supply piping.
- Boiler-feedwater suction piping.
- Vent piping.
- Overflow piping.
- Drain piping.
- Chemical-treatment connections.
- Instrument connections.
Special attention should be given to the boiler-feedwater pump suction piping.
Restrictions, incorrect routing, unnecessary pressure loss, or improper modifications can negatively affect pump suction conditions.
Never modify a feedwater suction arrangement simply because another routing appears easier to fabricate.
Thermal Expansion
The deaerator operates hot.
Connected piping can therefore experience significant thermal movement between shutdown and normal operation.
Supports, guides, anchors, spring hangers, expansion arrangements, and nozzle loading all matter.
Pipefitters should never force piping into alignment with deaerator nozzles.
The piping system must fit the equipment while respecting the engineered support and flexibility design.
Deaerator Safety
A deaerator contains hot pressurized water and steam.
That combination stores substantial thermal energy.
Potential hazards include:
- Pressurized steam.
- Hot boiler feedwater.
- Flashing liquid.
- Hot surfaces.
- Chemical-treatment exposure.
- Elevated work.
- Confined-space hazards during internal inspection.
- Heavy internal components.
- Stored pressure.
- Unexpected water release.
Before opening the vessel or connected piping, workers must follow facility isolation, depressurization, drainage, lockout/tagout, and confined-space procedures as applicable.
Never assume that water is harmless simply because it is water.
Hot pressurized water can rapidly flash into steam when pressure is released.
Troubleshooting Example
Suppose testing indicates elevated dissolved oxygen in boiler feedwater downstream of the deaerator.
The deaerator becomes an obvious area for investigation, but the equipment should not automatically be blamed.
Operators and maintenance personnel may investigate:
- Incoming water temperature.
- Steam supply.
- Deaerator pressure.
- Deaerator temperature.
- Vent operation.
- Water flow rate.
- Spray-nozzle condition.
- Tray condition.
- Instrument accuracy.
- Condensate-return conditions.
- Makeup-water rate.
The lesson is the same throughout refinery troubleshooting:
Understand the entire system before replacing equipment.
Important Deaerator Terminology
- Deaeration: Removal of dissolved gases from water.
- Boiler Feedwater (BFW): Treated water supplied to a boiler.
- Condensate: Water formed when steam gives up heat and condenses.
- Makeup Water: Treated replacement water added to compensate for water lost from the steam cycle.
- Non-Condensables: Gases such as oxygen that do not condense under the deaerator’s operating conditions.
- Oxygen Pitting: Localized corrosion associated with dissolved oxygen.
- Saturation Temperature: Temperature at which water boils at a particular pressure.
- NPSH: Net Positive Suction Head, an important consideration for pump suction performance.
- Cavitation: Formation and collapse of vapor bubbles caused by inadequate local pressure conditions.
- Vent: Connection allowing removed gases to leave the deaerator.
- Storage Section: Portion of the deaerator containing the inventory of treated boiler feedwater.
Field Rules
- Know why the deaerator is elevated. The elevation helps provide suction head to boiler-feedwater pumps.
- Protect the vent system. Removed gases need a controlled escape path.
- Remember that hot water can flash. Depressurizing hot feedwater can produce steam rapidly.
- Never force connected piping into alignment. Protect vessel nozzles from excessive loads.
- Watch boiler-feedwater pump suction piping carefully. Pressure loss matters.
- Temperature and pressure work together. Proper deaeration depends on both.
- Treat oxygen control seriously. Small concentrations can cause significant long-term damage.
- Trace the complete cycle. Condensate → deaerator → feedwater pump → boiler → steam → process → condensate.
Knowledge Check
- What is the primary purpose of a deaerator?
- Which dissolved gas is particularly damaging to boiler systems?
- Why does heating water help remove dissolved gases?
- What is the purpose of the deaerator vent?
- Why are spray nozzles or trays used?
- What happens to the deaerated water after it enters the storage section?
- Why are deaerators commonly installed at an elevated location?
- What is NPSH?
- Why can hot feedwater create cavitation concerns?
- What is oxygen pitting?
- What two major water streams commonly enter a deaerator?
- What major equipment is normally downstream of the deaerator?
Practical Exercise
Using a refinery steam-system drawing, locate the deaerator and trace the feedwater circuit.
Follow:
Condensate Return + Makeup Water → Deaerator → Boiler Feedwater Pump → Economizer → Boiler
Then identify:
- Steam entering the deaerator.
- Non-condensable gases leaving through the vent.
- The feedwater storage section.
- Boiler-feedwater pump suction piping.
- Overflow and drain connections.
- Relevant pressure, temperature, and level instrumentation.
Next, look at the physical elevation difference between the deaerator and boiler-feedwater pumps.
Ask yourself why that elevation exists.
Once you understand the relationship between temperature, pressure, gas removal, elevation, and pump suction, the entire deaerator system becomes much easier to understand.
The Big Picture
The deaerator may not receive the same attention as a massive distillation column or fired heater, but it protects one of the refinery’s most important utility systems.
Its mission is straightforward:
Heat the Water → Release Dissolved Gases → Vent the Gases → Store Hot Feedwater → Supply the Boiler
By removing oxygen and other unwanted gases before the water enters the boiler system, the deaerator helps protect feedwater piping, economizers, boiler components, and other equipment from corrosion.
And its elevated storage tank provides another critical benefit: helping deliver stable suction conditions to the boiler-feedwater pumps.
Understanding the deaerator means understanding that protecting a boiler starts before the water ever reaches the boiler.