Understanding Deaeration for Boiler Feedwater
To meet stringent industrial standards for oxygen content and allowable metal oxide levels in boiler feedwater, nearly complete oxygen removal is essential. This critical process is achieved through efficient mechanical deaeration, often supplemented by a precisely controlled oxygen scavenger.
Deaeration is governed by fundamental principles:
- The solubility of any gas in a liquid is directly proportional to the partial pressure of that gas at the liquid surface.
- Gas solubility decreases with increasing liquid temperature.
- The efficiency of gas removal is significantly enhanced when the liquid and gas are thoroughly mixed.
The primary purpose of a deaerator is to reduce dissolved gases, particularly oxygen and free carbon dioxide, to very low levels. Additionally, deaerators improve plant thermal efficiency by raising the water temperature, provide essential feedwater storage, and ensure proper suction conditions for boiler feedwater pumps.
Methods of Deaeration
Deaeration can be achieved through various mediums:
- Physical Mediums:
- Deaerating heaters (e.g., tray type, spray type)
- Vacuum deaerators
- Membrane contractors (increasingly utilized)
- Chemical Mediums:
- Oxygen scavengers (often used as a polishing treatment)
- Catalytic resins
Carbon dioxide is frequently removed using physical deaeration methods.
Pressure Deaerators
Pressure deaerators are broadly categorized into two main types: tray type and spray type. Both operate by heating water to its saturation temperature, causing dissolved gases to become insoluble and vent off.
Tray Type Deaerating Heaters
Tray type deaerating heaters consist of:
- A robust shell, typically constructed from low carbon steel.
- Spray nozzles, often made from corrosion-resistant stainless steel, to distribute and spray incoming water.
- A direct contact vent condenser.
- Multiple tray stacks.
- Protective interchamber walls.
Process Overview:
- Incoming water is sprayed into a steam atmosphere, where it is rapidly heated to within a few degrees of the steam's saturation temperature.
- Most non-condensable gases (primarily oxygen and free carbon dioxide) are released as the water is sprayed into the unit.
- Seals prevent recontamination of the water in the tray stack by gases from the spray section.
- Water cascades from tray to tray, breaking into fine droplets or films, which intimately contact the incoming steam.
- This intimate contact heats the water to the steam saturation temperature and removes the last traces of oxygen.
- Deaerated water then falls into the storage space below, where a steam blanket protects it from recontamination. It is usually stored in a separate tank.
Steam Flow Dynamics:
- Steam enters the deaerator through ports in the tray compartment and flows down through the tray stack, often parallel to the water flow.
- A small amount of steam condenses as the water temperature rises to saturation.
- The remaining steam vigorously scrubs the cascading water.
- Before exiting the tray compartment, the steam flows upward between the shell and interchamber walls to the spray section.
- Most of this steam condenses, becoming part of the deaerated water.
- A small portion of steam, laden with the released non-condensable gases, is continuously vented to the atmosphere. Adequate venting is crucial for complete deaeration.
- Steam flow through the tray stack can be cross-flow, counter-current, or co-current to the water.
Spray Type Deaerating Heaters
Spray type deaerating heaters comprise:
- A shell, typically low carbon steel.
- Spring-loaded inlet spray valves, often stainless steel.
- A direct contact vent condenser section, also stainless steel.
- A steam scrubber for final deaeration.
Process Overview:
- Incoming water is sprayed into a steam atmosphere and heated to within a few degrees of the steam's saturation temperature.
- Most non-condensable gases are released to the steam.
- The heated water falls through water seals and drains to the lowest section of the steam scrubber.
- In the scrubber, the water is thoroughly scrubbed by a large volume of steam and heated to the prevailing saturation temperature.
- As the water-steam mixture rises in the scrubber, the deaerated water's temperature slightly exceeds the saturation temperature due to a minor pressure loss. This induces a small amount of flashing, which aids in releasing dissolved gases.
- The deaerated water then overflows from the steam scrubber to the storage section below.
Steam Flow Dynamics:
- Steam enters the deaerator through a side chest and flows to the steam scrubber.
- After flowing through the scrubber, it passes upward into the spray heater section to heat the incoming water.
- Most of the steam condenses in the spray section, becoming part of the deaerated water.
- A small portion of the steam, containing the non-condensable gases, is continuously vented to the atmosphere.
Vacuum Deaeration
Vacuum deaeration is employed at temperatures below the atmospheric boiling point, primarily to reduce corrosion rates in water distribution systems.
Mechanism:
- A vacuum is applied to the system, bringing the water to its saturation temperature at the reduced pressure.
- Spray nozzles break the water into small particles, facilitating gas removal.
- Incoming water enters through spray nozzles and falls through columns packed with materials like Raschig rings or other synthetic packing.
- This process reduces water to thin films and droplets, promoting the release of dissolved gases.
- The released gases and water vapor are continuously removed by the vacuum, which is maintained by steam jet eductors or vacuum pumps, depending on the system size.
Efficiency: Vacuum deaerators generally remove oxygen less efficiently than pressure deaerators.
Corrosion Fatigue in Deaerators
Corrosion fatigue, particularly at or near welds, is a significant problem in deaerators. It results from a combination of mechanical factors and operational issues:
- Mechanical Factors: Manufacturing procedures, poor weld quality, and lack of stress-relieved welds.
- Operational Problems: Water hammer or steam hammer events can induce significant stress.
AquaChain Engineering Tip
Regularly inspect deaerator welds and internal surfaces for signs of corrosion fatigue, especially in areas prone to stress concentrations. Implement a robust preventative maintenance schedule that includes non-destructive testing (NDT) to detect early signs of cracking and ensure proper venting to prevent gas blanketing, which can exacerbate corrosion.
Frequently Asked Questions
Q1: Why is deaeration considered crucial for boiler feedwater? A1: Deaeration is crucial because it removes dissolved oxygen and carbon dioxide, which are highly corrosive to boiler components. By removing these gases, it prevents pitting corrosion, reduces the formation of metal oxides, and significantly extends the lifespan of the boiler and associated piping, while also improving thermal efficiency.
Q2: Can chemical oxygen scavengers completely replace mechanical deaeration? A2: No, chemical oxygen scavengers are typically used to supplement mechanical deaeration, not replace it. Mechanical deaerators remove the bulk of dissolved gases, bringing oxygen levels down to very low parts per billion (ppb) ranges. Chemical scavengers then "polish" the water, removing the last traces of oxygen that mechanical methods cannot eliminate, ensuring optimal protection.
Q3: What are the primary advantages of using a pressure deaerator over a vacuum deaerator? A3: Pressure deaerators generally achieve lower dissolved oxygen levels (often < 7 ppb or < 0.007 mg/L) compared to vacuum deaerators, making them more effective for high-pressure boiler systems. They also heat the feedwater, contributing to improved thermal efficiency, and provide a positive suction head for boiler feed pumps, which vacuum deaerators do not.