Question

Boilers are used to convert water to steam in electric power plants. Dissolved oxygen in the water promotes corrosion of the steel used in boiler parts. Explain how dissolved oxygen is removed from the water in boilers.

Short answer

Remove dissolved oxygen using heating and mechanical venting in a deaerator, and add oxygen scavenger chemicals if needed.

Step-by-step solution

- Overview of the Problem

Understand that dissolved oxygen can cause corrosion in boiler parts made of steel, which is a significant issue in power plants.

- Deaeration Process

Deaeration is the primary process used to remove dissolved gases, including oxygen, from boiler feedwater. This involves heating the water to reduce the solubility of gases and then mechanically venting them.

- Applying Heat

Heat the water in the deaerator to near its boiling point. The high temperature decreases the solubility of dissolved gases, causing oxygen and other gases to come out of the solution.

- Mechanical Venting

Use a venting system to physically remove the released gases. The deaerator has spray nozzles and trays to maximize the surface area between water and steam, facilitating the escape of dissolved gases.

- Chemical Deaeration

In some cases, a chemical treatment is used in addition to mechanical deaeration. Commonly, an oxygen scavenger like sodium sulfite is added to the water to react with and remove any remaining dissolved oxygen.

- Monitoring and Maintenance

Regular monitoring of the dissolved oxygen levels and maintenance of the deaeration system are essential for effective corrosion control in boilers.

Key concepts

dissolved oxygen

Dissolved oxygen (DO) is oxygen that is mixed into water molecules. In a boiler, this dissolved oxygen can cause serious problems, such as corrosion. Corrosion is the gradual destruction of materials, usually metals, by chemical reactions with their environment. When dissolved oxygen is present in the water, it reacts with the steel in the boiler parts, forming oxides or rust. This weakens and damages the metal over time. Understanding dissolved oxygen is the first step in effectively managing and preventing boiler corrosion.

High levels of dissolved oxygen are detrimental in power plant operations. Thus, various methods are employed to eliminate it from boiler feedwater to protect the equipment and ensure its longevity.

corrosion prevention

Preventing corrosion in a boiler is vital to maintaining its efficiency and longevity. Corrosion, often caused by dissolved oxygen in the water, damages boiler components. To prevent this damage, several strategies are used:

• Deaeration: This process reduces the amount of dissolved oxygen and other gases in water.
• Chemical Treatment: Adding specific chemicals to react with and remove dissolved oxygen.
• Regular Monitoring: Continuously checking dissolved oxygen levels to ensure they remain low.
• Maintaining Deaeration Systems: Keeping the deaeration system in good working order to ensure its effectiveness.

Incorporating these strategies can significantly reduce the risk of corrosion, protecting the boiler and extending its operational life.

Maintenance and consistent monitoring are crucial in catching early signs of corrosion, allowing for timely interventions to prevent further damage.

deaeration

Deaeration is a key process for removing dissolved oxygen from boiler feedwater. This technique utilizes heat and mechanical methods to strip gases from the water. The process involves several critical steps:

• Applying Heat: Heating water to nearly its boiling point reduces gas solubility, causing gases to escape.
• Mechanical Venting: Utilizing spray nozzles and trays inside a deaerator to enhance gas removal by maximizing water and steam surface contact.

By achieving near-zero dissolved oxygen levels through deaeration, the risk of corrosion is significantly minimized.

The deaeration process is essential for preventing gas-induced damage in power plant boilers. Ensuring proper operation and maintenance of the deaeration system guarantees effective removal of dissolved gases.

chemical treatment

Chemical treatment is often used in conjunction with mechanical deaeration to remove any remaining dissolved oxygen. This method involves adding chemicals to the boiler feedwater that react with the oxygen, forming harmless compounds. One common chemical used is an oxygen scavenger like sodium sulfite. Sodium sulfite reacts with dissolved oxygen to form sodium sulfate, which does not cause corrosion.

• Adding Oxygen Scavengers: Chemicals such as sodium sulfite and hydrazine are used to neutralize oxygen.
• Ensuring Proper Dosage: Correct dosing is critical for the chemicals to effectively remove dissolved oxygen.

Chemical treatment provides an additional layer of protection, ensuring that any dissolved oxygen missed by mechanical methods is also addressed.

Using chemical treatments effectively requires regular monitoring of water chemistry and adjustments of chemical dosages to ensure optimal results. This multi-faceted approach keeps the boiler system running efficiently and safely.