Wet and Dry Preservation of Boilers: Methods, Importance, and Best Practices
Boilers are critical equipment in power plants, refineries, and industrial facilities, designed to generate high‑pressure steam at elevated temperatures. When a boiler is taken out of service for a longer period, special care must be taken to prevent corrosion of pressure parts. Boiler tubes corrode rapidly in the presence of oxygen and moisture, so preservation methods are essential to ensure safety, reliability, and long service life.
There are two primary methods of boiler preservation: Dry Preservation and Wet Preservation.
🔹 Why Boiler Preservation is Necessary
- Prevents corrosion and pitting in boiler tubes and pressure parts.
- Ensures long‑term reliability when boilers are idle.
- Reduces maintenance costs and downtime.
- Extends the lifespan of economisers, superheaters, and steam drums.
🔹 Dry Preservation of Boilers
Definition
Dry preservation involves keeping the boiler tubes moisture‑free during shutdown.
Process
- The boiler is completely drained of water.
- Dry air is circulated continuously through the empty boiler tubes.
- Moisture is eliminated to prevent corrosion.
Advantages
- Effective for short‑term shutdowns.
- Prevents moisture‑induced corrosion.
Limitations
- Difficult to maintain in modern boilers with complex designs and multiple bends.
- Not always practical for long‑term preservation.
🔹 Wet Preservation of Boilers
Definition
Wet preservation eliminates oxygen from the boiler tubes by filling them with treated water.
Process
- Boiler pressure parts (economiser, steam drum, water wall, and superheater) are filled with feedwater containing high concentration of hydrazine (200 ppm).
- Hydrazine ensures no dissolved oxygen remains in the feedwater.
- The boiler is kept under pressure so that atmospheric air cannot enter the pressure parts.
Advantages
- More effective and easier than dry preservation.
- Suitable for long‑term shutdowns.
- Prevents both oxygen corrosion and moisture damage.
Limitations
- Requires chemical treatment (hydrazine or equivalent oxygen scavenger).
- Needs careful monitoring of water chemistry.
🔹 Wet vs Dry Preservation: Comparison
📌 Key Takeaways
- Dry Preservation keeps boiler tubes moisture‑free using dry air circulation.
- Wet Preservation eliminates oxygen using hydrazine‑treated feedwater under pressure.
- Wet preservation is preferred in modern boilers due to complex tube designs.
- Proper preservation ensures safety, efficiency, and extended boiler life.