Wet and Dry Storage Methods

Boilers are important components in many industrial settings and ensuring their longevity and efficiency requires proper maintenance, even during out-of-service periods. To prevent corrosion and other forms of damage when a boiler is offline, it is important to protect both the fireside and waterside of the boiler. There are two methods in achieving this – the dry method and the wet method.

Dry Storage

When a boiler is expected to be out of operation for an extended period of time and needs to be ready for service quickly, dry storage is advised. The dry storage method for industrial boilers involves draining, thoroughly cleaning, drying, and sealing the boiler to prevent moisture and air entry during extended periods of inactivity.

NOTE: If you are placing your boiler in storage for less than one month, follow steps 1 and 2 only. Be sure to follow your company guidelines and, at a minimum, label it as an oxygen-free atmosphere.

The following general procedure is advised for placing a unit into dry storage:

1. Shut down and secure the boiler. Once the pressure decreases to 20 psig, drain the boiler and header valves under air. Once the pressure is reduced to 0, open the drums to allow air to circulate and facilitate the drying of all internal surfaces.

2. If the unit is cold and filled with water, drain it under air, blow out non-drainable tubes with compressed air, and use external heat sources to aid in drying internal surfaces. Install non-porous trays with moisture absorbent (preferably silica gel) into the drums, ensuring they don't touch the drum's metal surface. Use at least one pound of absorbent per 1,000 pounds per hour of steam capacity.

3. Attach nitrogen to the steam drum vent, close all other vents, and pressurize the boiler to reach 10-15 psig with nitrogen, adjusting the amount based on the unit's volume.

4. With the boiler and superheater pressurized, alternately open all drains to purge air until the pressure drops to zero, repeating as needed to minimize oxygen levels.

5. The unit should now be stored with a nitrogen pressure of 5-10 psig in the steam drum. To maintain this pressure, ensure all connections and valves are sealed or tightly closed, and check the gas pressure daily for proper protection.

6. Boiler inspections are recommended every 3 months to ensure no corrosion is occurring and to replenish the absorbent as needed. During these inspections, air will enter the unit so you will need to repeat steps 3 and 4 to expel the air.

Wet Method

Wet storage prevents corrosion by keeping the boiler fully submerged with the right chemicals. Volatile chemicals are recommended to avoid increasing dissolved solids in the water.

The following general procedure is advised for placing a unit into wet storage:

1. Fill the boiler with deaerated, demineralized water treated with 200 ppm hydrazine and enough ammonia to reach a pH of 10.

2. Pre-mixing chemicals with water is recommended, using the blend-fill method to ensure uniform distribution in the boiler. Simply adding chemicals through the drum manhead may not adequately disperse them unless the boiler is heated to induce circulation.

3. Fill the unit with treated, demineralized water up to the normal centerline of the steam drum, then stop the filling process.

4. Refill the superheater with treated demineralized water until the steam drum level rises. Keep filling until water starts to flow from the steam drum vents. Once the filling is complete, make sure all connections are securely sealed.

5. Attach a low-pressure nitrogen source to the steam drum to maintain 5 to 10 psig and prevent air from entering the unit during storage.

6. When the boiler is ready for use, drain the superheater and the boiler drum to the normal start-up water level and resume operation.

7. Analyze the treated demineralized water weekly and add chemicals as needed through the chemical feed line to maintain recommended levels. Water samples can be collected from the continuous blowdown line or a suitable drain.

8. A boiler should not be stored wet if there is any risk of temperatures dropping to freezing, unless adequate heating is provided. Adequate heating will prevent water from freezing and avoid damage to the pressure parts.

Protection of External Surfaces:

Protection of external surfaces is also necessary during periods of extended downtime. Fireside corrosion in idle boilers can cause damage to components due to sulfuric acid from flue dust and ash. To mitigate this, remove deposits using air blasting, washing, scraping, or brushing, and neutralize any residual acid with alkaline water until the water is fully neutralized. Once flushing is complete, fill the boiler with treated water and fire it with low sulfur fuel to ensure complete drying.

Auxiliary heaters and blowers may be needed during storage to keep the metal temperature above the dew point and prevent freezing, especially with wet storage in cold conditions. Additionally, protect machined surfaces with preservative coatings and ensure there are no fuel leaks by disconnecting all oil and gas connections.

The choice between dry and wet storage depends on different factors. Although wet storage is often preferred, factors such as water quality, weather conditions, storage duration, and heat availability may make dry storage a more practical option. Be sensible when considering the conditions and needs of your boiler and choose the best storage method to ensure your equipment remains in excellent condition and is ready for use when needed.