Protection of Idle Boilers
Unless proper storage procedures are followed, severe corrosion may occur in idle boilers. The method of protecting idle boilers depends primarily on the length of downtime. Atmospheric corrosion of ferritic materials rapidly in the presence of oxygen and moisture. The oxides produced are objectionable and can be transported to critical heat transfer areas as well as to the turbine. Also in the inner surface of the tubes, pitting can take place due to corrosion.
Boiler with numerous complex circuits and bends, it is usually impossible to completely dry a boiler in preparation for storage. Draining all circuits while hot may temporarily dry the surfaces; however, unless dry air can be continuously circulated to eliminate all the water vapour from the unit, re-condensation will again result in moist conditions. For this reason, wet lay-up normally offers the most positive method of protection for components in the boiler.
Wet Method
- Check all access doors for the effectiveness of the seal
- Clean and examine all tubes, brickwork, refractory work and internal baffles, ash hoppers, and fuel bunkers (if applicable) for signs of erosion & free from ash deposition
- Clean and lubricate all valve spindles, damper, and gate. Remove gland packing from valve spindles and lubricate before repacking
- All instrument pipework, which is liable to contain condensate, must be drained as a precaution
- Empty the boiler, flush it with demineralized water of pH approximately 9.0 until clear water drains out, then completely drain the boiler
- Close all boiler drains including superheater drains. Open all boiler and superheater vents
- Fill the DM tank/feed transfer tank with DM water and add Oxygen scavengers such as Hydrazine to maintain about 200 PPM Oxygen scavenger concentrations
- Add Trisodium Phosphate (TSP) pre-dissolved in hot water outside to maintain a phosphate level of about 100 PPM
- Add pH boosters such as Morphelene OR Ammonia solution to boost up the pH from about 11.0 to 11.5
- Mix the solution thoroughly. A pump may be used to recirculate water back to the tank for proper mixing
- Inject the aforementioned chemicals into the boiler water and fill it until water emerges from all vents, resembling a hydro test; subsequently, seal off all vents and increase the boiler pressure by approximately 10 to 15 kg/cm2
- Halt the pump and sustain pressure; upon pressure dropping below 3 kg/cm2, restart the pump to elevate pressure, thereby preventing air from entering the boiler
- The chemical concentration must be checked daily for the first three days and based on experience the frequency may be reduced to 3 to 6 days
When the boiler is returned to service, the complete water of the boiler should be drained out and the boiler should be cleaned with a high-pressure water jet. After cleaning boiler treated water should be filled up to the normal water level.
Dry Method
- Fill the boiler including the Superheater (with DM Water of pH about 11.0 to 11.5 and an Oxygen scavenger to maintain about 200 PPM concentrations) till water freely starts coming out just like filling for a hydro test
- Connect the vents of the boiler with Nitrogen or other inert gas cylinders and raise the pressure to 10 PSIG
- Slowly start draining the boiler keeping the pressure around 10 PSIG
- Empty the entire boiler till nitrogen starts coming out of the drains
- Now close the drains and maintain the pressure between 5 to 10 PSIG
Wet Method with Capping
With capping fill the water with chemicals as explained in the dry method, till water starts coming out of all vents. Close all drains and connect the vents of the boiler with Nitrogen or other inert gas cylinders raise the pressure to 5-10 PSIG and maintain the pressure. Any loss of water (leakage etc.) will be replenished by Nitrogen.
Author
Nilesh Patil
Boiler Operation Engineer
Tata Steel
