Steam boilers are imperative equipment for the process and manufacturing industries. Therefore, boiler manufacturers in India suggest various ways to maintain the seamless operations of the boiler system. Boiler issues hinder the process operations that cause severe monetary loss for the industries. Among various techniques to avoid boiler problems, boiler shutdown is a prominent one. Boiler preservation is performed when the boiler system is out of service or in shutdown condition for a long duration to protect the boilers’ internal surface from corrosion and atmospheric oxygen.
What is Boiler Preservation?
When a steam boiler is not in use, out of service, or in shutdown condition for a longer period, precaution measures are necessary to prevent corrosion and protect internal metal surfaces from water and residual contaminants.
The preservation method opted varies on the length of the shutdown period and the characteristics of the concerned process plant. In most cases, due to the operational conditions, particularly in power plants, boilers need to be maintained in ready-to-start conditions. Therefore, wet conservation is the ideal solution. But, when the unit is out of service for an extended period, dry conservation is required.
There are two types of boiler preservation methods that are used in specific conditions.
Wet preservation of boiler
Dry preservation of boiler
Wet Preservation of Boiler:
Steam boilers are required in stand-by mode for sudden and quick steam demands are preserved through the wet method. The method is also used for large boiler systems that are difficult to drain.
Boiler pressure parts include economizer, steam drum, water walls, and superheater tube, filled with feed water having a high concentration of Hydrazine (200 PPM). The high hydrazine concentration ensures that the absence of dissolved oxygen is available in feed water. The boiler is kept under pressure to avoid atmospheric air entering into boiler pressure parts.
Steps for wet preservation of boiler:
The boiler should include feedwater to the normal water level than 90% water level of the gauge glass.
Maintain pH up to 9.5-11.
Hydrazine 200 PPM is to be doused with the water. Ensure uniform concentration.
In the case of N2H4 level, if the results of the boiler water sample are low from 100ppm, it requires injecting of N2H4 and in case of N2H4 level above 200ppm requires dilution process by feeding water into the boiler and by taking blowdown frequency.
Sodium sulfite (Na2SO3) 300-400 PPM can also be used in the unavailability of Haydazine.
Avoid using the wet preservation method in a very cold environment or under freezing water conditions.
Dry Preservation of Boiler:
The method is selected when the boiler needs to be shut down for a long period. Dry preservation requires eliminating all water and air moisture to maintain a relative humidity of preferably less than 40%. It prevents corrosion through hygroscopic salts. Dry-air storage is highly effective and easy to monitor. Dry layup enables easy and safe access for maintenance without any suffocation or exposure to toxic chemicals.
Steps for dry preservation of boiler:
The water must be drained out of the boiler. The drum manhole, superheater, blowdown, air cocks, gauge glass, and waterwall box drains need to be opened.
The internal surface of the boiler should be dried entirely with hot air.
After the silica gel on the activated alumina dries, it should be placed inside the drum.
Close all the pressure parts to prevent any leakages of moisture and air.
Boiler inspection should be performed once a month.
Replace the lime trays if required.
How to Shutdown the Steam Boiler?
During the scheduled shutdown, double the normal blowdown rate for two weeks before the shutdown and increase the sludge conditioner dosage to 1 1/2 to 2 times than normal.
With the boiler taken out of service, perform the blowdown several times from all the blowdown outlets, including headers. Follow the guidance of the boiler manufacturer for taking the boiler system out of service. Do not use cold water for cooling the boiler as it leads to leaks in rolled joints.
For preventing the suspended matter (sludge) in the boiler water from baking on the tubes, avoid draining water from a hot boiler. Wash it down after draining the water from the tubes. Flush out all feed and control lines to get rid of any sludge accumulations.
Flush the water-side surfaces with a high-pressure hose and assess to determine if further cleaning is required before lay-up. The slag, soot, and fly ash should be cleaned from the fireside surfaces. The deposits can lead to severe corrosion on the fireside. To protect the fireside from corrosion, it can be coated with light oil after cleaning.
Rakhoh Boilers is one of the trusted boiler manufacturers in Pune since 1983. With 38+ years of experience and expertise in thermal solutions, we deliver highly efficient and reliable industrial steam boilers, waste heat recovery systems, thermic fluid heaters, and boiler accessories to over 20 process industries in 26 countries worldwide. We provide excellent boiler services to boost the efficiency and productivity of the boilers.
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