Methods of maintaining industrial boilers when not in use
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To ensure the performance of industrial boilers
Industrial boilers should also be maintained during shutdown ~
Why do I need maintenance after shutting down the boiler?
After the boiler stops operating, a large amount of air from the outside will enter the steam and water system of the boiler, especially when the boiler is not dried after cooling and water discharge, a layer of water film will be attached to the metal surface, and oxygen in the air is easily dissolved in this water film and reaches saturation, and after the corrosion reaction between oxygen and iron, oxygen in the air will be quickly and continuously replenished, which can easily lead to metal oxidation corrosion. Corrosion can be even worse when chloride or sulfate ions in the boiler water remain on the metal surface. To prevent oxygen corrosion, the boiler must be maintained when it is shut down.
Oxygen corrosion on metal
Four basic principles of shutdown maintenance
1. Prevent outside air from entering the inactive boiler steam system;
2. Keep the metal surface in the steam system dry;
3. Form a protective film layer on the inner surface of the metal to isolate the air;
4. Immerse the metal surface in an environment containing oxygen scavenger or other protective chemicals.
Shutdown maintenance methods
Through industry practice, common maintenance methods today can generally be divided into two types: "dry" maintenance and "wet" maintenance, including methods such as dehumidification, drying, nitrogen filling, pressure maintenance, alkali method, ammonia method, and ammonia-hydrazine method. Among them, the following methods are very simple and practical, suitable for commonly used shell gas boilers, and can be referred to by enterprises for self-maintenance:
Gas-fired boiler
1. Dehumidification method
The dehumidification method is a "dry" maintenance method and is suitable for protecting boilers with long shutdown times (more than three months).
1. When the temperature of the boiler water drops to 100 ~ 120 °C, all water in the boiler is drained, and the metal surface is dried using the waste heat in the boiler, and deposits and slags settling at the bottom of the boiler water are removed.
2. Place desiccant in a suitable container and place it at different positions of the boiler, then close the steam valve immediately to prevent outside air from entering.
3. Desiccants commonly used are: anhydrous calcium chloride (CaCl2), quicklime (CaO), silica gel, moisture-absorbing agents that can keep the metal surface dry and corrosion-resistant.
4. It should be noted in this method that the desiccant should be checked regularly after being placed, and if the desiccant is found to be liquid, it needs to be replaced immediately.
2. Dry drying method
The drying method is a "dry" maintenance method and is often used to prevent corrosion during boiler maintenance. When the temperature of the boiler water drops to around 100°C, all boiler water is drained, and the metal surface of the boiler is dried using the waste heat of the boiler.
3. Alkali solution method
The alkali solution method belongs to "wet" maintenance, meaning the method of adding alkali solution is applied, and the boiler shutdown time is less than three months.
1. After stopping the boiler, first remove dirt and water scale inside and outside the boiler, block the pipes connected to the outside world, remove all copper parts in the steam and water system or seal and insulate them.
2. A mixture of sodium hydroxide and trisodium phosphate is commonly used. Sodium hydroxide (dosage: 4kg/m3) and trisodium phosphate (dosage: 1.5kg/m3) are dissolved in soft water or demineralized water and added to the boiler, and the alkali solution in the boiler is circulated evenly by a pump. The boiler is filled with water solution with a pH value greater than 10 to passivate the metal surface and prevent metal corrosion by dissolved oxygen.
3. It is best to heat the boiler water to about 105 °C to remove oxygen and other gases in the water. Then close all doors and valves and keep them from leaking.
4. Throughout the maintenance period, the fire furnace should be ignited regularly to keep the outer surface dry; the pump should be turned on regularly to circulate water so that the concentration of the solution is suitable everywhere; the solution should also be checked regularly, and if the alkalinity decreases, it should be supplemented. In winter, anti-freezing measures should be taken.
4. Pressure maintenance method
The pressure maintenance method is a "wet" maintenance method and is often suitable for boilers with shutdown times of less than a week.
1. Fill the water into the boiler water system before stopping the boiler, use the excess pressure in the boiler to maintain it at a level of 0.05 ~ 0.1MPa, and the temperature of the boiler water slightly higher than 100 °C, this not only makes the boiler water not contain oxygen but also prevents air from entering the boiler.
2. To maintain the temperature of the boiler water, a slight flame can be generated in the boiler regularly, and steam from adjacent boilers can also be used to heat the boiler water regularly.
Industrial boiler maintenance methods above.
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