Analysis on Corrosion of Hot Water Boiler

Hot water boilers have been the main heating source in recent years. Using hot water for heating, the room temperature is suitable, and it has obvious coal saving effect than steam heating, so it is very popular among heating users. However, through years of work experience, internal and external inspections after the boiler shutdown have found that some hot water boilers have very serious corrosion conditions, which deserve attention.

The reasons for boiler corrosion are as follows:

?. Does not control the amount of make-up water. Article 100 of the Hot Water Boiler Safety Supervision Regulation stipulates: “The leakage of the hot water system is generally not more than 1% of the water capacity of the system.” However, some units mistakenly believe that with a hot water boiler, it is convenient to use hot water. The hot water is used for bathing, washing clothes, etc. A 2.8MW hot water boiler can supply nearly 100 tons of water per day.

?. The “low-pressure boiler water quality standard” (hereinafter referred to as the standard) is not carefully implemented. The standard clearly stipulates that when the water supply temperature is less than or equal to 95°C, the circulating water should be controlled to a pH value of 10-12. This is a very important indicator, but some units do not implement it. However, these units pay more attention to the hardness of the make-up water, and some have installed a fluid bed water treatment to meet the needs of supplementary softened water. They mistakenly supply hot water boilers according to the water standard for steam boilers, and think that the softened water is qualified. Will scale and corrode.

?. The installation of the heating network pipeline is unqualified, and the circulating water cannot be sent to the users at the end of the system. In order to avoid freezing, some users increase the number of air discharges, and the drain will be hot, and if it is not drained, it will be cool. As a result, the amount of leakage is increased.

?. The constant pressure of the expansion tank is inconsistent with that of the boiler. The expansion water tank of a certain unit is set up on a four-story building, its height is less than 15 meters, but the boiler has a constant pressure of 2.5kg/cm2. As a result, the furnace workers frequently make up water in order to maintain the working pressure index of 2.5kg/cm2. The water is unknown. The white ground overflows into the trench, which wastes water and coal.

?. The furnace is shut down without maintenance. After the heating period, the boilers were shut down, temporary workers were dismissed, some boilers were filled with softened water, and some boilers were exposed to the atmosphere and lacked necessary maintenance.

The above situation can clearly show that the corrosion of hot water boilers is caused by users’ lack of knowledge about hot water heating. Among them, a large amount of supplementary water is the most harmful. The treatment equipment is overloaded. To ensure the water supply, the unqualified water containing high chloride is often added to the system. When the pH value of the boiler water is only 7-8, hydrogen ions, oxygen ions, and chloride ions It is very active as a corrosive medium. Due to the low pH value of the boiler water and the high concentration of hydrogen ions, hydrogen will undergo cathodic reaction under the corrosion products of the ulcer. When there is dissolved oxygen in the water, the presence of chloride will greatly increase the corrosion rate of iron, which is caused by water chlorine. Ions are easily absorbed by the oxide film on the metal surface and replace the oxygen ions in the oxide film, thereby forming soluble chlorides, destroying the oxide film, and making the metal surface continue to be corroded. Based on the above knowledge, excessive supply of water to the hot water heating system is the key to causing corrosion of the hot water boiler and reducing the service life.

The ideal hot water boiler operating system should be basically tight and non-leakage. There is a fixed amount of water in the system that circulates over and over again, carrying out the process of absorbing heat and releasing heat. After the water is circulated, all the dissolved oxygen in it has been absorbed by the iron and becomes oxygen-free water, which will not cause serious corrosion to the boiler. It is reasonable for the hot water boiler regulations to require that the leakage rate not exceed 1% of the system’s water capacity.In order to prevent the hot water boiler from being corroded, the recommendations are as follows:

Ⅰ. For boilers whose water replenishment rate exceeds the standard, we must find out the cause and cure it symptomatically, cut off all the drain faucets, block all running, leaking, dripping, and leaking, increase the system’s automatic bleed valve, and strictly manage the system to ensure that the water replenishment rate reaches the standard operation.

Ⅱ. A small amount of make-up water is inevitable, but the quality of make-up water must be paid attention to. It is best to make up deoxygenated water. Boiler water heated separately can use the waste heat of the tail flue to preheat the cold water (soft water) to 70℃～80℃, and then add appropriate amount of trisodium phosphate and sodium sulfite into the boiler at the same time, which is beneficial to the boiler. harmless.

Ⅲ. Strictly control the pH value of the boiler water, and test the pH value regularly (two hours). When the pH value is lower than 10, the amount of trisodium phosphate and sodium hydroxide can be increased for adjustment.

Ⅳ. Do the maintenance for shutting down the furnace. After the hot water boiler is out of service, it is best to use dry maintenance. The water must be drained and the moisture must be baked with a small fire. Then add human stones or calcium chloride, and add 2 kg to 3 kg per cubic meter of boiler volume to ensure The inner wall of the pot is dry, which can effectively prevent corrosion during the non-use period.

If it is a new heating boiler, the author advocates building a steam boiler to heat hot water through the mixed heat exchange of steam and water. Such a heating system can not only effectively protect the boiler, extend the service life of the boiler, but also have multiple uses in one boiler. It can not only realize hot water heating, but also ensure all domestic hot water, and also save capital investment, which is impossible for hot water boilers. Comparable.