Anticorrosion and deaeration of the hottest hot wa

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Corrosion prevention and deoxidization of hot water boilers and heating systems (2)

3. Sponge iron powder filtration deoxidization technology needs to be standardized

sponge iron powder filtration deoxidization is a new process successfully researched by Professor Wang mengju of the former Wuhan Institute of water resources and electric power in 1992 on the basis of steel chip deoxidization and activated steel particle deoxidization

deoxidization of steel chips is to clean the oil stains on the cutting steel chips with alkaline solution, then treat the surface of steel chips with sulfuric acid, and put the treated steel chips into the deaerator for compression. The water flows through the steel chip surface in the deaerator to oxidize the steel chip surface and consume the oxygen in the water to achieve the purpose of deoxidization. The steel chip deaerator has the advantages of simple device, less investment and simple operation. However, it requires that the water temperature be higher than 70 ℃, and the water to be deaerated should be heated; Acid pickling should be used after the steel chip surface is oxidized. It is difficult to clean the compacted steel chip completely, resulting in the reduction of deaeration effect in the later stage of operation. Especially the steel chips pressed tightly, when they need to be replaced after all oxidation, the rusted steel chips are difficult to pull out, and the amount of labor is huge

in view of the shortcomings of deoxidization of steel chips, steel chips or steel materials are refined into molten steel according to the formula requirements, and cast into 3 ~ 5mm irregular approximately spherical steel particles. After cleaning the surface of steel particles with alkaline solution, put them into a tank similar to the sodium ion exchanger, and the surface of the tank is coated with enamel to prevent acid. The steel particles are soaked and activated in dilute hydrochloric acid in the tank of the deaerator. When the water flows through the activated steel particles and the water temperature is ≥ 40 ℃, the oxygen can be removed. This method avoids heavy physical labor when replacing steel chips; Lower water temperature requirements; The deaerator is irregular spherical, which also improves the flow characteristics of the bed. However, the casting and activation of steel particles are troublesome; Common pickling is still needed; The filter is made of heavy steel particles. Plastic water caps cannot be used for water outlet. Deaerator should also be specially manufactured, so it is difficult to promote

the sponge iron powder deoxidization technology still adopts the filter type, without steel particles, and uses sponge reduced iron powder with light bulk specific gravity as the deoxidizer. When oxygen in water passes through iron powder, the following reactions occur:

2 + 2H2O + O2 → 2Fe (OH) 2 ↓

because Fe (OH) 2 is unstable in oxygenated water, it will continue to be oxidized into trivalent iron hydroxide with very low solubility:

2Fe (o this polymer has a biological base content of up to 20 ⑵ 5% h) 2 + H2O + 1/2o2 → 2Fe (OH) 3 ↓

the concentration of dissolved oxygen in the deep filter layer is very low, so it is impossible to oxidize Fe (OH) 2 into Fe (oh oh) 3. In order to prevent scale or water slag from entering the boiler, the deaerated water then passes through the Na type strong acid cation resin of the sodium ion exchanger to remove the water:

+ 2nar → fer2 + 2Na +

therefore, the sponge iron powder filtration deaeration is often made into two tanks. The water first passes through the first tank as the deaerator, and then passes through the second tank as the sodium ion exchanger. Or it can be made into a single tank double chamber bed. The lower chamber is a deaerator, and the upper chamber is a sodium ion exchanger. Water passes through the lower chamber first, and then the upper chamber

this deaeration method has the following main characteristics: (1) it can deaerate at room temperature. The soft water entering the deaerator does not need to be heated, nor does it need to consider the cavitation of the water pump. The equipment is simple and energy-saving. (2) It can be arranged in a low position, with small volume, low initial investment cost, low operation cost and good economic benefits. (3) Sponge iron is non-toxic. It does not need to be degreased or activated. It only needs to be rinsed and replenished regularly. The operation is simple. (4) The deaeration effect is good and stable, and it can adapt to the change of heat load

the above characteristics make up for the weakness of other deaeration methods, so they are often used by hot water boilers and their heating systems recently. However, there are too many manufacturers, and even some "workshop" manufacturers are producing. Due to insufficient research and Discussion on some technical problems, some faults have also occurred. How to standardize the technology is the way to further improve this deaeration technology. Through the analysis and comparison of the technical performance and installation and operation instructions of nine manufacturers with strong technology and production capacity in China, many problems to be standardized are found, such as:

(1) the selection of iron powder. There are two specifications for the bulk specific gravity of the current iron powder, one is the specific gravity of 1.4 ~ 1.7, and the other is the specific gravity of 2 ~ 2.6. The understanding of which is better is very different. Some people think that the "sponge iron" with light specific gravity has a large specific surface area and is conducive to oxygen removal. The specific gravity is 2 ~ 2.6, which is similar to iron and belongs to general iron powder. Some people think that it is better than the actual effect of major and will not harden. Some manufacturers simply write the heap specific gravity of iron powder to 1.6 ~ 2.5

(2) there are also different opinions on whether to set up an ion exchanger to remove ferrous ions after deaeration. Some people think that Fe2 + must be removed; Some people think that iron should be removed for medium pressure boilers to make the feed water meet the standard. For hot water boilers and low-pressure steam boilers, the feed water has no requirements for iron content, so iron can not be removed; Some people think that there is not much Fe2 + in the water when ph>7, and there is less Fe2 + if the water temperature rises, so there is no need to remove iron

(3) there are a variety of equipment systems, most of which are single tank Deaerator and sodium ion exchanger as a combination, and only a few manufacturers produce single tank double chamber bed equipment. Many units connect the deaerator in parallel, forming a double tank combination, a three tank or four tank combination, and even a five tank combination. In some single tank double chamber bed equipment systems, the room is vacuumized before the backwashing of the lower chamber, so a vacuum pump is set

in the equipment system, some manufacturers set deaeration water tanks and put forward measures to seal the water tanks. Some manufacturers also advocate that there is no deaerator tank to eliminate the difficulty of sealing the tank. The water of the deaerator is directly connected with the make-up water pump. Before measurement, a standard block is needed to calibrate the hardness tester

due to the good market sales of sponge iron powder deoxidation equipment in recent years, many manufacturers are competing for production. There are hundreds of manufacturers, and generally non professional "workshops" are also producing. The quality of equipment provided in the market varies greatly, and there is a lack of product quality standards

(4) different operation and control parameters, lack of optimization values. For example, 12m/h is used for the lowest flow rate of filtering empty tank, and 30m/h is used for the highest flow rate; The minimum backwashing intensity is about 7l/m2 s. Up to 18l/m2 s。 Some people suggest that the working temperature must be higher than 3 ℃, some suggest that it must be higher than 5 ℃, and others believe that as long as there is no ice, there is no need to set a lower limit. The upper temperature limit is set at 40 ℃ or 50 ℃, and some are set at 80 ℃

almost all manufacturers have not specified the requirements of influent water quality. Only one manufacturer proposed in the manual that if NaOH is added to the circulating water before entering the deaerator, a bypass pipe should be set. Add in the bypass pipe. Because there is no clear requirement on the quality of incoming water, a heat supply station will first add trisodium phosphate to the water, and then enter the deaerator, so that the iron powder is passivated, and the deaeration effect is extremely poor. The equipment system of one manufacturer is to first enter the untreated raw water into the deaerator, and then flow through the sodium ion exchanger, softening and removing ferrous iron in the exchanger at the same time

in order to prevent the divalent iron from polluting the resin during the reduction of sodium ion exchanger, it is advocated to add Na2SO3 equivalent to 1% of the CI amount of Na to apply 750N disassembly force to the handle sleeve in the salt solution for the handle sleeve disassembly experiment, or add 1 ~ 2% NaHSO3 at the same time; Some advocate adding Na2SO3 only, and adding NaHSO3 again after 4 ~ 6 cycles; Some people think it is unnecessary to add

(5) there are different opinions on the causes and prevention methods of iron powder hardening. Some people think it is iron powder problem, and high-quality iron powder will not harden; Some people think that iron powder larger than iron powder is not hardened; Some people think that the iron powder with a large proportion and a small proportion of 30% is the most difficult to harden. Some people think that it is an operation problem. Hardening is caused by the presence of Fe (OH) 3 in the iron powder layer. As long as we strengthen the backwashing intensity or the backwashing frequency, it can be solved; Some people think that it will not harden as long as it is filled with water and sealed under pressure when it is out of service, and no air is allowed in. Some people think that it is a problem of equipment. For example, the divided format deaerator for grid backwashing is not easy to harden. Some people also believe that the causes of hardening are multifaceted, and the above measures should be comprehensively adopted in order to be well solved

in conclusion, it can be seen that there are many problems that need to be standardized in the sponge iron powder filtration type deaeration technology. These problems need a lot of experiments and investigations, and the analysis optimization should be realized in the injection molding process

4. Water quality monitoring of heating system

water quality monitoring is a necessary means to ensure safe and economic operation. Recently, the Danish Institute of engineering and technology studied and improved the water quality monitoring of the largest heating system in Copenhagen. According to the large amount of data of water quality sampling and analysis of the system pipes and equipment for five years, the author makes analysis and research, and puts forward opinions and suggestions. Two of its monitoring indicators are related to corrosion prevention, namely pH value and dissolved oxygen. For the monitoring of these two indicators, the following opinions are put forward:

(1) these two indicators are the main indicators of corrosion prevention, which should be changed from sampling analysis to monitoring. Because the sampling analysis is static, we can only see the water quality at the time of sampling, but not the change of water quality

(2) the value of dissolved oxygen is the main index to prevent corrosion, but the degree of corrosion cannot be determined by the content of dissolved oxygen. Oxygen corrosion is mainly local corrosion. In addition to oxygen corrosion, biological corrosion is also common in pipe systems. Biological corrosion is caused by bacteria in water. About 95% of bacteria can float freely to the water surface in water, and only 5% of bacteria are in water. Bacteria on the surface of water form a film called biofilm. After biofilm is formed on the surface of pipes and heat exchangers, the effect of bacteria causes changes in water quality, resulting in rust spots or dents on the surface of pipes or equipment. This corrosion has nothing to do with dissolved oxygen content, so it is recommended to install a "corrosion monitoring device" as shown in the figure on the bypass of the return pipe of the heating system. The return water flows into the device from the inlet, and there are circulating flow channels and test pieces in the device. The materials of the test pieces are the same as those of the equipment or pipeline being tested. There is a rotating cylinder inside. Adjust its speed to make the water flow speed reach 0 ~ 2 m/s to simulate the actual flow situation. After a certain time, stop rotating, disconnect the device from the pipe, open the movable cover and take out the test piece. Various corrosion conditions including spot corrosion, surface corrosion, corrosion crack and biological corrosion dent can be observed through the test piece; Measure or evaluate the corrosion rate and degree. If necessary, more complex analytical tests can be carried out. In short, the real corrosion can be completely monitored

the corrosion monitoring device is simple in structure and can be designed and manufactured by itself without expensive initial investment. I think it can be used for reference

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