The circulating water treatment system (CTE) of nuclear power plant uses electrolytic seawater to make chlorine, which is used to prevent the growth of Marine organisms in the seawater system. The side effects of electrolytic process lead to the formation of scale products in the pipeline, and the phenomenon of scale products in many power plants is blocked, which greatly affects the normal operation of CTE system. Based on the investigation of the actual operation data of several nuclear power plants, this paper analyzes the formation mechanism of scaling products, and discusses the reasons for the blockage of dosing pipes. It is also suggested that gravity dosing units should be changed into more dosing methods, or scale inhibitors should be adopted to improve the water quality, or the operation process should be optimized to solve the problem of pipe blockage.
In July 2015, the reading of the residual chlorine detector downstream of the dosing pipeline of the circulating water treatment system (CTE) of the nuclear power plant was abnormal, and no flow display was found in the flowmeter during the adjustment of dosing. Subsequently, the ultrasonic flowmeter was used to preliminarily judge that there was no liquid flow in the pipeline, and the downstream isolation valve of the flowmeter was opened, and a large amount of white residue was found, as shown in Figure 1. Through investigation, similar phenomenon of sodium hypochlorite scaling and pipe blocking has also occurred in many nuclear power plants, which has greatly affected the normal operation of CTE system.
This paper analyzes the root cause of sodium hypochlorite plugging through the composition and related detection of pipeline scaling products, and puts forward corresponding solutions based on the actual cases and theoretical data on site.
1. Scaling product detection
1.1 Macro Analysis
There is a thick white scale attached to the surface of the dosing tube. When you twist it by hand, the texture is soft and the scale forms into a mass, as shown in Figure 2. The scale is insoluble in water and becomes a powder after drying, as shown in Figure 3. From the perspective of morphology, it is a typical scale formation of dosing pipeline equipment. After scaling is scraped off, the surface of the dosed tube is polytetrafluoroethylene primary color, and there is no obvious corrosion phenomenon.
1.2 Composition analysis of scaling substances
The white scale was scraped from the surface of the pipeline and X-ray analysis was carried out. The analysis results were shown in Figure 4, and the content of clogging substances was shown in Table 1. According to the analysis results, the blocking material is mainly (Mg0.03Ca0.97)CO3, the content accounts for 93.9± 0.5%, followed by Fe3O4, the content is 4.9 %, and also contains FeO(OH), SiO2, etc., the total is about 1.0 %.
2. Analysis of scaling causes
The Ling Lier saturation index (L.S.I index) is generally used to judge whether the water quality has scale inclination (L.S.I>0 is scale type water quality, L.S.I≤0 is stable water quality):
L.S.T= pH – pHs
pHs=(9.7+ A+ B)-(C+D)
Where :pH is the actual pH value; pHs is the saturation pH value. A is the total amount of dissolved solids in water; B is the temperature coefficient of electrolytic seawater; C is the hardness coefficient of seawater; D is the alkalinity coefficient of seawater.
Based on the analysis of the water quality of sodium hypochlorite in CTE system of nuclear power plant, the total amount of dissolved solids is 325. The circulating water is an open cycle, and its temperature is normal temperature 25 ℃. The hardness of seawater is 166; The alkalinity of sea water is 108.
According to the calculation
PHs = (9.7 + 0.15 + 1.98) – (2.24 + 2.04) = 7.55
L.S.I = 7.92-7.55 = 0.37 > 0
It can be judged that the water quality is scale type water quality, when the seawater electrolysis chlorine, there will be scale precipitation.
The solubility of calcium and magnesium carbonate is extremely low, and the by-product calcium and magnesium deposits of electrolysis are partly attached to the cathode plate, and the rest flow into the outdoor sodium hypochlorite storage tank together with sodium hypochlorite. Some precipitate in the storage tank, and some precipitate in the dosing pipeline or directly flow into the circulating water.
The scale also contains a certain amount of Fe304 and FeO(OH), mainly because a certain amount of oxygen is dissolved in seawater, and the oxygen reacts with the pipe to produce Fe304 and FeO(OH). Small amounts of impurities, such as Si02, may also be introduced from the atmosphere and rainwater.
3. Investigation and analysis of pipe blocking phenomenon
From the above analysis, it can be seen that the existence of magnesium carbonate scaling products must be caused by the existence of electrolytic side reactions, but scaling does not mean that the tube will be blocked. According to the investigation, several nuclear power plants reported that the dosing pipeline of CTE system was blocked, and the blocked location was the dosing branch pipe. However, there are also some units did not occur plugging phenomenon. At present, none of the nuclear power plants using pump dosing have clogging, so it can be seen that the pipe clogging has a direct relationship with the dosing method.
(1) Gravity dosing refers to the use of high liquid level gravity in the CTE system sodium subaerate buffer tank to dosing the downstream pipeline. Its driving force is related to the liquid level, and the driving force that can be provided is relatively limited (the height of the tank does not exceed 10 m). When continuous dosing is used, in general, continuous fluid impact can enable scaling products to be quickly transported to the CRE seawater flow channel, and the probability of tube blockage is small. In the case of non-continuous dosing, the adhesion continues to increase due to the gradual deposition and solidification of scale products in the local area during downtime. The gravity-driven flow rate during dosing may not be able to remove the scale that has formed. The continuous accumulation of scaling products leads to reduced flow in the pipeline, weakened velocity, and more serious scaling accumulation. After scaling to a certain extent, the fluid resistance is greater than the driving force, which will lead to pipeline blockage.
(2) Pump dosing refers to the downstream of the sodium hypochlorite buffer tank, adding a platform dosing pump to send sodium hypochlorite solution to the dosing pipeline. No matter continuous dosing or non-continuous dosing, the unit using pump dosing did not have pipe blocking phenomenon. This is because the continuous and impact power provided by the pump is relatively large, even if a certain amount of crystalline scaling is accumulated in the pipeline, its impact force can still ensure sufficient flow rate for dosing, the scale precipitated in the dosing pipeline will be washed away or can only maintain a certain amount of scaling, so it is difficult to form blockage in the pipeline, BLUEWAV uses the dosing pump dosing device.
4. Suggested Measures
4.1 Changing the dosing method Changing the dosing method of sodium hypochlorite is the most thorough transformation method, that is, changing the gravity dosing method to the pump dosing method. At present, there is no report on the influence of velocity and scaling characteristics in sodium hypochlorite seawater environment. Some literature indicates that the relationship between scaling characteristics and velocity of urban secondary effluent is non-monotonic and there is a limit velocity value. When the limit flow rate is below, the increase of the flow rate will promote the dirt growth, while when the limit flow rate is above, the increase of the flow rate will show an obvious inhibition effect. It is recommended that the selection of dosing pump, referring to the engineering experience of a well-run power plant, the flow rate of the dosing pump is recommended to be not less than 258myh(can refer to the unit dosing demand), and the head is not less than 100 m.
4.2 Add Scale inhibitor to treat water quality by adding scale inhibitor, which is also an optional treatment measure, the scale inhibitor commonly used in daily life can be divided into condensed phosphates, organophosphates, surfactants, amino polycarboxylate, polymer categories.
4.2.1 Polyphosphate condensation
Inorganic polyphosphate is a good scale inhibitor, and the addition of condensed polyphosphate to water inhibits and interferes with the normal growth process of calcium carbonate and other crystals, thus preventing the formation of calcium carbonate scale. Sodium tripolyphosphate and sodium hempetaphosphate typically have long chain anions, and sodium hempetaphosphate has no fixed melting point, and its solubility in water is large, but not constant. A serious disadvantage of polyphosphates is hydrolysis (above 50 ° C), resulting in a hydrolyzed product that is more precipitable as a calcium phosphate scale.
4.2.2 Organophosphates
Organophosphates inhibit scale by slowing down crystal growth and character distortion. In contrast, polyphosphate has good chemical stability, is not easy to be destroyed by acid and base, is not easy to hydrolysis and degradation, can withstand higher water temperature and oxidation resistance, and will not cause excessive algal reproduction due to hydrolysis to orthophosphate in use. At the same time, it has a critical value effect, only a few mg/l of organic phosphoric acid can prevent hundreds of mg/l of calcium carbonate precipitation, and its scale inhibition performance is better than polyphosphates. However, when there is suspended matter, the scale inhibition effect will become worse.
4.2.3 Surfactants
The anti-scale effect of surfactants stems from their adsorption. On the one hand, due to the adsorption of surfactants, the normal coalescence state of the scale surface is disturbed, and the continuous growth of scale crystals is inhibited or partially inhibited, so that scale ions are in a saturated state or loose scale is washed away by water. On the other hand, due to the adsorption of surfactants on the scale surface, a diffused double electric layer is formed, which makes the scale surface charged and inhibits the coalescence between the scale crystals, resulting in the scale crystals existing in the solution in a dispersed state.
4.2.4 Amino polycarboxylate
Amino polycarboxylate compounds have good scale inhibition effect on calcium carbonate and other scale, but also have critical value effect, high dosage, high cost, and few applications. The main disadvantage is that it is easy to bridge with calcium ions in water to generate a polymer type of gel, which not only reduces the active component of the scale inhibitor, but also makes the water cloudy in serious cases.
By contrast, polyphosphates are easily hydrolyzed to produce polyphosphate anionic polymers, which may lead to algal proliferation. Amino-polycarboxylate scale inhibitors produce colloids and reduce scale inhibitor activity; Surfactants and polymers are relatively rare in practical applications. Combined with the above indicators and the demand for sodium hypochlorite in the operation of nuclear power plants, organophosphorus scale inhibitors can be considered for water quality treatment in subsequent improvements or CTE systems in new power plants.
4.3 Optimization of daily operation process The daily operation process management of the optimization system can reduce the possibility of pipeline scaling through a variety of measures. The measures that can be implemented include :(1) Regular discharge of sodium hypochlorite storage tank, increasing the frequency of discharge, from the original once/day to 3 times/day. The regular bottom of the storage tank is easy to accumulate electrolytic sediment, and after regular cleaning, the amount of sediment is reduced from the source, which helps to reduce the sediment scaling in the pipeline when sodium hypochlorite is added. (2) At least once a month, the main dosing pipeline should be washed with large flow of seawater, increase backwashing, improve the flow rate of water in the pipeline, and effectively reduce the deposition of scale. Every Monday, Wednesday and Friday, the small diameter pipeline of the SEC system is washed. The small diameter pipeline is more prone to scaling, and the daily flushing frequency is increased, so that the attached slight scaling can be washed and removed in time. Through the improvement of the above daily operation and maintenance management, the pipeline blockage situation has been significantly changed, although it can not completely eradicate pipeline scaling, at least it can meet the daily operation requirements of the unit.
BLUEWAV Technology CO., LTD
The article is reprinted in Guangdong Chemical Industry, volume 46, Issue 15