The reasons for the failure of rupture of boiler desuperheating water pipes
The reducers of the desuperheating water pipe of a thermal power boiler were ruptured. Through sampling of the reducers, macroscopic inspection, chemical composition analysis, hardness testing, and energy spectrum analysis were carried out to analyze the causes of the reducer’s ruptures. The results show that flow accelerated corrosion inside the reducers, resulting in thinning of the wall thickness, which makes their strength unable to meet requirements for the pressure within the pipeline, ultimately leading to insufficient strength and rupture under high temperature and pressure. The influencing factors and preventive measures in pipelines were discussed based on the above reasons for failures, and specific suggestions were put forward to prevent failures in power station boiler pipelines.
 
2. Analyzing causes for failures
It can be seen from the above physical and chemical tests that the metallographic structure of the reducers is normal, and the chemical composition and mechanical properties are in compliance with relevant standards, which can rule out problems with the material itself. The wall thickness of the reducer is significantly thinned, resulting in insufficient strength, which is the direct cause of the rupture of the reducer. There is obvious plastic deformation in the macroscopic view of the fracture, and the microscopic morphology under the electron microscope is characterized by directional tensile dimples, which also indicates that the rupture of the reducer is a ductile tear caused by insufficient strength. The surface of the inner wall of the reducers has a densely horseshoe-shaped pit or fish scale, which is a typical feature of accelerated corrosion of carbon steel pipes due to flow. Flow accelerated corrosion (FAC) is pipe wall thinning caused by the flow of single-phase fluid or vapor-liquid two-phase fluid in carbon steel or low alloy steel pipelines. The occurrence process can be briefly summarized as follows: the carbon steel matrix undergoes oxidative corrosion under the action of little amounts of dissolved oxygen in the fluid, forming a loose and porous Fe3O4 oxide film. Because of the presence of H+ in the solution, the part of the oxide film in contact with the aqueous solution will partially dissolve. The dissolved Fe2+ will gradually diffuse into the main solution under the diffusion driving force and be taken away with the flow of the solution. Fe2+ is a product of the above chemical reaction. As the fluid flows, the concentration decreases. Therefore, the dissolution and formation of the oxide film will continue. As a result, a horseshoe pit or fish scale pit appears on the inner surface, and the wall thickness thins. Combined with the surface energy spectrum analysis results of the thinned area of the inner wall surface of the reducers, the chemical composition of the surface layer is mainly Fe and O, and the O element content is low; the atomic mass ratio Fe:O is about 4:1, indicating that the iron oxide layer of the thinned inner wall surface is very thin, which is consistent with the characteristics of FAC. It can be seen that the reason for the significant thinning of the inner wall of the reducers this time is the occurrence of FAC in the carbon steel pipeline.
 
Table 3 Energy spectrum analysis results of the inner wall surface near the rupture

 
3. Influencing factors and prevention of FAC
For thermal power boilers, flow-accelerated corrosion (FAC) usually occurs in the feed water and drain systems of super or supercritical boilers. To prevent the occurrence of FAC, we should start with the main factors that affect the FAC rate, including material factors, fluid dynamics factors, and environmental factors.
 
3.1 Materials
Flow accelerated corrosion (FAC) mainly occurs in carbon steel pipes and low alloy steel pipes with low alloy content. For pipe fittings with sudden flow changes such as elbows, tees, and reducers that are prone to FAC, low alloy steel containing Cr should be selected. The occurrence of FAC can be slowed down to a certain extent. The literature points out that when the Cr content in carbon steel is greater than 0.1%, the FAC rate of single-phase fluid can be greatly reduced; when the Cr content reaches 1%, the FAC rate is extremely low and can be ignored. It can be seen that for pipe fittings with sudden flow changes such as elbows, tees, and reducers that are prone to FAC, replacing carbon steel pipes with low-alloy steel containing more than 0.1% Cr can slow down or inhibit the occurrence of FAC.
 
3.2 Fluid dynamics
Fluid dynamics that affect FAC include fluid flow rates, roughness and geometry of pipes. At low and high flow rates, the rate of pipe wall thinning is linearly related to the flow rate; at medium flow rates, the rate of pipe wall thinning is linearly related to the cube of the flow rate. At the same fluid flow rate, the greater the roughness of the pipe wall is, the higher the FAC rate of the pipe wall becomes. The pipe’s geometry also has a large impact on FAC. Elbows, tees, valves, orifices and reducers in pipelines are areas where the fluid cross-section changes suddenly. The flow velocity increases, the turbulence intensifies, and the effect of peeling on the pipeline also increases; FAC is easier to occur compared with straight pipe sections where the fluid is stable. In the unit’s maintenance process, ultrasonic thickness measurement is performed on the above-mentioned components that are prone to FAC, and the thickness measurement data in different periods are compared and analyzed. The corrosion rate of the FAC at this location can be calculated. Combined with the final thickness measurement results, make adjustments in advance. Replace severely thinned parts in advance.
 
3.3 Environment
The pH value, oxygen content, temperature, and pressure of the aqueous solution will all affect the rate of FAC. When the pH of the boiler feed water is less than a certain critical value, the H+ content in the feed water increases significantly, which accelerates the dissolution of the Fe3O4 protective film and the FAC rate increases sharply; when the oxygen content in the feed water increases, the loose and porous Fe3O4 protective film will be oxidized into a dense Fe2O3 protective film, and the FAC rate decreases; FAC usually occurs at temperatures between 90 and 230°C, and the FAC rate reaches the maximum at 150 and 175°C; under conditions of a certain pH, oxygen content and temperature, the pressure, as the value increases, the FAC rate increases. When the FAC rate increases, the Fe ion concentration in the water supply system increases. Therefore, changes in Fe ion content in the system can be monitored in real time to predict the FAC occurrence rate of the system. For boiler pipes, the temperature and pressure of the internal medium (solution) are related to the boiler design. Under normal working conditions of constant temperature and pressure, FAC rates are mainly controlled by controlling the pH value and oxygen content of the solution. The commonly used method is to add ammonia to the feed water to increase the pH of the feed water to above 9.0, and then add oxygen to the feed water to increase the oxygen content. Fully Protected Oxygen Treatment (FPOT) technology is currently a relatively advanced technology for controlling the oxygen content of water supply. FPOT technology accurately controls the amount of oxygen added so that the added oxygen only exists on the water supply side, while the steam side is oxygen-free. It effectively solves the FAC of the water supply system and at the same time avoids falling off of the oxide scale caused by excess oxygen entering the steam side.
 
4. Conclusions and suggestions
Through experimental analysis of samples of broken reducers, the following conclusions are drawn:
(1) The metallographic structure of the failed reducer is normal, and the chemical composition and hardness value meet the requirements of relevant standards. The failure of the reducer has nothing to do with the material itself.
(2) The reason for the failure of the reducers is that the wall thickness is thinned due to flow accelerated corrosion (FAC), so that its strength cannot meet the requirement for pressure within the pipeline, which ultimately leads to its insufficient strength and rupture under high temperature and high pressure.