Why are there always pores in the welds of the elbow in the superheating section of steam air heaters?
Thread Content
It is an air heater, with the air coming in through the shell side, and finned tubes on the tube side. There are two types of steam used on the tube side: one is saturated steam (pressure 0.35–0.45 MPa, temperature 147–154°C), and the other is superheated steam (pressure 2.0±0.2 MPa, temperature 213–320°C). After operation, pores always appear at the welds of the superheated steam line, while the line using saturated steam functions normally; I can’t understand why this is the case. If it’s the large temperature difference causing deformation, then the weld seam should tear apart into cracks ； If cavitation erosion is responsible for the formation of pores, it should be on the back side of the elbow; so why are the pores located at the weld seam?
It is possible that during the welding process, the flow rate and pressure of the superheated steam are high, preventing the gases in the welding area from being properly expelled, which results in the formation of pores at the weld site. Furthermore, the temperature of the superheated steam is also relatively high, which can affect the material properties in the welding area; this may lead to unstable welding quality and defects such as pores. It is recommended to adopt stricter welding procedures and control measures during the welding process to ensure the quality and stability of the welds. .
The welding quality is inadequate; high temperatures amplify the welding defects
Let me tell you why. This happens because when using welding methods with a flame gun to melt the welding wire, in order to meet deadlines, the welder fails to melt the wire again at the end of the welding process, which prevents the weld from being more complete and smooth; as a result, full penetration is not achieved. This issue isn’t apparent during pressure testing, as pressure testing relies on air pressure measurements rather than temperature changes. However, under actual operating conditions, increased thermal stress leads to cavitation. You’re welcome to visit our company to see how we carry out welding operations. My phone number is 13921272663, Yang Jinxian
Your analysis should be the most reliable
The convection section of the heating furnace built back then had this very structure, and such a situation has never occurred before; the pressure and temperature were much higher than these values. It’s likely due to welding quality issues as analyzed upstairs – check if there are any X-ray films available to take a look
For welding quality issues, check whether the same person is responsible for welding on the saturated steam side and the superheated steam side, as well as what the status of welding procedures is
Very useful questions, and everyone did a great job explaining them. Another possibility for discussion is the issue of electrode material selection. If the expansion coefficient of the welding material is lower than that of the pipe material. In the case of superheated steam, temperature fluctuations as high as nearly 170°C cause the phenomenon of thermal expansion and contraction to occur repeatedly. When two materials with different coefficients of expansion are placed together, gaps are formed, and the material within these gaps can be dislodged due to the action of high-speed flow. The heat release of saturated steam occurs at a constant temperature, allowing the latent heat to be released and preventing similar problems from arising.