1. Analysis of the causes of weld cracking
The stress generated by the pipeline when thermal expansion, cold contraction and other displacements are constrained is called secondary stress). The allowable stress value is 52.00MPa [from the "Technical Regulations for Stress Calculation of Steam and Water Pipes in Thermal Power Plants") (SDGJ6-90). The primary stress at the small head of the big and small head is 35.71MPa. This shows two points: ①The intermediate support is suspended, and the primary stress of the temperature reducer exceeds the standard, and the pipeline is prone to cracks after the water spray valve; ②The intermediate support and hanger is suspended, and the primary stress at the small head of the small and small heads is not large ( The first-time stress is qualified), and the piping stress is not the main reason for cracks when the support is suspended.
2. Analysis of the causes of suspension and simplified bending of supports and hangers
The temperature at the upper part of the pipe section is higher than the temperature at the lower part, which will cause axial bending and deformation of the pipe, which is consistent with the theoretical analysis of actual pipe deformation, indicating that the temperature difference between the upper and lower sections of the pipe section is the main reason for the bending and deformation of the pipe and the suspension of the support.
3. The thermal stress at the lower part of the section of the desuperheater and pressure reducer causes the weld to crack
There is a large temperature difference between the upper and lower pipes, especially when a certain working condition is switched, there may be condensed water in the pipe when driving again. High-temperature steam passes through the pipe to quickly heat up the water-free pipe wall, while the water-storing part of the pipe wall has a slower temperature rise. This results in higher axial tensile stress at low temperatures, and this higher thermal stress or thermal fatigue is the main cause of weld cracking.
4. Countermeasures
(1) Simplified increase of drainage pipelines to improve drainage conditions. That is, a 620 drain pipe is added after the 6# bracket, and the drain pipe is turned on during the heating and hot standby of the temperature reducer, so as to reduce the temperature difference between the upper and lower sides, so that the upper and lower temperature difference of the pipe section under various working conditions can be controlled, thereby Reduce pipe bending and eliminate weld cracks.
(2) In order to solve the problem of the suspension of the sliding support and hanger and the bulge in the middle of the pipeline, the Xi'an Thermal Power Research Institute redesigned and calculated the system pipeline and selected the support and hanger to make the 6# and 8# supports bear the load, 5#, 7 #, 9# The sliding bracket is suspended in the air. In this way, the suspension space problem of the support and hanger is solved, and the structural stress of the temperature and pressure reducer is improved.
(3) Optimize the water spray device. The original water spraying location was at the 6# support and hanger, and the water spraying device was placed at the pressure reducing valve after the transformation. Make the mixing of desuperheating water and steam more even and reduce temperature difference.