Pressure pipe wall thickness select and stress

It is the most basic and common problem to choose the wall thickness for pressure pipe. but in the actual design process is very confusing, often messed SCH table, not through calculus free to apply some manual data phenomenon empirically estimate , as well as the pipe wall thickness, the bigger the better, free to expand the pipeline wall thickness. Unreasonable choice of pipe wall thickness, not only poses a potential threat to security, but also contributed to the improvement of the construction costs and material waste.

Due to the the pipeline standard range for domestic use, the pipe wall thickness series and calculated selection methods are used in the actual design process tend to be confused. Also often exist in the actual engineering design, the thicker the better misconceptions tube, free to expand the behavior of the pipe wall thickness, so that not only the increase in construction costs also bring insecurity to the pipeline.

Due to historical reasons, the domestic pipe production standard European system, the U.S. Department into two categories, tube production by foreign standards, along with the deepening of opening up to the outside world, in the country also began large-scale use. Domestic often use the tube, only the tubes produced by SH3045 and ASME B3611 SCH table SCH table is wall thickness series table. SCH is calculated as follows:

SCH = P/δ *1000

The formula: P — operating pressure, MPa

δ — material allowable under the operating temperature stress, MPa

If we carefully analyze this formula, only considering the stress factors, other factors are not taken into account, so SCH only as a rougher pipe wall thickness.

Related defects generated during the manufacturing process in order to effectively prevent the above problems, it is necessary using reasonable processing and timing. First, we need to strictly ensure the tube plate hole size to meet the design requirements, immediately after the completion of the tube plate processing group of welding; carefully measure the heat transfer tubes extending the length does not meet the requirements need to be adjusted immediately; Secondly, welding with a wire brush to clean up or compressed air blowing, the area to be welded to the exposed metallic luster, good welding before the clean-up work to be welding. Manual welding process, in strict accordance with the requirements of the welding parameters welding process card reasonable choice of welding voltage, welding current and welding speed, and each electrode welding integrity of welds in the welding procedure used in the four-hole intermediate triangle arc and arc-quenching; For automatic welding required to strictly ensure the weld start and end points of the different layers must not overlap, in order to avoid the generation of welding defects and stress concentration. Double welded carbon steel heat exchanger should be used after the completion of the tube plate and the first layer of the heat transfer tube weld, clean the surface of the weld, according to process requirements using direct visual inspection or coloring method checks to ensure defect-free after the second-layer welding seam. Reasonable welding process, on the one hand, the welded joints size to meet the design requirements, check through the middle, you can also effectively eliminate weld defects hidden, so as to ensure the quality of the welded joints of the heat exchanger tube plate. For some of the good corrosion resistance of stainless steel and TIG to ensure weld quality heat exchangers should choose reasonable welding process and to ensure the quality of welded joints.

Post time: Sep-18-2021