Pipeline replacement issue

Is the design wall thickness of the pipe minus the corrosion allowance the wall thickness that needs to be replaced? I would like to ask what is meant by “wall thickness to be replaced”? I don’t understand what the original poster’s question is

What the original poster said makes sense. However, the actual situation is often much worse than this; despite having no chance to make a replacement, people still persist.

SH3509 specifies that the service life of pipes is 15 years; in reality, the service lives of various types of pipes in China vary. According to API581, pipes for which there is no apparent mechanism of degradation can be designed with a service life of 40 years. In the case of degradation mechanisms such as corrosion and material deterioration, it is necessary to take these mechanisms into account during design and to establish safety margins based on normal conditions. How many years are needed to ensure safety throughout the usage period? The actual service life of a pipeline can be determined through multiple inspections and evaluations.

The design thickness is actually greater than what the original poster thinks it is. This is because procurement – namely, the selection of pipes – requires standardization. For example, for an NPS 8\" pipe, the minimum designed wall thickness is 3.5 mm, with a corrosion allowance of 3 mm; thus the actual wall thickness of the pipe chosen may not be 6.5 mm, but rather 8.18 mm. That is for NPS 8\" sch40

The design thickness is actually greater than what the original poster thought it would be. This is because standardization is required for pipe selection during procurement. For example, for NPS 8” pipes, the minimum designed wall thickness is 3.5 mm, with a corrosion allowance of 3 mm; thus, the actual wall thickness of the selected pipe may not be 6.5 mm, but rather 8.18 mm. In the case of NPS 8” sch40 pipes, even after 3.5 mm of corrosion occurs, the remaining wall thickness is 4.6 mm, which is still greater than the minimum designed thickness of 3.5 mm, allowing the pipe to continue to be used

This is because the Special Inspection Agency determines whether the expiration date has been reached by subtracting the corrosion allowance from the design thickness specified in the drawings

The most important thing is to understand the corrosion condition of the pipeline caused by the medium inside it, as well as the pressure, in order to determine the wall thickness of the pipeline based on the design requirements.

The wall thickness to be replaced is the one that reaches the minimum design thickness

Does it mean that the ultimately selected wall thickness or the wall thickness purchased is greater than the design wall thickness plus the corrosion thickness, or that the final design wall thickness is greater than the calculated wall thickness plus the corrosion thickness? The latter can be understood as being due to manufacturers not meeting standards, but in the case of the former, is the reason the same?

Generally speaking, in terms of EPC project management: the wall thickness of the materials used in procurement and actual construction is 8.18 mm, which is greater than the minimum wall thickness specified for the design life period (including a margin for corrosion), which is 6.5 mm; it is also greater than the minimum wall thickness required for calculating the design safety strength, which is 3.5 mm； So, as technicians in the “operation and maintenance” phase, including those at inspection agencies, how do they determine when it is necessary to replace pipes? After the unit has been in operation for many years, as in this case, online inspections revealed that the actual wall thickness was less than the minimum wall thickness of 3.5 mm calculated based on design requirements; only then was it necessary to replace the pipeline. Instead of replacing it when the wall thickness is (8.18-3=5.13) mm.