On the relationship between pipeline cold tightening and stress
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After reviewing relevant literature and forum materials, I found two somewhat contradictory opinions. Both agree that it can reduce the stress on the pipe end supports; however, one opinion states that cold tightening can reduce the stress under operating conditions, while the other claims that it cannot change the results of the primary and secondary stress evaluations – in other words, if the stress evaluation fails, cold tightening still cannot make any difference. What exactly is this theory? As I understand it, according to the stress formula for thermal expansion, the thermal stress in pipes depends only on temperature and the properties of the pipe material; it has nothing to do with length or wall thickness. So at first I thought that cold tightening wouldn’t change the stress level. But then I realized that the company in question calculated values without taking cold tightening into account. If cold tightening is applied now, then there will be stress present from the beginning – will this have an impact on the secondary stress under hot conditions?
In fact, cold straining of the pipeline can effectively reduce the stress on the pipe end supports, thereby lowering the stress level of the pipeline. However, the verification results for primary stress and secondary stress cannot be directly changed by using cold tightening treatment. This is because the initial stress is applied during the construction phase, while the cold straining treatment is carried out during the operation phase; the two are not synchronized. At the same time, secondary stress generally arises only when the temperature of the pipeline changes, and cold straining does not alter the temperature characteristics of the pipeline; therefore, it also has no direct effect on the magnitude of secondary stress. However, cold straining can affect the thermal expansion properties of pipes by altering their initial state, thereby indirectly influencing the stress levels in those pipes. Specifically, cold straining can reduce the deformation of pipes at the initial stage of operation, thereby lowering the thermal stress and stress levels experienced by the pipes during operation. Therefore, when designing pipelines, cold straining treatment should be taken into account, and the decision to employ this strategy should be made based on the actual conditions and requirements of the pipeline. .
I would like to ask what the term “cold tightening” means in practical applications. Thank you. :)
If stress levels can be reduced, then why is it said that the verification results cannot be changed? What exactly does this stress level mean?
Could you indicate the source of this information? It is true that cold pressing cannot change the primary stresses, but it can affect the results of secondary stress calculations; one can use relevant formulas to conduct a rough calculation to reinforce understanding.
It is included in the training materials for pressure pipelines