For pipelines that are already in operation at increased pressure, can a strength test be conducted instead of a flaw detection test?
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As the title indicates, the pipeline that has been constructed is already in operation (D762×10, with a design pressure of 1.6 MPa; the original design called for 100% UT testing and 20% RT testing). Currently, the owner plans to increase the operating pressure to 3.2 MPa. The medium transported is natural gas, and the wall thickness and strength meet the required standards after verification. For such large-diameter pipelines that are of significant importance, it is standard practice to carry out 100% testing after increasing the pressure. I would like to ask experts whether, in cases where the original testing requirements for such pipelines were not met after the pressure increase, pressure testing alone can be used to ensure the safety of the system. After all, the pipelines are already buried underground, so it’s not practical to dig them up for testing. However, there is concern that pressure testing alone may not be sufficient to determine whether there are any potential defects in the original welds. Could someone with expertise take a look? If it can be replaced, please provide the relevant standards as support. Thank you!
TSG D0001-2009 \"Regulations on Safety Supervision of Pressure Pipelines – Industrial Pipelines\": to be determined by the design unit~
:'(I’m the design agency; I want to know if this has actually been done in practice, because the design calculations get stuck at the edges.)
The design pressure is 1.6 MPa; is it possible to increase it to 3.2 MPa?
The original design pressure is 1.6, so what was the original test pressure? Can the original test pressure meet the test pressure after it is increased to 3.2 MPa? If that’s possible, it means the pressure testing and flaw detection have already been carried out at 3.2 MPa ； If not, pressure testing and flaw detection must be carried out at the new design pressure in accordance with the specifications.
Assume that this is a newly constructed pipeline, designed for 1.6 MPa; construction is complete, and pressure testing and flaw detection have been carried out； At this point, recalculation at 3.2 MPa yields a satisfactory result; the inspection has already been completed earlier (the requirements for inspection before and after should be consistent). A pressure test is conducted again at 3.2 MPa, and if it passes, the pipeline is considered to meet the standards for 3.2 MPa. But this is an underground pipeline that has been built and in use for some time now, and it’s unclear whether the welds are corroded; therefore, I believe it cannot be used directly.
The pressure increase after operation should be considered a technical modification rather than a design change. In addition to strength, valve grade selection, choice of flanges and gaskets, and bolt matching,
Taking 304 as an example, a stainless steel seamless tube with dimensions of 762*10 can withstand a pressure of 5.46 MPa; therefore, as long as the joints of the tube are properly treated, there should be no major issues with the tube
Firstly, the previous inspection rate did not meet the requirements set after the increase in pressure; in itself, this constitutes a failure. For example, if a pipeline system requires an inspection rate of 10%, but only 5% was carried out, then it is certainly not acceptable. Secondly, even if the previous flaw detection met the requirements after pressure application, it cannot be used unless it can be proven that the welds are unchanged compared to when they were inspected previously. But this is impossible, because the pipeline has been in use for some time and is likely to have started corroding.
I think it makes sense; save money where it’s necessary, and don’t save where it’s not necessary