Regarding the selection of pressure for hydrotesting
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I am a PIPING engineer at a design institute, and I have been in disputes with the EPC team recently regarding the pressure levels to be used for hydraulic testing and strength tests. According to the standards, whether they are national or international standards, the design pressure is set at 1.5 times the value. The question is, which design pressure should be used? My opinion is, of course, to take 1.5 times the design pressure calculated based on the wall thickness of the material, but EPC insists on using 1.5 times the process design pressure. For example, in a closed-system, if the design pressure for the process is 0.6, but the pipe grade is 1.6, then the hydrostatic test pressure I would apply would naturally be 2.4. However, EPC has objections. I also recall having read literature from foreign engineering companies, which stated that Chinese companies generally use insufficient strength levels for hydraulic testing, usually only reaching 90% of the yield strength, while European and American companies use a strength level of 100% of the yield strength, or even 105%–110% of it. Personally, I think it is reasonable to conduct hydraulic strength tests based on the yield limit, as the yield value is after all the fundamental property of a material.
The hydraulic test of the pipelines on-site is at 1.5 times the process design pressure.
The design pressure is a parameter specified by the process; the 1.6 you mentioned refers to the pipe grade, not the design pressure
On-site tests for steel pipes are conducted at 1.5 times the process pressure, while tests carried out by pipe manufacturers are done at the calculated pressure
The water pressure is always maintained at 1.5 times the design pressure, as specified in the standards as well
The problem is that my pipes are rated for 1.6 pressure levels, with a process design pressure of 0.6. If I use 1.5 times that value, I get 0.9. My pipes can handle 1.6 pressure without issue, so what’s the point of using a pressure level of 0.9?
What is the purpose of a hydrostatic test? Is it meaningful to use a pipe thickness calculated for 1.6 MPa for the hydrostatic test, when the test is conducted at 0.9 MPa?
So, fundamentally speaking, what is the purpose of a hydrostatic test? Is it to test the process design pressure? Still testing the pipeline material properties? I lean toward the latter; therefore, the hydrostatic test pressure should be 1.5 times the design pressure calculated based on the pipe wall thickness. The purpose of the hydrostatic strength test is to fill minor cracks, improve ovality, and relieve residual stresses from cold working.
What is the purpose of using a process pressure that is 1.5 times higher in the field tests? The pipes I use at 1.6 megapascals can also be used in situations with 0.6 megapascals pressure; what practical meaning does it have if the water pressure reaches 0.9 megapascals in such cases?
You lean towards the latter, but the latter is not a system design pressure at the root level; the system design requirements are defined in HG20570. The strength test of pipes is carried out at the pipe manufacturer according to the grade you select, while the system strength test is conducted based on the system’s design pressure.