Small question about pipeline pressure testing
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This post was last edited by mu*ao2000 on 2009-11-18 at 20:37. For the drilling and penetration section of a long-distance pipeline, the design pressure is 4.0 MPa; it is located in zone 3 with low temperatures. The drawings specify that the hydraulic test pressure should be 1.4 times the design pressure, which equals 5.6 MPa, over a period of 4 hours (according to standard 50369-2006). However, due to the extremely low temperatures, a pneumatic test was used instead, with the strength pressure still set at 1.4 times the design pressure, That is, 5.6 MPa. 4 hours. However, taking into account the temperature difference between inside and outside the pipeline, the on-site thermometer indicated a maximum temperature of 18 degrees inside the pipeline, while the ambient temperature was -12 degrees. Using the formula PV/T=C, the pressure resulting was 6.3 megapascals. 4 hours. Is it okay to do this? Does this count as overpressure testing? What is the basis for this? Please advise
The first question is whether the temperature difference between inside and outside the tube is 30 degrees. In other words, if the temperature of the gas used is 18 degrees while the ambient temperature is -12 degrees, it’s very likely that after 4 hours of maintaining pressure, the temperature inside the tube will also drop to -12 degrees. If understood in this way, the pressure used for testing should still be 5.6 MPa. After 4 hours, if there is no leakage, the pressure will certainly decrease as the temperature inside the pipe drops. All you need to do is calculate the pressure corresponding to 5.6 MPa at that temperature after 4 hours and compare it with the actual pressure to see whether it meets the requirements; there is no need to first convert the pressure to 6.3 MPa for testing.
This post was last edited by ms3210 on 2009-12-3 at 18:21. First, I would like to ask why the medium in the main pipeline is gas phase? When using gas for applying pressure, it is not possible to use the same test pressure as that used with liquids. Moreover, the method used by the original poster for converting the material’s test pressure is incorrect. The correct formula should be: (Allowable stress of the material at ambient temperature / Allowable stress of the material at its design temperature) * 1.4 * Design pressure. When gas is used in place of liquid, the test pressure should be 1.15 times the design pressure, rather than 1.4 times. Please recalculate it. Additionally, using gas tests is very dangerous; in principle, they should only be used when the design pressure is below 0.6 MPa
2# guanwutang: I think what you say makes sense; otherwise it would be considered a destructive test. But do you have any standard criteria to back this up?
Do not perform any conversion; after filling the tube with gas, wait until the temperature of the gas inside the tube is equal to the ambient temperature before conducting the test. Remember: The test temperature must be 30°C higher than the material’s brittle transition temperature.
It is necessary to calculate this temperature in order to compare the stress on the material at that experimental pressure with the material’s allowable stress