Regarding design pressure and test pressure

I passed by the study spot; I’m waiting for the answers from the seniors, but I guess it’s option 2.5

The design pressure of 2.5 MPa is determined based on process requirements; the pipeline pressure class can be selected as 4.0 MPa； The hydrostatic test is not a long-duration process; short-term pressure resistance poses no problem.

Operating pressure is 2.2 MPa, design pressure is 2.5 MPa, and the test pressure under hydraulic conditions is 3.75 MPa. Once the experiment is complete, there’s no need to worry about it anymore; the pressure of the materials during actual operation will not exceed the designed pressure. In cases where it might exceed that level, safety valves are in place to prevent problems.

If the design pressure chosen is 2.5 MPa, should the pipe grade selected be 2.5 or 4.0? I think the pipeline grade should not be lower than the test pressure grade

The design pressure is 2.5 MPa, so the pipe class must be 4.0 MPa. Please take a close look at the pipe class table – both the design pressure and design temperature need to be taken into consideration. In other words, the pipe grade is a temperature and pressure grade, and it is certainly greater than the design pressure.

The design pressure is a theoretical value; the actual value chosen must not be lower than this design pressure. But the test pressure is sometimes higher than what is actually used; it depends on the situation. The actual pressure rating selected has nothing to do with the test pressure. For example, if you choose a 2.5 MPa flange and valve, that is its maximum operating pressure, but its test pressure is higher than 2.5 MPa.

The design pressure and test pressure serve different purposes; the design pressure is determined by taking into account various factors such as corrosion, service life, and material variations to ensure normal operation. The test pressure is used to conduct a comprehensive inspection of the pipeline’s performance after its construction. The strength criteria for test pressure and design pressure are different; the test pressure calculated based on the design pressure needs to be verified, but it generally meets the requirements.

The grade of a pipe is determined by the most severe conditions resulting from the combination of pressure and temperature during operation. The theoretical wall thickness of the pipe in design is calculated taking into account the worst operating conditions; in practice, the actual wall thickness chosen also takes into account the amount of corrosion expected over the designated service life, as well as any positive or negative deviations in pipe manufacturing, so that an integer value is selected as the standard thickness. During pressure testing, the temperature is usually at room temperature, around 20 degrees Celsius, and there is no rapid corrosion during such tests. Therefore, the allowable pressure calculated by reversing the process using the standard thickness chosen for the pipe generally can reach 1.5 times the design pressure. Of course, some large pipes are exceptions; the larger the pipe, the lower the allowable pressure calculated, and it gets closer to the calculated pressure.

S=(P*D)/(2*σ*Ф)+G1+G2, where p is the design pressure, D is the outer diameter of the pipe, σ is the allowable stress of the material at the design temperature, Ф is the weld coefficient, G1 represents the corrosion factor, G2 refers to the negative deviation in the pipe wall thickness

There is also a negative wall thickness deviation expressed as a percentage, which will not be listed here