The principle of pressurizing pipes with water and some basic questions
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The pressure that can be applied to pipes can reach any value (as long as it is within the pressure limits specified by the equipment design), and the reason for this is simple: gases can be compressed, whereas liquids cannot. So why is it possible to apply a water pressure of 10 kilograms to some pipes, or even 20 kilograms, 30 kilograms, or even 40 kilograms? (1.5 times the design pressure for piping) What is the principle behind this? Also, is it true that when the pressure in the pipeline is too high, water pressure testing is carried out to ensure safety and assess the strength of the pipeline? You can’t increase air pressure.
The first question is hard to explain, but saying that liquids or water are incompressible is also relative. The second issue is that pressure testing affects the strength of the pipelines or the condition of their welds. It is not appropriate to use air pressure for brittle materials, as failure in such cases leads to an explosion; with water pressure, it results in a sudden release of water.
Since the liquid is not pure but contains dissolved gases, this results in an increase in pressure. Get a bottle of Sprite, Coke, or something like that, open it – doesn’t pressurized carbon dioxide come out? It’s basically the same principle.
Water can also be compressed, though to different degrees. There are standard requirements for both airtightness and strength tests; for the projects we work on, the strength test for gas pipelines is required to be at 1.1–1.25 times the design pressure. The oil pipeline has a design pressure of 1.25 times!
The pressure for leak detection is the design pressure!
1. The first question is not very clear, but the liquid oil streams in petrochemical plants can all be pressurized using pumps. 2. The second question: as mentioned on the second floor, water is used as the medium for pressure testing, and if the pipeline ruptures during the test, it’s just water that flows out ； However, when the medium is air and the pipe ruptures, it’s like an explosion, which can easily injure people. The standards specify that air pressure can be used only when water pressure is truly problematic, but proper protection measures must be in place and the pressure should not exceed 1.6 Mpa. 3. The leak test is also conducted using gas, and the test pressure is the design pressure of the pipeline; therefore, the pressure during the leak test can exceed 1.6 Mpa. In my understanding, the leak test is conducted after the pressure test, that is, the strength test; by that point the safety of the pipe fittings has already been ensured. The leak test is simply used to check the tightness of seals such as those on valve gaskets and flange surfaces. The pressure test commonly referred to is the strength test of piping systems; therefore, the two are different.
Firstly, it should be explained using Pascal’s principle: liquids transmit pressure, and even in the case of incompressible liquids, it is the pressure generated by the testing pump that is transmitted. What is called pressure is actually stress. Secondly, it is explained using the internal energy of the pressure testing medium. It is related to volume, pressure, temperature, velocity, and so on. Compressibility is much lower for liquids compared to gases.