Pressure test for pressure vessel manufacturing

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Part 01 Introduction 1. Purpose of pressure testing a. To evaluate the overall strength, stiffness, and stability of the container (for components that may suffer from local instability due to internal pressure)； b. Check the density of the welded joint; c. Verify the sealing performance of the sealing structure ； d. Eliminate or reduce welding residual stresses and the peak stresses in local discontinuities ； e. It has a closing effect on microcracks, blunting their tips. 2. Types of tests The pressure resistance tests for internal-pressure vessels include hydraulic testing, pneumatic testing, and combined hydraulic-pneumatic testing. The gas-liquid combination test is primarily designed to address the issue where, for certain large pressure vessels, hydraulic testing cannot be carried out due to load-bearing constraints during pressure resistance tests; as a result, gas testing is used, but it is both dangerous and time-consuming. From a safety perspective during testing, the standards specify that the minimum test pressure for the gas-liquid combination used in containers should be the same as the minimum pressure for gas testing; moreover, the requirements regarding container design, manufacturing, non-destructive testing, and safety measures must also be identical to those for gas testing. 3. Pressure test of externally pressurized vessels: Externally pressurized vessels are subjected to a pressure test using internal pressure. The main purpose of this is to check the integrity of the weld joints and verify the sealing performance of the sealing structures, rather than to assess the stability of the vessel. The main failure mode of pressure vessels subjected to external pressures is instability. For pressure vessels constructed in full compliance with standards, they will not experience instability failure under the design external pressure; therefore, there is no need to conduct external pressure tests to assess their stability. Only a pressure test using internal pressure is required. Check the sealing performance of the sealing structure and identify potential risks that could develop into penetrating defects. Ⅱ Part 02 Requirements 1. Test Conditions All manufacturing work has been completed, except for those processes that cannot be carried out before the test. All testing has been completed, except for the tests required after the experiment. The equipment must be manufactured and pass inspections according to the specified criteria before a hydrostatic test can be conducted. Before pressure testing, all debris inside the equipment such as slag, welding rods, loose scale, slag, metal shavings, welding spatter, oil, and greases must be thoroughly removed. The washers should be of the same type as those used with the product. 2. Test temperature: When conducting hydraulic tests on containers made of Q345R, Q370R, and 07MnMoVR, the temperature of the liquid must be no lower than 5°C; for containers made of other carbon steels and low-alloy steels, the liquid temperature must be no lower than 15°C. For hydraulic tests on containers designed for low-temperature operations, the liquid temperature must be at least 20°C higher than the impact test temperature of the container’s material and welding joints, whichever is higher. If factors such as plate thickness cause the material’s plastic transition temperature to increase, the test temperature must be raised accordingly. When supported by experimental data, it is possible to use a liquid at a lower temperature for the tests, but it must be ensured that the test temperature (the temperature of the metal walls of the container) is at least 30°C higher than the temperature at which plastic deformation of the metal walls of the container occurs. 3. Test medium a. Any non-hazardous liquid can be used for hydraulic testing at temperatures below its boiling point. Flammable liquids with a flash point below 43°C, such as petroleum distillates, can only be used in tests at near-room temperature. To minimize the risk of brittle fracture, it is recommended that during the hydrostatic test, the metal temperature be maintained at least 17°C above the minimum design metal temperature, but not exceeding 48°C. b. The liquid used for hydrostatic testing should be fresh water treated with a corrosion inhibitor. For stainless steel containers, the chloride content in the water used for testing must not exceed the standard requirements. 【See: Why control the chloride ion concentration in hydrostatic testing】 4. Pressure gauges: If pressure gauges are used to measure the test pressure, two gauges with identical readings and that have passed calibration checks should be employed. The scale range of these pressure gauges must comply with the relevant construction standards. The pressure gauges must be calibrated within their valid period, and whenever there is any reason to believe that they may be inaccurate, they must be recalibrated. 5. Airtightness test of reinforcement plates and saddle gaskets: The openings in the container, along with the reinforcement rings and saddle gaskets, should have their joint quality checked using specified compressed air before testing. Compressed air and soap bubble tests are conducted through the inspection holes to determine the sealability of the welds (GB/T150.4, etc.). Before conducting the pressure test on the container, compressed air at a pressure of 0.4MPa to 0.5MPa should be introduced into the reinforcement ring in order to check the quality of the welds; hydraulic testing is strictly prohibited. The signal hole of the reinforcement ring must be at least M10 in size and located at the lowest point of the reinforcement ring (HG/T20584.17). 6. Test pressure: The test pressure for the product shall be determined in accordance with the requirements specified in the drawings. 7. Preparations before the hydrostatic test a. Before the pressure test, the fasteners at all connections of the container must be fully installed and properly tightened; temporary pressure-bearing elements installed for the purpose of the pressure test shall have appropriate measures taken to ensure their safety. b. The pressure gauge for testing should be installed at the top of the location where the container under test is placed. c. The pressure gauge calibration certificate, water quality analysis report, and hydrostatic test record sheet should be prepared. Part 03 Process 1. Hydrostatic testing process and requirements a. Water should be slowly injected into the container to prevent water hammer effects, while simultaneously all air in the system must be thoroughly removed. b. The gas inside the test container should be completely removed and replaced with liquid; during the test, the observation surface of the container must remain dry. c. The pressure is gradually increased to the test pressure; any leaks or issues detected should be marked and recorded, and repairs should be carried out only after the pressure has been completely released. d. It shall be maintained at the hydrostatic test pressure for the time required by the specifications (GB/T150.4 specifies that this period should generally be no less than 30 minutes). e. When a leak is detected, the location should be marked; the pressure should be released gradually, the container should be drained of water, and appropriate repairs and replacements should be carried out. The affected container should then be tested again at its original pressure. f. There must be no sources of fire in the vicinity of the testing area, and appropriate fire-fighting equipment must be available. After the pressure test, if the repair depth exceeds one-half of the thickness of the base material, the pressure test must be repeated. g. During the pressure holding phase of the withstand voltage test, continuous pressure application to maintain the test pressure is not allowed; during the test, fasteners must not be tightened under pressure, nor shall any external force be applied to the components under stress. 2. Acceptance criteria for hydraulic testing: a. During the testing, the container shall show no leaks, no visible deformation, and no abnormal noises. b. Inspections shall be carried out at a pressure not less than the design pressure (GB/T150.4). Inspect all welds, joints, and sealing surfaces; no leakage is allowed during visual inspection. However, leakage is permissible at joints where openings intended for welding are temporarily sealed for pressure testing. Leaks from the temporary closure should be directed outside to avoid obscuring leaks at other connections. c. During testing, the pressure should be increased only after the container temperature is roughly equal to the liquid temperature, with the temperature not exceeding 48°C. d. Unless the user or their designated agent permits it prior to the hydraulic test, neither the inner nor the outer surface of the pressure-welded seams of the container shall be painted or coated ； When paint or coating is permitted prior to the pressure test, or when an internal non-metallic lining is allowed, the pressure-welded joints shall first undergo a leak test. With the consent of the user or their designated agent, this test can exempt them from requirement a at the end of Part 04 of IV. After completing the test, fill out the test record. b. All product components shall be properly installed, all temporary blind flanges shall be removed, and all filling lines and fittings shall be drained of water and then dried with air or nitrogen. c. Unless otherwise specified, all washers shall be replaced with new ones after testing.