Why do the valves keep failing? Is it a quality issue or incorrect operation?

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Why do valves always fail? Every step in the entire process – design, manufacturing, installation, operating conditions, operation, and maintenance – must be handled with care. How can it be determined whether there are any problems with a valve before it leaves the factory or after it has been fully installed? This requires visual inspection as well as certain performance tests. Through these test results, defects can be identified and corresponding adjustments made; the system can only be put into use after all tests have passed. So, what details should be paid attention to during visual inspection? What does performance testing include? Visual Inspection: 1. Check whether there are any defects such as sand holes or cracks on the inner and outer surfaces of the valve body. 　　2. Check whether the valve seat is firmly bonded to the valve body, whether the valve core fits well with the valve seat, and whether there are any defects in the sealing surfaces. 　　3. Whether the connection between the valve stem and the valve core is flexible and reliable, whether the valve stem is bent, and whether the threads are damaged or corroded. 　　4. Whether the packing and washers are aged or damaged, and whether the valve opens smoothly, etc. 　　5. The valve body shall be equipped with a nameplate, which shall include information such as the manufacturer’s name, the name of the valve, nominal pressure, and nominal diameter. 　　6. The open and closed positions of valves during transportation shall meet the following requirements: (a) Valves such as gate valves, globe valves, throttle valves, butterfly valves, bottom valves, and control valves shall be in the fully closed position. 　　(b) The shut-off elements of the plug valves and ball valves shall be in the fully open position. 　　(c) The diaphragm valve should be in the closed position, but it must not be closed too tightly to prevent damage to the diaphragm valve. 　　(d) The valve disc of the check valve shall be closed and secured. 　　7. Spring-type safety valves shall be equipped with lead seals, while lever-type safety valves shall have a weight positioning device. 　　8. The valve disc or plug of the check valve must operate smoothly and accurately, without any eccentricity, displacement, or skewing. 　　9. The inner surfaces of valves lined with rubber, enamel, or plastic should be smooth and even; the lining should be firmly bonded to the substrate, without any defects such as cracks or bulging. 　　10. The flange sealing surface must meet the requirements and shall have no radial scratches. 　　11. The valve must not have any damage, missing parts, corrosion, or detached nameplates, and the valve body must be free from dirt. 　　12. The valves should be equipped with protective covers on both ends, and the handles or handwheels must operate smoothly without any sticking. 　　13. The valve quality certificate shall include the following contents: (a) Manufacturer’s name and date of manufacture. 　　(b) Product name, model, and specifications. 　　(c) Nominal pressure, nominal diameter, applicable medium, and applicable temperature. 　　(d) The standards applied, the inspection conclusions, and the inspection date. 　　(e) Factory serial number, inspector’s name, and signature of the person responsible for inspection. 　　Performance tests Shell strength tests Valves can be regarded as pressure vessels, and therefore must be able to withstand the pressure of the fluid without leaking. As such, the blanks for components such as the valve body and valve cover should not contain any defects that could affect their strength, such as cracks, porosity, or inclusions. 　　Strength tests are generally carried out after final assembly. Tests are usually carried out at room temperature, and to ensure safety in use, the test pressure P is generally 1.25–1.5 times the nominal pressure PN. During the test, the valve is in the open position, one end is sealed, and a medium is injected from the other end while pressure is applied. The exposed surfaces of the housing (body, cover) shall be inspected; no leakage is allowed during the specified test duration (usually not less than 10 minutes), for which case the valve can be considered to have passed the strength test. To ensure the reliability of the tests, strength tests should be conducted before painting the valve; when water is used as the medium, the air inside the cavity must be completely removed. 　　For valves with leaks, if welding repairs are permissible according to the technical specifications, such repairs can be carried out in accordance with those specifications; however, a strength test must be conducted again after the repair, with the test duration being appropriately extended. 　　Sealing test: Whether it is a shut-off valve or a control valve, it must have a certain degree of sealing capability when closed. Therefore, each valve must undergo a sealing test before leaving the factory; valves that already have seals in place also need to go through an additional sealing test. 　　Tests are usually carried out at room temperature at the nominal pressure PN, while Su valves are tested at 1.1 times PN pressure. When water is used as the testing medium, it can cause corrosion in the valves; therefore, the water quality must be controlled in accordance with technical requirements, and any remaining water should be blown dry or dried after the test. 　　Gate valves and ball valves require a two-way sealing test because they have two sealing pairs. During the test, first open the valve to block one end of the channel, and introduce pressure from the other end. Once the pressure reaches the specified value, close the valve, then gradually release the pressure at the blocked end, and conduct inspections. The same experiment was repeated on the other end as well. Another testing method for gate valves is to maintain the test pressure inside the chamber and simultaneously check the dual sealing performance of the valve from both ends of the channel. 　　When testing a check valve, pressure should be applied from the outlet side, with inspections being carried out at the inlet. 　　During the sealing test, the closing torque of the valve shall be determined based on the nominal pressure and nominal diameter. Manual valves are usually allowed to be closed only with normal physical strength, without the use of any auxiliary equipment; however, when the diameter of the handwheel is 320 mm or more, two people may be used to close it. Valves with driving mechanisms. Testing should be conducted with the drive device in use. If a closing torque requirement is specified in the technical requirements, a torque wrench must be used to measure the closing torque. 　　The sealing test should be conducted after the strength test following the assembly of the valve, as it is necessary to verify not only the sealing performance of the valve when it is closed, but also the sealing performance of the packing and the gaskets in the middle flange. 　　The upper seal test is usually carried out simultaneously with the strength test. During testing, raise the valve stem to its limit position so that it makes tight contact with the sealing surface of the valve cover; then loosen the packing gland and check its sealing performance. 　　Valves used in gas media, or those specified in technical drawings to require a low-pressure gas seal test, must be tested in accordance with the relevant standard specifications, using nitrogen or dry, clean air as the testing medium. The test pressure is 0.6 MPa. 　　Dynamic performance test: The test medium is the same as that used in the shell strength test and the sealing test, and it is conducted after those tests are successful. 　　Manual valve operation performance test: The valve is in the open position, and the valve chamber is pressurized to the test pressure. The valve is closed using a specified torque, and then the pressure on one side of the valve disc is reduced to create a pressure difference in the direction most unfavorable for opening the valve. Subsequently, the valve is opened using the specified torque. This process is repeated at least three times in full cycles under load, in order to check whether the valve opens and closes properly, whether its operation is smooth, and whether the position indicators for opening and closing are accurate. 　　Check valve operating performance test: Conduct valve opening tests at the specified pressure difference, with no fewer than 3 tests. 　　Tests on the operating performance of electric and pneumatic valves are carried out in accordance with the specifications provided in the valve technical data sheet. If such specifications do not give clear guidelines, the valve should be operated using the rated actuator to complete three full cycles under load. Throughout the test, the valve must operate smoothly and flexibly; it must open and close completely, and the position indication must be accurate. 　　Vacuum sealing test: A highly sensitive method for testing seals. 　　Vacuum testing is usually carried out after the valve’s strength and sealing tests have passed. To ensure the accuracy of the tests, the valve under test should have a high level of cleanliness and finely processed sealing surfaces. Moreover, the valve body and valve cover should generally be made of forgings. 　　Helium mass spectrometry leak detection: After the valve under test is pumped to a specified vacuum level using a vacuum pump, helium gas is applied to the area of the valve that is to be tested. If there is a leak, helium will enter the part of the valve that is under inspection, and the helium mass spectrometer detector in the system can detect this, allowing the leak rate to be calculated. 　　Micro-leakage test: In recent years, as environmental awareness has increased, various organizations around the world have imposed stricter requirements on the sealing performance of valves, especially when these valves are used with highly corrosive, highly radioactive, or highly toxic substances. One of such requirements is the minimal leakage rate for valves. The detection of micro-leakages in valves involves checking the level of minor leaks at the flanges and packing boxes within the valve; it is a type of seal test for the valve housing. 　　The basic principle of valve micro-leak detection is as follows: when the valve is in a semi-open state, helium at a specified pressure is introduced into its interior, and a helium mass spectrometer leak detector equipped with a suction probe and having a calibrated leak rate is used to examine the interior cavity and the packing area to determine whether these areas meet the leak rate requirements specified by the user.