Valve manufacturing is a complex yet simple process. Production cycles of ordinary valves such as
gate valves, ball valves, globe valves, butterfly valves and check valves are generally three days. The finished valves must go through a variety of performance tests, and the most important test is the pressure test. Pressure tests are to test whether pressure values which valves can bear meet production requirements or not. Generally, we should comply with the following principles and precautions for valve pressure tests.
(1) In general,
valves are not required to have strength tests, but there should be strength tests for valve bodies and valve bonnets which are patched or corrosively damaged valve bodies and valve bonnets. For safety valves, their level pressure, reseating pressure and other tests ought to conform to their specifications and relevant regulations.
(2) There should be strength tests and leakage tests before installing valves. We need to spot check 20% of low pressure valves; if they are not qualified, we will need to check all the low pressure valves; we should fully check high pressure valves.
(3) Installation positions of
valves should be in the direction where we can easily examine valves when we test them.
(4) For valves with welded connections, if blind flanges don't work, we need to adopt cone seals or O ring seals for pressure tests.
(5) We need to remove air in the
valves as much as possible when we have hydraulic tests for the valves.
(6) We should gradually increase pressure in the test; rapidly and suddenly pressurize valves are not allowed.
(7) Strength tests and leakage tests generally should last for two to three minutes, and the tests for important & special valves should last for five minutes. Testing time for valves with small diameters can be correspondingly shorter, while valves with big diameters can be correspondingly longer. If you have doubts, you can extend the testing time in the process. Drips or leakages of valve bodies and valve bonnets are not allowed in strength tests. The general valves only need to have leakage tests once, while safety valves, high pressure valves and other valves should be tested twice. Unimportant valves such as low pressure valves, valves with big diameters as well as valves that are allowed to leak due to regulations can have little leakage; we need to implement the relevant regulations of leakage due to different leakage requirements of general valves, power station valves, marine valves and other valves.
(8) There is no need to have leakage tests for valve closures of throttle valves, but there should be strength tests and leakage tests for packing and gaskets.
(9) In the pressure test, the maximal valve closing force is one person's normal physical strength; we can not use tools like levers (except torque wrenches) to increase the force; when the hand wheels' diameters are greater than or equal to 320mm, two people are allowed to close valves together.
(10) We should take out packing of valves with upper seals for the leakage tests. Check whether there are leakages or not after upper seals are closed.
(11) For
valves with actuating devices, we should close valves by actuating devices and then have leakage tests. For actuating devices with manual operation, we also need to close the actuating devices by hands and then have leakage tests.
