Generally, industrial valves are not subjected to strength tests when in use, but the valve body and valve cover after repair or the valve body and valve cover damaged by corrosion should be subjected to strength tests. For safety valves, the set pressure and return seat pressure and other tests should comply with the provisions of their instructions and relevant regulations. Before installing the valve, the valve strength test and valve sealing test should be carried out on the valve hydraulic test bench. 20% of low-pressure valves are randomly inspected, and if they are unqualified, they should be inspected 100%; medium and high-pressure valves should be inspected 100%. Common media for valve pressure testing include water, oil, air, steam, nitrogen, etc. The pressure test methods for various industrial valves including pneumatic valves are as follows:
1. Pressure test method for stop valves and throttle valves
For the strength test of stop valves and throttle valves, the assembled valves are usually placed in the pressure test stand, the valve disc is opened, the medium is injected to the specified value, and the valve body and valve cover are checked for sweating and leakage. Strength test can also be carried out on a single piece. The sealing test is only performed on stop valves. During the test, the valve stem of the stop valve is in a vertical state, the valve disc is opened, and the medium is introduced from the bottom end of the valve disc to the specified value, and the packing and gasket are checked; after passing the test, the valve disc is closed, and the other end is opened to check for leakage. If both the valve strength and sealing tests are to be performed, the strength test can be performed first, and then the pressure can be reduced to the specified value of the sealing test to check the packing and gasket; then the valve disc is closed, and the outlet end is opened to check whether the sealing surface is leaking.
2. Pressure test method of gate valve
The strength test of the gate valve is the same as that of the stop valve. There are two methods for the sealing test of the gate valve.
① Open the gate to raise the pressure in the valve to the specified value; then close the gate, immediately remove the gate valve, check whether there is leakage at the seals on both sides of the gate, or directly inject the test medium into the plug on the valve cover to the specified value, and check the seals on both sides of the gate. The above method is called intermediate pressure test. This method is not suitable for sealing tests on gate valves with a nominal diameter of less than DN32mm.
② Another method is to open the gate to raise the valve test pressure to the specified value; then close the gate, open the blind plate at one end, and check whether the sealing surface is leaking. Then turn it over and repeat the above test until it is qualified.
The sealing test of the packing and gasket of the pneumatic gate valve should be carried out before the sealing test of the gate.
3. Pressure test method of ball valve
The strength test of the pneumatic ball valve should be carried out when the ball valve is half-open.
① Floating ball valve sealing test: put the valve in a half-open state, introduce the test medium at one end, and close the other end; turn the ball several times, open the closed end when the valve is in the closed state, and check the sealing performance of the packing and gasket at the same time. There must be no leakage. Then introduce the test medium from the other end and repeat the above test.
② Sealing test of fixed ball valve: rotate the ball several times without load before the test, the fixed ball valve is in the closed state, and the test medium is introduced from one end to the specified value; use a pressure gauge to check the sealing performance of the introduction end, and use a pressure gauge with an accuracy of 0.5 to 1 level and a range of 1.6 times the test pressure. If there is no pressure drop within the specified time, it is qualified; then introduce the test medium from the other end and repeat the above test. Then, put the valve in a half-open state, close both ends, fill the inner cavity with medium, and check the packing and gasket under the test pressure. There must be no leakage.
③ The three-way ball valve should be tested for sealing at various positions.
4. Pressure test method of plug valve
① When the plug valve is tested for strength, the medium is introduced from one end, the remaining passages are closed, and the plug is rotated to each fully open working position for testing. If no leakage is found in the valve body, it is qualified.
② During the sealing test, the straight-through plug valve should keep the pressure in the cavity equal to that in the passage, rotate the plug to the closed position, check from the other end, and then rotate the plug 180° to repeat the above test; the three-way or four-way plug valve should keep the pressure in the cavity equal to that at one end of the passage, rotate the plug to the closed position in turn, introduce the pressure from the right-angle end, and check from the other ends at the same time.
Before the plug valve test bench, it is allowed to apply a layer of non-acidic thin lubricating oil on the sealing surface. If no leakage or enlarged water droplets are found within the specified time, it is qualified. The test time of the plug valve can be shorter, generally specified as 1 to 3 minutes according to the nominal diameter.
The plug valve for gas should be tested for air tightness at 1.25 times the working pressure.
5. Pressure test method of butterfly valve
The strength test of pneumatic butterfly valve is the same as that of stop valve. The sealing performance test of the butterfly valve should introduce the test medium from the medium flow end, the butterfly plate should be opened, the other end should be closed, and the injection pressure should be up to the specified value; after checking that there is no leakage in the packing and other seals, close the butterfly plate, open the other end, and check that there is no leakage in the butterfly plate seal to be qualified. The butterfly valve used for regulating flow can be exempted from the sealing performance test.
6. Pressure test method of diaphragm valve
The strength test of diaphragm valve introduces the medium from either end, opens the valve disc, and closes the other end. After the test pressure rises to the specified value, check that there is no leakage in the valve body and valve cover to be qualified. Then reduce the pressure to the sealing test pressure, close the valve disc, open the other end for inspection, and check that there is no leakage to be qualified.
7. Pressure test method of check valve
Check valve test state: the axis of the valve disc of the lifting check valve is in a position perpendicular to the horizontal; the axis of the channel and the axis of the valve disc of the swing check valve are in a position approximately parallel to the horizontal line.
During the strength test, the test medium is introduced from the inlet end to the specified value, and the other end is closed. Check that there is no leakage in the valve body and valve cover to be qualified.
The sealing test introduces the test medium from the outlet end, and checks the sealing surface at the inlet end. If there is no leakage at the packing and gasket, it is qualified.
8. Pressure test method of safety valve
① The strength test of the safety valve is the same as other valves, and water is used for testing. When testing the lower part of the valve body, the pressure is introduced from the inlet I=I end, and the sealing surface is closed; when testing the upper part of the valve body and the valve cover, the pressure is introduced from the outlet El end, and the other ends are closed. If there is no leakage in the valve body and the valve cover within the specified time, it is qualified.
② The sealing test and constant pressure test generally use the following media: saturated steam is used as the test medium for steam safety valves; air is used as the test medium for ammonia or other gas valves; water is used as the test medium for water and other non-corrosive liquid valves. Nitrogen is often used as the test medium for safety valves in some important positions.
The sealing test is carried out with the nominal pressure value as the test pressure, and the number of times is not less than two times. If there is no leakage within the specified time, it is qualified. There are two methods for leak detection: one is to seal all the joints of the safety valve, and use butter to seal thin paper on the outlet flange. If the thin paper bulges, it is leaking, and if it does not bulge, it is qualified; the other is to seal a thin plastic plate or other plate on the lower part of the outlet flange with butter, fill water to seal the valve disc, and check that there is no bubbling in the water to be qualified. The number of fixed pressure and return seat pressure tests of the safety valve shall not be less than 3 times, and it shall be qualified if it meets the requirements.
For various performance tests of safety valves, see GB/T 12242 "Safety Valve Performance Test Method".
9. Pressure test method of pressure reducing valve
① The strength test of the pressure reducing valve is generally assembled after a single piece test, and it can also be tested after assembly. Strength test duration: 1min for DN<50mm; more than 2min for DN65~150mm; more than 3min for DN>150mm.
After the bellows is welded to the assembly, 1.5 times the highest pressure after the pressure reducing valve is applied and air is used for strength test.
② The sealing test is carried out according to the actual working medium. When testing with air or water, the test is conducted at 1.1 times the nominal pressure; when testing with steam, the test is conducted at the maximum working pressure allowed at the working temperature. The difference between the inlet pressure and the outlet pressure is required to be no less than 0.2MPa. The test method is: after the inlet pressure is adjusted, gradually adjust the adjusting screw of the valve so that the outlet pressure can change sensitively and continuously within the maximum and minimum value range, and there must be no stagnation or blocking. For steam pressure reducing valves, after the inlet pressure is adjusted away, close the shut-off valve behind the valve, and the outlet pressure is the highest and lowest value. Within 2 minutes, the rise of its outlet pressure should meet the regulations. At the same time, the volume of the pipeline behind the valve meets the regulations to be qualified; for water and air pressure reducing valves, after the inlet pressure is adjusted, when the outlet pressure is zero, close the pressure reducing valve for a sealing test, and no leakage within 2 minutes is qualified.