Under normal circumstances, industrial valves are not subjected to strength tests when in use, but after repairing the valve body and bonnet or the valve body and bonnet damaged by corrosion, the strength test should be carried out. For safety valves, the constant pressure and return pressure and other tests shall comply with the specifications and relevant regulations. The valve strength test and valve sealing test should be done on the valve hydraulic test bench before the valve is installed. Low-pressure valves should be spot-checked 20%, if they are unqualified, 100% should be checked; medium and high-pressure valves should be 100% checked. Commonly used media for valve pressure testing include water, oil, air, steam, nitrogen, etc. The pressure testing methods for various industrial valves including pneumatic valves are as follows:
Pressure test method for globe valve and throttle valve
In the strength test of the stop valve and the throttle valve, the assembled valve is usually placed in a pressure test rack, the valve disc is opened, the medium is injected to the specified value, and the valve body and valve cover are checked for sweat and leakage. The strength test can also be carried out in a single piece. The tightness test is only for shut-off valves. During the test, the valve stem of the shut-off valve is in a vertical state, the valve flap is opened, and the medium is introduced from the bottom end of the valve flap to the specified value. Check the packing and gasket; after passing the test, close the valve flap and open the other end to check for leakage. If both the valve strength and tightness test are to be done, the strength test can be done first, then the pressure is reduced to the specified value of the tightness test, and the packing and gaskets are checked; then the valve flap is closed and the outlet end is opened to check whether the sealing surface is leaking.
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 increase the pressure in the valve to the specified value; then close the gate and immediately take out the gate valve to check whether there is any 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 , Check the seals on both sides of the gate. The above method is called intermediate pressure test. This method is not suitable for the sealing test of gate valves with a nominal diameter below DN32mm.
② Another method is to open the gate to increase the test pressure of the valve to the specified value; then close the gate and open the blind plate at one end to check whether the sealing surface is leaking. Reverse the head and repeat the above test until it is qualified.
The sealing test of the packing and gasket of the pneumatic gate valve shall be carried out before the sealing test of the gate.
Pressure test method of ball valve
The strength test of the pneumatic ball valve should be carried out with the ball valve half open.
① Floating ball valve tightness test: Put the valve in a half-open state, introduce the test medium at one end, and close the other end; rotate the ball several times, open the closed end when the valve is in the closed state, and check the sealing performance at 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 Tightness 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; the pressure gauge is used to check the sealing performance of the inlet end, and the accuracy of the pressure gauge is 0 ．5~1 level, measuring range is 1.6 times of the test pressure. Within the specified time, if there is no depressurization phenomenon, 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, with both ends closed, and the inner cavity filled with medium. Check the packing and gasket under the test pressure, and there should be no leakage.
③ The three-way ball valve should be tested for tightness at various positions.
Pressure test method of plug valve
① During the strength test of the plug valve, the medium is introduced from one end, the remaining passages are closed, and the plug is rotated to the fully open working positions for the test. The valve body is qualified if no leakage is found.
② During the sealing test, the straight-through plug should keep the pressure in the cavity equal to that of 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; three-way or four-way plug valve The pressure in the cavity and one end of the passage should be kept equal, the stopper should be rotated to the closed position in turn, the pressure should be introduced from the right-angle end, and the other end should be checked simultaneously.
Before the plug valve test bench, it is allowed to apply a layer of non-acidic dilute lubricating oil on the sealing surface, and no leakage or enlarged water droplets within the specified time are qualified. The test time of the plug valve can be shorter, generally 1~3min according to the nominal diameter.
The plug valve for gas shall be tested for air tightness at 1.25 times the working pressure.