Pressure Test Methods for Various Industrial Valves
October 7, 2020
In normal conditions, industrial valves are not subjected to a strength test while in operation, but after maintenance, the corrosion-damaged valve body and cover may be subjected to a strength test. In the case of safety valves, steady pressure, return pressure and other checks should meet with the standards and associated regulations. The valve strength test and the valve sealing test should be conducted on the hydraulic test bench before the valve is fitted back. Twenty percent of low-pressure valves must be tested randomly. If not qualified, all the valves should be tested; the medium and high-pressure valves should be checked 100 percent. Water, oil, air, steam, nitrogen, etc. are widely used as media for valve pressure testing.
Pressure testing techniques for different industrial valves, including pneumatic valves, which are as follows:
1. Pressure test method for globe valve and throttle valve.
The valve is normally placed in a pressure test rack during the strength test of the stop valve and the throttle valve, the valve disc is opened, the medium is injected to the stated value, and the valve body and valve cover are tested for sweating and leakage. The strength test can be carried out on a single piece as well. For the sealing test, only stop valves are being used. The valve stem of the shut-off valve is in a vertical state during the test, the valve flap is opened, and the medium is added to the stated value from the bottom end of the valve flap. Check the packing and gasket; after passing the inspection, close the valve flap and open its other end to check for leakage.
The strength test should be performed first if both valve strength and tightness tests are to be performed, then the pressure is lowered to the required tightness test value, and the packaging and gaskets are checked; then the valve flap is closed and the output end is opened to check if the sealing surface has leakages.
2. Pressure test method of gate valve.
The gate valve’s strength test is the same as that of the globe valve. There are two techniques for the gate valve sealing test.
- To increase the pressure inside the valve to the stated value, open the gate; then close the gate and quickly remove the gate valve to inspect the seals for leakage on both sides of the gate or inject the test medium to the stated value directly into the plug on the valve cover. The method above is called the test of intermediate pressure. This method is not appropriate for the sealing test of a nominal diameter gate valve below DN32 mm.
- In order to raise the test pressure of the valve to the defined value, another approach is to open the gate, now close the gate and open the blind plate at one end to examine if the sealing surface is leaking. Reverse the head, repeat the above examination until it is qualified. Before the gate sealing test, a pneumatic gate valve packing and gasket sealing test must be conducted.
3. Pressure test method of a ball valve.
The pneumatic ball valve strength test should be conducted with the ball body half-open.
- Floating ball valve tightness test: place the valve in a half-open state, insert the test media at one end and close the other end; rotate the ball many times, open the closed-end when the valve is closed, and at the same time check the sealing efficiency at the packaging and gasket. There must be no leak. Then enter the test medium from the other end and repeat the test mentioned above.
- Fixed ball valve sealing test: rotate the ball several times until the load-free test, the fixed ball valve is closed, and the test medium is applied from one end to the stated value; check the sealing efficiency of the inlet end with a pressure gauge, and the accuracy of the pressure gauge is 0.5 -1, the measurement range is 1.6 times the test pressure. There is no decompression phenomenon as qualified within the period specified; then enter the test medium from the other end and repeat the test mentioned above. Then position the valve in a half-open state, closed at both ends, and the inner cavity is filled with a medium. Check the package and the gasket under the test pressure, and there should be no leakage.
- The three-way ball valve must be checked at each position for tightness.
4. Pressure test method of plug valve.
- The medium is inserted from one end during the strength test of the plug valve, the other passages are closed, and the plug is turned to fully open working position for the test. If there is no leakage, then the valve body qualifies.
- At the time of the sealing test, the straight-through cock must maintain the pressure in the cavity similar to that of the passage, turn the plug to the closed position, inspect from the other end, and then turn the 180 ° plug to repeat the above test; the three-way or four-way plug valve. Pressure in the cavity and one end of the passage should be maintained at the same time, the plugs should be turned to the closed position, the pressure from the right-angle end, as well as the other end, should be examined at the same time. Stop the test bench before allowing the sealing surfaces coated with a non-acid dilute lubricant to be sealed within a predetermined time without leaking and the expansion of the water droplets is permissible. The test time of the plug valve may be shorter, normally 1~3min depending on the nominal diameter.
- The gas plug valve shall be checked for airtightness at 1.25 times the operating pressure.
5. Pressure test method of the butterfly valve.
The pneumatic butterfly valve strength test is the same as the globe valve test. The sealing performance test of the butterfly valve will require the test medium from the inflow end of the media, the butterfly plate must be kept open, its other end shut, and the injection pressure should meet the specified value; after testing the sealing and other seals without leakage, close the butterfly plate, open the other end and check the butterfly plate. If there is no leakage at the seal it qualifies. The butterfly valve that is used to adjust the flow cannot be checked for sealing.
6. Pressure test method of the diaphragm valve.
For the diaphragm valve strength test, the media is added from any one of the ends, the valve flap needs to be opened and the other end should be closed. After the test pressure is raised to the stated value, it’s qualified to see if the valve body and the valve cover have no leakage. Reduce the pressure to the tightness test pressure, shut the valve flap, and open the other end for inspection. No leakage, the valve will get qualified.
7. Pressure test method of a check valve.
Test condition of the check valve: the disc axis of the lift check valve is perpendicular to the horizontal; the channel axis of the swing check valve and the disc axis is roughly parallel to the horizontal line.
In the sealing test, from the outlet end, the testing medium is applied and the sealing surface is tested at the inlet end. No leakage at the packing and gasket, the valve gets qualified.
8. Pressure test method of a safety valve.
- The safety valve strength test is similar to other valves. Water is used as testing media. The pressure from the inlet end and the sealing surface is closed when the lower part of the valve body is tested; the pressure from the outlet end and the other ends are closed when the upper part of the valve body and the bonnet are tested. For the specified duration without leakage, the valve body and cover get to qualify.
- The common medium for sealing and constant pressure testing is: steam safety valve uses liquid steam as the test medium; ammonia or other gas valves use air as the test medium; water and other non-corrosive liquid valves use water as the test medium. As the testing medium, nitrogen is also used for safety valves in a critical position.
The sealing test shall be carried out as the test pressure with a nominal pressure value and the number of times shall not be less than twice, and no leakage shall be found to be eligible within the stated duration.
There are two ways to perform leak detection: one is to seal the safety valve joints, and the thin paper on the outlet flange is coated with butter. It is leaky if the thin paper is bulging, and the thin plastic is not bulging. To the lower part of the outlet flange, the plate or other plate seals are attached, and the valve disc is sealed by water filling. If the water does not bubble, it gets qualified. The number of tests of the safety valve’s constant pressure and return pressure shall not be less than 3 times and shall be qualified only if it passes the requirements.
9. Pressure test method of pressure reducing valve.
The pressure reduction valve strength test is normally assembled after a single component test, and after configuration, it can also be tested. Duration of the strength test: more than 1 min for DN<50 mm; more than 2min for DN65 – 150 mm; more than 3min for DN>150 mm. Apply 1.5 times the maximum pressure after the pressure relief valve and do a strength test with air after the bellows and the parts are welded.
The sealing test is performed based on the actual work medium. Test at 1.1 times the nominal pressure when measured with air or water; test at the maximum operating pressure permissible under the working temperature when measured with steam. The gap must not be less than 0.2MPa between the inlet pressure and the outlet pressure.
The test method is: gradually adjust the adjusting screw of the valve after the inlet pressure is set so that the outlet pressure will change sensitively and continuously without stagnation or jamming within the range of the maximum and minimum values. For valves that reduce water and air pressure, when the inlet pressure is adjusted and the outlet pressure is zero, close the valve that reduces pressure and perform a tightness test. If no leakage occurs within 2 minutes, it is accepted.