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2021

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07

Comprehensive pressure testing methods for various valves

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Under normal circumstances, industrial valves do not undergo strength tests when in use, but the valve body and valve cover after repair or those damaged by corrosion should undergo strength tests. For safety valves, their set pressure, backseat pressure and other tests should comply with the instructions and relevant regulations. The strength and tightness test should be carried out before the valve is installed. 20% of low-pressure valves shall be randomly inspected. If they are unqualified, 99.99% shall be inspected; medium and high-pressure valves shall be inspected 99.99%. 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:

Under normal circumstances, industrial valves do not undergo strength tests when in use, but the valve body and valve cover after repair or those damaged by corrosion should undergo strength tests. For safety valves, their set pressure, backseat pressure and other tests should comply with the instructions and relevant regulations. The strength and tightness test should be carried out before the valve is installed. 20% of low-pressure valves shall be randomly inspected. If they are unqualified, 99.99% shall be inspected; medium and high-pressure valves shall be inspected 99.99%. 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:

 

  1. Pressure test method for ball valves
  The strength test of pneumatic ball valves should be carried out with the ball in a half-open state.
  ① Floating ball valve sealing test: Put the valve in a semi-open state, introduce the test medium at one end, and close the other end; rotate the ball several times, open the closed end for inspection 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.
  ②Fixed ball valve sealing test: Before the test, rotate the ball several times without load, the fixed ball valve is in a closed state, and introduce the test medium 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 .Level 5~1, the measuring range is 1.5 times of the test pressure. Within the specified time, if there is no pressure drop phenomenon, it is qualified; then introduce the test medium from the other end and repeat the above test. Then, put the valve in a semi-open state, seal both ends, and fill the inner cavity with medium. Check the packing and gasket under the test pressure to see that there must be no leakage.
  ③ The three-way ball valve should be tested for sealing at various positions.

 

  2. Check valve pressure test method
  Check valve test status: the lift check valve disc axis is in a position perpendicular to the horizontal; the swing check valve channel axis and valve disc axis 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. If there is no leakage in the valve body and valve cover, it is qualified.
  In the sealing test, the test medium is introduced from the outlet end, and the sealing surface is checked at the inlet end. If there is no leakage at the packing and gasket, it is qualified.

 

  3. Pressure test method of pressure reducing valve
  ① The strength test of the pressure reducing valve is generally performed as a single piece and then assembled, or 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 and components are welded, use 1.5 times the higher pressure after the pressure reducing valve to perform a strength test with air.
  ② The sealing test shall be carried out according to the actual working medium. When testing with air or water, the test is performed at 1.1 times the nominal pressure; when testing with steam, the test is performed at the higher working pressure allowed at the working temperature. The difference between the inlet pressure and the outlet pressure is required to be not less than 0.2MPa. The test method is: after the inlet pressure is set, gradually adjust the adjusting screw of the valve so that the outlet pressure can change sensitively and continuously within the maximum and minimum range without stagnation or jamming. For the steam pressure reducing valve, after the inlet pressure is adjusted away, the valve is closed and the valve is cut off. The outlet pressure will be higher and lower values. Within 2 minutes, the increase in outlet pressure shall comply with the provisions in Table 4.176-22. At the same time, the valve The pipeline volume is qualified if it meets the requirements in Table 4.18; for water and air pressure reducing valves, when the inlet pressure is adjusted and the outlet pressure is zero, close the pressure reducing valve and conduct a sealing test. If there is no leakage within 2 minutes, it is qualified.

 

  4. 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 reach 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 the butterfly valve. If there is no leakage at the plate seal, it is qualified. Butterfly valves used to regulate flow do not need to undergo sealing performance testing.

 

  5. Pressure test method of plug valve
  ① When the plug valve is subjected to strength test, the medium is introduced from one end, the rest of the passage is closed, and the plug is rotated to the fully open working position in sequence for testing. If no leakage is found in the valve body, it is qualified.
  ② During the sealing test, the straight-through plug should keep the pressure in the cavity and the passage equal, 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 valves The pressure in the cavity and one end of the passage should be kept equal, the plug should be rotated to the closed position in sequence, the pressure should be introduced from the right-angle end, and the inspection should be carried out from the other end at the same time.
  Before the plug valve test, it is allowed to apply a layer of non-acidic thin lubricating oil on the sealing surface. If no leakage or expanding water droplets are found within the specified time, it is qualified. The test time of the plug valve can be shorter, generally 1 to 3 minutes according to the nominal diameter.
  Plug valves for gas use should be tested for air tightness at 1.25 times the working pressure.

 

  6. Pressure test method of diaphragm valve

  The diaphragm valve strength test introduces medium from either end, opens the valve disc, and closes the other end. After the test pressure rises to the specified value, the valve body and valve cover are deemed to be qualified if there is no leakage. Then reduce the pressure to the sealing test pressure, close the valve disc, and open the other end for inspection. If there is no leakage, it is qualified.

 

  7. Pressure test method of stop valves and throttle valves. For
  the strength test of stop valves and throttle valves, usually place the assembled valve in the pressure test stand, open the valve disc, inject the medium to the specified value, and check the valve body and valve body. Cover for sweating and leakage. Strength testing can also be performed on a single piece. The sealing test is only conducted 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. Check the packing and gasket; after passing the test, close the valve disc and open the other end to check whether there is leakage. If both the strength and sealing tests of the valve are required, the strength test can be done first, then the pressure is reduced to the specified value of the sealing test, and the packing and gasket are inspected; then the valve disc is closed, and the outlet end is opened to check whether the sealing surface is leaking.

 

  8. Pressure test method of gate valve:
  The strength test of gate valve is the same as that of stop valve. There are two methods for gate valve sealing test.
  ① Open the gate to raise the pressure in the valve to the specified value; then close the gate, take out the gate valve immediately, 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 , 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 nominal diameters below DN32mm.
  ② Another method is to open the gate to increase 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. Turn the head back again and repeat the above test until it passes the test.
  The sealing test of the pneumatic gate valve packing and gasket should be carried out before the gate sealing test.

 

  9. Pressure test method of safety valve
  ① The strength test of the safety valve is the same as other valves, using water. When testing the lower part of the valve body, the pressure is introduced from the inlet end and the sealing surface is closed; when testing the upper part of the valve body and valve cover, the pressure is introduced from the outlet end and the other ends are closed. The valve body and valve cover are deemed to be qualified if there is no leakage within the specified time.
  ② For sealing test and constant pressure test, the generally used media are: safety valves for steam use saturated steam as the test medium; valves for ammonia or other gases use air as the test medium; valves for water and other non-corrosive liquids use water as the test medium medium. Nitrogen is commonly used as the test medium for safety valves in some important locations.
  The sealing test is carried out with the nominal pressure value as the test pressure, and the number of tests is not less than twice. If there is no leakage within the specified time, it is qualified. There are two methods for leak detection: one is to seal each connection of the safety valve, and use butter to seal the tissue paper on the outlet flange. If the tissue paper bulges, it is a leak, and if it does not bulge, it is qualified; the other is to use butter to seal the thin paper. A plastic plate or other plate seal is attached to the lower part of the outlet flange, and water is poured into the valve to seal it. If the water does not bubble, it is considered qualified. The number of constant pressure and seat back pressure tests of the safety valve shall be no less than 3 times, and it shall be considered qualified if it meets the regulations. For various performance tests of safety valves, see GB/T 12242-1989 Safety Valve Performance Test Methods.


  Valve pressure test management regulations
  1. All valves should be visually inspected after unpacking, including complete nameplates on the valve body, product quality certification documents, valves in a closed state, no flange or other damage, no dirt, and protective covers on both ends. wait.
  2. The operator should be familiar with the operation of the pressure test bench.
  3. The valve pressure test JHA is complete. Operators should be familiar with the safety hazards of valve pressure test, reverse operation and other processes and have corresponding preventive measures.
  4. The valve pressure test process is: valve shell strength test, upper seal test, seal test.
 (1) The valve shell strength test medium uses water, the pressure is 1.5 times the nominal pressure of the valve, the pressure holding time is 5 minutes, and it is qualified if there is no dripping or moisture.
 (2) The sealing test medium on the valve is water, the pressure is 1.5 times the nominal pressure, the pressure holding time is 15s for DN≤50, 60s for 65≤DN≤300, and 120s for DN≧350. No leakage is considered qualified.
 (3) Valve sealing test: Carry out a low-pressure sealing test. The test medium is air (gate valve), the pressure is 0.6MPa, the pressure holding time is 15s for DN≤50, 60s for 65≤DN≤150, 120s for DN≧200, and the penetration amount complies with Requirements are qualified.
 (4) After passing the pressure test, compressed air should be used to blow dry the water in the valve cavity in time.
 (5) Valves that pass the pressure test will be marked with √ on the valve body, and valves that fail to pass the pressure test will be marked with ×.
 (6) After pressure testing, valves should be stacked separately according to device and model specifications, and unqualified products should be stacked separately; clear signs should be erected in each stacking area. After the pressure test, the valve should be covered and protected with colored strips of cloth in the stacking area.
 (7) After the valve pressure test, the "Valve/Pipe Fitting Test Record" should be filled in according to the device in a timely manner, and the purchase contract number on the valve body should be filled in the model column.
 (8) For the distribution of valves that have passed the test, the valve "Valve Distribution Record" must be filled in and signed by both the issuing and receiving parties.
 (9) Unqualified valves need to establish a separate "Unqualified Valve Information Account"