Why is the valve always malfunctioning, is it a quality problem or an operation error?
Why is the valve always malfunctioning?
In the entire process of design, manufacture, installation, working conditions, operation and maintenance, every step cannot be relaxed. Before the valve leaves the factory or complete installation, how to determine if there is a problem? This requires inspection through visual inspection and certain performance tests. Through these test results, the defects can be exposed and adjusted accordingly. Only after all the tests are qualified can they be put into use. So, what kind of details need to pay attention to appearance inspection? What are the performance tests? Let’s take a look with the little one!
1. Whether there are defects such as sand holes or cracks on the internal and external surfaces of the valve.
2. Whether the joint between the valve seat and the valve body is firm, whether the valve plug and the valve seat are consistent, and whether the sealing surface has defects.
3. Whether the stem and the valve plug are connected flexibly and reliably, whether the stem is bent or not, and whether the thread is damaged or corroded.
4. Whether the packing and gasket are aged and damaged, and whether the valve opening is flexible.
5, the body should have a nameplate, body and nameplate should include: manufacturer name, valve name, nominal pressure, nominal diameter and other signs.
6. The opening and closing position of the valve during transportation shall meet the following requirements: (a) Valves such as gate valves, globe valves, check valves, butterfly valves, safety valves, and regulating valves shall be in fully closed positions. (b) The plug valve and ball valve closure should be in the fully open position. (c) The diaphragm valve should be in the closed position and must not be closed too tightly to prevent damage to the diaphragm valve. (d) The flap of the check valve should be closed and fixed.
7. The spring-type safety valve shall have a seal, and the lever-type safety valve shall have a positioning device for the weight.
8. The valve flap or spool movement of the check valve should be flexible, accurate, without eccentricity, displacement or skew.
9. The inner surface of the rubber lined liner, lining enamel, and lining plastic should be smooth and smooth, and the lining layer should be firmly bonded to the substrate without defects such as cracks and blisters.
10. The flange sealing surface shall meet the requirements without radial scratches.
11, the valve must not have damage, missing parts, corrosion, the nameplate off and so on, and the valve body must not be dirty.
12. Both ends of the valve should be protected by a protective cover. The handle or hand wheel should be operated flexibly without any jamming.
13. The valve quality certificate shall contain the following contents: (a) Manufacturer’s name and date of manufacture. (b) Product name, model number and specifications. (c) Nominal pressure, nominal diameter, applicable medium, and applicable temperature. (d) Based on criteria, inspection conclusions and inspection dates. (e) Factory serial number, inspection personnel, and inspection person’s signature.
Shell strength test
The valve can be regarded as a pressurized container, and the solid needs to meet the requirements of receiving medium pressure without leakage. Therefore, the blanks of the valve body, bonnet and other parts should not have defects such as cracks, loose pores, and slag that affect the strength.
The strength test is generally performed after final assembly. The test is usually conducted at room temperature. To ensure safety, the test pressure P is usually 1.25-1.5 times the nominal pressure PN. During the test, the valve was opened and one end was closed. The medium was injected from the other end and pressure was applied. Examine the exposed surface of the housing (body, cover) and require that there be no leakage during the specified test duration (generally not less than 10 minutes) before the valve strength test is considered acceptable. In order to ensure the reliability of the test, the strength test should be conducted before the valve is painted. When using water as the medium, the air in the cavity should be drained.
Leaky valves, such as technical conditions allow repair welding can be repaired according to technical specifications, but repair welding must be re-strength test, and appropriate extension of the test duration.
Outside the throttle valve, whether it is to cut off the valve or the adjustment valve, should have a certain closing seal, solid valve before the factory one by one to carry out the sealing test, with a sealed valve but also on the sealing test.
The test is usually conducted at room temperature at a nominal pressure of PN and the Su valve at a pressure of 1.1 times PN. When water is used as the test medium, it is easy to cause rust on the valve. Usually, the water quality should be controlled according to technical requirements, and the residual water should be dried or dried after the test.
The gate valve and the ball valve have two sealing pairs, so a two-way sealing test is required. During the test, the valve is first opened to seal one end of the channel, and the pressure is introduced from the other end. When the pressure rises to the specified value, the valve is closed, and then the pressure at the plugging end is gradually unloaded and checked. The other test is repeated at the other end. Another test method for the gate valve is to maintain the test pressure in the body cavity and to check the double tightness of the valve from both ends of the channel.
When testing the check valve, pressure should be introduced from the outlet end and checked at the inlet.
Sealing test, the valve closing torque should be determined by the nominal pressure and nominal diameter. Manual valves are normally only allowed to close with normal body force, and must not be assisted by other auxiliary instruments. Two people are allowed to close when the handwheel diameter is greater than or equal to 320mm. There is a valve for the drive. It should be tested with the use of a drive. If the technical requirements stipulate that the closing torque is required, the measuring torque wrench shall be used to measure the closing torque.
The sealing test should be carried out after the final strength test of the valve, because not only the closing tightness of the valve but also the seal of the packing and the middle flange gasket should be checked.
The upper seal test is usually performed together with the strength test. During the test, the valve rod is raised to the limit position, so that the valve stem is in close contact with the sealing surface of the valve cap, and the packing gland is released to check its tightness.
Valves or drawing specifications for gas mediums require that the valves used for low-pressure gas-tight tests must be tested in accordance with test standards. The test medium is either nitrogen or dry clean air. The test pressure is 0.6 MPa.
Action performance test
Test medium with the shell strength test and seal test, after the shell strength test and seal test pass.
Manual valve performance test
The valve is open, the valve chamber is filled with pressure to the test pressure, the valve is closed with the specified torque, decompression on the side of the valve, to establish the pressure difference in the most unfavorable direction to open the valve, and then open the valve with the specified torque, At least three or more full load-carrying cycles are performed in this way to check whether the valves are open and closed, whether the operation is flexible, and whether the opening and closing position indications are correct.
Check valve performance test
The valve opening test was performed under the specified pressure difference, and the number of tests was not less than 3 times.
The performance test of electric and pneumatic valves is carried out according to the specifications of the valve specifications. If there is no specific specification in the valve technical specifications, the valve shall be operated by the rated actuator to complete three complete loading cycles. During the entire test, the valve must be operated. Smooth and flexible, the valve must be opened and closed, and the position indication must be correct.
Vacuum seal test
A highly sensitive seal test method.
The vacuum test is usually performed after the valve strength and seal test pass. In order to ensure the accuracy of the test, the valve under test should have a high degree of cleanliness and process a fine sealing surface. And the valve body and valve cover should generally use forgings.
Mass spectrometer leak detection: After the valve to be tested is pumped to the specified vacuum level by a vacuum pump, helium gas is applied outside the measured position of the valve. If there is a leak, helium enters the measured part of the valve and the helium mass spectrometer leak detector in the system can be displayed. According to this, the leak rate can be calculated.
Micro leak test
In recent years, with the strengthening of people’s awareness of environmental protection, various institutions in the world have imposed more stringent requirements on the sealing of valves, especially when the use of media is strongly corrosive, strongly radioactive, and highly toxic. The micro leak requirement of the valve is one of them. The micro leak detection of the valve is mainly to check the degree of leakage of the flange and the stuffing box in the valve, which belongs to the valve housing sealing test.
The basic principle of valve micro-leakage detection is: when the valve is in half-closed state, the inside of the valve is ventilated with the specified pressure, and the helium mass spectrometer leak detector with suction probe with adjusted leak rate is used to align the cavity. Test the stuffing box and see if the part meets the user specified leak rate.
Source: China Valves Manufacturer – Yaang pipe fitting Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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