Wenzhou Unique Valve Co., Ltd.
Inspection and Quality Analysis of Valves in Large Petrochemical Devices (Part Three)
May 24, 2021
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3.2 Matters needing attention
(1) Before the pressure test for the valve, a suitable pressure test machine should be selected according to the structural characteristics of the tested valve, and the clamping force should be preset according to the nominal diameter and nominal pressure of the valve to avoid excessive external force on the valve, resulting in deformation or difficulty in opening and closing of the disc and affecting the test results.
(2) Before the valve is clamped, the protective cover of the valve end, the shield of the sealing surface of the check valve, protective oil paper of the exposed valve stem, dirt and foreign matter in the valve cavity should be removed.
(3) Valves with hand wheels usually only allow one person's normal physical strength to open and close. The correct operation is to hold the two sides which is symmetrical on hand wheels and then evenly apply the force. Because the packing is too tight, the new valve can be opened with a suitable torque wrench.
(1) Before the pressure test for the valve, a suitable pressure test machine should be selected according to the structural characteristics of the tested valve, and the clamping force should be preset according to the nominal diameter and nominal pressure of the valve to avoid excessive external force on the valve, resulting in deformation or difficulty in opening and closing of the disc and affecting the test results.
(2) Before the valve is clamped, the protective cover of the valve end, the shield of the sealing surface of the check valve, protective oil paper of the exposed valve stem, dirt and foreign matter in the valve cavity should be removed.
(3) Valves with hand wheels usually only allow one person's normal physical strength to open and close. The correct operation is to hold the two sides which is symmetrical on hand wheels and then evenly apply the force. Because the packing is too tight, the new valve can be opened with a suitable torque wrench.
(4) In the pressure test, if water is used as the test medium, it should be clean water. If the material of the inspected valve is austenitic stainless steel, ensure that the chloride content in the water does not exceed the standard based on the corresponding technical standards.
(5) In the whole testing process, the operator should do a good job in safety protection. Observe the pressure change in real time, and the valve pressure rise should be slow.
It is forbidden to pressurize rapidly and suddenly to prevent the pressure from rising to excessively high pressure.
(6) When liquid is used as the test medium, the air in the cavity should be removed as much as possible to reduce the test risk. The residual medium should be exhausted timely and use compressed air to blow the valve after the pressure test is done.
Take a batch of new valves replaced by a large-scale petrochemical enterprise in Fujian Province as an example. The batch of valves is supplied by 12 Chinese valve manufacturers, totaling 8,853 sets, which include gate valves, globe valves, check valves, ball valves, butterfly valves, cryogenic valves, regulating valves and many other types. Through a 100% visual inspection and pressure test of the entire batch of valves, it was found that 147 valves have hidden safety hazards, and the unqualified rate reached 1.7%. After returning to the factory for repair and inspecting the valves again, it was found that 3 valves still had problems. On-site inspectors usually analyze the causes of unqualified items first, and then try their best to repair the valve on-site, such as removal of foreign matter on the surface, pre-tightening of the middle flange and packing gland bolts, appropriate auxiliary opening and closing of the torque wrench, etc. The valve will be inspected by other inspectors. If it is not qualified, the judgment will be made. The above data is obtained based on the on-site inspectors not being able to perform easy repairs. The actual first inspection rejection rate is much higher than the statistical value. According to statistics, the unqualified items and their quantity are as follows: leakage on sealing surface of 107 valves, leakage on a middle flange of 25 valves, leakage on stem packing of 6 valves, perforation on casting bodies of 4 valves, cracks on hand wheels of 2 valves, fracture on eyelet bolts of one valve, and 2 valves not being able to be opened. The specific typical defects are as follows:
It is forbidden to pressurize rapidly and suddenly to prevent the pressure from rising to excessively high pressure.
(6) When liquid is used as the test medium, the air in the cavity should be removed as much as possible to reduce the test risk. The residual medium should be exhausted timely and use compressed air to blow the valve after the pressure test is done.
Take a batch of new valves replaced by a large-scale petrochemical enterprise in Fujian Province as an example. The batch of valves is supplied by 12 Chinese valve manufacturers, totaling 8,853 sets, which include gate valves, globe valves, check valves, ball valves, butterfly valves, cryogenic valves, regulating valves and many other types. Through a 100% visual inspection and pressure test of the entire batch of valves, it was found that 147 valves have hidden safety hazards, and the unqualified rate reached 1.7%. After returning to the factory for repair and inspecting the valves again, it was found that 3 valves still had problems. On-site inspectors usually analyze the causes of unqualified items first, and then try their best to repair the valve on-site, such as removal of foreign matter on the surface, pre-tightening of the middle flange and packing gland bolts, appropriate auxiliary opening and closing of the torque wrench, etc. The valve will be inspected by other inspectors. If it is not qualified, the judgment will be made. The above data is obtained based on the on-site inspectors not being able to perform easy repairs. The actual first inspection rejection rate is much higher than the statistical value. According to statistics, the unqualified items and their quantity are as follows: leakage on sealing surface of 107 valves, leakage on a middle flange of 25 valves, leakage on stem packing of 6 valves, perforation on casting bodies of 4 valves, cracks on hand wheels of 2 valves, fracture on eyelet bolts of one valve, and 2 valves not being able to be opened. The specific typical defects are as follows:
(1) Leakages on the sealing surface
The main reasons for leakages on the sealing surface are the wear of the sealing surface, foreign matter on the sealing surface, the incorrect closing position of the opening and closing parts, and not good machining accuracy on the sealing surface, as shown in Figure 1.
Figure 1 Leakage on the sealing surface
Figure 1 Leakage on the sealing surface
(2) Leakage on the middle flange
The main reasons for leakage on the middle flange are insufficient or uneven pre-tightening force of bolts, failure of the middle flange's gasket, etc., as shown in Figure 2.
Figure 2 Leakage on the middle flange
Figure 2 Leakage on the middle flange
(3) Leakage on the stem's packing
The main reason for the leakage on the stem packing is that the pre-tightening force of the packing gland bolt is not enough and the packing is not put properly, as shown in Figure 3.
Figure 3 Leakage on the stem's packing
(4) Perforation of the casting shell
The main reason is the casting defects of the valve body and bonnet, such as sand holes and gas holes, as shown in Figure 4.
Figure 4 Perforation of the casting shell
(5) Cracks of the casting handwheel
The main cause of the casting defects of handwheels and other accessories, such as cracks, shrinkage cavity, etc., as shown in Figure 5.
Figure 5 Cracks of casting hand wheels
(6) The fracture of eyelet bolt
The main reason is the quality problems of bolts, nuts and other parts, as shown in Figure 6.
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Figure 6 The fracture of eyelet bolts
(7) The valve not being able to be opened
The main reason is that the packing is too tight and the valve body is deformed; the stem and the stem nut are not correctly engaged or jammed, as shown in Figure 7.
Figure 7 The valve not being able to be opened
5. Conclusion
(1) Visual inspection and pressure tests are the most basic inspection items for valves, which are required before the valve leaves the factory and before installation. The visual inspection focuses on signs and quality of surfaces, and the main basic standards implemented are GB/T 12220-2015 and JB/T7927-2014 respectively. Commonly used valve pressure test standards are usually the two systems represented by API 598-2016 and ISO5208-2015. Different pressure test standards can be used as complements to each other. Inspectors should choose the appropriate implementation standard based on actual conditions. The inspection of the valve used for the project before installation is mainly carried out according to the construction specification of the relevant field.
(2) Various inspections should be done before the valve's pressure test, especially to avoid the influence of excessive external force on the valve pressure test result. Safety protection should be done during the pressure test. The pressure test should be done with liquid as the medium. The air in the valve cavity should be discharged as much as possible. The pressure rise should be slow. The valve should be dried with compressed air in time after the pressure test is completed.
(3) Through the inspection of 8853 valves in the petrochemical industry, it was found that many defects such as leakage on the sealing surface, leakage on the flange, leakage on packing of valve stems and other defects are on 147 valves. This not only eliminates hidden safety hazards for enterprises, but also reflects that quality inspection is very important.
Figure 3 Leakage on the stem's packing
(4) Perforation of the casting shell
The main reason is the casting defects of the valve body and bonnet, such as sand holes and gas holes, as shown in Figure 4.
Figure 4 Perforation of the casting shell
(5) Cracks of the casting handwheel
The main cause of the casting defects of handwheels and other accessories, such as cracks, shrinkage cavity, etc., as shown in Figure 5.
Figure 5 Cracks of casting hand wheels
(6) The fracture of eyelet bolt
The main reason is the quality problems of bolts, nuts and other parts, as shown in Figure 6.
Figure 6 The fracture of eyelet bolts
(7) The valve not being able to be opened
The main reason is that the packing is too tight and the valve body is deformed; the stem and the stem nut are not correctly engaged or jammed, as shown in Figure 7.
Figure 7 The valve not being able to be opened
5. Conclusion
(1) Visual inspection and pressure tests are the most basic inspection items for valves, which are required before the valve leaves the factory and before installation. The visual inspection focuses on signs and quality of surfaces, and the main basic standards implemented are GB/T 12220-2015 and JB/T7927-2014 respectively. Commonly used valve pressure test standards are usually the two systems represented by API 598-2016 and ISO5208-2015. Different pressure test standards can be used as complements to each other. Inspectors should choose the appropriate implementation standard based on actual conditions. The inspection of the valve used for the project before installation is mainly carried out according to the construction specification of the relevant field.
(2) Various inspections should be done before the valve's pressure test, especially to avoid the influence of excessive external force on the valve pressure test result. Safety protection should be done during the pressure test. The pressure test should be done with liquid as the medium. The air in the valve cavity should be discharged as much as possible. The pressure rise should be slow. The valve should be dried with compressed air in time after the pressure test is completed.
(3) Through the inspection of 8853 valves in the petrochemical industry, it was found that many defects such as leakage on the sealing surface, leakage on the flange, leakage on packing of valve stems and other defects are on 147 valves. This not only eliminates hidden safety hazards for enterprises, but also reflects that quality inspection is very important.
