Wenzhou Unique Valve Co., Ltd.
Inspection and Quality Analysis of Valves in Large Petrochemical Devices (Part Two)
May 20, 2021
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2.2 Pressure tests of valves
Pressure tests generally should be carried out for all valves before installation for petrochemical enterprises. Pressure tests include shell tests, upper sealing tests, high pressure sealing tests, low pressure sealing tests. The test personnel should determine the pressure test items according to the valve type, structural characteristics and implementation standards. For example, traps only require shell tests, while safety valves require set-pressure adjustment.
Valves are required to be subjected to a pressure test, and they should be qualified before leaving the factory. The main execution standards of pressure tests for valves are GB/T 26480-2011, GB/T 13927-2008, API 598-2016 and ISO 5208-2015. According to regulations, valves that have been pressure tested and witnessed by the user or the third part in the factory and whose corresponding information can be provided don't need to be pressure tested on-site again. However, from the perspective of safety production, most users will require to perform the pressure test for the valve before installation. For petrochemical, natural gas and other large engineering projects, the main implementation standards for pressure testing of valves are SH 3518-2013 and SY/T 4102-2013.
Commonly used pressure test standards for valves can be divided into two major systems: The API 598-2016 Standard of American Petroleum Institute, which corresponds to GB/T 26480-2011 standard in China; ISO 5208-2015 of the international standard, corresponding to Chinese standard GB/T13927-2008. The requirements for working pressure at room temperature under different systems are the same, which all represent the maximum allowable working pressure at -20°C to 38°C. The test pressure specified in GB/T 13927-2008 and GB/T 26480-2011 is the maximum allowable working pressure at 20°C and 38°C times the corresponding times. ASME 16.34-2017 stipulates that the allowable stress of metal materials within -29°C to 38°C is the same, so strictly distinguish the maximum allowable working pressure at 20°C and 38°C is of little significance.
However, there are certain differences in the standards under different systems. Take the Chinese standard as an example for comparison. GB/T 26480-2011 stipulates that the pressure for the low-pressure sealing test pressure is from 0.4MPa to 0.7MPa; the temperature of the test medium is between 5°C and 50°C, and special regulations are given for the pressure of shell test pressure of iron valves. The pressure for high pressure sealing tests of check valves is the maximum allowable working pressure at room temperature. The upper sealing test must be performed for gate valves and globe valves with upper sealing structure. The maximum allowable leakage rate of the sealing test has 10 levels and regulations are given. GB/T 13927-2008 stipulates that the low-pressure sealing test pressure is 0.6 MPa±0.1MPa and the test medium temperature is between 5°C and 40°C. The test pressure of iron valves and check valves is 1.1 times the maximum allowable working pressure at room temperature. The upper sealing test of gate valves and globe valves can be optional. The maximum allowable leakage rate of the sealing test is divided into 4 items according to the nominal size and the regulations are given respectively.
The regulations of the same system are slightly different, mainly as follows: API 598-2016 stipulates that the temperature of the test medium is from 5°C to 38°C. The chloride content in the water required by the test of austenitic stainless steel valve does not exceed 50ppm. The high-pressure sealing test pressure of the steel check valve is the maximum allowable working pressure at room temperature. GB/T 26480-2011 stipulates that the temperature of the test medium is between 5°C and 50°C. The chloride content in the water required for the test of the stainless steel valve does not exceed 100ppm, and the high-pressure sealing test pressure of the steel check valve is 1.1 times higher than the maximum allowable working pressure at room temperature. In contrast, GB/T 26480-2011 has fewer test requirements for the nodular cast iron with the pressure of CL300 and gray cast iron CL250 based on the API 598-2016. ISO 5208-2015 stipulates that the accuracy of pressure measurement should reach the required test pressure of ±5%, and GB/T 13927-2008 requires that the accuracy of pressure measuring instruments is not less than 1.6.
By comparing the relevant valve pressure test standards at home and abroad, it is also found that when liquid is used as the shell test and sealing test medium, most standards are based on the maximum allowable working pressure at room temperature and it multiplies by 1.5 times and 1.1 times respectively, except for iron valves and check valves which have special regulations in some standards. However, SH 3518-2013 still uses the nominal pressure as a benchmark. In addition, GB 50184-2011 and GB 50235-2017 both stipulate that the pressure holding time of all test items is not less than 5 minutes, which is stricter than the requirements for common pressure test standards, and the pressure holding time specified by TSG D7002-2006 is relatively long.
3. Test procedures and precautions
3.1 Test procedures
(1) Shell tests
Each end of the valve is closed and the opening and closing parts are partially opened. The inner cavity is filled with the test medium and the air is exhausted. The pressure is slowly increased to the test pressure. The pressure is maintained until the specified test time. There should not be structural damage for the shell, no visible leakage on the shell wall, middle flange connections and sealing parts of packing of valve stems.
Pressure tests generally should be carried out for all valves before installation for petrochemical enterprises. Pressure tests include shell tests, upper sealing tests, high pressure sealing tests, low pressure sealing tests. The test personnel should determine the pressure test items according to the valve type, structural characteristics and implementation standards. For example, traps only require shell tests, while safety valves require set-pressure adjustment.
Valves are required to be subjected to a pressure test, and they should be qualified before leaving the factory. The main execution standards of pressure tests for valves are GB/T 26480-2011, GB/T 13927-2008, API 598-2016 and ISO 5208-2015. According to regulations, valves that have been pressure tested and witnessed by the user or the third part in the factory and whose corresponding information can be provided don't need to be pressure tested on-site again. However, from the perspective of safety production, most users will require to perform the pressure test for the valve before installation. For petrochemical, natural gas and other large engineering projects, the main implementation standards for pressure testing of valves are SH 3518-2013 and SY/T 4102-2013.
Commonly used pressure test standards for valves can be divided into two major systems: The API 598-2016 Standard of American Petroleum Institute, which corresponds to GB/T 26480-2011 standard in China; ISO 5208-2015 of the international standard, corresponding to Chinese standard GB/T13927-2008. The requirements for working pressure at room temperature under different systems are the same, which all represent the maximum allowable working pressure at -20°C to 38°C. The test pressure specified in GB/T 13927-2008 and GB/T 26480-2011 is the maximum allowable working pressure at 20°C and 38°C times the corresponding times. ASME 16.34-2017 stipulates that the allowable stress of metal materials within -29°C to 38°C is the same, so strictly distinguish the maximum allowable working pressure at 20°C and 38°C is of little significance.
However, there are certain differences in the standards under different systems. Take the Chinese standard as an example for comparison. GB/T 26480-2011 stipulates that the pressure for the low-pressure sealing test pressure is from 0.4MPa to 0.7MPa; the temperature of the test medium is between 5°C and 50°C, and special regulations are given for the pressure of shell test pressure of iron valves. The pressure for high pressure sealing tests of check valves is the maximum allowable working pressure at room temperature. The upper sealing test must be performed for gate valves and globe valves with upper sealing structure. The maximum allowable leakage rate of the sealing test has 10 levels and regulations are given. GB/T 13927-2008 stipulates that the low-pressure sealing test pressure is 0.6 MPa±0.1MPa and the test medium temperature is between 5°C and 40°C. The test pressure of iron valves and check valves is 1.1 times the maximum allowable working pressure at room temperature. The upper sealing test of gate valves and globe valves can be optional. The maximum allowable leakage rate of the sealing test is divided into 4 items according to the nominal size and the regulations are given respectively.
The regulations of the same system are slightly different, mainly as follows: API 598-2016 stipulates that the temperature of the test medium is from 5°C to 38°C. The chloride content in the water required by the test of austenitic stainless steel valve does not exceed 50ppm. The high-pressure sealing test pressure of the steel check valve is the maximum allowable working pressure at room temperature. GB/T 26480-2011 stipulates that the temperature of the test medium is between 5°C and 50°C. The chloride content in the water required for the test of the stainless steel valve does not exceed 100ppm, and the high-pressure sealing test pressure of the steel check valve is 1.1 times higher than the maximum allowable working pressure at room temperature. In contrast, GB/T 26480-2011 has fewer test requirements for the nodular cast iron with the pressure of CL300 and gray cast iron CL250 based on the API 598-2016. ISO 5208-2015 stipulates that the accuracy of pressure measurement should reach the required test pressure of ±5%, and GB/T 13927-2008 requires that the accuracy of pressure measuring instruments is not less than 1.6.
By comparing the relevant valve pressure test standards at home and abroad, it is also found that when liquid is used as the shell test and sealing test medium, most standards are based on the maximum allowable working pressure at room temperature and it multiplies by 1.5 times and 1.1 times respectively, except for iron valves and check valves which have special regulations in some standards. However, SH 3518-2013 still uses the nominal pressure as a benchmark. In addition, GB 50184-2011 and GB 50235-2017 both stipulate that the pressure holding time of all test items is not less than 5 minutes, which is stricter than the requirements for common pressure test standards, and the pressure holding time specified by TSG D7002-2006 is relatively long.
3. Test procedures and precautions
3.1 Test procedures
(1) Shell tests
Each end of the valve is closed and the opening and closing parts are partially opened. The inner cavity is filled with the test medium and the air is exhausted. The pressure is slowly increased to the test pressure. The pressure is maintained until the specified test time. There should not be structural damage for the shell, no visible leakage on the shell wall, middle flange connections and sealing parts of packing of valve stems.
(2) Upper sealing tests
All ends of the valve should be closed; the packing gland is loosened, and the inner cavity is filled with a test medium. When the air is exhausted, fully open the valve. Slowly increase the pressure to the test pressure and maintain the pressure for the specified test time. Make sure that there is no visible leakage at the packing of the valve stem.
(3) Sealing tests
For two-way sealed valves, each end should be closed and the opening and closing parts are partially opened; the inner cavity is filled with test medium and the pressure is slowly increased to the test pressure; the opening and closing parts are completely closed. One end is used to release pressure and pressure on the other end is maintained until the specified test time. Check the leakage of the end used for pressure relief. Repeat the above operation. Perform the test from the other way and check for leakage. For the one-way sealed valve with flow directions, the opening and closing parts are completely closed. The inlet end is filled with the test medium and the air is exhausted. Increase the pressure slowly to the test pressure and maintain the pressure till the specified test time. Check for leakages at the outlet ends. The reverse pressure is adopted for the check valve. Fill the outlet end with the test medium and exhaust the air. Increase the pressure slowly to the test pressure and maintain it till the specified test time. Check for leakages at the inlet ends.
(3) Sealing tests
For two-way sealed valves, each end should be closed and the opening and closing parts are partially opened; the inner cavity is filled with test medium and the pressure is slowly increased to the test pressure; the opening and closing parts are completely closed. One end is used to release pressure and pressure on the other end is maintained until the specified test time. Check the leakage of the end used for pressure relief. Repeat the above operation. Perform the test from the other way and check for leakage. For the one-way sealed valve with flow directions, the opening and closing parts are completely closed. The inlet end is filled with the test medium and the air is exhausted. Increase the pressure slowly to the test pressure and maintain the pressure till the specified test time. Check for leakages at the outlet ends. The reverse pressure is adopted for the check valve. Fill the outlet end with the test medium and exhaust the air. Increase the pressure slowly to the test pressure and maintain it till the specified test time. Check for leakages at the inlet ends.
