Because of the special structure of cryogenic valves, the installation of cryogenic valves also has special requirements. The direction of the valve stem must be within a vertical angle of 45 degrees when the cryogenic valve is installed due to the structural characteristics of its long-neck bonnets, and installation on vertical pipelines should be avoided as much as possible. Otherwise, the low-temperature medium will fill the extended part of the valve bonnet, causing the valve packing to fail, transferring the cold to the valve handle, and causing injury to the operator. For cryogenic valves with a pressure relief structure, when installing the valve, pay special attention to the requirements for the direction of the valve pressure relief. The direction of valve pressure relief should be marked on the process flow chart and reflected in the axonometric(al) drawing of the pipeline.
The reasons for the leakage of the cryogenic valve
1. Internal leakages: The main reason for the internal leakage of the valve is the deformation of the sealing pair in the low temperature state. When the temperature of the medium drops to a phase change of the material, the volume changes, which causes the sealing surface with high grinding accuracy to warp and deform, resulting in poor low-temperature sealing.
2. External leakages: There are two causes for the external leakages. First, when the valve and pipeline are connected by flanges, a leakage occurs due to the unsynchronized contraction of the gasket, connecting bolts, and connecting parts at low temperatures. Therefore, the connection between the valve body and pipeline can be changed from the flange structure to the welding structure to avoid low-temperature leakages. Second, a leakage occurs at the valve stem and packing.
Manufacturing of cryogenic valves
Strict manufacturing processes and special equipment are adopted for cryogenic valves, and strict quality control is performed on the processing of parts. After special low-temperature treatment, the rough machined parts are placed in the cooling medium for several hours (2 to 6 hours) to relieve stress, ensure the low-temperature performance of the material, and make sure the fine finishing size prevents the valve from deforming and leaking due to temperature changes in low-temperature conditions. The assembly of the cryogenic valve is also different from that of the ordinary valve. The parts need to be strictly cleaned to remove any oil stains to ensure performance.
The test and inspection of cryogenic valves
The main components of the cryogenic valve are treated at low temperatures and each batch is sampled for the low-temperature impact test to ensure that the valve is not brittle under low-temperature conditions and can withstand the impact of low-temperature media.
Carry out the following tests on each valve:
(1) Perform shell strength tests at normal temperatures, low-pressure upper sealing tests at normal temperatures and low-pressure sealing tests at normal temperatures.
(2) Conduct low-temperature upper seal air-tight tests (When there is an upper seal.) and low-temperature air-tight tests to ensure that the entire cryogenic valve meets the requirements for the standard.
(3) Carry out low-temperature treatment for the main parts and low-temperature impact tests for each batch of samples to ensure that the valve is not brittle under low-temperature conditions and can withstand the impact of low-temperature media.
(4) Low-temperature valves or cryogenic valves should be subjected to low-temperature treatment and impact tests in accordance with the corresponding material specifications.
(5) The conduction resistance between the valve body and valve stem or the internal parts and valve body is less than 1 Ohm.
The test equipment for the cryogenic valve is as follows: test devices for cryogenic valves, storage devices for liquid nitrogen and cryogenic treatment tanks.
Test methods of cryogenic valves
(1) Preparation before the test
Remove the oil stains of the valve parts, wipe them and assemble them in a clean, dust-free and oil-free environment; tighten the bolts to the predetermined torque value and tension value, and record the value; use a suitable thermocouple to connect the valve so that the temperature of the valve can be monitored throughout the test.
Install the valve in the test container and connect it well. Make sure that the valve packing is at the top of the container where there is no vaporized gas. Perform the initial system verification test with the specified medium gas and test pressure of the large valve seat at room temperatures to ensure that the valve is in a suitable place. Then, conduct the test. Immerse the valve in liquid nitrogen for cooling. The level of the liquid should at least cover the connection part between the valve body and valve bonnet, and helium was supplied to the valve in the entire cooling process. In the cooling process, the temperature of the valve is monitored by a thermocouple installed in the appropriate position. The valve should be stable at the test temperature. Use a thermocouple to measure the temperature of the valve to make sure that it is uniform. Perform an initial verification test with helium with test pressure for a large valve seat at the test temperature. Perform a pressure test for the valve seat on the valve's inlet side. For a valve with bidirectional sealing, test the two valve seats separately. Keep the valve in the open position, and close the needle valve on the outlet side of the valve; increase the pressure in the valve cavity to the test pressure of the valve seat. Keep the pressure at the specified requirements, and check whether there is a leakage at the valve's packing and connection part between the valve body and bonnet. There should be no leakage. Let the temperature of the valve return to the room temperature, and then perform the sealing test at room temperatures. After the test is completed, the valve shall be cleaned, blown, and then it should pass the inspection before leaving the factory.
The reasons for the leakage of the cryogenic valve
1. Internal leakages: The main reason for the internal leakage of the valve is the deformation of the sealing pair in the low temperature state. When the temperature of the medium drops to a phase change of the material, the volume changes, which causes the sealing surface with high grinding accuracy to warp and deform, resulting in poor low-temperature sealing.
2. External leakages: There are two causes for the external leakages. First, when the valve and pipeline are connected by flanges, a leakage occurs due to the unsynchronized contraction of the gasket, connecting bolts, and connecting parts at low temperatures. Therefore, the connection between the valve body and pipeline can be changed from the flange structure to the welding structure to avoid low-temperature leakages. Second, a leakage occurs at the valve stem and packing.
Manufacturing of cryogenic valves
Strict manufacturing processes and special equipment are adopted for cryogenic valves, and strict quality control is performed on the processing of parts. After special low-temperature treatment, the rough machined parts are placed in the cooling medium for several hours (2 to 6 hours) to relieve stress, ensure the low-temperature performance of the material, and make sure the fine finishing size prevents the valve from deforming and leaking due to temperature changes in low-temperature conditions. The assembly of the cryogenic valve is also different from that of the ordinary valve. The parts need to be strictly cleaned to remove any oil stains to ensure performance.
The test and inspection of cryogenic valves
The main components of the cryogenic valve are treated at low temperatures and each batch is sampled for the low-temperature impact test to ensure that the valve is not brittle under low-temperature conditions and can withstand the impact of low-temperature media.
Carry out the following tests on each valve:
(1) Perform shell strength tests at normal temperatures, low-pressure upper sealing tests at normal temperatures and low-pressure sealing tests at normal temperatures.
(2) Conduct low-temperature upper seal air-tight tests (When there is an upper seal.) and low-temperature air-tight tests to ensure that the entire cryogenic valve meets the requirements for the standard.
(3) Carry out low-temperature treatment for the main parts and low-temperature impact tests for each batch of samples to ensure that the valve is not brittle under low-temperature conditions and can withstand the impact of low-temperature media.
(4) Low-temperature valves or cryogenic valves should be subjected to low-temperature treatment and impact tests in accordance with the corresponding material specifications.
(5) The conduction resistance between the valve body and valve stem or the internal parts and valve body is less than 1 Ohm.
The test equipment for the cryogenic valve is as follows: test devices for cryogenic valves, storage devices for liquid nitrogen and cryogenic treatment tanks.
Test methods of cryogenic valves
(1) Preparation before the test
Remove the oil stains of the valve parts, wipe them and assemble them in a clean, dust-free and oil-free environment; tighten the bolts to the predetermined torque value and tension value, and record the value; use a suitable thermocouple to connect the valve so that the temperature of the valve can be monitored throughout the test.
Install the valve in the test container and connect it well. Make sure that the valve packing is at the top of the container where there is no vaporized gas. Perform the initial system verification test with the specified medium gas and test pressure of the large valve seat at room temperatures to ensure that the valve is in a suitable place. Then, conduct the test. Immerse the valve in liquid nitrogen for cooling. The level of the liquid should at least cover the connection part between the valve body and valve bonnet, and helium was supplied to the valve in the entire cooling process. In the cooling process, the temperature of the valve is monitored by a thermocouple installed in the appropriate position. The valve should be stable at the test temperature. Use a thermocouple to measure the temperature of the valve to make sure that it is uniform. Perform an initial verification test with helium with test pressure for a large valve seat at the test temperature. Perform a pressure test for the valve seat on the valve's inlet side. For a valve with bidirectional sealing, test the two valve seats separately. Keep the valve in the open position, and close the needle valve on the outlet side of the valve; increase the pressure in the valve cavity to the test pressure of the valve seat. Keep the pressure at the specified requirements, and check whether there is a leakage at the valve's packing and connection part between the valve body and bonnet. There should be no leakage. Let the temperature of the valve return to the room temperature, and then perform the sealing test at room temperatures. After the test is completed, the valve shall be cleaned, blown, and then it should pass the inspection before leaving the factory.
Next: Low-temperature Physical Property Analysis of Key Materials of Valves
Previous: The Design of Petrochemical Cryogenic Valves (Part Two)
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