Key points of supervision in the manufacture of low temperature valves
Ultra low temperature valves and supporting devices are widely used in industrial enterprises such as electric power, petrochemical, and LNG. They have the characteristics of large usage, long manufacturing cycles, and complex process, making them difficult to supervise the manufacture of valve products. Based on the characteristics of low-temperature valve equipment manufacturing, this article expounds the supervision and control methods from four aspects: cryogenic treatment of raw materials and parts, normal temperature pressure test, and low-temperature test, and puts forward some suggestions for ensuring the quality, safety control, and orderly inspection of low-temperature valves.
In recent years, China’s petroleum refining enterprises, power industry, and LNG project construction stable development, even during the new crown epidemic, is unaffected. The valve is an important part of these new plants, (ultra) low-temperature valve is a particularly important part of the valve. (Ultra) low-temperature valves have high processing accuracy requirements, raw material stability, and other characteristics. At present, only some domestic valve manufacturers with mature R & D, manufacturing, and inspection capabilities, these companies are becoming increasingly sophisticated in new methods of manufacturing processes, which put forward higher requirements for equipment supervision units and supervisory engineers; equipment supervisors can be from the valve raw materials, parts of the deep-freezing treatment, room temperature pressure test, low-temperature test these four important control points to keep.
1. Raw material control
Cryogenic valve raw materials must be selected according to the design requirements; the manufacturing process control is the most important to ensure the safety of ultra-low temperature valves and consider the cost. Equipment supervision engineers should be familiar with the technical agreement provided by the party, review drawings, material reports, material re-inspection reports, and other information is complete, check the qualifications of the manufacturing plant to understand the level of technical management, factory equipment capabilities, operator qualifications, heat treatment equipment and measuring instruments to ensure the quality of cryogenic valves. For the site inspection, to check the material test pieces and test process is real and effective. Valve markings site review to prevent the manufacturing process material mix. Audit the corresponding test reports, technical agreements, and other documents for review so that the site documents technical indicators and the commissioner’s document requirements to maintain consistency. Cryogenic materials are mainly austenitic, austenitic a ferritic (two-way steel) materials, according to “ASME material iron-based-2019 A 351 pressure components with austenitic, austenitic a ferritic (dual-phase) castings” “ASME material iron-based-2019 A 352 low-temperature pressure parts with ferritic and martensitic steel castings” standard to review the relevant parameters of the material acceptance.
2. Deep cooling treatment of parts
For -100 ℃ -40 ℃ below the working conditions of the valve for low-temperature valves, -100 ℃ below the working conditions of the valve for ultra-low temperature valves, low-temperature valve working temperature depends on the medium temperature, its bonnet, valve body, valve seat, spool, fasteners, valve stem and other parts in the finishing process should be done before the deep cooling treatment; the parts should be put into the liquid nitrogen tank cooling, until the temperature of the part reaches -196 ℃, holding time of not less than 2h, remove the parts to the liquid nitrogen tank outside to room temperature state, cycle 2 times. In the first time down to ambient temperature up to -196 ℃, due to permanent metallographic organization changes, part of the austenite metallographic organization volume will be slightly increased in the parts before finishing deep cooling treatment will reduce the metal deformation caused by temperature differences, and changes in the metallographic organization whether the deep cooling treatment of the parts according to the standard to do is a more important part of the supervision and manufacturing process.
3. Normal temperature pressure test
Pressure test includes shell strength test, upper seal test (with upper seal test structure) high, pressure liquid seal test, and low-pressure gas seal test, according to technical requirements and API 598-2016 “valve inspection and testing” and related standards to test, see Table 1, according to the type of valve to choose the pressure test, sealing test method. Table 2 shows the pressure holding time, the shell strength test after the structure should not be damaged, and the valve shell does not allow visible leakage. Pressure test such as leakage is not allowed to use hammering, plugging, or dipping methods to plug leaks, for the shell strength test in the case of destructive damage to the valve to be rejected.
Table.1 Pressure test
|Test Description||Caliber||Pressure Rating||Gate Valve||Globe Valves And Parallel Single Valve Plate Gate Valves||Plug Valve||Check Valve||Floating Ball||Butterfly Valve And Fixed Ball Valve|
|Upper Seal||Whole||Whole||Necessary||Necessary||Not Applicable||Not Applicable||Not Applicable||Not Applicable|
|Low Pressure Seal||≤DN100(NPS4)||<1500||Necessary||Optional||Necessary||Optional||Necessary||Necessary|
|High Pressure Seal||≤DN100(NPS4)||≤1500||Optional||Optional||Optional||Necessary||Optional||Optional|
A: All valves with an upper sealing structure shall undergo an upper sealing test, except for bellows valves;
B: When optional tests are specified by the Buyer, they shall be conducted as necessary;
C: After the high-pressure sealing test of an elastic sealing valve, the sealing performance of the valve under low-pressure conditions may be reduced;
D: High-pressure sealing test for power-operated and manually-operated gear unit stop valves, including check-type reducing valves;
The pressure should be taken as 110% of the design differential pressure used to determine the size of the operating device;
E: All valves specified as dual block and discharge type shall be subject to a high-pressure sealing test unless otherwise specified by the Buyer;
F: For oil-sealed plug valves, the high-pressure seal test is mandatory, and the low-pressure seal test can be an option.
Table.2 Minimum pressure holding time
|Valve specification||Minimum test duration/s|
|DS||NPS||Casing||Upper sealing test (valve with upper sealing structure)||Check valve sealing test||Other valve sealing tests|
|The test duration refers to the inspection time after the valve is fully prepared and the pressure rises to the specified value.|
4. Low-temperature test
The cryogenic test must be carried out after the completion of the pressure test process at room temperature; at this stage, the main standard for the implementation of the cryogenic test is JB/T 7749-1995 “Cryogenic Valve Technical Conditions,” BS 6364-1998 “Cryogenic Valve,” before the cryogenic test to remove grease and moisture from the valve, grease, and moisture in the low-temperature solid-state environment will form a hard substance, the valve structure caused Damage. Degreasing is the specification of the SGT28-7200 ultrasonic cleaning agent and efficient water-based metal cleaning agent for parts degreasing. For stainless steel valves, the chloride ion content of water should be below 30×10-6.
The cleaning process is to put the parts into the ultrasonic cleaning tank, time 5min – 10min; the process should change position at the right time, should be regularly replenished with water and cleaning agent, and the valve body into the rinse tank with flowing water for rinsing, especially the seating ring and the gap between the valve body, with clean compressed air to blow the valve body dry, the seating ring and the gap between the valve body with a hot air gun blow dry. Use a syringe to inject acetone into the gap between the seat and the valve body, clean compressed air to blow the gap, the valve body cavity with a white lint-free cloth dipped in acetone for scrubbing, and finally check with ultraviolet light; there should be no residue, the clean valve body into the shovel pad with a clean paper pad, and covered with a plastic bag.
Do the initial inspection test with helium at room temperature and maximum working pressure to ensure all parts are connected properly. The entire test is done in the test tank with the valve immersed in liquid nitrogen and the liquid level reaching above the neck of the valve cover. During the test, the purity of the helium is maintained. During the cooling period, the low temperature of the valve body and the valve cover is measured with uniformly arranged thermocouples. The test can begin when the temperature at each location reaches the required level. The relevant standards perform the process of low-temperature testing. Check the sealing effect and performance of the valve in the low-temperature state, and do several switching operations in the middle. Do the valve sealing pressure test according to the medium flow direction of the valve; for a two-way seal valve, both sides of the seal valve should be tested separately. Table 3 shows pressure increments to test until the valve set sealing test pressure.
Table.3 Pressure increment diagram
After the test, disassemble the valve in a clean and dust-free environment. Check the ease of disassembly and the wear and damage of each part, and review the test report to see if it meets the requirements of the technical agreement and relevant standards.
In summary, in the production of valves, the key point in the process of supervision and manufacture is to ensure that the valve is the reliable, stable, and safe operation of the key factors. Considering the special characteristics of low-temperature fluids and low-temperature conditions on the valve material, material deformation after low-temperature treatment, special treatment of the strength of low-temperature valve components, low-temperature resistance, and durability of the valve in use are guaranteed. Cryogenic valves in the simulation of the actual working conditions using the helium leak detection method to do a relatively accurate leakage measurement. It ensures the normal operation of the valve under actual working conditions. In future supervision, work should also focus on the mechanical properties of the valve sealing material and processing accuracy and control of the deformation under low-temperature conditions to better ensure the quality and duration of the product and promote the improvement of valve quality.
Author: Wang Peng, Wu Xingguang
Source: China Ultra Low Temperature Valve Manufacturer – Yaang Pipe Industry Co., Limited (www.sfutube.com)