What is a low temperature valve
Low temperature valve refers to the valve whose medium temperature is – 40 ℃ ~ – 196 ℃. Common low temperature valves include low temperature ball valve, low temperature solenoid valve, low temperature gate valve, low temperature stop valve, low temperature safety valve, low temperature check valve, low temperature butterfly valve, low temperature needle valve, low temperature throttle valve, low temperature pressure reducing valve, low temperature electric regulating valve In addition to liquid nitrogen and other liquid inert gases, most of the media controlled by low-temperature valves such as low-temperature pneumatic control valves are not only flammable and explosive, but also vaporized during heating or flash, resulting in rapid volume expansion, which is easy to lead to leakage and explosion. Based on the characteristics of the medium and the requirements for the use of the valve at low temperature, the design, manufacture, test and installation methods of the low-temperature valve are different from those of the ordinary valve.
Standards and definitions for cryogenic valves
Cryogenic valve plays an important role in modern energy technology transmission pipeline. It is mainly used in storage tanks or pipelines of special low-temperature media, such as LNG (liquefied natural gas), LO2 (liquid oxygen), LN2 (liquefied nitrogen), LPG (liquefied petroleum), CNG (compressed natural gas), liquid argon, carbon dioxide, low ethylene and aviation fuel, as media regulation and shutdown. Low temperature medium is not only flammable and explosive, but also has strong permeability. When heating up, it needs to be gasified. During gasification, the volume expands several times. Therefore, in the manufacturing process of low-temperature valve, from material selection to processing and then to low-temperature treatment is a very severe and harsh process flow. Due to the special low-temperature medium, it is also necessary to make comprehensive consideration in the selection of valve material. First, the synthetic elements, hardness and toughness of the material.
Different standards have different definitions for low temperature valves
- (1) British Valve Standard BS 6364 valves for low temperature service is applicable to the medium temperature range of – 50 ℃ – 196 ℃;
- (2) The low temperature in American Standard MSS SP-134 requirements for low temperature valves and their body/bonnet extensions is interpreted as “for standard practice, the low temperature range is – 100 ℃ – 195 ℃”;
- (3) The Chinese national standard GB/T 24925 technical conditions for low temperature valves is applicable to the medium temperature range of – 29 ℃ – 196 ℃;
- (4) Shell valve standard shell MESC SPE77/200 valves for low temperature and ultra-low temperature is applicable to the medium temperature range of – 30 ℃ – 196 ℃.
- (5) According to the liquefaction temperature of different gases under atmospheric pressure, it is generally divided into six temperature levels abroad.
- Level I: 0-46 ℃;
- Secondary temperature: – 47-60 ℃;
- Grade III: – 61-70 ℃;
- Grade IV: – 71-101 ℃;
- Grade V: – 102-196 ℃;
- Grade VI is below – 253 ℃;
- Generally, – 46-150 ℃ is called low temperature, and below – 150 ℃ is called ultra-low temperature.
The definition of low-temperature valve in the petrochemical industry is defined according to the design temperature of the transmission medium. Generally, the valve applied to the medium temperature below – 40 ℃ is called low-temperature valve, and the valve applied to the medium temperature below – 101 ℃ is called ultra-low temperature valve.
Design features of low temperature valve
Material selection of low temperature valve
The working medium of low-temperature valve is not only low in temperature, but also mostly or, and has strong permeability. Therefore, many special requirements for valve materials are determined. The mechanical properties of steel at low temperature are different from those at normal temperature. In addition to strength, the important index of low temperature steel is its low temperature impact toughness. The low-temperature impact toughness of materials is related to the brittle transition temperature of materials. The lower the brittle transition temperature of materials, the better the low-temperature impact toughness of materials. Low temperature cold embrittlement exists in metal materials with body centered cubic lattice such as carbon steel, while the impact toughness of metal materials with face centered cubic lattice such as austenitic stainless steel is basically not affected by low temperature.
Low temperature valve body, bonnet and other pressure parts are usually made of ductile materials with good low temperature strength. At the same time, factors such as weldability, machining performance, stability and economy should be considered. In design, three low temperature levels – 46 ℃, – 101 ℃ and – 196 ℃ are commonly used- Low temperature carbon steel is generally selected for 46 ℃ low temperature class, and 300 series austenitic stainless steel is generally selected for – 101 ℃ and – 196 ℃ low temperature class. This stainless steel has moderate strength, good toughness and good processability.
Structural design of petrochemical low temperature valve
(1) Structural design of valve cover
A notable feature of low-temperature valves is that their bonnet is generally of long neck structure. It is also clearly stipulated in GB/T 24925 technical conditions for low-temperature valves that “the bonnet of low-temperature gate valve, globe valve, ball valve and butterfly valve shall be designed into a long neck bonnet structure for easy cold insulation according to different service temperature requirements, so as to ensure that the temperature at the bottom of stuffing box is maintained above 0 ℃. The design of the extended bonnet structure is mainly to keep the valve operating handle and stuffing box structure away from the low-temperature area, which can not only avoid the frostbite of operators caused by too low temperature, but also ensure that the stuffing box and pressure sleeve can be used at normal temperature, prevent the reduction of the sealing performance of the packing and prolong the service life of the packing. In the low temperature state, with the decrease of temperature, the elasticity of the packing gradually disappears and the leak proof performance decreases. Due to the medium leakage, the packing and the valve stem freeze, affecting the normal operation of the valve stem. At the same time, the packing will be scratched due to the up and down movement of the valve stem, resulting in serious leakage. Therefore, low temperature valve must adopt long neck bonnet structure. In addition, the long neck structure is also convenient for winding cold insulation materials to prevent cold energy loss. As the low-temperature pipeline generally has a thick cold insulation layer thickness, the long neck valve cover is convenient for cold insulation construction, and the packing gland is outside the cold insulation layer, which is conducive to fastening the gland bolts or adding packing at any time when necessary without damaging the cold insulation layer.
Bonnet extensions are specified in BS 6364, MSS sp-134 and shell MESC SPE77/200. Among them, BS 6364 specifies the extension size of 15 ~ 500 with cold box, and specifies that the small extension length of non cold box shall be 250mm; MSS SP-134 includes the lengthening size requirements of 15 ~ 300 with cold box and non cold box. In comparison, the lengthening size of non cold box is longer than that specified in BS 6364, and the lengthening size of cold box is shorter than that specified in BS 6364. Shell MESC SPE77/200 does not distinguish between cold box and non cold box, and specifies the length of 15 ~ 1200 in different temperature ranges.
Comprehensively considered, shell MESC SPE77/200 has a wide selection range of extended length, which is convenient and reliable to use. If it is used in low-temperature key occasions, it can be designed with reference to shell MESC SPE77/200 standard or according to the special length requirements of the design unit. In addition, when selecting the length, it is also necessary to consider whether the thickness of the design cold insulation layer is greater than the length. If so, it should be lengthened to match the cold insulation thickness.
(2) Structural design of drip plate
Since the transmission in the valve is low-temperature medium, in order to avoid or reduce the transmission of medium temperature to the valve stem and its upper filling materials and prevent these materials from failure due to freezing, a drip plate structure can be added to the valve. Some research institutions have experimentally verified the valve with drip plate structure, and proved that the upper end temperature of the valve cover with drip plate is higher. Due to the low temperature at the upper part of the extended bonnet, the valve is usually exposed to the air, and the water vapor in the air will liquefy into water droplets when encountering low temperature. The diameter of the drip plate exceeds the diameter of the middle flange, which can prevent the low-temperature liquefied water vapor from dripping on the middle flange bolts and prevent the bolt rust candle from affecting the on-line maintenance. In addition, the drip plate shall be set outside the cold insulation layer to prevent condensed water from falling onto the cold insulation layer and the upper part of the valve body, protect the cold insulation layer and prevent the loss of cooling capacity.
(3) Structural design of pressure relief components
For the low-temperature valve with closed middle chamber structure, when applied to flammable, explosive and easily gasified media, there are special requirements for the valve sealing structure. The volume of some low-temperature media will increase after vaporization. For example, the volume of liquefied natural gas after vaporization is more than 600 times that of liquid. When the valve is closed and the ambient temperature is relatively high, the low-temperature media in the valve body will absorb the heat in the environment and gradually vaporize, and its volume will rise rapidly, resulting in overpressure in the valve and even threatening the safety of the valve, In order to ensure the safety of valves and factories, such valves are required to be equipped with a middle chamber self pressure relief structure to realize automatic relief in case of abnormal overpressure in the inner chamber of the valve. Such as low-temperature gate valve and ball valve, there will be obvious differences in pressure relief design due to different valve sealing principles. However, different manufacturers have different characteristics in the design of pressure relief structure.
(4) Anti static and fireproof structure design
As low-temperature valves are generally used in flammable and explosive media, anti-static design and fire protection design are particularly important. The anti-static design is mainly to conduct the valve rod and valve body in a current guiding mode similar to the lightning rod, so as to export the static electricity to eliminate potential safety hazards and ensure the supply safety of the whole system. For example, GB / T 24925 clearly stipulates that “for valves with soft seat or soft closing insertion parts for flammable vapor or liquid, the design shall ensure that the valve body and valve stem have conductive continuity, and the large discharge path resistance shall not exceed 10 Ω”. The design of fireproof structure is mainly aimed at the problem of medium leakage caused by drastic temperature change. The design of fireproof structure is similar to the design requirements of ordinary valves.
(5) Valve body
The valve body shall be able to fully withstand the expansion and contraction caused by temperature change. Moreover, the structure of the valve seat will not be deformed due to temperature changes.
(6) Valve disc
The gate valve adopts flexible ram or open ram; The flat seat and needle valve of stop valve adopt plug shaped disc. No matter how the temperature changes, these structural forms can maintain a reliable seal.
(7) Valve stem
The valve rod shall be chrome plated, nickel plated or nitrided to improve the surface hardness of the valve rod and prevent the valve rod from biting with the packing and packing gland (gland), damaging the sealing packing and causing the leakage of the stuffing box.
The low temperature performance of gasket material shall be considered in gasket selection, such as compression resilience, preload, fastening pressure distribution and stress relaxation characteristics.
(9) Stuffing box and packing
The stuffing box shall not be in direct contact with the low temperature section, but shall be set at the top of the long neck bonnet to make the stuffing box far away from the low temperature and work at a temperature above 0 ℃. In this way, the sealing effect of the stuffing box is improved. In case of leakage, or when the sealing effect is reduced due to the direct contact of low-temperature fluid with the packing, grease can be added from the middle of the stuffing box to form an oil seal layer and reduce the differential pressure of the stuffing box as an auxiliary sealing measure. Stuffing box mostly adopts two-stage packing structure with intermediate metal isolation ring. However, other types such as general valve stuffing box structure and double stuffing box structure with self tightening valve stem are also adopted.
(10) Upper seal
Low temperature valves are equipped with upper sealing seat structure, and the upper sealing surface shall be overlaid with cobalt chromium tungsten carbide and ground after finishing.
(11) Sealing surface of valve seat and disc (RAM)
The closing part of low-temperature valve adopts cobalt chromium tungsten cemented carbide surfacing structure. Due to the large expansion coefficient of PTFE and embrittlement at low temperature, the soft sealing structure is only suitable for low-temperature valves with a temperature higher than – 70 ℃, but polytrifluoroethylene can be used for low-temperature valves with a temperature of – 162 ℃.
(12) Middle flange bolt
The bolt should have sufficient strength because the bolt often breaks due to fatigue when working under repeated load. Because the bolt is easy to cause stress concentration at the root of the thread, the bolt with full thread structure is adopted.
Installation requirements for low temperature valves
Because of the special structure of low-temperature valve, the installation of low-temperature valve also has its special requirements. Because of the structural characteristics of the long neck bonnet of the low-temperature valve, the stem direction of the low-temperature valve must be within 45 degrees of the vertical upward angle during installation, and the installation on the vertical pipeline should be avoided as far as possible. Otherwise, the low-temperature medium will fill the extension of the valve cover, resulting in the failure of the valve packing, and will transfer the cooling capacity to the valve handle, causing personal injury to the operator. For low-temperature valves with pressure relief structure, special attention shall be paid to the requirements of valve pressure relief direction when installing the valve. The direction of valve pressure relief shall be marked on the process flow diagram and reflected in the pipeline isometric diagram.
There are two main reasons for the leakage of low-temperature valve: Internal leakage and external leakage.
Internal leakage of valve
The internal leakage of the valve is mainly caused by the deformation of the sealing pair at low temperature. When the medium temperature drops to make the material phase change, the volume changes, warping and deformation of the sealing surface with high grinding accuracy, resulting in poor low-temperature sealing. We conducted a low-temperature test on DN250 valve. The medium was liquid nitrogen (- 196 ℃) and the butterfly plate material was 1Cr18Ni9Ti (without low-temperature treatment). It was found that the warpage deformation of the sealing surface was about, which was the main cause of internal leakage.
The newly developed butterfly valve is changed from plane seal to conical seal. The valve seat is an inclined conical elliptical sealing surface, which forms a sealing pair with a positive circular elastic sealing ring embedded on the butterfly plate. The sealing ring can float upward in the inner diameter of the butterfly plate groove. When the valve is closed, the elastic sealing ring first contacts with the short shaft of the elliptical sealing surface. With the rotation of the valve stem, the sealing ring is gradually pushed inward, forcing the elastic ring to contact with the long shaft of the inclined conical surface, resulting in all contact between the elastic sealing ring and the elliptical sealing surface. Its sealing is achieved by the deformation of the elastic ring. Therefore, when the valve body or butterfly plate deforms at low temperature, it will be absorbed and compensated by the elastic sealing ring without leakage and jamming. When the valve is opened, this elastic deformation disappears immediately, and there is basically no relative friction in the opening and closing process, so the service life is long.
External leakage of valve
First, when the valve and pipeline are connected by flange, the leakage is caused by the unsynchronized shrinkage of the connecting gasket, connecting bolt and connecting piece at low temperature. Therefore, we changed the connection mode of valve body and pipeline from flange connection to welding structure to avoid low-temperature leakage. The second is the leakage at the valve stem and packing. Generally, F4 is used as the packing of most valves, which is widely used because of its good self sliding performance, small friction coefficient (F = ~ 0.1 for steel) and unique chemical stability. However, F4 also has some shortcomings. First, it has a large tendency of cold flow; Second, the linear expansion coefficient is large, resulting in cold shrinkage at low temperature, resulting in leakage, resulting in a large amount of ice at the valve stem, so that the valve fails to open. Therefore, the developed low-temperature butterfly valve adopts self shrinking sealing structure, that is, it can be sealed at room temperature and low temperature through the reserved gap by taking advantage of the large expansion coefficient of F4.
Strict manufacturing process and special equipment are formulated for the low-temperature valve, and strict quality control is carried out for the processing of parts. After special low-temperature treatment, the rough machined parts are placed in the cooling medium for several hours (2-6 hours) to release the stress, ensure the low-temperature performance of the material and ensure the finish machining size, so as to prevent the leakage caused by the deformation caused by the temperature change of the valve under low-temperature working conditions. The assembly of the valve is also different from the ordinary valve. The parts need to be strictly cleaned to remove any oil stain to ensure the service performance.
- 1. Valve body and bonnet: LCB (- 46 ℃), LC3 (- 101 ℃), CF8 (304) (- 196 ℃);
- 2. Ram: stainless steel surfacing cobalt based cemented carbide;
- 3. Valve seat: stainless steel surfacing cobalt based cemented carbide;
- 4. Valve stem: 0Cr18Ni9.
Main material of low temperature valve
Low temperature valve material is very important. Unqualified material will cause external or internal leakage of shell and sealing surface; The comprehensive mechanical properties, strength and rigidity of parts can not meet the use requirements or even fracture.
1. Factors to be considered in the selection of main materials of low temperature valves
Considering metallography, in addition to austenitic steel, copper and aluminum with face centered cubic lattice, general steel will be brittle at low temperature, thus reducing the strength and service life of the valve. When selecting the main material, first select the material suitable for working at low temperature. Aluminum will not appear low-temperature brittleness at low temperature. However, due to the low hardness of aluminum and aluminum alloy and the poor wear and scratch resistance of aluminum sealing surface, there are certain restrictions on the use of aluminum in low-temperature valves. It is only selected in low-pressure and small-diameter valves. Materials working at low temperature shall ensure the low-temperature performance of low-temperature valve, mainly to ensure its cold impact strength. Low temperature valve internals must be properly selected to have sufficient cold impact strength in order to prevent fracture. The impact strength of alloy steels of C and CR will soon lose when it is lower than – 20 ℃, so the service temperature is limited to – 30 ℃ and – 50 ℃ respectively. Nickel steel containing 3.5% Ni can be used up to – 100 ℃, and nickel steel containing 9% Ni can be used up to – 192 ℃. Austenitic stainless steel, nickel, Monel alloy, Hastelloy, titanium, aluminum alloy and bronze can be used at lower temperatures (- 273 ℃). In addition, the material selection of low-temperature valves shall also consider the following factors:
- (1) Minimum service temperature of the valve;
- (2) The mechanical properties of metal materials required to maintain working conditions at low temperature, especially impact toughness, relative elongation and microstructure stability;
- (3) It has good friction resistance at low temperature and no oil lubrication;
- (4) Good corrosion resistance.
2. Selection of valve body, bonnet, valve seat and disc (gate) materials
The selection principle of main parts and materials of low-temperature valve is roughly as follows: ferritic steel shall be selected when the temperature is higher than – 100 ℃; Austenitic steel shall be selected when the temperature is lower than – 100 ℃; Low pressure and small diameter valves are optional.
3. Material selection of valve rod and fastener
When the temperature is higher than – 100 ℃, the valve rod and bolt materials are made of Ni,, Cr Mo and other alloy steels, which are subject to appropriate heat treatment to improve the tensile strength and prevent thread bite. When the temperature is lower than – 100 ℃, it shall be made of austenitic stainless acid resistant steel. However, the hardness of 18-8 acid resistant steel is low, which will cause mutual scratch between the valve stem and the packing, resulting in leakage at the packing. Therefore, the valve stem surface must be plated with hard chromium (coating thickness 0.04-0.06mm), or nitrided and nickel phosphorus treatment to improve the surface hardness. In order to prevent the nut from seizing with the bolt, the nut is generally made of Mo steel or Ni steel, and the thread surface is coated with molybdenum disulfide.
Structure of casting blank
Most of the valve blanks are thin shell castings with complex structure, which requires not only good appearance quality, but also dense internal quality and good metallographic structure, without defects such as pores, shrinkage cavities, sand inclusions and cracks. Therefore, the casting process is complex and the heat treatment technology is difficult. In the machinery industry, the casting of pressure bearing thin shell castings of valves is much more complex and difficult than that of other mechanical components.
Machining process of cryogenic valves
Due to the poor cutting performance of most high-strength, high hardness and high corrosion-resistant materials, such as high alloy stainless steel and acid resistant steel, which have the disadvantages of high toughness, high strength, poor heat dissipation, large chip viscosity and strong work hardening tendency, it is difficult to achieve the required dimensional accuracy and finish, which brings some difficulties to the machining tools, processes and equipment. In addition, the valve sealing surface has high requirements for machining accuracy, matching angle, finish and matching sealing pair, which brings great difficulty to machining.
Process arrangement of valve parts
The number of main parts of the valve is small, the structure is relatively simple, the machining accuracy of most dimensions is not high, and the outside is rough, which gives people the impression that it belongs to simple machinery. In fact, the heart sealing part of the valve is extremely precise. The “three degrees” (flatness, smoothness and hardness) of the sealing surface is very high, and the coincidence of the sealing pair composed of two sealing surfaces must reach zero to zero, so as to meet the zero leakage of the air tightness test. This rough benchmark to ensure the precise zero to zero requirements of the heart is the biggest process difficulty of valve processing.
Valve test and inspection
Valve is an important opening, closing and regulating element of pressure pipeline, and the service conditions of pressure pipeline are very different, high temperature and high pressure, low temperature and deep cold, flammable and explosive, highly toxic and corrosive. However, the test and inspection conditions of valve manufacturing can not meet the same requirements of working conditions. Various international and domestic valve test standards stipulate that the test is carried out with gas or water as the medium under the condition close to normal temperature. There is a fundamental hidden danger, that is, the valve products that pass the normal factory test may be difficult to meet the use requirements due to problems such as material selection, casting quality and seal damage under harsh actual working conditions, and major quality accidents will occur. No wonder some old valve experts who have worked all their lives are more cautious and worried as they get older.
Design requirements for valve body, valve stem and bushing.
Structure and shape of low temperature valve shell
Whether the material selection is correct or not is of great significance to the normal and reliable operation of the valve. Compared with stop valve and gate valve, the structural characteristics of butterfly valve not only avoid the deformation caused by irregular shape, uneven shell wall thickness, cold shrinkage at low temperature and temperature difference stress, but also due to the small volume of butterfly valve, the shape of valve body is basically symmetrical, so the heat capacity is small; The consumption of precooling capacity is also small; The shape is regular and convenient for the cold insulation measures of the valve. For example, the newly developed yy363h butterfly valve is designed and manufactured in full accordance with the particularity of the low-temperature valve in order to ensure the reliable use of the valve at low temperature. For example, 1Cr18Ni9Ti austenitic stainless steel with cubic lattice is selected as the shell material.
The selection of valve stem bushing
According to the user’s feedback, some low-temperature valves are in operation, and the rotating parts of the valves are sticky and occluded from time to time. The main reasons are: the selection of matching materials is unreasonable, the reserved cold clearance is too small, and the machining accuracy. When developing low-temperature valves, a series of measures were taken to prevent the above phenomena. For example, SF-1 composite bearing with small friction coefficient and self-lubricating performance is selected for the upper and lower bushings of the valve rod, which can be suitable for some special needs of low-temperature valves.
Test and inspection of low temperature valve
The main parts of the low-temperature valve shall be subject to low-temperature treatment, and each batch shall be sampled for low-temperature impact test, so as to ensure that the valve is not brittle under low-temperature working conditions and can withstand the impact of low-temperature medium.
Perform the following tests on each valve
- (1) Normal temperature shell strength test; Sealing test at room temperature and low pressure; Normal temperature and low pressure sealing test;
- (2) Low temperature upper seal air tightness test (with upper seal); Low temperature gas seal test, etc. to ensure that the whole low temperature valve meets the provisions of the standard;
- (3) Low temperature treatment shall be carried out for main parts and low-temperature impact test shall be carried out for each batch of sampling to ensure that the valve is not brittle under low-temperature working conditions and can withstand the impact of low-temperature medium;
- (4) Low temperature (cryogenic) valves shall be subject to low temperature treatment and impact test according to corresponding material specifications;
- (5) The on resistance between valve body and valve stem or between internals and valve body is less than 1 OHM.
Test equipment of low temperature valve
- Cryogenic valve test device;
- Liquid nitrogen storage device;
- Cryogenic treatment tank.
Test method of low temperature valve
(1) Preparation before test
Remove the oil stains on the valve parts, wipe them clean, and assemble the valve in a clean environment without dust and oil stains; Tighten the bolt to the predetermined torque value and tension value, and record the value; Connect the valve with a suitable thermocouple so that the temperature of the valve can be monitored throughout the test.
Install and connect the valve in the test vessel, ensure that the valve packing is at the position where there is no vaporized gas on the top of the vessel, conduct the initial system verification test with the specified medium gas at the large valve seat test pressure at room temperature to ensure that the valve is in the appropriate state, and then start the test, immerse the valve in liquid nitrogen for cooling, The liquid level shall at least cover the connection between the valve body and the bonnet, and helium shall be supplied to the valve throughout the cooling process. During the cooling process, the temperature of the valve is monitored by thermocouples installed in appropriate positions. The valve is stable at the test temperature. Measure the temperature with a thermocouple to make sure that the temperature of the valve is uniform. At the test temperature, the initial verification test shall be conducted with helium at large valve seat test pressure, and the valve seat pressure test shall be conducted at the inlet side of the valve. For the valve that can be sealed in both directions, the two valve seats shall be tested respectively. Keep the valve in the open position, close the needle valve on the outlet side of the valve, and raise the pressure in the valve chamber to the test pressure of the valve seat. Maintain the pressure to the specified requirements, and check whether there is leakage at the valve packing and the connection between the valve body and the cover. Restore the valve to room temperature, and then conduct normal temperature sealing test. After the test, clean and dry the valve, and leave the factory after passing the inspection.
In addition to the normal temperature and low temperature test, the low temperature valve shall also be inspected as follows:
- 1. The main parts of ultra-low temperature valves shall be subject to cryogenic treatment (how to cryogenic treatment? We will talk about it later. The figure below shows the cryogenic treatment of valves).
- 2. Low temperature impact test shall be conducted for main parts and welds to ensure that the valve is not brittle under low temperature conditions;
- 3. The valve must be subject to normal temperature test, first conduct hydraulic strength test at 1.5 times the nominal pressure, and then conduct sealing test at 1.1 times the nominal pressure;
- 4. After the hydrostatic test, the valve must be cleaned of moisture, free of grease and kept dry;
- 5. Each batch of low-temperature valves shall be sampled according to a certain proportion for low-temperature test. The low-temperature test must be carried out after 1.5 times of hydraulic strength test;
- 6. After the low temperature test of the valve, air shall be prevented from entering the valve;
- 7. After the low temperature test of the valve, it shall be heated naturally or blown by a fan. It is strictly prohibited to accelerate the temperature rise of the valve by heating and other methods;
- 8. Tighten the valve fasteners again after the valve low temperature test and natural temperature rise;
- 9. When the valve is put into storage, the valve switch must be checked to ensure that the valve is in a reasonable state;
- 10. During the storage and transportation of valves, attention shall be paid to the separation of stainless steel valves and carbon steel valves, and dust, water, oil and collision prevention measures shall be taken;
- 11. The flow direction, pressure relief direction and nameplate of the valve shall be inspected according to the requirements of the design unit.
Application of low temperature valve
Low temperature valve refers to low-temperature ball valve, low-temperature gate valve, low-temperature stop valve, low-temperature check valve, etc. it is mainly used for fluid control on low-temperature storage tanks and tank cars with medium of ethylene, liquefied natural gas, liquid oxygen, liquid nitrogen, liquid argon, carbon dioxide, pressure swing adsorption oxygen production and other devices. Low temperature valve refers to the ultra-low temperature valve whose temperature is between – 40 ℃ – 120 ℃ and – 120 ℃ – 196 ℃. Weidie low temperature valve is subjected to special low-temperature treatment, and the rough machined parts are placed in the cooling medium for several hours (2-6 hours) to release stress, ensure the low-temperature performance of materials and ensure the finishing size, so as to prevent the valve from being in low-temperature working conditions, Leakage due to deformation caused by temperature change. The assembly of the valve is also different from the ordinary valve. The parts need to be strictly cleaned to remove any oil stain to ensure the service performance.
How to select low temperature valve
Low temperature valve in low temperature conditions, the valve with the usual working temperature lower than minus 40 ℃ is called low temperature valve. Low temperature valve is an indispensable and important petrochemical, air separation, natural gas and other industrial equipment. The quality determines whether it is safe, economic and sustainable production. With the development of modern science and technology, it has become an increasingly common low-temperature valve, and the demand for low-temperature valve is also increasing. Therefore, how to select the appropriate low-temperature valve is the urgent concern of the industry. The selection of low-temperature valve should be considered from the aspects of material, purpose and so on.
Materials of low temperature valves
The materials of low temperature valves mainly have the following requirements
- (1) Low temperature brittle fracture shall not occur at normal working temperature;
- (2) It should be stable at normal working temperature and its organizational structure to prevent the volume change of phase change materials;
- (3) Adopt welding structure, welding performance and low-temperature welding must be reliable;
- (4) operate frequently under low-temperature conditions, and the decorative materials shall be free of interference, bite and scratch.
In addition, the space for the materials of low-temperature valves and the materials in the above table, such as the service temperature and chemical composition of low-temperature valves commonly used in non-ferrous materials such as forged steel and copper, is limited, so they will not be listed one by one here. Ferritic low temperature pressure components generally need to do the temperature pendulum impact test required by the standard, and the average and minimum impact energy to meet the requirements of relevant standards. Austenitic stainless steel has good low-temperature toughness and generally does not need low-temperature impact test. The austenitic stainless steel valve uses a portable spectrometer for chemical analysis (PMI). Due to the low temperature required for expensive non-ferrous materials, many domestic material manufacturers generally do not need to add sufficient chromium, nickel or molybdenum. The result of the usual chemical analysis is that the content of some non-ferrous metals is at the lower limit of the content specified in the standard, Or just below the standard. I have tested austenitic stainless steel products with magnets. Without exception, there is a larger magnetic field in domestic valve manufacturers, indicating that austenitic materials contain more ferrite, and the ferrite content will significantly affect the low-temperature properties of the materials. Austenitic stainless steel containing more ferrite: high carbon content, low non-ferrous metal content or inadequate solution treatment.
Usually, all low-temperature metal materials must be subjected to cryogenic treatment or cold treatment to minimize the distortion of the low-temperature of the material. The upcoming workpiece is immersed in cryogenic liquid (cold treatment, frozen with dry ice, cold treatment with liquid nitrogen) for a certain time (usually 2H), and then restored to room temperature at the temperature of air. The role of low-temperature immersion treatment is to reduce residual stress and improve the dimensional stability of the workpiece. In advance, the defects of the valve that may occur in low-temperature operation are exposed in the manufacturing stage. Some manufacturers sometimes omit the safe use of valves due to cost or delivery considerations, which poses a threat, so it is necessary that the purchase quality inspection plan will be included in the supervision and inspection procedures.
Packing and gasket
The packing and gasket of low temperature valve shall remain stable under the sealing state. Common expanded graphite filler or impregnated PTFE filler, common gasket material expanded graphite or impregnated PTFE and stainless steel wound gasket system. Low temperature between the body and the ball floating ball valve sealing material commonly used fluoroplastics. Please note that fluoroplastics at low temperature have strong shrinkage, reduce the sealing pressure and affect the sealing effect. In general, pure tetrafluoroethylene can only be used for sealing. Under the following conditions of – 70 ℃ and – 70 ℃ at the lowest temperature, the low-temperature shrinkage of multi modified tetrafluoroethylene seal will be changed. Modified tetrafluoroethylene and tetrafluoroethylene materials have a certain price. Domestic manufacturers still use a lot of pure PTFE seal-70e low temperature, so many domestic valves pass the test at room temperature and show their original shape at a lower temperature, exceeding the internal leakage. Supervision inspectors tend to attach great importance to metal materials and ignore the control of sealing materials, which must be pointed out here.
General requirements for selection of low temperature valves
Low temperature valves should pay attention to the selection of valve materials
Generally, the use of low-temperature valves shall meet the following requirements: the whole valve shall be able to work in low-temperature medium and ambient temperature for a long time during assembly; The low-temperature medium of valve phase should not become a significant heat source code, and excessive heat inflow may also lead to the temperature in the valve of medium vaporization, resulting in abnormal pressure and danger; The high-temperature medium around the valve shall meet the relevant explosion-proof and fire-proof requirements for the performance and sealing performance of hand wheel operation, direct contact and low-temperature medium.
Gate valve, ball valve, butterfly valve and other valves with almost similar properties should be subdivided to help make choices. The gate valve has the advantages of convenient use, simple structure, small fluid resistance, unlimited flow direction of medium, but it is easy to damage, strong pressure resistance, difficult to repair, long opening and closing time, and is generally used less considering its comprehensive performance; The ball valve has many advantages, good sealing, low flow resistance is almost zero, compact structure, light weight, strong compression and damage resistance, and its wide application is almost unlimited. However, the ball valve has poor sensitivity to cold flow, poor thermal conductivity, easy aging, and its performance price ratio is high in all aspects; Butterfly valve has the advantages of light weight, simple structure, rapid start-up and low resistance. It is widely used in many fields of metallurgy and petrochemical industry, but its flow range is small and its sealing performance is poor; When selecting two eccentric or three eccentric, the sealing effect must be considered. Generally, the sealing effect of three eccentric butterfly valve is better.
Liquid cryogenic valve should pay attention to the selection of valve structure
Generally, LNG and low-temperature ethylene liquid low-temperature valves are slightly different from traditional low-temperature valves. In addition to the overall requirements of valve structure, the following problems need to be considered, but the following problems also need to be solved: when the low-temperature valve is closed, the temperature of low-temperature gasification medium remaining in the valve body rises rapidly and the internal pressure in the valve body; Adverse effect of low temperature stuffing box on sealing performance; Harmful effects of parts and valves; Cold deformation required for explosion-proof of low temperature medium components. Specific structural requirements for specific LNG cryogenic liquid ethylene process valve: first, these requirements are refined. As a basic part of the cryogenic valve body, when it is under low temperature conditions, the valve body can withstand the expansion and contraction of additional stress under low temperature stress without deformation and maintaining the sealing pair of the valve. The valve body must have certain rigidity and hardness; Secondly, the low temperature valve long neck bonnet structure is the basic means to reduce the external input means, and the heat should be ok to ensure that the temperature of some stuffing boxes is above 0 ℃, so that the stuffing can work normally; Prevent frost or freezing in the dry part of the supercooled filling part of the stuffing box components and the part of the valve cover; When the pressure relief part of the low-temperature valve should meet certain requirements, due to the external pressure expansion valve, the material of the pressure relief valve must make the valve work normally to avoid the internal valve flushing out. Due to the excessive pressure, this part of the packing and flange gasket can be relieved in the shortest time. Finally, the upper sealing surface material of the common low-temperature ethylene liquid valve must be chrome finishing Tungsten carbide made by grinding at low temperature. In general, liquid terminal valves for different purposes should vary according to their purposes and cannot be generalized.
General problems in the selection of low temperature valves
The quality of the valve determines the selection space
At present, there are many different kinds of low-temperature valves circulating in the domestic market. All of these valves are of high quality and can be used for a long time through the advertising of merchants. However, it has been proved that the goods that cost a lot of money are not necessarily free of quality defects. Generally, the valve of liquid wharf is determined by the following factors: the name of low-temperature medium, the temperature of low-temperature medium, the length of long neck of low-temperature valve, the pressure of low-temperature valve, the sealing and connection mode of low-temperature valve, etc. Excellent quality valves are nothing more than that these requirements have been met or basically met, but in the face of a wide range of commodities, the quality problem of low-temperature valves is still worth studying. According to incomplete statistics, the quality problems of low-temperature valves are mainly in the following aspects: first, the low-temperature refrigeration system of valves is often paralyzed; Second, the pressure relief system of the valve often fails; Third, the overall compressive capacity of the valve is poor. If the quality problem of valve selection is to be fundamentally solved, the buyer must have a detailed understanding of the overall performance of the valve in order to avoid selecting the wrong product.
The inspection problem of the valve determines the selection space
The inspection of valves shall not only be carried out at normal temperature, but also at low temperature. The normal temperature inspection mainly includes shell hydraulic strength test, hydraulic and pneumatic sealing test, upper sealing test, opening and closing and torque test. The main purpose of low temperature test is to test the operation performance and sealing performance of low temperature valve under low temperature. The operating performance requires that the valve can be opened and closed flexibly, and the moving parts and sealing pairs shall not be scratched and bitten. Sealing performance requires that the leakage of valve sealing surface shall be less than the allowable leakage. At present, some low-temperature valve manufacturers are most likely to deceive consumers, so that the links that can muddle through the inspection of the valve are generally the upper sealing device and hydraulic strength test. The core part of the low-temperature valve is its internal low-temperature device, which can adjust the working state of the valve neck cover at any time under the action of pressure, so as to complete the general working process of the valve. The upper sealing device cooperates with the pressure relief device to control the pressure inside the valve. When the external pressure is high enough, the upper sealing device will bear most of the pressure from the valve, and some non nickel alloy materials can also achieve the expected effect during the valve quality inspection, but these materials can not work normally for a long time. As for the hydraulic strength test, in the inspection of valves, all kinds of shoddy valves will also be replaced by various informal materials to complete the inspection. In addition, the expansion coefficient of the main components of the valve must also be included in the formal inspection process. We know that the key of the low-temperature valve is cryogenic treatment, so as to ensure that all parameters of the low-temperature valve meet the requirements, especially the expansion coefficient, so as not to cause various valve jamming in use.
The selection of low-temperature valve is a matter that can be heavy or light. In the actual selection, we must carefully consider and select appropriate commodities in combination with the actual use of the valve. After fully understanding the structure, materials and other basic parameters of the valve, we can make appropriate and rational choices.
Source: China Low Temperature Valve Manufacturer – Yaang Pipe 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.)
If you want to have more information about the article or you want to share your opinion with us, contact us at email@example.com
Please notice that you might be interested in the other technical articles we’ve published:
- Sealing Tests for Valves
- The Origin and Development of the Valve
- Nine Anti-Corrosion Measures of Valves
- The Selection of Control Valve
- Operation and Maintenance Manuals for Gate Valves
- When the control valves are used in liquid service
- Choosing Valves Based on Industry Characteristics
- Designs and Uses of Ball Valves for Submarine Pipelines
- Corrosion Fatigue of Valves
- How to choose valves
- What are valves