In demanding applications, cutoff requirements will be imposed on the regulating valve. Therefore, the shutoff function is an important function of the control valve and an intrinsic quality index (for the division of cutoff levels and standards, see related data). In the field, it often happens that the regulating valve is cut off badly and the leakage is big. Except for the increase in leakage caused by jamming, allowing the pressure difference to be too small and being opened, etc., most of them are caused by structural defects and improper selection of regulating valves.
First, the choice of structure type
One of the major drawbacks of double-sealing type regulating valves (such as double-seat valves, double-sealed sleeve valves, etc.) is the large leakage. However, prior to the 1980s, due to the prevalence of sleeve valves at that time, however, many manufacturers took great efforts in the sleeve valve to improve the structure to reduce the leakage, making it evolve into a very complex structure. As a result, this has brought a lot of deficiencies: many parts, poor reliability, difficult spare parts, difficult maintenance, and unsatisfactory cutting results. In fact, this is a misunderstanding in design thinking. Instead of making such complex changes in the "heart" of the valve body, it is better to achieve external adjustments. Therefore, a single seal type valve should be selected. In this way, the problems of sealing, reliability, maintenance and spare parts have all been solved. At this time, the focus of resolving the contradiction turns into a problem that the imbalance force of the medium to the valve increases. To solve it, only the powerful piston actuator and the thickened valve stem are required. This approach to solving problems through the outside world is obviously much easier than through internal and complex improvements. It is not so much a solution as it is a matter of thinking. In the 1990s, people began to look for shut-off valves with simpler structures and more reliable seals. At this time, it was discovered that the quarter-turn valve was superior to the straight-stroke valve (single seat valve, sleeve valve, and gate valve), and thus a three-eccentric cut butterfly valve, a full-function shut-off valve, and a hard-seal cut-off ball valve were produced.
Second, the choice of sealing surface (1) face to face sealing:
Common plunger valve, the sealing surface of the small cone surface of 60 °C, the valve seat is a small cone surface 60 °C, the width of the small cone is usually between 0.5mm ~ 2mm, to seal it must ensure that two Cone surface is in good contact. In fact, it is always affected by machining errors (like heart, out-of-roundness, inclination, etc.) and its sealing effect is not ideal. The leak rate of this type of valve is usually 10-4, and after precision grinding up to 10-6, it can only achieve a better sealing level.
(2) Spherical seal:
The spherical rotation of the valve core is tangent to the fixed valve seat small cone surface, which is line contact between them, which is better than the above face to face seal effect. Hualin's full-featured ultra-lightweight valve and spherical seal butterfly valve are manufactured using this idea. The leakage rate can reach 10-6 to 10-8, and the high-performance triple eccentric butterfly valve can reach as high as 10-8. Zero leakage.
Third, the choice of sealing material (1) soft seal
In addition to corrosion-resistant lining valves, general soft-sealing valves are valves in which the valve plug or valve seat is a non-metallic material (mainly PTFE, rubber, etc.) in the form of a seal. The soft seal valve has the best sealing effect, but it may leave more or less unclean debris (such as welding slag and iron swarf) in the pipeline installation and system cleaning. When they flow through the regulating valve, they are easy to handle. Soft seal seats or valve cores are scratched to increase leakage and seal reliability. Therefore, the choice of soft seal structure must consider the strict cleaning of the pipeline before the cleaning and commissioning of the medium.
(2) Hard seal
Hard seal and resurfacing wear-resistant alloys are the best choice for shut-off valves. This method considers the sealing property and also considers the service life and reliability. Although the factory specification is only 10-6 to 10-8, the effect of the zero leakage of the soft seal can not be met, but it is sufficient to meet the strict and close requirements, and it Durable and more economical.
Fourth, sealing reliability considerations
(1) Soft seals have a good cutting effect, but are not suitable for use in granular media. Once scratched, the leakage will increase dramatically. Therefore, use a hard seal when possible. Full function ultra-light valve hard seal up to 10-7 ~ 10-8, triple eccentric butterfly valve can reach zero leakage.
(2) Considerations for seal wear resistance: In addition to the selection of hard seals, cut-off type control valves requiring a leakage of 10-6 or more should (or must be) build-up welding wear-resistant alloys to improve seal wear resistance and resistance. Erosion, scratch resistance and increased valve life.
(3) The high-temperature medium cut-off regulator valve is assembled at room temperature and operates at a high temperature, and the temperature changes by several hundred degrees. Thermal expansion will change the seal specific pressure established during assembly. If it is too tight, the sealing surface will get stuck. When it is in motion, it will be seriously scratched and pulled. If it is too loose, the sealing pressure is not enough. In both cases, the leakage will increase rapidly. The best solution is to use elastic seats to absorb the difference caused by this change. There are usually three types of elastic solutions: 1 spring type; 2 thin flexible metal rings; 3 flexible seats with softness and softness. The latter is best, that is, it has the flexibility to overcome the thermal expansion, and has rigidity and builds up the wear-resistant alloy to improve the reliability of the wear-resistant surface.
(4) For the erosive medium containing particles, the direct erosion of the valve core to avoid high-speed medium should be considered. Straight stroke valves and butterfly valves do not have this function, while the full-function ultra-light valve eccentric rotation has this feature. In this case, the latter two valves should be selected.
(5) For hard seals, there must be a sufficient hardness difference, the same hardness material "do not give each other", easy to scratch.
(6) The thrust and stiffness should be increased for the assigned actuator to make it move freely.
V. The main valve type of shut-off valve
According to the above analysis, the main valve type of the shut-off valve is as follows. Selecting the shut-off valve should comprehensively consider various parameters such as leakage rate, medium type, medium temperature, flow coefficient, and cost performance. In the preferred order below, consider a hard seal before considering a soft seal.
(1) Full-featured ultra-lightweight control valve (2) Three-eccentric metal hard-seal butterfly valve (3) Double-eccentric spherical face seal butterfly valve (4) Ball valve (5) Eccentric rotary valve (6) Single-seat valve (7) Single seat sleeve valve (8) a soft sealing valve <br> <br> six, high temperature, high pressure, high pressure, large diameter cut valve selection
High temperature, high pressure, large pressure difference, and large diameter cutting are common problems. The main reason is the lack of valves that allow large pressure difference, good cutting and high temperature expansion, such as high pressure steam adjustment and cutting, due to large pressure difference. (1 ~ 3MPa or more), large diameter (DN200 ~ 300), had to use a balanced double seal structure (such as double seat valve or double seal sleeve valve), after special treatment, the leak rate of up to 10-4 , It is difficult to improve, and the full-featured ultra-light control valve has good overall performance and can solve this problem better. The leakage rate can reach 10-6 to 10-7 or more.
First, the choice of structure type
One of the major drawbacks of double-sealing type regulating valves (such as double-seat valves, double-sealed sleeve valves, etc.) is the large leakage. However, prior to the 1980s, due to the prevalence of sleeve valves at that time, however, many manufacturers took great efforts in the sleeve valve to improve the structure to reduce the leakage, making it evolve into a very complex structure. As a result, this has brought a lot of deficiencies: many parts, poor reliability, difficult spare parts, difficult maintenance, and unsatisfactory cutting results. In fact, this is a misunderstanding in design thinking. Instead of making such complex changes in the "heart" of the valve body, it is better to achieve external adjustments. Therefore, a single seal type valve should be selected. In this way, the problems of sealing, reliability, maintenance and spare parts have all been solved. At this time, the focus of resolving the contradiction turns into a problem that the imbalance force of the medium to the valve increases. To solve it, only the powerful piston actuator and the thickened valve stem are required. This approach to solving problems through the outside world is obviously much easier than through internal and complex improvements. It is not so much a solution as it is a matter of thinking. In the 1990s, people began to look for shut-off valves with simpler structures and more reliable seals. At this time, it was discovered that the quarter-turn valve was superior to the straight-stroke valve (single seat valve, sleeve valve, and gate valve), and thus a three-eccentric cut butterfly valve, a full-function shut-off valve, and a hard-seal cut-off ball valve were produced.
Second, the choice of sealing surface (1) face to face sealing:
Common plunger valve, the sealing surface of the small cone surface of 60 °C, the valve seat is a small cone surface 60 °C, the width of the small cone is usually between 0.5mm ~ 2mm, to seal it must ensure that two Cone surface is in good contact. In fact, it is always affected by machining errors (like heart, out-of-roundness, inclination, etc.) and its sealing effect is not ideal. The leak rate of this type of valve is usually 10-4, and after precision grinding up to 10-6, it can only achieve a better sealing level.
(2) Spherical seal:
The spherical rotation of the valve core is tangent to the fixed valve seat small cone surface, which is line contact between them, which is better than the above face to face seal effect. Hualin's full-featured ultra-lightweight valve and spherical seal butterfly valve are manufactured using this idea. The leakage rate can reach 10-6 to 10-8, and the high-performance triple eccentric butterfly valve can reach as high as 10-8. Zero leakage.
Third, the choice of sealing material (1) soft seal
In addition to corrosion-resistant lining valves, general soft-sealing valves are valves in which the valve plug or valve seat is a non-metallic material (mainly PTFE, rubber, etc.) in the form of a seal. The soft seal valve has the best sealing effect, but it may leave more or less unclean debris (such as welding slag and iron swarf) in the pipeline installation and system cleaning. When they flow through the regulating valve, they are easy to handle. Soft seal seats or valve cores are scratched to increase leakage and seal reliability. Therefore, the choice of soft seal structure must consider the strict cleaning of the pipeline before the cleaning and commissioning of the medium.
(2) Hard seal
Hard seal and resurfacing wear-resistant alloys are the best choice for shut-off valves. This method considers the sealing property and also considers the service life and reliability. Although the factory specification is only 10-6 to 10-8, the effect of the zero leakage of the soft seal can not be met, but it is sufficient to meet the strict and close requirements, and it Durable and more economical.
Fourth, sealing reliability considerations
(1) Soft seals have a good cutting effect, but are not suitable for use in granular media. Once scratched, the leakage will increase dramatically. Therefore, use a hard seal when possible. Full function ultra-light valve hard seal up to 10-7 ~ 10-8, triple eccentric butterfly valve can reach zero leakage.
(2) Considerations for seal wear resistance: In addition to the selection of hard seals, cut-off type control valves requiring a leakage of 10-6 or more should (or must be) build-up welding wear-resistant alloys to improve seal wear resistance and resistance. Erosion, scratch resistance and increased valve life.
(3) The high-temperature medium cut-off regulator valve is assembled at room temperature and operates at a high temperature, and the temperature changes by several hundred degrees. Thermal expansion will change the seal specific pressure established during assembly. If it is too tight, the sealing surface will get stuck. When it is in motion, it will be seriously scratched and pulled. If it is too loose, the sealing pressure is not enough. In both cases, the leakage will increase rapidly. The best solution is to use elastic seats to absorb the difference caused by this change. There are usually three types of elastic solutions: 1 spring type; 2 thin flexible metal rings; 3 flexible seats with softness and softness. The latter is best, that is, it has the flexibility to overcome the thermal expansion, and has rigidity and builds up the wear-resistant alloy to improve the reliability of the wear-resistant surface.
(4) For the erosive medium containing particles, the direct erosion of the valve core to avoid high-speed medium should be considered. Straight stroke valves and butterfly valves do not have this function, while the full-function ultra-light valve eccentric rotation has this feature. In this case, the latter two valves should be selected.
(5) For hard seals, there must be a sufficient hardness difference, the same hardness material "do not give each other", easy to scratch.
(6) The thrust and stiffness should be increased for the assigned actuator to make it move freely.
V. The main valve type of shut-off valve
According to the above analysis, the main valve type of the shut-off valve is as follows. Selecting the shut-off valve should comprehensively consider various parameters such as leakage rate, medium type, medium temperature, flow coefficient, and cost performance. In the preferred order below, consider a hard seal before considering a soft seal.
(1) Full-featured ultra-lightweight control valve (2) Three-eccentric metal hard-seal butterfly valve (3) Double-eccentric spherical face seal butterfly valve (4) Ball valve (5) Eccentric rotary valve (6) Single-seat valve (7) Single seat sleeve valve (8) a soft sealing valve <br> <br> six, high temperature, high pressure, high pressure, large diameter cut valve selection
High temperature, high pressure, large pressure difference, and large diameter cutting are common problems. The main reason is the lack of valves that allow large pressure difference, good cutting and high temperature expansion, such as high pressure steam adjustment and cutting, due to large pressure difference. (1 ~ 3MPa or more), large diameter (DN200 ~ 300), had to use a balanced double seal structure (such as double seat valve or double seal sleeve valve), after special treatment, the leak rate of up to 10-4 , It is difficult to improve, and the full-featured ultra-light control valve has good overall performance and can solve this problem better. The leakage rate can reach 10-6 to 10-7 or more.
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