How to achieve the shut-off function of a control valve

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How to achieve the shut-off function of control valves: In industrial control processes, in applications where high standards are required for the manufacturing process, there is a need for control valves to have a shut-off function; therefore, this shut-off function is an important operational feature and an essential quality indicator for control valves. In field use, faults such as poor shut-off of control valves and excessive leakage often occur. Apart from increased leakage caused by blockages, excessively low allowable pressure differences that lead to forced opening, etc., the vast majority of cases are due to structural defects in the control valve and improper selection. I. Selection of structural type: One of the main disadvantages of double-seal control valves (such as two-seat valves and double-seal sleeve valves) is high leakage. Before the 1980s, since sleeve valves were widely used at that time, many manufacturers invested a great deal of effort in improving their design in order to reduce leakage, which resulted in very complex structures. As a result, this brings about many drawbacks: numerous parts, poor reliability, difficulty in obtaining spare parts, hard maintenance, and unsatisfactory cutting performance, among others. In fact, this reflects a misunderstanding in the design approach; rather than making such complex changes inside the \"heart\" of the valve body, it would be better to achieve the desired result through external adjustments. Therefore, a single-seal control valve should be selected. In this way, the sealing issues, reliability problems, as well as maintenance and spare parts issues have all been resolved one by one. At this point, the key to resolving the conflict becomes the issue of increased unbalanced force exerted by the medium on the valve; addressing this can be achieved simply by using a powerful piston actuator and thickening the valve stem. This approach of resolving the issue externally is clearly much easier than making complex internal improvements; it’s less of a matter of method and more of a matter of mindset. Entering the 1990s, people began seeking cut-off valves with simpler structures and more reliable sealing. At this point, it was found that angle-type valves had advantages over linear-type valves (single-seat valves, sleeve valves, gate valves), which led to the development of three-eccentric cut-off butterfly valves, all-functional cut-off valves, and hard-sealed cut-off ball valves. II. Selection of sealing surfaces: (1) Face-to-face sealing: In common plunger valves, the sealing surface is a small cone with an angle of 60°, and the valve seat also has a small cone with a 60° angle; the width of these cones is usually between 0.5 mm and 2 mm. Good sealing requires proper contact between the two conical surfaces. But in reality, it is always affected by processing errors (such as concentricity, out-of-roundness, inclination, etc.), resulting in less than ideal sealing performance. The leakage rate of such valves is typically 10-4; with precise grinding, it can be reduced to 10-6, achieving only a good level of sealing. 　(2) Face sealing between the small conical surface of the ball and the valve seat: By utilizing the rotational movement of the ball on the valve core together with its fixed position, a linear contact is established between them, which results in better performance compared to the face sealing mentioned above. 　 III. Selection of Sealing Materials (1) Soft Sealing Apart from lined valves that are resistant to corrosion, ordinary soft-sealing valves refer to those in which either the valve core or the valve seat is made of a non-metallic material (mainly polytetrafluoroethylene, as well as rubber, etc.) for sealing purposes. Soft-sealed valves offer the best sealing performance, but during pipeline installation and system cleaning, various types of contaminants (such as weld slag and iron shavings) may remain. When these contaminants pass through the valve, they can easily scratch the soft seat or valve core, leading to an increase in leakage and reduced reliability of the seal. Therefore, when choosing a soft-seal design, it is necessary to take into account the cleanliness of the medium and to thoroughly flush the pipes before operation. (2) Hard seal: A hard seal combined with the welding on of wear-resistant alloys is the best option for cut-off valves. This approach takes into account both sealing performance as well as service life and reliability. Although the factory-specified leakage rate is only 10-6 to 10-8, which does not achieve zero leakage like soft seals, it is sufficient to meet the requirements for tight shut-off. Moreover, it is durable, making it more cost-effective from an economic perspective. IV. Considerations regarding sealing reliability (1) Soft seals provide good shut-off performance, but they are not suitable for media containing particles; once scratched, leakage will increase significantly. Therefore, hard sealing should be preferred whenever conditions permit. 　 (2) Considerations regarding sealing wear resistance: In addition to choosing a hard seal, for cut-off control valves with leakage requirements of 10–6 or better, it is necessary (or essential) to weld a wear-resistant alloy on to improve the sealing material’s resistance to wear, erosion, and scratches, thereby extending the valve’s service life. (3) The cut-off control valve for high-temperature media is assembled at room temperature and operates at high temperatures, with temperature changes of several hundred degrees. Thermal expansion changes the sealing specific pressure established during assembly. If it’s too tight, the sealing surface will get stuck, causing severe abrasion and tearing during movement ； If it is too loose, the sealing pressure will be insufficient; in both cases, leakage will increase sharply. The best way to address this is to use a flexible valve seat to absorb the differences caused by these changes. There are usually three types of elastic solutions: ① spring-type ； ②Sheet-type flexible metal ring ； ③An elastic valve seat that combines firmness and flexibility. The latter is the best option: it offers flexibility to counteract thermal expansion, as well as stiffness, and uses wear-resistant alloys to enhance the reliability of the wear-resistant surface. (4) For erosion media containing particles, it is necessary to ensure that the valve core is protected from direct erosion by the high-speed medium. Both linear stroke valves and butterfly valves do not possess this function, whereas eccentric rotation has this characteristic. 　 (5) For hard seals, there must be a sufficient difference in hardness; materials with the same hardness level will not yield to each other, making scratches more likely to occur. (6) The actuator used should have increased thrust and stiffness to ensure smooth operation. V. Main types of shut-off valves Based on the analysis above, the main types of shut-off valves are as follows. When selecting a shut-off valve, it is necessary to take into account various parameters such as leakage rate, type of medium, medium temperature, flow coefficient, as well as cost-effectiveness. In the following preferred order, hard seals are considered first, followed by soft seals. VI. Selection of Cut-Off Valves for High Temperature, High Pressure, Large Pressure Differences, and Large Diameter Applications The challenge of working under high temperature, high pressure, large pressure differences, and with large diameters is quite common. The main reason for this is the lack of valves that can handle large pressure differences, provide effective cutting action, and resist thermal expansion. In applications such as high-pressure steam regulation and cutoff, where the pressure difference is high (1–3 MPa or more) and the diameter is large (DN200–300), balanced double-seal designs (such as double-seat valves or valves with double-seal sleeves) are necessary. With special treatment, these valves can achieve a leakage rate of 10–4; achieving even lower leakage rates proves difficult.