How to choose a cut-off valve correctly

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In applications with high requirements, there is a need for the control valve to be capable of shut-off; therefore, the shut-off function is an important operational feature and an essential quality indicator for control valves (for the classification and standards related to shut-off levels, please refer to relevant documentation). In field use, problems such as poor shut-off of control valves and significant leakage often occur. Apart from increased leakage caused by factors such as blockages and excessively low allowable pressure differences, the vast majority of cases are due to structural defects in the control valves and improper selection of these valves. 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. However, before the 1980s, since sleeve valves were widely used at that time, many manufacturers invested considerable 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, the design approach is flawed; rather than making such complex changes inside the \"heart\" of the valve body, it would be better to make adjustments from the outside. 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 time, it was found that angle-seat valves had advantages over linear-motion valves (single-seat valves, sleeve valves, gate valves), which led to the development of triple-eccentric shut-off butterfly valves, all-function shut-off valves, and hard-sealed shut-off ball valves. II. Selection of sealing surfaces (1) Face-to-face sealing: In common plunger valves, the sealing surface is a small conical surface with a 60° angle, and the valve seat also has a small conical surface with a 60° angle. The width of these conical surfaces is usually between 0.5 mm and 2 mm; good sealing requires that these two conical surfaces make good contact. 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) Spherical sealing: The spherical surface of the valve core rotates and comes into contact with the fixed conical surface of the seat; the contact between them is linear, which results in better performance compared to the face-to-face sealing mentioned above. The fully functional ultra-lightweight valves and spherical-sealed butterfly valves developed by Hualin Company are manufactured using this approach; their leakage rate can reach 10‑6 to 10‑8. High-performance triple-eccentric butterfly valves can achieve a leakage rate as low as 10‑8, or even zero leakage. III. Selection of sealing materials (1) Soft seals: Apart from lined valves that are resistant to corrosion, ordinary soft-seal valves are 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 welding slag and iron shavings) may remain. When these contaminants pass through the control valve, they can easily scratch the soft-sealed seat or valve element, resulting in increased leakage and reduced reliability of the seal. Therefore, when choosing a soft-seal structure, it is necessary to consider the cleanliness of the medium and to thoroughly flush the pipes before operation. (2) Rigid seal: A rigid seal with wear-resistant alloys welded on is the best choice for cut-off valves. This approach takes into account both sealing performance as well as service life and reliability. Although the factory specifications are only 10‑6 to 10‑8, which do not achieve zero leakage like soft seals, it is sufficient to meet the requirements for tight sealing. 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 sharply. Therefore, hard sealing should be preferred whenever conditions permit. Fully functional ultra-lightweight valves with hard sealing can achieve a leakage rate of 10‑7 to 10‑8, while triple-eccentric butterfly valves can achieve zero leakage. (2) Considerations regarding sealing wear resistance: In addition to choosing hard seals, for cut-off control valves with leakage requirements of 10–6 or better, it is necessary (or essential) to weld wear-resistant alloys on them in order to enhance the sealing material’s wear resistance, erosion resistance, and scratch resistance, thereby prolonging 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 strain 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 hardness and flexibility. The latter is the best option: it has flexibility to overcome thermal expansion, as well as stiffness, and a wear-resistant alloy 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 fully functional ultra-lightweight valves feature eccentric rotation. In such cases, the latter two types of valves should be chosen. (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. (1) Full-function ultra-lightweight control valve (2) Triple-eccentric metal hard-sealed butterfly valve (3) Double-eccentric spherical-sealed butterfly valve (4) Ball valve (5) Eccentric rotary valve (6) Single-seat valve (7) Single-seat sleeve valve (8) Soft-sealed valve. VI. Selection of cut-off valves for high temperature, high pressure, large pressure difference, and large diameter applications. High temperature, high pressure, large pressure difference, and large diameter conditions present a common challenge; the main issue is the lack of valves that can simultaneously accommodate large pressure differences, provide effective cut-off performance, and resist thermal expansion. In applications such as high-pressure steam regulation and cut-off, where the pressure difference is high (1–3 MPa or more) and the diameter is large (DN200–300), balanced double-seal structures (such as double-seat valves or double-seal sleeve valves) must be used. With special treatment, the leakage rate can reach 10–4, but further reduction in leakage is difficult. The full-function ultra-lightweight control valve, with its excellent overall performance, can effectively address this issue, achieving a leakage rate of 10–6 to 10–7 or lower.