When in use, control valves often experience malfunctions such as poor shut-off and excessive leakage. Besides leakage caused by blockage, insufficient allowable pressure differential, etc., the vast majority of these problems are due to structural defects and improper selection of the control valve. Modern industry places increasingly higher demands on control valves, and high-demand applications require specific shut-off capabilities. Therefore, the shut-off function is a crucial operational feature and intrinsic quality indicator of a control valve (for classification and standards of shut-off grades, see relevant documents). Therefore, special attention should be paid to this aspect.

I. Selection of Structural Type
One of the main disadvantages of double-sealed control valves (such as double-seated valves, double-sealed sleeve valves, etc.) is excessive leakage. However, before the 1980s, sleeve valves were prevalent. Sleeve valves, however, had many shortcomings: numerous parts, poor reliability, difficult spare parts procurement, difficult maintenance, and unsatisfactory shut-off performance. In reality, this was a flawed design approach; it would be better to achieve the desired result through external adjustments rather than making such complex modifications within the valve body’s “heart.” Therefore, single-sealed control valves should be chosen. This solves the sealing, reliability, maintenance, and spare parts problems. At this point, the focus shifts to addressing the increased unbalanced force of the medium on the valve. This can be solved simply by using a powerful piston actuator and a thicker valve stem. This external solution is clearly much easier than complex internal modifications; it’s more a matter of approach than a matter of method. By the 1990s, people began to look for shut-off valves with simpler structures and more reliable sealing. At this time, it was discovered that rotary valves were superior to linear valves (single-seated valves, sleeve valves, gate valves), leading to the development of triple-eccentric shut-off butterfly valves, full-function shut-off valves, and hard-sealed shut-off ball valves.

II. Selection of Sealing Surfaces
(1) Face-to-face sealing:
In common plunger valve cores, the sealing surface is a small 60° conical surface, and the valve seat is also a small 60° conical surface. The width of this small conical surface is usually between 0.5mm and 2mm. To ensure good sealing, the two conical surfaces must be in good contact. However, in reality, it is always affected by machining errors (such as concentricity, out-of-roundness, and tilt), resulting in less than ideal sealing performance. The leakage rate of this type of valve is usually 10⁻⁴, and can reach 10⁻⁶ after precision grinding, only achieving a relatively good sealing level.
(2) Spherical sealing:
This utilizes the spherical rotation of the valve core to tangent with the small conical surface of the fixed valve seat, resulting in line contact between them, which is better than the face-to-face sealing described above. The full-function ultra-lightweight valves and spherical sealing butterfly valves launched by Hualin Company are manufactured using this approach, with leakage rates reaching 10⁻⁶ to 10⁻⁸, and high-performance triple-eccentric butterfly valves reaching as high as 10⁻⁸, or even zero leakage.

III. Selection of Sealing Materials
(1) Soft sealing
Except for corrosion-resistant lined valves, general soft-sealed valves refer to valves where either the valve core or the valve seat uses non-metallic materials (mainly polytetrafluoroethylene, but also rubber, etc.) for sealing.
(2) Hard sealing

Hard sealing with wear-resistant alloy overlay welding is the best choice for shut-off valves.
(1) Soft sealing provides good shut-off performance, but is not suitable for media containing particles. Once scratched, leakage will increase sharply. Therefore, hard sealing should be used whenever possible. The hard sealing of full-function ultra-lightweight valves can reach 10⁻⁷ to 10⁻⁸, and triple-eccentric butterfly valves can achieve zero leakage.
(2) Consideration of sealing wear resistance: In addition to selecting hard sealing, for shut-off type control valves requiring a leakage rate of 10⁻⁶ or higher, wear-resistant alloy overlay welding should (or must) be applied to improve the wear resistance, erosion resistance, and scratch resistance of the seal, thereby increasing the valve’s lifespan.
(3) Shut-off control valves for high-temperature media are assembled at room temperature and operate at high temperatures, with temperature changes of several hundred degrees. Thermal expansion will change the sealing pressure established during assembly. If too tight, the sealing surface will seize, causing severe abrasion and tearing during movement; if too loose, the sealing pressure is insufficient. In both cases, leakage will increase sharply. The best solution is to use an elastic valve seat to absorb the differences caused by this change. There are usually three types of elastic sealing solutions: ① spring type; ② flexible metal ring type; ③ rigid-flexible elastic valve seat. The latter is the best, as it combines flexibility to overcome thermal expansion with rigidity and wear-resistant alloy to improve the reliability of the wear surface.
(4) For erosive media containing particles, the valve core should be designed to avoid direct erosion from the high-speed medium. Straight-stroke valves and butterfly valves do not have this function, while the full-featured ultra-light valve with eccentric rotation does. In this case, the latter two types of valves should be selected.
(5) For hard seals, there must be a sufficient difference in hardness; materials of the same hardness will “not yield” to each other and are prone to scratching.
(6) The actuator should have increased thrust and rigidity to ensure smooth operation.

V. Main Valve Types for Shut-off Valves
Based on the above analysis, the main valve types for shut-off valves are as follows. When selecting a shut-off valve, various parameters such as leakage rate, medium type, medium temperature, flow coefficient, and cost-effectiveness should be considered comprehensively. In the following order of preference, hard-seal control should be considered first, followed by how the shut-off function of the control valve is achieved, and then soft seals.
(1) Full-featured ultra-light control valve
(2) Triple-eccentric metal hard-seal butterfly valve
(3) Double-eccentric spherical seal butterfly valve
(4) Ball valve
(5) Eccentric rotary valve
(6) Single-seat valve
(7) Single-seat sleeve valve
(8) Soft-seal valve