Valve selection tips and considerations

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Tips and considerations for valve selection. The main types of control valves are as follows; when selecting a control 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) Fully functional ultra-lightweight control valve; (2) Three-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. I. Considerations for selecting control valves: (1) Determine the nominal pressure – it is not necessary to use Pmax to determine PN; instead, the appropriate PN value should be found from tables based on temperature, pressure, and material characteristics, ensuring that it meets the requirements of the selected valve. (2) Determine the valve type such that its leakage rate meets the process requirements. (3) Determine the valve type, ensuring that its operating pressure difference is less than the valve’s allowable pressure difference; if this is not possible, special considerations must be given or another valve should be selected. (4) The temperature of the medium is within the operating temperature range of the valve, and the ambient temperature meets the requirements. (5) Consider the issue of valve clogging based on the cleanliness of the medium. (6) Consider the corrosion resistance of the valve based on the chemical properties of the medium. (7) Consider the erosion and wear resistance of the valve based on the pressure difference and the medium containing hard particles. (8) Performance-to-price ratio considering comprehensive economic effects. Three issues need to be considered: a. Simple structure (the simpler, the higher the reliability), easy maintenance, and availability of spare parts ； b. Service life ； c. Price. (9) Preferred order. Butterfly valve – Single-seat valve – Double-seat valve – Sleeve valve – Angle valve – Three-way valve – Ball valve – Eccentric rotary valve – Diaphragm valve. 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 the 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. 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 dirt and debris (such as weld slag and iron shavings) may remain. When these substances pass through the control valve, they can easily scratch the soft-sealed seat or valve core, leading to an increase in leakage and reduced reliability of the seal. Therefore, when choosing a soft-seal structure, the cleanliness of the medium must be taken into account, and the pipes must be thoroughly flushed before operation. (2) Rigid seal: A rigid seal with wear-resistant alloys welded on it is the best choice for control 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 precise regulation. Moreover, it is durable, making it more cost-effective from an economic perspective. IV. Considerations regarding sealing reliability (1) Soft seals offer good adjustment capabilities, but they are not suitable for use with media containing particles; once scratched, leakage will increase significantly. 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 a hard seal, for control valves with leakage requirements of 10–6 or higher, it is necessary (or required) to weld a wear-resistant alloy on to improve the seal’s ability to resist wear, erosion, and scratches, thereby extending the valve’s service life. (3) Regulation of high-temperature media: The control valve 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 ； ② Sheet-type flexible metal ring ； ③ An 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 the fully functional ultra-lightweight valve features 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.