Selecting the appropriate valve requires a systematic and thorough process, ensuring that each step is carefully examined to meet the functional requirements and operational tasks. The first step involves understanding the characteristics and primary functions of different types of valves. This foundational knowledge is crucial for making an informed decision. A summary of these characteristics, classifications, and main functions is provided in Table I and Table II for reference.
Next, the valve’s size or flow capacity must be determined. Table III presents the nominal diameter of the valve, which may not always match the actual flow path diameter. To ensure proper sizing, the Cv value—representing the flow capacity at a pressure drop of 1 psi with water at 60°F—should be calculated based on the fluid conditions. This Cv value is then used to select the appropriate valve size from the manufacturer's catalog. The formula for calculating Cv is:
Q = Cv × √(ΔP)
Where Q is the flow rate, ΔP is the pressure drop across the valve, and P is the liquid density in lb/ft³.
Another important consideration is the relationship between temperature and pressure. For pressure valves, the permissible and safe operating pressure at a given temperature is specified. This pressure represents the maximum allowable pressure without causing damage.
Material selection for the valve body and other pressure-containing components is critical to its performance and longevity. Key factors include fluid temperature and pressure, corrosion resistance, and erosion resistance. High-temperature environments can reduce material strength, while low temperatures may cause embrittlement. Corrosion can occur due to fluid type, concentration, and temperature, leading to various forms of degradation such as pitting, intergranular corrosion, or cracking.
For internal components like the seat, stem, and bushings, additional considerations apply, including temperature, corrosion, erosion, wear, and anti-stick properties. These parts must be selected carefully to ensure reliable operation under specific service conditions.
The bonnet type also plays a role in the valve’s design. Common options include screw-on, bolted, sealed welded, and pressure-sealed bonnets, each suited to different installation and operational requirements. Special designs, such as fire-safe, extended, noise-reducing, or anti-leakage features, may be necessary depending on the application.
Finally, the mode of operation should be considered. While electric and pneumatic actuators are common, especially with modern control systems, manual operation via handwheels or gearboxes is still widely used due to cost-effectiveness and reliability. The choice depends on the installation environment, operational needs, and frequency of use.
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