Key points for selecting valves
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Key points for selecting valves: To choose the appropriate type of valve, it is necessary to follow the steps below and conduct a thorough check; only in this way can the selected valve fulfill its functions and carry out the tasks assigned to it. 1) Characteristics and main functions of valves. Understanding the characteristics and functional roles of various types of valves is the first step in selecting the appropriate ones. The characteristics, classifications, and main functions of these different valve types were discussed in Sections 2 and 3 earlier; they are now summarized in Tables 1 and 2 for reference in making selections. 2) Diameter or flow (capacity) “as shown in Table 3”. A. The nominal diameter of the valve is not necessarily the same as the diameter of the flow channel; the appropriate size is determined by calculating the required Cv value based on the conditions of the fluid being transported, and then the suitable valve diameter is selected using that Cv value (referencing the manufacturer’s catalog). B. The Cv value is defined as the number of U.S. gallons per minute measured at 60 degrees Fahrenheit, when there is a pressure drop of 1 unit as water flows through the valve. This value represents the Cv of that valve. C. The Cv value can be used to calculate the flow rate (Q) passing through the valve. Q = CvΔP = 7.9Cv; P = density of the liquid, LB/FT3; ΔP = pressure drop after passing through the valve, PSI
3) Temperature VS Pressure: The “temperature VS pressure” of a valve refers to the allowable and safe pressure at a certain temperature (though this is the maximum allowable pressure in the absence of shocks). 5. Structural materials of valves: The choice of materials used in the structure of valves is extremely important for determining their functional lifespan. Generally, the structural materials of valves can be divided into two main categories: A. Materials for the pressure-bearing parts: valve body, valve cover, bottom cover, bolts, etc. Main factors considered in the selection of material for pressure vessels: a. Temperature and pressure of the fluid: A) Use at high temperatures: In high-temperature environments, the tensile strength and service life of metal materials generally decrease as temperature rises, and their creep strength is also affected. B) Use at low temperatures: In applications where metal materials are used at low temperatures, their toughness decreases sharply, leading to the phenomenon of low-temperature brittleness. b. Corrosion resistance of materials: Reasons for corrosion of the metals in valve structures: (A) Type of fluid; (B) Concentration; (C) Temperature. Forms of corrosion damage: (A) Uniform corrosion; (B) Pitting; (C) Zinc loss and delamination; (D) Intergranular corrosion; (E) Cracking. c. Erosion resistance of materials: Materials resistant to erosion generally need to possess the following characteristics: A) Materials with a strong oxide film; B) Materials with a high yield point and strength; C) Materials with a high fatigue limit; D) Materials with a high hardness value
B. Main component materials (TRIIM): Materials for the valve seat surface, stem, guide sleeves, and other internal components. Factors considered in selecting these materials include: a) temperature of the fluid, b) corrosion resistance of the material, c) erosion resistance of the material, d) wear resistance and anti-stick properties of the material. 6) Valve cover types: There are several ways in which the valve cover can be connected to the valve body; the appropriate method is chosen based on factors such as the size of the valve, operating conditions, temperature and pressure, as well as the risks associated with leakage. A. Screwed bonnet type B. Bolted bonnet type C. Seal welded bonnet type D. Pressure seal bonnet type E. Union type 7) Special requirements for valve structure vary depending on the operating temperature and environment; these are listed as follows: A. Fire and static electricity protection design – special attention must be paid to this during the design and use of ball valves. B. Extended bonnet design – applied to refrigeration valves and liquefied gas transport valves. C. Limits of noise and cavitation — special considerations especially for the design and use of control valves. D. Design for incorporating expansion bladders to prevent filler leakage — absolute sealing to prevent leakage must be considered. 4.8 Operation methods are generally limited by the installation environment, operating and running conditions, or the number of operations; electric and pneumatic drive devices are taken into consideration. With the widespread use of computers, it has now evolved to include the networking and monitoring of the entire system process, eliminating the need for manual operation. However, due to the cost-effectiveness and durability of wheels or gear reduction devices, they are still widely preferred by most people.