Valve Cv value and opening degree

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The Cv value of a valve and its opening degree are two different concepts; abroad, it is commonly referred to as Cv, while in China it is usually called Kv. Kv represents the flow capacity of the valve, and it is defined as the volume of fluid that passes through the valve per hour, measured in m3/h or t/h, under conditions where the control valve is fully open, the pressure difference ΔP across the valve is 100 KPa, and the density of the fluid is 1 gf/cm3 (i.e., normal water temperature). (For example, a control valve with Kv=50 means that when the pressure difference across the valve is 100 KPa, the water flow rate is 50 m3/h.) The valve opening degree refers to the position of the valve core (or valve plate) when it moves in order to change the throttling area of the flow passage during regulation; it is generally expressed as a percentage, with 0% indicating a closed state and 100% indicating a fully open state. For butterfly valves, manufacturers sometimes provide a Cv–opening curve; in this case, Cv represents the flow capacity of the valve at different opening degrees. The flow rate value Cv on the valve: For liquid flow, Q = Cv*sqr(△P/s) or △P = s*(Q/Cv)^2. Here, Q represents the flow rate in gallons per minute; △P is the pressure drop across the valve in psi; and S is the specific gravity of the fluid. This equation applies to liquids with turbulent flow and viscosity similar to that of water. (Cv refers to the flow rate per minute of water at a medium temperature of 60 o F, when a pressure drop of 1.0 psi is created by the valve.) ) (At this time, the specific gravity of water is 1. ) Warning: The fluid flow coefficient contained in this table is a calculated value. Therefore, they are approximate values and cannot be used in calculations of very important flow rates and pressure drops. For flow measurement and detection that require extremely high precision, the specific operation of any valves mentioned in this sample is necessary. It is not recommended to use a ball valve for throttling when the valve is opened to less than 45°. Cv value: The flow rate when the pressure drop of water at 20°C through the valve body is 1 bar is given by Cv = 6.6Q / √△P …………………………….(1) Where Q is the flow rate in liters per minute, SG is the density of water, and △P is the pressure difference across the valve body in bars. △P = SG × [6.6Q / Cv]. A higher Cv value indicates a greater flow rate, which means that the resistance across the valve body is low. Valve selection: The Cv value of the selected valve must be equal to or greater than its rated Cv value. Factors affecting the Cv value: * Too small an inlet diameter of the pipe * Length of the pipe * Opening size of the valve body * Turbulence * Being too close to the ends of the elbow fittings * Shape of the valve body inlet. Part 1: Calculation of the Cv value and selection of the diameter for control valves. 2. Calculation of the Cv value and selection of the diameter. The flow coefficient Cv is an important parameter for control valves; it reflects the capacity of these valves. The nominal diameter of a control valve is determined based on the value of Cv. The Cv value is defined as the number of US gallons per minute of clean water at 60°F (15.6°C) that pass through the valve, under the condition that the valve is fully open and the pressure difference across it is 1 pound per square inch. China’s flow coefficient is defined according to the metric system. The symbol is Kv, and the relationship between Kv and Cv is Cv=1.17Kv. 1. Calculation for liquid media: (Imperial) (Metric) ……………………. (1) …………… (1′) Where Q = maximum flow rate, in gpm (gallons per minute)；Q = maximum flow rate, in m3/h. G = specific gravity (water = 1); P1 = inlet pressure, in psi; P1 = inlet pressure, in 100 kPa (kgf/cm2). P2 = outlet pressure, in psi; P2 = outlet pressure, in 100 kPa (kgf/cm2). ΔP = P1 – P2. Note: P1 and P2 refer to the pressures at the maximum flow rate. (1) Viscosity correction: If the viscosity of the liquid is greater than 100 SSU (Stokes seconds) or greater than 20 CST (centistokes), i.e., 20 mm2/s, then the Cv value required for calculation should be adjusted using the viscosity correction method as follows. 1) Without considering the effect of viscosity, calculate Cv using equation (1) or (1′). 2) Calculate the coefficient R using equations (2) and (3) or equations (2′) and (3′). 3) From the viscosity correction curve in Figure 2-1, determine the correction factor for Cv corresponding to the coefficient R. 4) Multiply the Cv obtained in the first step by this correction factor. 5) Then, select the appropriate control valve diameter from the Cv value table. Formulas for calculating the coefficient R (Imperial) (Metric) ………………. (2′) (3′) Where Q = maximum flow rate in gpm; Q = maximum flow rate in m3/h. Mcs – dynamic viscosity coefficient of the liquid at the inlet temperature in cst. Cv – uncorrected Cv value. Mssu – viscosity of the liquid at the inlet temperature in SSU (Sébaud seconds). Note: If the liquid viscosity is ≥ 200 SSU, use formulas (3) or (3′) for calculation; if the viscosity is less than 200 SSU, convert the viscosity value from SSU to cst before using formulas (2) or (2′) for calculation. (2) Flash correction: When a liquid at saturation temperature or near saturation temperature passes through the throttle opening of a control valve, the increased flow velocity leads to a drop in the liquid’s pressure, resulting in instantaneous and rapid evaporation within the liquid. That is, the liquid will produce a large amount of vapor. In such a case, it is no longer correct to use the basic laws (formulas) of fluid flow; a (pressure difference) correction must be applied. The correction methods are as follows: △T＜2.8℃ (5℉) → △Pc=0.06P1 …………………..(4) △T＞2.8℃ (5℉) → △Pc=0.9(P1-Ps)…………… (5) Where △T is the difference between the saturated temperature of the liquid at the inlet pressure and the inlet temperature; △Pc is the allowable pressure difference for calculating the flow rate, equal to 100 kPa (kgf/cm2) abs; Ps is the absolute saturated pressure of the liquid at the inlet temperature, also 100 kPa (kgf/cm2) abs. Only when the △Pc calculated using equation (4) or (5) is less than the actual pressure difference △P on the control valve, must equation (1) or (1′) use △Pc instead of △P. 2. Calculation for gaseous (general gas) media: If the maximum flow rate under standard conditions, namely 760 mmHg (14.7 psia) and 15.6°C (60°F), is known, the following formulas can be used directly without any adjustments. (1) When △P＜ ………… (6) … (6′) (2) When △P＞ …………. (7) …… (7′) Where Q = maximum flow rate under standard conditions, in ft3/h; Q = maximum flow rate under standard conditions, in m3/h. G = specific gravity (air = 1); G = specific gravity (air = 1). T = fluid temperature, in ℉; T = fluid temperature, in ℃. P1 = absolute inlet pressure, in Psia; P1 = absolute inlet pressure, in 100 Kpa (kgf/cm2). P2 = absolute outlet pressure, in Psia; P2 = absolute outlet pressure, in 100 Kpa (kgf/cm2). △P = P1 – P2, in Psia; △P = P1 – P2, in 100 Kpa (kgf/cm2). CV/KV has no direct relationship with the diameter of the control valve; factors such as the valve’s mechanism and whether it operates in a flow-open or flow-close mode must also be taken into consideration. When the process conditions remain constant, if the CV value is set too high, the valve may not open far enough to achieve the flow rate required by the process; this not only makes adjustment difficult but also affects the lifespan of the control valve. If the CV value is set too low, the control valve may not be able to reach the specified flow rate even when it is fully open. After the Cv value is calculated, it must be rounded to the nearest value provided by the manufacturer; simultaneously, the flow coefficient of the control valve in imperial units, Cv, needs to be determined. This value is defined as the number of US gallons per minute that flow through the control valve when water at a temperature of 60°F (15.6°C) passes through it under a pressure drop of IIb/in (7 kPa). China’s flow coefficient is defined according to the metric system. The symbol is Kv; the relationship between Kv and Cv is Cv = 1.17Kv. The value of Cv chosen should be higher than the calculated value. Additionally, a verification of the valve opening degree is necessary – it is appropriate to keep the opening degree of the control valve between 30% and 85%. However, if a too high value for Cv is selected, this results in the use of a valve with a large diameter on a small pipeline