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Practical Technology: Introduction to the Calculation Method of CV Value of Control Valve
1915 FISHERGOVERNER company in the United States according to design conditions accumulated charts, according to the chart first caliber. As the cost of regulating the valve was reduced in this way, the life of the valve was extended and was therefore well received. But according to the selected caliber than calculated now more. Later, according to the selection method of liquid, gas, steam and various forms of valve for further algorithm research. Until 1930 the FOXBORO companies in the United States, ROLPHRJOKWELL and DR. E. MASON, initially used the CV values ​​for the following V-shaped (equal percentage) valves and published the CV calculation formula. In 1944 the United States MASON-NELLANREGULATOR company ROKWELL and MAXON merged into MASON-NEILAN, published @ V calculation formula. In 1945, the SONALDEKMAN company in the United States published a formula similar to that of MASON-NELLAN. However, the study on the relative relationship between flow area and flow coefficient was carried out. In 1962 FLIID ONTROLSINSTITUTE published the FCI58-2 flow measurement method and published the caliber calculation of control valve. The CV calculation is still in use, but the KV calculation is also published in VDI / VDE of FCI62-1. In 1960, the same as FCI62-1, only the unit was changed to metric. From 1966 to 1969, the Japan Society of Mechanical Engineers' Branch on Control Valve Basic Investigation conducted a study on the caliper calculation, specification and use of the definition block. But it is not over yet. In 1977 the United States ISA (INSTRUMENTSOCIETYOFAMERICA) published a standard S39.1 "Compression Fluid Computation" formula. 1977 ~ 1978 American ANSI / ISA standard, S75.01 on May 15, 1979 published NO \\ 0046-79, for the engineering services report. Adjust and flow capacity of the calculation, the instrument factory is currently using FCI recommended CV value formula as shown in Table 1 formula pressure conditions formula △ P <21> △ P ≥ P1 / 2 liquid with the left gas at room temperature (0 ~ 60C) temperature correction > 60 ° C) Saturated steam Table in the various types of general use to meet. However, for high pressure differential, high viscosity close to the saturated liquid and other occasions, especially the butterfly valve, ball valve and other low pressure recovery factor of the valve, the error is very large, and must be amended. 80 years Japan individual companies have begun to use the following coefficients to be amended. Cavitation coefficient: When the liquid passes through the control valve, when the pressure in the contraction section is lower than the saturated vapor pressure PV at the valve inlet temperature, a part of the liquid rapidly vaporizes and the phenomenon that the liquid passing through the valve becomes a gas-liquid two-phase flow is referred to as Flash. After the contraction of the liquid pressure gauge gradually restored mixed bubble in the liquid broken in the bubble caused by the pressure rise, the pressure sometimes up to thousands of kgf / cm2, the role of local pressure in the spool surface Metal flaking causes damage, a phenomenon known as cavitation. When this happens, the flow through the valve will not increase as the pressure difference increases to produce a clogged flow (CHOKOD FLOW) as the valve inlet pressure difference DR = R1-P2 increases to a certain value. At this time the formula in Table 1 does not apply, and must be amended. That can not simply use △ P = P1-P2 to calculate the valve flow capacity, but must make the fluid in the valve contraction part of the pressure is not less than PV. As a result of a variety of valve pressure recovery factor is not the same, can be seen from Figure 2, the butterfly valve, ball valve and other high pressure to restore the adjustment of the boil more likely to produce steam and cavitation. Different control valve forms have different pressure recovery factors, and the pressure recovery factor directly affect the flashover, cavitation easier, so the introduction of cavitation coefficient KC. P1 - Valve Inlet Pressure; P 2 - Valve Outlet Pressure; PV - Saturated Vapor Pressure; DRCV - Contraction Section Differential Pressure; DR = R1 - R2 KC is defined as: / (P1-PV) KC value is determined by the structure of the control valve itself, reflecting the valve pressure recovery level, as DR = KC · DR0 that is P1-P2 = KC The maximum allowable valve pressure drop, DRCRI, at which the pressure of the partial pressure is lower than the PV, ie, does not generate cavitation, is ΔPCri = P1-P2 = KC (P1-PV)