The control valve is used to adjust the flow, pressure and liquid level of the medium. According to the signal of the adjustment position, the opening of the valve is automatically controlled to achieve the adjustment of the medium flow, pressure and liquid level. The regulating valve is divided into electric regulating valve, pneumatic regulating valve and hydraulic regulating valve.
The regulating valve consists of two parts: electric actuator or pneumatic actuator and regulating valve. The adjustment is usually divided into two types, straight-through single-seater and straight-through two-seater. The latter has the characteristics of large flow capacity, small imbalance and stable operation, so it is usually especially suitable for occasions with large flow, high pressure drop and less leakage.
The flow capacity Cv is one of the main parameters for selecting the control valve. The flow capacity of the control valve is defined as: when the control valve is fully opened, the pressure difference between the two ends of the valve is 0.1MPa, and the fluid density is 1g/cm3, the flow diameter per hour The flow rate of the regulating valve is called the flow capacity, also called the flow coefficient, expressed in Cv, the unit is t/h, and the Cv value of the liquid is calculated by the following formula.
According to the flow capacity Cv value, the nominal diameter DN of the regulating valve can be determined.
The flow characteristic of the regulating valve is the relationship between the relative flow of the medium flowing through the regulating valve and its opening under the condition that the pressure difference between the two ends of the valve remains constant. The flow characteristics of the regulating valve include linear characteristics, equal percentage characteristics and parabolic characteristics. The meanings of the three fluence characteristics are as follows:
(1) Equal percentage characteristics (logarithmic)
The relative stroke and relative flow of equal percentage characteristic are not in a linear relationship. The change in flow caused by the change of unit stroke at each point of the stroke is proportional to the flow at this point, and the percentage of flow change is equal. So its advantage is that when the flow is small, the flow change is small, and when the flow is large, the flow change is large, that is, it has the same adjustment accuracy at different opening degrees.
(2) Linear characteristics (linear)
The relative stroke of the linear characteristic is linear with the relative flow. The flow change caused by the change of unit stroke is constant. When the flow is large, the relative value of the flow changes little, and when the flow is small, the relative value of the flow changes greatly.
(3) Parabolic characteristics
The flow rate changes in proportion to the two sides of the stroke, and generally has an intermediate characteristic between linear and equal percentage characteristics.
From the analysis of the above three characteristics, it can be seen that in terms of its adjustment performance, the equal percentage characteristic is the best, its adjustment is stable and the adjustment performance is good. The parabolic characteristic is better than the linear characteristic. Any one of the flow characteristics can be selected according to the requirements of the use occasion.
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