Must the characteristics of the control valve body and the positioner in the data sheet be identical?

The output of the potentiometer is related to the diaphragm head of the control valve, and has nothing to do with the structure of the valve body; this can be determined from its working principle

The selection is made based on the requirements of the actual operating conditions. The flow characteristic when the pressure difference across the control valve remains constant is known as the ideal flow characteristic, or inherent flow characteristic. The relationship between flow rate and valve stem displacement depends entirely on the structural parameters of the valve. 1. Linear flow characteristic. The relative flow rate of a control valve is in a linear relationship with its relative opening degree; that is, the change in flow rate resulting from a unit change in the valve stem’s displacement is constant. When a linear control valve operates at low or high opening degrees, its control performance is poor, and it is not suitable for use in situations with large load variations. 2. Equal percentage flow characteristic (logarithmic flow characteristic). Since the flow rate increases by 3.4% for every 1% increase in valve stem displacement, it is referred to as equal percentage flow characteristic. The equal percentage flow characteristic means that the change in relative flow caused by a change in relative stroke is proportional to the relative flow at that point; in other words, the amplification factor K_v of the control valve varies and increases as the relative flow increases. Since the amplification factor K_v of the equal percentage valve increases as the relative opening degree increases, the equal percentage valve is beneficial for automatic control systems. At low opening degrees, the amplification factor of the equal percentage valve is small, resulting in smooth and gentle control ； At large opening angles, the amplification factor is high, ensuring sensitive and effective control. 3. Fast-opening flow characteristic. The fast-opening flow characteristic features a high flow rate even at low valve openings; as the opening increases, the flow rate quickly reaches its maximum value, and further increases in opening result in only minimal changes in flow rate, which is why it is called a fast-opening characteristic. If the valve seat diameter is D, then its stroke is generally within D/4; if the stroke increases and the flow area of the valve does not increase as well, it will lose its control function. Therefore, quick-opening control valves are mainly suitable for cut-off or on/off control systems that require rapid opening and closing. 4. Parabolic flow characteristic. It means that there is a parabolic relationship between q/q_max and l/L, which corresponds to a parabola in Cartesian coordinates.

The flow characteristics of a control valve and those of a positioner are two distinct concepts. The ideal flow characteristic of a control valve refers to its flow behavior when the pressure difference before and after the valve remains constant, and it is expressed as the ratio of the relative flow through the valve element to the relative change in the control signal. The flow characteristics of a control valve depend on the shape of its valve element, with four typical inherent flow characteristics: linear, equal percentage, parabolic, and quick opening. 　　The flow characteristic of a positioner refers to the relationship between the positioner’s input signal and the valve opening degree. For a given valve, its structural properties remain constant; that is, the ratio of the relative change in flow rate to the relative displacement of the valve stem remains unchanged. An intelligent valve positioner can modify the flow characteristics of a control valve by changing the ratio between the relative displacement of the valve stem and the relative change in the control signal. 　　In the process of implementing the flow characteristic of the positioner, based on the given flow characteristic data, a curve fitting method is used to obtain the equation of the flow characteristic curve, and programming is employed to establish the relationship between the input signal and the valve opening degree. When the positioner is installed on the control valve, the user can select different flow characteristic curves to achieve flow control of the control valve.

These are theoretical materials that can be used for reference

Basically, the flow curve of most valves is fixed at the initial stage of production design; it is determined by the operating conditions. If there is a need to change the location of use or the operating conditions later on, it is necessary to replace the valve with one that has an appropriate flow curve, or use the function of an intelligent valve positioner to adjust the valve’s opening degree in order to obtain an approximate flow curve. Modern intelligent positioners are capable of doing this; one can refer to the manuals provided by various manufacturers, where detailed explanations of this function are available.

If the flow characteristics of the valve body and those of the actuator are two distinct concepts, with the actuator installed on the valve body, then if the characteristics chosen for the valve body and the actuator do not match, which ones will prevail in determining the overall flow control characteristics – those of the valve body or those of the actuator? After all, we are only using control valves; we are not involved in product design, so we don’t know much about the details or the results of experimental tests.

The flow characteristics controlled by the positioner are determined by the valve by default; if the valve’s flow curve is linear, then the output controlled by the positioner will also be linear. The valve plays the primary role, while the positioner serves only as an auxiliary element.

Of course, if the function for modifying the flow curve using a positioner is used, it will also cause changes in the flow curve output by the control valve. Under normal operation, there is no need to consider this flow curve function, unless the valve’s flow curve does not match the operating conditions.