The design of actuators and positioners must be considered together. The combination of these two devices greatly influences the static performance (dead zone) and dynamic response of the regulating valve (control valve) components and the total air consumption of the valve. Today, positioners are used for most specified regulator (control valve) applications. When used with a common digital control system, the positioner can provide high positioning accuracy and a more rapid response to process disturbances. As people increasingly focus on the economics of process control, positioners must be considered for every valve application where process optimization is important. The most important characteristic of a good positioner in reducing process deviation is that it is a high gain device. The gain of the positioner consists of two parts: static gain and dynamic gain. Static gain is the sensitivity of a device that can sense small changes in the input signal (0.125% or less). Unless this device is sensitive to these small signal changes, it cannot respond to disturbances in the process variables. The high static gain of the positioner is achieved by a preamplifier. This amplifier is functionally similar to the preamplifier included in the hi-fi system. In many pneumatic positioners, a nozzle-flapper or similar device has the same effect as this high static gain preamplifier. Once the change in process variable is sensed by the high static gain preamplifier, the positioner must enable the shut-off element to move quickly to provide a timely corrective action on the process variable. This requires a great deal of power to quickly move the actuator and valve assembly to a new location. In other words, the positioner must quickly provide a large amount of air to the actuator so that it can respond quickly. This ability comes from the high dynamic gain of the positioner. Although a preamplifier can provide high static gain, it typically does not have enough power to provide the required power. In this way, the function of the preamplifier must be compensated by a high dynamic gain power amplifier. And this high dynamic gain power amplifier can quickly provide the required air flow as needed. The function of this power amplifier is generally provided by an amplifier or a spool valve. Spool valve positioners are relatively common due to their simplicity. Unfortunately, many spool valve positioners achieve this simplicity by eliminating high-gain preamplifiers at design time. The input stage of these positioners is usually a low static gain converter module. This converter module converts the input signal (electrical or pneumatic) into the movement of the slide valve, but this device is usually of low sensitivity to small signal changes. The result is a large delay time and very slow overall response time of the control valve (control valve) assembly. Some manufacturers try to compensate for the poor performance of these devices by using spools that are larger in diameter and have less channel overlap. This does increase the dynamic power gain of the device. This method will improve performance to some extent if it is well matched to the actuator, but it greatly increases the air consumption of these high gain spools. Many high gain spool positioners have more than 5 times the static meter air consumption than a typical high performance secondary positioner. A typical secondary positioner uses a pneumatic amplifier during the power amplification stage. Amplifiers are favored because they provide high power gain. This high power gain produces the best dynamic performance with minimal steady air consumption. In addition, they are less susceptible to fluid contamination. With the increasing popularity of microprocessor devices (see Chapter 4), the positioner design is undergoing dramatic changes. These microprocessor-based positioners provide the same dynamic performance as the best common secondary positioners. They also provide valve monitoring and diagnostic capabilities that help ensure that the initial good performance does not decline with use. In summary, a high-performance positioner with both high static and high dynamic gain can provide optimum overall performance in terms of process deviation for any given valve assembly. 30-40Nm³/H Oxygen Generator,Oxygen Generators With Inbuilt Compressor,Psa Oxygen Generator With Compressor,Oxygen Generator To Fill Tanks Jiangsu Minnuo Group Co.,Ltd , https://www.jsmngroupcn.com