How to Choose the Right Regulating Valve? (Part 3)

Jul. 28, 2020

2 choice of accessories

The accessories of regulating valve mainly include valve positioner, valve position switch, pneumatic position retaining valve, pneumatic relay, solenoid valve, air filter pressure reducer, handwheel mechanism, valve position transmitter, and converter, etc. Among them, the valve positioner includes electric valve positioner and pneumatic valve positioner, which are mainly used to improve the working characteristics of the control valve, achieve correct positioning, improve the linearity of the control valve position, reduce the transmission delay of the control signal, and change the flow characteristics of the control valve. , Change the response range of the regulating valve to the signal pressure to achieve split-range control and correct positioning. It is one of the most important accessories of the regulating valve, and its quality will directly affect the performance and quality of the regulating valve and the regulating system. The following focuses on several main factors that need to be considered when choosing a valve positioner:

1) Whether the valve positioner can realize the "split range" function, that is, the valve positioner only responds to a certain range of the input signal. If the valve positioner can achieve this function, one input signal can be used to control two or more regulating valves according to actual needs;

2) Whether it is easy to adjust the zero point and range, whether the calibration is independent, and how stable is it;

2) Whether it is easy to adjust the zero point and range, whether the calibration is independent, and how stable is it;

3) How accurate is the valve positioner? Under ideal working conditions, corresponding to a certain input signal, the internal parts of the regulating valve (including the valve core, stem, seat, etc.) of the regulating valve should be accurately positioned at the required position every time, regardless of the direction of the stroke Or how much load the internal parts of the regulating valve bear;

4) What is the speed and frequency characteristics of the valve positioner? Because the valve positioner can constantly compare the input signal and the valve position, and adjust its own output according to the deviation between them. If the valve positioner responds quickly to this deviation, then the flow of the medium per unit time is greater, and the faster the response of the adjustment system to the set point and load changes, that is, the smaller the system error, the better the control quality. good. Generally speaking, the higher the frequency characteristic, the higher the sensitivity to frequency response, the better the control performance. It should be noted that the evaluation of frequency characteristics should adopt a combination of experiment and theory, rather than relying solely on theory, and in the evaluation, the experimental method must be stable and scientific, and the valve positioner and actuator should be considered together;

Single Seat Regulating Valve

Single Seat Regulating Valve

5) After the valve positioner is combined with the regulating valve, how does its positioning resolution change? The positioning resolution has a very obvious effect on the control quality of the control system, because the higher the resolution, the closer the positioning of the control valve is to the ideal value, and the fluctuations caused by the over-adjustment of the control valve can be effectively suppressed, and finally achieved The purpose of limiting the periodic change of the adjusted amount.

6) Whether the maximum rated air supply pressure of the valve positioner matches the rated operating pressure of the actuator, whether it is convenient to install and connect, how much maintenance and the degree of maintenance, etc.

In addition to the valve positioner, the other types of accessories are relatively simple, and do not need to be repeated here. All accessories play the role of supplementary function and ensure the correct operation of the regulating valve. The principle to be grasped when selecting is to increase necessary and discard unnecessary, otherwise, it will only increase the operating cost of the control system and reduce the reliability.

3 conclusion

The correct selection of the control valve is the first step in the application of the control valve, and it is also the most critical step. The selection of the control valve directly affects the use effect of the control valve and then affects the adjustment quality of the system. Of course, the selection work is more complicated, and at the same time, it is a subject that requires continuous exploration and summary in practical applications. Therefore, it is necessary for us to master certain methods and skills in the selection of single seat regulating valve, on the premise of familiarity with relevant professional knowledge, and only in this way can we truly play its role in industrial process automation control. The role of "hands and feet".