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日期: 2019-03-20

Control Valve: Guide to Valve Positioner Selection

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In many control applications, valve positioner is one of the most important accessories of control valve. Especially for a specific application, if you want to choose the most suitable valve positioner, you should pay attention to the following factors:

Regulating valve regulator valve positioner (also known as pneumatic valve positioner) is the main accessory of regulating valve. It is usually used with pneumatic valve. It receives the output signal of the regulator, and then controls the pneumatic regulating valve with its output signal. When the regulating valve operates, the displacement of the valve stem is fed back to the valve positioner through the mechanical device, and the status of the valve position is transmitted to the upper system through the signal. Unification.

In many control applications, valve positioner is one of the most important accessories of control valve. Especially for a specific application, if you want to choose the most suitable valve positioner, you should pay attention to the following factors:

1) Is it easy and convenient for the valve positioner to realize 'Split_ranging'? Having 'split' function means that the valve positioner only responds to a certain range of input signals (e.g. 4-12mA or 0.02-0.06MPaG). Therefore, if 'split' can be achieved, two or more control valves can be controlled successively with only one input signal according to actual needs.

2) Is it easy and convenient to adjust the zeros and ranges of the locator? Is it possible to adjust the zeros and ranges without opening the lid of the box? But it is worth noting that sometimes, in order to avoid incorrect (or illegal) operation, this arbitrary way of adjusting needs to be prohibited.

3) How stable is the zero point and range of the valve positioner? If the zero point and range easily drift with the change of temperature, vibration, time or input pressure, then the valve positioner needs to be adjusted frequently to ensure that the stroke action of the control valve is correct.

4) Valve positioner accuracy under ideal conditions, corresponding to an input signal, the trim Parts (including sphere/spool, stem, seat, etc.) of the control valve should be positioned accurately at the required position each time, regardless of the direction of the stroke or the load on the inner parts of the control valve.

5) What is the requirement of air quality for valve positioners? Because only a few air supply devices can provide air that meets the requirements of ISA standard (air quality standard for instruments: ISA standard F7.3), for pneumatic (or electric-gas) valve positioners, if they are to withstand the test of real environment, they must be able to withstand a certain amount of dust, water vapor and oil pollution.

6) Is the calibration of zeros and ranges interacting or independent? If they are interacting, the calibration of zeros and ranges will take more time, because the calibrators must adjust these two parameters repeatedly in order to achieve the accurate setting step by step.

7) Does the valve positioner have 'bypass' that allows input signals to act directly on the control valve? This 'bypass' can sometimes simplify or omit the verification of actuator assembly settings, such as 'support assembly settings' and 'spring seat load settings' - this is because in many cases, the pneumatic output signals of some pneumatic regulators and actuator settings.“ The support assembly setting 'matches perfectly, and there is no need to set it again (in fact, in this case, the valve positioner can be completely eliminated. Of course, if you choose, you can also use the 'bypass' of the valve positioner to make the pneumatic output signal of the pneumatic regulator act directly on the control valve. In addition, the 'bypass' sometimes allows online valve positioner for limited adjustment or maintenance (that is, using the 'bypass' of the valve positioner to keep the valve in normal operation, without forcing the valve off-line).

8) Is the function of valve positioner fast? The larger the air flow (valve positioner constantly compares the input signal and valve position, and adjusts its output according to the deviation between them. If the valve positioner responds quickly to this deviation, the air flow per unit time will be large, and the response of the regulating system to the setting point and load changes will be faster - which means that the smaller the system error (lag) is, the better the control quality will be.

9) What are the frequency characteristics (or frequency response, Frequency Response, or G (j), steady-state response of the system to sinusoidal input) of the valve positioner? Generally speaking, the higher the frequency characteristics (i.e., the higher the sensitivity to frequency response), the better the control performance. However, it must be noted that the frequency characteristics should be determined by stable experimental rather than theoretical methods, and the valve positioner and actuator should be combined when evaluating the frequency characteristics.

10) What is the maximum rated pressure of the valve positioner? For example, the maximum rated pressure of some valve positioners is only 501b/in2 (i.e. 50psi, lpsi = 0.070kgf/cm2_6.865kPa). If the rated operating pressure of the actuator is higher than 501b/in2, the valve positioner becomes the restricting factor of the output driving force of the actuator.

11) When the control valve and valve positioner are assembled and combined, how about their position resolution? This 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. The fluctuation caused by the overshoot of the control valve can be restrained, so that the periodic change of the regulated quantity can be limited finally. Purpose.

12) Is the positive and negative conversion of valve positioner feasible? Is the conversion easy? Sometimes this function is necessary. For example, to change the way of 'signal increase - valve close' to 'signal increase - valve open', the positive and negative conversion function of valve positioner can be used.

13) What is the complexity of internal operation and maintenance of valve positioners? It is well known that the more components, the more complex the internal operation structure, the more valve technical training for maintenance (repair) personnel, and the more spare parts in stock.

14) Steady-state gas consumption of valve positioners is a key parameter for some plant installations and may be a limiting factor.

15) Of course, other factors should also be taken into account when evaluating and selecting valve positioners. For example, the Feedback Linkage of the valve positioner should be able to truly reflect the position of the valve core; in addition, the valve positioner must be strong and durable, have the ability to resist environmental protection and corrosion, and easy to install and connect.

 


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