Valve positioner (also known as: pneumatic valve positioner) is the main control valve accessories, usually supporting the use of pneumatic control valve, which accepts the output signal controller, and then its output signal to control the pneumatic control valve, when valve action, but also by mechanical displacement of the valve stem to the valve positioner feedback device, the valve position signal to pass through the upper state system.
In many control applications, the valve positioner is one of the most important accessories. Especially for some specific applications, to select the most appropriate valve positioner, then they should pay attention to consider the following factors:
1) The positioner can be achieved "Split (Split_ranging)" to achieve "Split" is easy and convenient? Have the "Split" function means that the valve positioner input signal only a range (such as: 4 ~ 12mA or 0.02 ~ 0.06MPaG) a response. Therefore, if we can "Split", then it can be based on actual needs, with only one input signal can have two or more control valves.
2) Locator zero and span adjustment is easy and convenient? Is not can be done without opening the lid to adjust the zero and span? But it is worth noting that: Sometimes in order to avoid incorrect (or illegal) operation, this adjustment can be carried out at random the way need to be banned.
3) the valve positioner to the stability of zero and span? If zero and span is easy with the temperature, vibration, time, or changes in input pressure drift, then the valve positioner often need to be re-tuned to ensure the accuracy of the control valve stroke action.
4) the accuracy of the valve positioner in ideal conditions, corresponding to an input signal, the control valve trim (TrimParts, including ball / spool, stem, seat, etc.) should be accurately positioned in each of the requirements of the position, regardless of the direction of travel or the control valve trim bear much of the load.
5) The valve positioner how the air quality requirements? Because only a very small number of gas supply device can provide to meet ISA standards (related to air quality standards for Instrument: ISA standard F7.3) under the air, therefore, pneumatic ( or electric - gas) valve positioner, to withstand the test of reality, to be able to withstand a certain amount of dust, water vapor and oil.
6) The zero and span calibration of the two affect each other or independent of each other? If the interaction is zero and span adjustments need to spend more time, because the adjustment must be repeated to adjust these two parameters in order to gradually achieve accurate settings.
7) whether the valve positioner with "bypass" that allows the input signal directly on the valve? This "bypass" Sometimes the actuator assembly to simplify or eliminate calibration set, such as: implementing agencies " bearing component set "and" load setting spring seat "- this is because in many cases, some pneumatic regulator pneumatic output signal with the implementing agencies of the" bearing component set "completely consistent match, with vain to conduct its setting (in fact, in this case, completely eliminates the need for valve positioner not. Of course, if selected, then the valve positioner can also use the "bypass" the air regulator pneumatic output signal directly on the control valve.) In addition, with the "bypass" sometimes allows the valve positioner online for a limited adjustment or repair and maintenance (namely, the use of the valve positioner "bypass" the control valve to maintain normal work, not mandatory regulation valve off.)
8) whether the role of valve positioner fast? Air flow rate (Airflow) larger (valve positioner continuously compare the input signal and valve position, and according to the deviation between them, adjust their own output. If the valve positioning rapid response to such bias device, then the unit of time on the big volume of air flow), conditioning on the set point and load changes in response to the more quickly - which means that the system error (lag) the smaller, more quality control good.
9) the frequency characteristics of the valve positioner (or frequency response, FrequencyResponse - namely, G (jω), the steady-state system to sinusoidal input response) What is it? In general, the higher the frequency (ie, the frequency responseThe higher sensitivity), control performance, the better. It must be noted: the frequency characteristics of the experimental methods should be used rather than the theory of stable method to determine, and the determination of frequency characteristics in the assessment, should the valve positioner and the implementing agencies to consider merging.
10) The maximum rated valve positioner supply pressure? Example: Some of the valve positioner maximum rated air pressure is only rated as 501b/in2 (ie: 50psi, lpsi = 0.070kgf/cm2 ≈ 6.865kPa), If the executive body of the rated operating pressure is higher than 501b/in2, then the valve positioner to become the driving force implementation of the output constraints.
11) When the control valve and the valve positioner assembly combination, their positioning resolution (PositioningResolution) How? This quality control system to control has a very significant role, because the higher the resolution, the closer positioning of control valve ideal value, due to valve off the fluctuations caused by changes in tone can be curbed, which ultimately limit the amount of periodic change is the purpose of regulation.
12) positive and negative effect of the valve positioner conversion feasible? Conversion is easy? Sometimes this feature is necessary. For example, to a "signal to increase - the valve off" approach to "increase the signal - the valve opening" approach, you can use the positive and negative effect of the valve positioner conversion.
13) valve positioner operation and maintenance of the internal complexity of how? We all know, the more components, the internal operation more complex the structure of the maintenance (repair) personnel of the valve the more technical training, and the more spare parts inventory .
14) valve positioner parameters of steady-state air consumption of this device is critical for some plants, and may be a limiting factor.
15) Of course, in the evaluation and selection of valve positioner, other factors should also be considered. For example: valve positioner feedback linkage (FeedbackLinkage) give a true response to the position of the spool; In addition, the positioner must be durable, with anti-environmental protection and corrosion resistance, and easy installation and connection convenience. Water Pumping Station and our abrasion status quo.