Pneumatic control valve pneumatic valve positioner selection guide

Pneumatic valve positioner is a major attachment of pneumatic control valve, which is equipped with pneumatic pneumatic - valve, it accepts the regulator output signal, and then to its output signal to control the pneumatic pneumatic control valve, when the pneumatic control valve, the valve stem of the displacement feedback through mechanical device to valve positioner, valve position condition through the electrical signal to the upper system. In many control applications, the valve pneumatic control valve positioner is one of important accessories. Especially for a particular application, if you want to choose a suitable valve positioner, then it should be paid attention to consider the following factors: the valve positioner can be fulfilled 'cheng ( Split_ranging) 'Implementation' cheng 'is easy, convenient? Have the 'cheng' means that the valve positioner only to a certain range of the input signal ( Such as: 4 ~ 12 ma or 0. 02 ~ 0. 06MPaG) Have a response. So, if you would 'cheng', can according to actual needs, use only one input signal to realize successively control two or more pneumatic control valve. Locator of the zero and span adjustment is easy and convenient? If don't have to open the lid can complete zero and span adjustments? But it is worth noting that sometimes in order to avoid incorrect ( Or illegal) Operation, the calibration can be carried out on the casual way need to be banned. The valve positioner of the stability of the zero and span? If zero and range easily with the change of temperature, vibration, time or input pressure and drift, then the valve positioner need often set-up again, to ensure that the pneumatic control valve action accurate itinerary. The precision of the valve positioner under ideal conditions, the number one input signal, pneumatic control valve internal parts ( TrimParts, including sphere/valve core and valve stem, seat, etc. ) Every time should be to accurately locate the position of the required, regardless of the direction of travel or stand pneumatic - valve internal parts of the load. Valve positioner requirement for air quality? Because only a handful of air supply device can provide meet the ISA standards ( About instrument air quality standard: ISA standard F7. 3) By air, therefore, for pneumatic ( Or electricity - Gas) Valve positioner, if want to withstand the test of reality, it must be able to bear the corresponding amount of dust, vapor and oil. Zero and range of the calibration are both influence each other or are independent of each other? If the mutual influence, the zero and span adjustments will need to spend more time, this is because the set-up personnel should be on these two parameters adjustment, so as to gradually achieve accurate setting. If a valve locator 'bypass', allow the input signal directly ACTS on the pneumatic control valve? This 'bypass' sometimes can simplify or save actuator assembly set of calibration, such as: actuator 'bearing component set' and 'spring seat load set' - this is because, in many cases, some pneumatic regulator of pneumatic output signal and the actuator bearing component set is match, no need to set it again on the other hand, have 'bypass' is also sometimes allow online the valve positioner for calibration or repair of the limits of maintenance. The role of the valve positioner is fast? The air flow 气流) The more ( Valve positioner constantly compare input signal and valve position, according to the deviation between them, adjust the output of its own. If the valve locator for this kind of deviation of rapid response, the unit of time in the air flow is big) , adjust the system response is to change the set point and load faster - which means that the error of the system ( Lag) The less, the quality control. Frequency characteristics of the valve positioner is what? In general, the higher the frequency characteristic and control performance is better. But must pay attention to: frequency stability experiment method should be adopted instead of theoretical method to determine, and in the measurement of frequency characteristic, valve positioner and actuator should be combined to consider. Valve positioner Z big rated gas supply pressure is how many? For example: some valve positioner Z big gas supply pressure rating of the calibration for 501 b/in2 ( Namely: 50 psi, lpsi = 0. 070kgf /cm2≈6. 865 kpa) , if the actuator rated operating pressure is higher than 501 b/in2, the valve locator will become a restriction factor of actuator output drivers. When the pneumatic control valve and the valve positioner assembly after combination, their positioning resolution ( PositioningResolution) How to? It's very clear for control quality of the regulating system of the role, because the higher the resolution, pneumatic control valve position is more close to the ideal value, the fluctuations caused by pneumatic - valve over-modulation changes can get stem, thereby Z will achieve the goal of limiting the amount be adjusted periodically change. Positive role conversion of valve positioner is workable? Easy to convert? This function is sometimes necessary. For example, to put a 'signal increase - valve' instead of 'signal increases, the valve to open', can use positive role transformation function of the valve positioner. Inside the valve positioner the complexity of the operation and maintenance? As is known to all, components, the more the more complex internal operating structure, the maintenance ( Fix) The more the valve technology training, and the more inventory of spare parts. Valve positioner steady gas consumption this is a key parameter for some factory equipment, and can be a limiting factor. Of course, choose in evaluation and valve positioner, also should consider other factors. Such as: valve positioner can feedback linkage to real reaction the position of the valve core; In addition, valve positioner shall be durable, resistance to environmental protection and anticorrosion ability, and easy installation and simple connection.