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Valve positioners are among the most important accessories for regulating valves in a wide range of control applications. In particular, for a particular application, consider the following if selecting the most suitable (or best) valve positioner: 1) Whether the positioner can achieve "Split_ranging" Is it easy and convenient to implement "splitters"? With the "splitter" function, the positioner responds only to a range of input signals (eg 4 to 12 mA or 0.02 to 0.06 MPaG). Therefore, if we can "split", then according to actual needs, only one input signal has been achieved to control two or more control valve. 2) Is it easy and convenient to adjust the zero and span? Is it possible to adjust the span and span without opening the lid? It is worth noting, however, that sometimes it is sometimes necessary to avoid incorrect (or illegal) operation The kind of method that can be adjusted at will needs to be banned. 3) What is the stability of the zero and span? If the zero and span easily drift with changes in temperature, vibration, time or input pressure, then the positioner needs to be recalibrated frequently to ensure that Travel is accurate. 4) What is the accuracy of the positioner? Ideally, Trim Parts (including spools, stems, seats, etc.) should be accurately positioned each time they are required for an input signal Regardless of the direction of travel or how much load the trim valve's inner part is subjected to. 5) What is the air quality required by the positioner? As only a handful of air supply units are capable of supplying air meeting the ISA standard (ISA standard F7.3 for air quality in instrumentation), for pneumatic (or Electrical - gas) valve positioner, if you want to withstand the test of the real environment, it must be able to withstand a certain amount of dust, water vapor and oil. 6) Is calibration of zero and span both interdependent or independent? If they affect each other, it takes more time to calibrate the zero and span because the operator must adjust these two parameters repeatedly , In order to gradually achieve the exact set. 7) Does the positioner have a "Bypass" that allows the input signal to act directly on the regulator? This "by-pass" can sometimes simplify or eliminate the check of the Actuator Settings, Such as: actuator "Benchset" and "Seat Load" ?? This is because in many cases, some of the pneumatic regulator pneumatic output signal and the implementation of the " Bearing assembly set "completely match, do not need to set it again (in fact, in this case, the valve positioner can save unnecessary .Of course, if you choose, you can also use the valve positioner "Bypass" allows the pneumatic actuator pneumatic output signal directly on the regulator valve). In addition, the presence of "by-pass" can sometimes allow limited on-line calibration or service maintenance of the positioner (ie bypassing of the positioner to keep the regulator in normal operation without forcing the regulator off-line ). 8) Does the positioner function quickly? The greater the Airflow (the positioner continuously compares the input signal to the valve position and adjusts its own output based on the deviation between them.) If the positioner is doing this The rapid response of the kind of deviation, then the flow of air per unit of time on the large), the adjustment system setpoint (Setpoint) and load changes in response faster ?? This means that the system error (hysteresis) smaller control quality The better. 9) What is the frequency response (Frequency Response, G (jω), the steady-state response of the system to the sinusoidal input)? Generally speaking, the higher the frequency characteristic (that is, the frequency response The higher the sensitivity), the better the control performance. It must be noted, however, that the frequency characteristics should be determined using the Consistent Test Methods rather than the theoretical method, and the valve positioner and actuator should be considered in evaluating the measured frequency characteristics. For example, some valve positioners have a maximum rated supply pressure of only 501b / in2 (ie 50psi, lpsi = 0.070kgf / cm2≈6.865kPa). If performed The rated working pressure of the mechanism is higher than 501b / in2, then the valve positioner becomes the limiting factor for the output of the actuator. 11) When the control valve and valve positioner assembly combination, their positioning resolution (Positioning Resolution)? This is the control system of the control system has a very significant role, because the resolution is higher, the closer the position of the control valve The ideal value, due to the control valve over-flow (Overshooting) caused by changes in the fluctuations can be strangled, so as to eventually limit the amount of adjustment of the cyclical changes in the purpose. Lining fluorine butterfly valve 12) positive and negative valve positioner conversion is feasible? Conversion is easy? Sometimes this function is necessary. For example, to change the way a "signal is increased by a valve" to "a signal is added by a valve," the positive and negative effects of the positioner can be used. 13) What is the complexity of the internal operation and maintenance of the positioner? As we all know, the more components, the more complex the internal operation structure, the more training (maintenance) personnel will be trained and the more inventory spare parts will be available. 14) What is the Steady-state Air Consumption for valve positioners? For some plant installations, this parameter is critical and may be a limiting factor. Of course, other factors should also be considered when evaluating and selecting valve positioners. For example, the feedback linkage of the positioner should reflect the position of the valve core. In addition, the positioner must be durable, anti-environmental protection and anti-corrosion, and the installation and connection are easy and convenient. China's hydropower station and pumping station water machine status quo One of the characteristics of the river sediment in China is large, with an average annual sediment discharge of more than 10 million t in the river there are 115 direct sediment in the sea amounted to 1.94 billion t, of which the Yellow River And its tributaries of the hydropower station, there are or will be faced with water erosion problems. According to Wang Zhigao, a senior engineer at Tianjin Institute of Water Resources and Hydropower Survey, Design and Research Institute, there are about 132 units of sediment erosion in China, accounting for more than 12 million kW (excluding the Three Gorges Project). The total installed capacity of small and medium hydropower stations is about 22 million kW 30% of the power plants with sediment erosion are about 6.6 million kW, a total of 13,000 units. In addition, the Yellow River pumping station 100,000 units, about 340 million kW pump to varying degrees, the existence of sand erosion damage, plagued the Yellow River pumping project safe operation. Abrasion of water machine is mainly manifested in shortening maintenance cycle, maintenance workload increased to the Yellow River Sanmenxia Hydropower Plant, for example, turbine operation 15000h must be expanded, of which 4 machine running over the flow of serious damage to components, the efficiency decreased by 8.7% and general Overhaul period of hydropower station in about 5 years. Gezhouba Hydropower Station No. 17 machine 20mm thick blade edge erosion rate of 3.5mm / 10000h, No. 15 turbine blade edge water erosion rate of 4.3mm / 10000h. The Yellow River across the Yellow River pumping station pump over-current components generally run 2000 ~ 4000h that scrapped or replaced, high lift pumps run only 1000 h reported. More Keywords Search: Valve
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