Attention to the correct selection of valves in building piping systems – A guide to the selection of key valve positioners in the control area

Attention to the correct selection of valves in building piping systems – A guide to the selection of key valve positioners in the control area

In building piping, valves play the role of fluid control. Because of the different structure and material, so the manufactured valves are not the same. In order to ensure that the piping system can achieve high efficiency, low cost and long service life, the correct selection of valves is very important.
The valve has four main functions: start and stop the flow of media; Adjust the medium flow; Prevents backflow or reflux and regulates or relieves fluid pressure. The choice of building piping system can be considered according to temperature, medium type, temperature and other factors. , for example, in the high-rise building fire hydrant valve control valve should be used signal, this is related to whether the fire hydrant system is the key to rational use, when the fire hydrant system control valves are set to signal valve, and valve open to display in the center of the fire control, in order to facilitate management inspection, although the cost has increased, however, The ratio of investment to the overall hydrant system is still very small, and it can make the overall hydrant system safer, which is worth the investment.

The type of valve USED in a building piping system must be selected according to the characteristics of the building. If the valve used is not suitable for the design characteristics of the building, a number of potential hazards will constantly arise.
The selection of valve positioner will directly affect the performance and quality of regulating valve and regulating system. So how to choose the valve positioner correctly and reasonably is particularly important in the control field. Key words: valve positioner selection guide in many control applications, valve positioner is one of the most important accessories. For a particular application, if you want to choose a proper (or good) valve locator, you should consider the following factors: 1) Can a valve locator be “split-ranging”? Is it easy and convenient to implement “splitting”? Having a “split” function means that the valve positioner responds only to a range of input signals (e.g., 4 to 12mA or 0.02 to 0.06MPaG). Therefore, if you can “divide”, you can according to the actual needs, only one input signal to achieve the control of two or more regulating valves. 2) Is the adjustment of zero point and range easy and convenient? Is it possible to adjust zero and range without opening the lid? However, it is important to note that sometimes such arbitrary tuning needs to be prohibited in order to avoid incorrect (or illegal) operations. 3) What is the stability of zero and range? If THE ZERO AND RANGE ARE PRONE TO DRIFT WITH CHANGES IN TEMPERATURE, VIBRATION, TIME, OR INPUT PRESSURE, THE VALVE POSITIONER WILL NEED TO BE RETUNED FREQUENTLY TO ENSURE ACCURATE TRAVEL OF THE REGULATOR. 4) How accurate is the valve positioner? Ideally, for an input signal, the Trim Parts of the valve (Trim Parts, including spool, stem, valve seat, etc.) each time should be accurately positioned in the required position, regardless of the direction of travel or regulating valve with how much load of the inner parts. 5) What is the air quality requirement of valve positioner? As only a very small number of air supply units can be supplied to meet ISA standards (air quality standards for instrumentation: ISA STANDARD F7.3) FOR AIR, THEREFORE, FOR AIR-MOBILIZED OR ELECTRIC-GAS (VALVE) POSITIONERS, IF THEY ARE TO WITHSTAND REAL-WORLD CONDITIONS, THEY MUST BE ABLE TO WITHSTAND A CERTAIN AMOUNT OF DUST, MOISTURE AND OIL. 6) Do the calibration of zero and range affect each other or are they independent? If they affect each other, the zeros and ranges take more time to adjust, because the tuner must repeatedly adjust these two parameters to gradually reach the exact setting. 7) Is the valve positioner equipped with a “Bypass” that allows the input signal to act directly on the regulator? This “bypass” can sometimes simplify or omit the calibration of the Actuator Settings, such as: The “Benchset setting” and “Seat Load setting” of the actuator — this is because in many cases, the aerodynamic output signal of some pneumatic regulators matches the “seat set” of the actuator exactly so that no further setting is required (in fact, in this case, Valve positioners can be eliminated entirely. Of course, if selected, then the valve positioner can also be used to “bypass” the pneumatic output signal of the pneumatic regulator directly on the regulator). In ADDITION, with the “bypass” sometimes can also allow a limited adjustment or maintenance of the valve positioner online (that is, the use of the valve positioner “bypass” so that the regulator continues to maintain normal work, without forcing the regulator offline). 8) Whether the function of valve positioner is fast? Airflow The greater the airflow (the valve locator constantly compares the input signal and the valve level and adjusts its output according to the difference. If the valve positioner responds quickly to this deviation, then more air flow per unit time), the faster the adjustment system RESPONDS to Setpoint and load variations — which means less system error (lag) and better control quality. 9) The Frequency characteristics of the valve positioner (or frequency Response, Frequency Response — G (jω), what is the steady-state response of the system to a sinusoidal input? Generally speaking, the higher the frequency characteristic (that is, the higher the sensitivity to the frequency response), the better the control performance. However, it must be noted that frequency characteristics should be determined by Consistent Test Methods rather than theoretical methods, and the valve positioner and actuator should be considered together when evaluating the frequency characteristics. 10) What is the maximum rated air supply pressure of the valve positioner? For example, some valve positioners have a larger rated air supply pressure of only 501b/in (i.e. 50psi, lpsi =0.07kgf/cm ≈ 6.865kpa), the valve positioner becomes a constraint to actuator output thrust if the actuator is rated to operate at pressures higher than 501b/in. 11) When the regulating valve and valve positioner are assembled and combined, how about their Positioning Resolution? This has a very obvious effect on the control quality of the regulating system, because the higher the resolution, the closer the positioning of the regulating valve is to the ideal value, and the fluctuation changes caused by Overshooting of the regulating valve can be controlled, so as to limit the periodic changes of the regulated quantity. 12) Whether the positive and negative conversion of the valve positioner is feasible? Is the transition easy? Sometimes this feature is necessary. For EXAMPLE, TO CHANGE a “SIGNAL INCREASe-VALVE CLOSE” mode TO “SIGNAL INCREASE-VALVE OPEN” MODE, YOU CAN USE the VALVE POSItioner’s positive AND NEGATIVE CONVERSION function. 13) How complex is the internal operation and maintenance of the valve positioner? As we all know, the more parts, the more complex the internal operation structure, the more training for maintenance (repair) personnel, and the more spare parts in stock. 14) What is the Steady-state Air Consumption of the valve positioner? For some plant installations, this parameter is critical and can be a limiting factor. 15) Of course, other factors should also be considered when evaluating and selecting valve positioners. For example, the Feedback Linkage of a valve positioner should reflect the position of the spool; In addition, the valve positioner must be strong and durable, with environmental protection and corrosion resistance, and easy to install and connect.