1 To determine the nominal pressure, instead of using Pmax to set the PN, find the corresponding PN from the table based on the three conditions of temperature, pressure, and material and satisfy the PN value of the selected valve. Valve type selection
Selection of valve type: (1) To determine the nominal pressure, instead of using Pmax to set the PN, find the corresponding PN from the table based on the three conditions of temperature, pressure, and material and satisfy the PN value of the selected valve. (2) The leakage amount of the determined valve type meets the process requirements. (3) The working pressure difference of the determined valve type should be smaller than the allowable pressure difference of the valve. If this is not possible, special considerations must be made or another valve must be selected. (4) The temperature of the medium is within the operating temperature range of the valve, and the ambient temperature meets the requirements. (5) Consider the anti-blocking problem of the valve according to the unclean condition of the medium. (6) Consider the corrosion resistance of the valve based on the chemical properties of the medium. (7) According to the pressure difference and medium containing hard objects, consider the erosion.
Selection of actuators: (1) The simplest is the pneumatic diaphragm type, followed by the piston type, and finally the electric type. (2) The main advantage of electric actuators is that the driving source (power supply) is convenient, but the price is high, and the reliability, waterproof and explosion-proof are not as good as pneumatic actuators, so pneumatic actuators should be preferred. (3) The old electric actuators are bulky and bulky. We already provide electronic precision, small and high-reliability electric actuators (the price is correspondingly high). (4) The old ZMA and ZMB membrane actuators can be eliminated and replaced by multi-spring lightweight actuators (performance improved, weight and height reduced by about 30%). (5) There are many varieties and specifications of piston actuators. It is recommended that the old, big and stupid ones be no longer used, and new light structures should be used.
Selection of materials: (1) The pressure rating, operating temperature and corrosion resistance of the valve body should not be lower than the requirements of the process connection pipeline, and the manufacturer's finalized products should be given priority. (2) Cast iron valves should not be used for water vapor or wet gases containing more water and flammable and explosive media. (3) When the ambient temperature is lower than -20°C (especially in the north), cast iron valves should not be used. (4) For media with severe cavitation and erosion, in the rectangular coordinates formed by temperature and pressure difference, when the temperature is 300°C and the pressure difference is 1.5MPa in the area outside the two-point line, the throttling sealing surface should be Use wear-resistant materials, such as cobalt-based alloys or surface surfacing Stellite alloys. (5) For highly corrosive media, when selecting corrosion-resistant alloys, appropriate corrosion-resistant materials must be selected based on
Selection of flow characteristics: The following is a preliminary selection. For detailed selection, please see the special information: (1) Select logarithmic characteristics when S>0.6. (2) Select logarithmic characteristics when working with small openings and large changes in unbalanced force. (3) When the required adjusted parameter reflection speed is fast, choose a straight line, and when it is slow, choose a logarithm. (4) Linear characteristics are optional for the pressure regulation system. (5) The liquid level adjustment system has optional linear characteristics.
Mode of action selection: (1) In foreign countries, faults are often expressed as open or closed, that is, fault open and fault closed, which is exactly the opposite of the air open and air closed expressions in our country. Fault open corresponds to an air closed valve, and failure close corresponds to an air open valve. (2) New light valves and fine small valves no longer emphasize the positive and negative effects of the execution machinery, so they must be marked on the endnote.
Selection of spring range: (1) First, select the spring range, and also determine the working spring range. (2) Determining the working spring range involves calculating the output force to overcome the unbalanced force. If there are difficulties, the conditions (mainly the pressure difference when the valve is closed) should be informed to the manufacturer to help calculate and adjust the spring and working range before leaving the factory (currently, many manufacturers do not do calculations at all).
Selection of flow direction: (1) At the orifice, the medium flowing in the opening direction of the valve core is called flow open, and the medium flowing in the closing direction is called flow closed. (2) The main choice of flow direction is single-seal regulating valves, which include single-seat valves, angle valves, and single-seal sleeve valves. It is based on specified flow direction (such as double-seat valve, V ball) and arbitrary flow (such as O ball). (3) When dg＞15, flow open is usually selected. When dg≤15, small-diameter valves, especially high-pressure valves, can choose flow closed to improve service life. (4) Flow closing is optional for the two-position switch valve. (5) If the closed-flow valve oscillates, change it and the open-flow valve will be eliminated.
Selection of packing: (1) The commonly used control valves are PTFE -shaped packing and graphite -shaped packing. (2) PTFE packing has low friction, but is resistant to temperature differences and has a short service life; graphite packing has high friction, but has good temperature resistance and long service life; graphite packing is recommended for high temperatures and valves with positioners. (3) If the PTFE packing is frequently replaced, graphite packing may be considered.
Selection of accessories: (1) The accessories of the control valve mainly include: positioner, converter, relay, booster valve, retaining valve, pressure reducing valve, filter, oil mist, travel switch, position transmitter , solenoid valve, handwheel mechanism. (2) Accessories serve to supplement functions and ensure valve operation. Increase what is necessary and do not increase what is unnecessary. Adding accessories when unnecessary increases price and reduces reliability. (3) The main function of the positioner is to increase the output force and movement speed. When these functions are not needed, it is not necessary to use the positioner. (4) For fast response systems, if the valve does not act quickly, a converter is optional. (5) In strict explosion-proof situations, optional: electrical converter + pneumatic positioner. (6) The solenoid valve should choose a reliable product to prevent it from not acting when it is required to act. (7) It is recommended not to use the handwheel mechanism in import.
