Control valve in the power plant automation system is the most common type of actuator, the general automatic control system consists of objects, monitoring instruments, controllers, actuators and other components. Correct selection of the structure type, flow characteristics, flow capacity and actuator output torque, thrust and travel of the control valve are very important to the safety, stability, economy and reliability of the automatic control system. If improper selection, will directly affect the control system performance, or even unable to achieve automatic control, thereby affecting the safety and economic operation of the entire unit. The survey found that about 95% of these cases were caused by improper selection. The calculation error caused less than 5% of the problems. Practice has proved that compared with the selection and selection, the selection of more difficult, there are more problems, which should pay special attention. A selection of the control valve 1.1 Selection of the main factors to be considered (1) To meet the production process temperature, pressure, level and flow requirements; (2) valve leakage and sealing requirements; (3) valve working pressure Difference <required pressure difference; (4) to improve the valve life and reliability considerations; (5) the valve speed, flow characteristics of the consideration; (6) the valve mode of action and flow considerations; Actuator type, output torque, stiffness and spring range considerations; (8) material and valve economy considerations (the wrong type of selection will be a difference of 3 to 4 times). 1.2 Selection of the general principles In the process to meet the requirements of the premise, the selected valve should be as simple, reliable, inexpensive, long life, easy maintenance and timely and reliable source of spare parts. Try to avoid simple pursuit of good structure, good material, more with attachments, while ignoring the reliability, economic considerations. From the reliability point of view, the simpler the structure, the higher its reliability; the material is too high, will result in unnecessary price investment. 1.3 Selection of process parameters and system requirements should be provided (1) Process parameters: temperature, pressure, pressure and normal flow when the pressure drop off. (2) fluid characteristics: corrosive, viscosity, temperature changes on the fluid properties. (3) System Requirements: leakage, adjustable ratio, speed and frequency of movement, linearity and noise. 1.4 Control Valve Classification and Selection Control valve according to the structural characteristics can be broadly divided into the following 9 categories: (1) Straight through the single seat control valve: The valve is the most widely used, with small leakage, allowable pressure difference, the flow path is complex, Simple structure, so it is suitable for clean media with strict requirements of leakage and working pressure difference, but the small size valve (DN <20mm) can also be used in occasions with large pressure difference. (2) through the two-seat control valve: And through the single-seat control valve on the contrary, with a large leakage, allowable pressure difference characteristics, it applies to the lax leakage, working pressure large clean media occasions, the selection should be Note that the valve leakage is large enough to meet the process control requirements. (3) sleeve valve: sleeve valve is divided into single and double sealed two kinds of structure, the former is similar to the single-seat valve, suitable for single-seat valve occasions. The latter is similar to a two-seater valve, suitable for two-seater valve applications. Sleeve valve also has good stability, easy loading and unloading features, but the price is more than single, double seat valve 50% to 200%, also need a special winding seal, is second only to the single, double seat valve is more widely used Valve. (4) Angle valve: Throttle type is equivalent to a single seat valve, but the valve body flow path is simple, suitable for small leaks, pressure is not large clean media occasions and require right-angle piping occasions. (5) three-way valve: with three channels, instead of two through the single-seat valve for shunt and confluence and two-phase flow temperature difference ≤ 150 ℃ occasions. When DN ≤ 80mm, confluence valve can be used for shunt occasions. (6) Diaphragm valve: flow path is simple, the diaphragm has a certain corrosion resistance, suitable for more dirty medium, weak corrosive medium cut off 2 occasions. (7) butterfly valve: equivalent to take straight pipe to do the valve body, and the valve body is equivalent to the valve seat, so "self-cleaning" performance, small size, light weight. For more filthy media and large diameter, large flow, pressure of the occasion. When DN> 300mm, usually have a butterfly valve to complete. (8) ball valve: "O" ball valve is open at noon control valve, "self-cleaning" the best performance, especially for dirty, containing fiber medium 2 cut occasions. "V" -shaped ball valve has approximately equal percentage adjustment characteristics, suitable for more dirty, containing fiber media adjustable relatively large adjustment occasions, the valve is more expensive. (9) eccentric rotary valve: between the butterfly valve and ball valve valve, "self-cleaning", good performance, but also cut off, it is more applicable to the medium than the dirty media, leakage requires a small regulatory occasions, but the valve is more expensive . The first six of these nine categories of products are direct stroke control valves and the last three are angle stroke control valves. As a user, you must understand its characteristics. 1.5 The right choice of a number of issues 1.5.1 Body Material Selection (1) body pressure level, temperature, corrosion resistance and other aspects should not be less than the process connection pipe, and preferred stereotypes products. (2) water vapor and moisture more wet gas medium, the ambient temperature is below -20 ℃, should not use cast iron valve. 1.5.2 valve trim material selection (1) non-corrosive media generally use 1Cr18Ni9Ti or other stainless steel. (2) cavitation, erosion is more serious, medium temperature and pressure in Cartesian coordinates, the temperature is 300 ℃, the pressure difference is 1.5MPa 2 points outside the area should choose wear-resistant materials such as Cobalt-based alloy or surface surfacing history too Lai alloy. (3) on the hard seal cut-off valve, in order to improve the reliability of the sealing surface, wear-resistant alloy should be used. When the sealing requirements are very strict, soft seals should be used, such as PTFE, rubber. 1.5.3 High and low temperature material selection When the medium temperature <-60 ℃, choose copper or 1Cr18Ni9Ti; when the temperature is 450-600 ℃, choose titanium and molybdenum stainless steel; when the medium temperature> 600 ℃, choose high temperature and high strength alloy Nile). 1.6 Packing and bonnet type selection (1) Under normal circumstances, the medium temperature <200 ℃, the choice of "V" PTFE packing, common type bonnet; medium temperature <450 ℃, the choice of "V" Filler, but must be thermal valve cover. (2) For straight-travel valve, if with locator attachment, the common type bonnet can still be used for medium temperature≤450 ℃ high temperature valve, but graphite packing must be used. (3) medium temperature> 400 ℃, the need to use thermal bonnet and graphite packing. (4) In order to increase the reliability of the valve stem seal, double-layer packing structure can be used. 1.7 Locator Selection Locator should be used in the following situations: (1) The electric instrument controls the pneumatic valve, and is a slow response system. (2) need to improve the film actuator output force of the occasion. (3) slow process need to improve the response speed of the valve occasions, such as temperature, level and analysis of parameters. (4) need to overcome the friction, reduce the backlash caused by poor quality control of poor occasions, such as low temperature or flexible graphite packing valve. (5) regulator with a very wide band, but also requires the valve small signal response, the use of spring-free actuator adjustment system. Valve with a positioner, usually selected 20 ~ 100kPa of the spring, but in order to improve the output force, the optional source pressure PS = 250kPa. On the gas valve, the choice of 60 ~ 180kPa spring, to increase the starting point of implementing agencies output force. The valve can be closed on the gas 20 ~ 100kPa spring, to increase the closing force of the implementing agency output. 2 regulating valve installation of a number of issues 2.1 General requirements for installation (1) The control valve should be vertical, is mounted on the level of the pipeline, the nominal through the Dg ≥ 50 control valve, the valve before and after the best on the pipeline there is a permanent Bracket. (2) The installation position of the control valve should be convenient for operation and maintenance so that personnel can carry out maintenance and operation. If necessary, the platform should be set. (3) The upper and lower parts of the control valve should have enough space to remove the lower flange and plug of the actuator and valve trim and valve during maintenance. (4) When the control valve is installed in a vibration occasions, should consider the anti-vibration measures. (5) Do not install the valve positioner valve, the film head is best to install a small pressure gauge indicating the control signal. (6) Control valve should be checked first, and installed in the pipeline after purging. 2.2 Safety Considerations (1) The valve in all aspects of the operation (that is, installation, testing, operation and maintenance), should first pay attention to the safety of personnel and equipment. (2) After the valve is cut off, the pressure in the valve can also be maintained for a period of time. Safety measures should be taken to reduce the pressure, such as installing a vent valve or a vent valve. (3) For liquid media, a venting valve capable of limiting flow should be installed to prevent the hazards caused by water hammer when the vent valve is opened too quickly. (4) on the steam line, near the upper and lower ends of the control valve should be heat. (5) Where pressure fluctuations are serious, a line buffer should be installed. 2.3 on the control valve performance considerations (1) Piping diameter try to be consistent with the valve diameter. (2) Regulating valve inlet straight pipe length of at least not less than 10 times the pipeline through. (3) control valve outlet straight pipe should be 3 to 5 times the pipe through. (4) import and export control valve take pressure point position for the valve before the 2 times through the pipe, valve 3 times the pipe diameter at. (5) must be installed according to the direction of flow arrow valve to avoid excessive installation stress. 2.4 Consideration of Manual Operation (1) The valve should be installed at a convenient location and should allow the operator to see the parameters displayed on the indicator (eg level gauge). (2) It should be considered to remove the control valve hand wheel mechanism, locator and other accessories side space. (3) for large diameter, high-altitude installation of the control valve, to take into account the maintenance of the operator's working position. 2.5 Regulating valve signal piping and wiring (1) Regulating valve piping and wiring scheme should meet the regulatory requirements of the system. (2) Regulating valve piping should adopt D6 × 1mm copper pipe; large diaphragm valve and pneumatic valve should adopt D8 × 1mm copper pipe. 3 main performance control valve on-site inspection valve performance indicators are many, the following items should be focused on testing and calibration. (1) The basic error: 20 ~ 100kPa signal steadily increase or decrease the input chamber (or locator), measuring the corresponding stroke value of each point to calculate the "signal - stroke" relationship between the theoretical value Of the point error, the maximum is the basic error. The test point should be 5% of the signal range of 0%, 25%, 50%, 75%, 100%, the basic error of the measuring instrument should be limited to the test valve basic error limit of 1/4. (2) back to the difference: the experimental method Ibid. In the same input signal measured on the positive and negative strokes of the maximum difference that backlash. (3) always point deviation: The experimental method is the same as above. The basic error at the upper limit (starting point) of the signal is the starting point deviation. The basic error at the lower limit (end point) of the signal is the ending point deviation. (4) Leakage test: The test medium is usually water at room temperature. When the pressure difference of the valve is less than 350kPa, the test pressure is to be made at 350kPa. When the working pressure difference of the valve is more than 350kPa, the allowable pressure difference shall be used. The test medium should flow into the valve as required and the outlet of the valve can be directly connected to the atmosphere or the low-pressure head measuring device connected to the outlet through the atmosphere. After confirming that the valve and the downstream connecting pipe are completely filled with the medium, the leakage can be measured. On the main valve, but also to do strong pressure test. (5) on the matching valve locator, installation, put into operation, should be on-site commissioning. 4 regulating valve on-site maintenance Regulating valve directly contact with the process medium, the performance of a direct impact on system quality and environmental pollution, so the valve must be regularly maintained and regularly overhaul, especially for harsh conditions and important occasions should pay attention to maintenance jobs. The focus of inspection and maintenance of parts: (1) For the use of high pressure and corrosive media in the case of regulating valve, the valve body wall, the diaphragm is often subject to media impact and corrosion, stress inspection should focus on the situation. (2) for the fixed valve seat thread, the inner surface of the valve seat susceptible to corrosion loosening, should focus on checking this site; work on the high pressure valve should also check the valve seat sealing surface erosion, cavitation. (3) The valve core is washed by the medium and the corrosion is the most serious. During maintenance, it is necessary to carefully check whether it is corroded or worn out, especially when the valve core is worn by the cavitation phenomenon even under high pressure drop. (4) Check the diaphragm, "0" ring and other gaskets are cracking or aging. (5) should pay attention to PTFE packing, sealing grease is aging, with the surface is damaged, if necessary, should be replaced. 5 common faults and on-site control valve regulating valve on-site common problem is not dead, can not open, back to the big difference, leakage, vibration, oscillation, etc., the treatment methods are as follows: (1) The valve solution is to increase the air pressure or adjust the spring preload (that is, to reduce the pressure outside the starting point of the air chamber). The solution to open the valve is to increase the spring preload, while increasing the source pressure. (2) push rod slow or not action: test diaphragm, rolling diaphragm, gasket aging, rupture caused by leakage. (3) large backlash: pushrod is bent, the packing gland is pressed too tight, especially graphite packing, valve-oriented for injury. The solution is to replace the valve stem, for packing, increase the guide gap, for strength enforcement agencies. (4) The valve full stroke is not enough: Loosen the stem connecting nut, the stem outward rotation or inward stretch. So that the entire stroke deviation exceeds the allowable value and then tighten the nut. (5) valve small opening stability is poor: the scene first check whether the flow of anti-installed, or valve too large, the solution is to open the flow of change, reduce spool size. (6) valve instability: locator fault, the output pipeline leakage, the actuator stiffness is too small fluid pressure changes caused by insufficient thrust. The solution is to repair the positioner and pipeline, use a rigid actuator. (7) large amount of leakage: first check the sealing surface is damaged, the valve seat and the valve stem thread is loose, the valve is closed, the pressure difference is greater than the output of the implementing agencies. The solution is to replace the sealing surface, and tight seat, replace the high output force of the implementing agencies. (8) Oscillation phenomenon: It is due to the valve in a small opening work or flow to flow closed type. The solution is to avoid a small degree of work, change the flow of open-type work. 6 Conclusion With the unit capacity increases and the degree of automation increased, the importance of regulating valve is more and more important. A satisfactory control valve can not only make the automatic control system run stably, reduce the labor intensity of the operator, but also effectively reduce the deviation of the operating parameters of the unit and improve the economy and power quality of the unit. There are often design and manufacture of quality control valve in use, but more problems related to the use and maintenance.
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