1. On-site measuring instruments are generally divided into four categories: temperature, pressure, flow and liquid level. A common fault analysis of temperature instrument system (1) Sudden increase in temperature: This fault is mostly caused by thermal resistance (thermocouple) disconnection, loose connection terminals, (compensation) wire breakage, and temperature failure. At this time, you need to understand the location of the temperature and the wiring layout. Using a multimeter's resistance (millivolt) file to measure several sets of data at different locations can quickly identify the cause. (2) Sudden decrease in temperature: This fault is mostly caused by thermocouple or thermal resistance short circuit, wire short circuit and temperature failure. Start with the weak points that are prone to failure, such as wiring ports and wire bends, and check them one by one. The on-site temperature rises, but the total control instructions remain unchanged, mostly due to the liquid (water) with a lower boiling point at the measuring element. (3) Large temperature fluctuations or rapid oscillations: At this time, the main operation of the process should be checked (the inspection and adjustment system involved in the adjustment). Two common faults and analysis of pressure instrument system (1) The pressure suddenly decreases, increases or the indication curve does not change: At this time, the pressure-increasing system of the transmitter should be checked to check whether the root valve is blocked, whether the pressure-inducing pipe is unblocked, whether there is any abnormal medium or drain wire Whether the clogging and sewage valve are leaking. Winter medium freezing is also a common phenomenon. The transmitter itself is unlikely to fail. (2) Large pressure fluctuations: This situation must first be combined with the process personnel, which is generally caused by improper operation. The parameters involved in the adjustment should mainly check the adjustment system. Three common faults and analysis of flow meter system (1) The minimum flow indication value: generally caused by the following reasons: the detection element is damaged (zero point is too low .; display is problematic; the circuit is short-circuited or broken; the positive pressure chamber is blocked or leaked; the system pressure is low; the parameters involved in the adjustment must also be checked Regulators, control valves and solenoid valves. (2) The maximum flow indication: the main reason is that the negative pressure chamber pressure system is blocked or leaking. The transmitter may not need to be adjusted. (3) The flow fluctuation is large: flow parameters do not participate in the adjustment, generally for process reasons; those involved in the adjustment can check the PID parameters of the regulator; parameters with isolation tank, check whether there are bubbles in the pressure tube, positive and negative pressure induced pressure Is the liquid in the tube the same height? Four common faults and analysis of liquid level instrument system (1) The liquid level suddenly becomes larger: mainly check whether the pressure system of the negative pressure chamber of the transmitter is blocked, leaked, gas-collected, or lack of liquid. The specific method of irrigation is: stop the meter first according to the order of stopping the meter; close the positive and negative pressure root valve; open the positive and negative pressure drain valve to release pressure; open the dual chamber balanced container irrigation plug; open the positive and negative pressure chamber drain plug; At this time, the liquid level indicator is the largest. Close the drain valve; close the positive and negative pressure chamber drain plugs; slowly fill the dual-chamber balance container with the same medium, at this time slightly open the drain plug to exhaust; until it is full, then open the positive pressure chamber plug, change The transmitter indicates that it should return to zero. Then use the transmitter in the order in which it was cast. (2) The liquid level suddenly becomes smaller: mainly check whether the pressure system of the positive pressure chamber is blocked, leaking, gas collection, liquid shortage, whether the balance valve is closed, etc. The specific method to check whether the impulse system is unblocked is to stop the transmitter, open the drain valve, and check the drain situation (except for media that cannot be leaked). (3) The indication of the general control room does not match the on-site liquid level: first determine whether the on-site liquid level gauge is faulty. At this time, the liquid level can be artificially increased or decreased, and the cause of the problem is specifically analyzed according to the on-site and general control instructions The root valve is closed, clogged, and leakage is likely to cause inaccurate on-site instructions). You can restore the normal liquid level by checking the zero point, range, and irrigation. If it is still not normal, you can notify the process personnel to monitor the site and remove the transmitter to adjust the pressure. (4) The liquid level fluctuates frequently: First, check the feeding and discharging conditions in conjunction with the process personnel, and after determining that the process status is normal, it can be stabilized by adjusting the PID parameters. The specific method is: the adjustment valve is put into the manual state, first adjust the set value to be consistent with the measured value, so that the fluctuation of the liquid level is stabilized, and then slowly adjust the opening of the adjustment valve, so that the liquid level slowly rises or falls to meet the process requirements, then adjust The set value is consistent with the measured value. After the parameters are stabilized, the control valve will automatically turn on. In short, once the instrument parameters are found to be abnormal, first combine with the process personnel, start from the process operating system and the field instrument system, comprehensively consider and carefully analyze, especially consider the relationship between the measured parameter and the control valve, and divide the fault With step-by-step judgment, it is easy to find out where the problem is, and solve the problem by taking the right medicine. 2. On-site control instruments are mainly valves The safety effect and use of valves can be divided into the following types 1. Exhaust valve: Eliminate excess gas in the pipeline, improve pipeline efficiency and reduce energy consumption. 2. Diversion valve: Distribute, separate or mix the medium in the pipeline. 3. Safety valve: prevent the medium pressure in the pipeline or device from exceeding the specified value, so as to achieve the purpose of safety protection. 4. Check valve: to prevent the medium from flowing back in the pipeline. 5. Shut-off valve: connect or cut off the medium circulation in the pipeline. 6. Adjustment valve: adjust the pressure, flow and other parameters of the medium. Now mainly introduce the self-supporting control valve and pneumatic control valve. One self-operated pressure regulating valve 1. Working principle of self-operated pressure regulating valve (pressure control behind the valve) The pre-valve pressure P1 of the working medium passes through the spool and the throttle behind the valve seat and becomes the post-valve pressure P2. P2 is input into the lower membrane chamber of the actuator through the control line and acts on the top plate. The generated force is balanced with the reaction force of the spring, which determines the relative position of the valve core and valve seat and controls the pressure behind the valve. When the pressure P2 increases after the valve, the force of P2 acting on the top plate also increases. At this time, the force of the top disc is greater than the reaction force of the spring, so that the valve core is closed to the position of the valve seat until the force of the top disc is balanced with the reaction force of the spring. At this time, the flow area between the valve core and the valve seat is reduced, and the flow resistance becomes larger, thereby reducing P2 to the set value. Similarly, when the pressure P2 decreases after the valve, the action direction is opposite to the above. This is the working principle of the self-operated (post-valve) pressure regulating valve. 2. Working principle of self-operated pressure regulating valve (pressure control before valve) The pre-valve pressure P1 of the working medium passes through the spool and the throttle behind the valve seat and becomes the post-valve pressure P2. At the same time, P1 is input to the upper chamber of the actuator through the control line and acts on the top plate. The generated force is balanced with the reaction force of the spring, which determines the relative position of the valve core and valve seat and controls the front pressure of the valve. When the pressure P1 before the valve increases, the force of P1 on the top plate also increases. At this time, the acting force of the top disc is greater than the reaction force of the spring, so that the spool moves away from the valve seat until the force of the top disc balances with the reaction force of the spring. At this time, the flow area between the valve core and the valve seat is reduced, and the flow resistance is reduced, thereby reducing P1 to the set value. Similarly, when the pressure P1 before the valve decreases, the direction of action is opposite to the above. This is the working principle of the self-acting (front valve) pressure regulating valve. 3. Working principle of self-operated flow regulating valve After the controlled medium is input into the valve, the pressure P1 before the valve is input into the lower membrane chamber through the control line, and the pressure Ps after being throttled by the throttle valve is input into the upper membrane chamber. . The difference between the thrust generated by P1 acting on the diaphragm and the thrust generated by Ps acting on the diaphragm is balanced with the spring reaction force to determine the relative position of the valve core and the valve seat, thereby determining the flow through the valve. When the flow through the valve increases, that is, △ Ps increases. As a result, P1 and Ps act on the lower and upper membrane chambers respectively, causing the valve core to move in the direction of the valve seat, thereby changing the flow area between the valve core and the valve seat. Ps increases, the increased thrust of Ps acting on the diaphragm plus the spring reaction force and the thrust of P1 acting on the diaphragm balances the new position to achieve the purpose of controlling the flow. On the contrary, it is the same. Two pneumatic regulating valve Pneumatic control valve uses compressed air as power source and cylinder as actuator, and uses electrical valve positioner, converter, solenoid valve, limit valve and other accessories to drive the valve to achieve on-off or proportional adjustment. The control signal of the automatic control system is used to complete the adjustment of the pipeline medium: flow, pressure, temperature and liquid level and other process parameters. 1. Classification of pneumatic control valves Pneumatic control valve can be divided into two types: air-open type and air-close type. Air to Open (Air to Open) is when the air pressure on the membrane head increases, the valve moves in the direction of increasing the opening. When the upper limit of the input air pressure is reached, the valve is fully open. Conversely, when the air pressure decreases, the valve moves in the closing direction, and when there is no air input, the valve is fully closed. Therefore, sometimes the gas-open valve is also called Fail to Close FC. The air-to-close type (Air to Close) moves in the opposite direction to the air-to-open type. When the air pressure increases, the valve moves in the closing direction; when the air pressure decreases or does not exist, the valve opens in the opening direction or fully open. Therefore, it is sometimes called Fail to Open FO. The pneumatic opening or closing of the pneumatic control valve is usually realized by the positive and negative effects of the actuator and the different assembly methods of the valve structure. 2. Several common terms The regulating valve consists of an actuator and a valve body part. The regulating valve generally adopts a pneumatic membrane actuator, and its action modes are positive and negative. When the signal pressure increases, the positive-acting actuator is pushed down, and when the signal pressure increases, the negative-acting actuator is pushed up. The valve body parts are divided into two types: positive and reverse. When the valve stem moves down, the flow area of ​​the valve core and valve seat is reduced to the front-mounted type, and the reverse is the reverse-mounted type. The mode of action of the regulating valve is divided into two types: air opening and air closing. The air opening and air closing are composed of the positive and negative effects of the actuator and the positive and negative assembly of the valve body components. The gas opening or gas closing of the regulating valve is considered in many aspects. First, the process safety is the main consideration. After determining the gas closing or gas opening, the role of the actuator is determined, and finally the positive and negative installation of the valve body is determined. The combination method is as described above. Positive-acting actuator means that when the gas pressure on the diaphragm increases, the actuator push rod moves toward the valve body; reverse-acting actuator means that when the gas pressure on the diaphragm increases, the actuator push rod moves away from the valve body; (air to open), air closed (air to close) valves are completely different concepts. Positive-acting actuators and valves installed (reverse mounted) get air-closed (air-open); conversely, reverse-acting actuators and valves installed (reverse mounted) can get air-closed (air-open). The positive and negative effects of the positioner correspond to the gas opening and gas closing of the control valve you choose. In other words, it is set to realize the negative feedback of the entire valve itself. The positive and negative effects of the regulator are used to set the negative feedback of the entire control loop. When the regulator is turned on automatically, the positive and negative effects of the regulator can be specifically reflected. The positive and negative effects of the valve positioner are determined according to the air opening and closing of the regulating valve. The positive and negative effects of the regulator are determined according to the characteristics of each link of the control loop. Ensure that the control loop meets the control requirements. For example, to realize negative feedback control, in the automatic control system, the adjusted parameter often deviates from the set value due to interference, that is, the adjusted parameter has a deviation: For the regulator, according to the unified regulations, if the measured value increases, the regulator output increases, and the regulator amplification factor Kc is negative, the regulator is called a positive-acting regulator; when the measured value increases, the regulator output decreases, Kc is regular. The regulator is called a reaction regulator. 3. Selection of pneumatic regulating valve Before any control system is put into operation, the positive and negative effects of the regulator must be correctly selected so that the direction of the control effect is correct. Otherwise, instead of negative feedback but positive feedback, it will continue to increase the deviation in the closed loop. Will lead the controlled variable to the highest or lowest limit value. In a single loop control system, as long as the product of the amplification factor Kc of the regulator, the amplification factor Kv of the regulator valve, and the amplification factor Ko of the controlled object is positive, negative feedback control can be achieved. The sign of the regulator, regulating valve and object amplification factor are specified as follows: (1) The sign of the regulator's amplification factor; for the regulator, according to the unified regulations, the measured value increases, the output increases, and the regulator's amplification factor Kc is negative, which is called a positive effect. The measured value increases, the output decreases, and Kc is positive, which is called reaction. (2) The positive and negative signs of the amplification factor of the regulating valve; the amplification factor Kv of the regulating valve is defined as that the air-opening valve Kv is positive and the air-closing valve Kv is negative. (3) The sign of the object amplification factor; the object amplification factor Ko is defined as: if the manipulated variable increases, the controlled variable also increases, Ko is positive; the manipulated variable increases, the controlled variable decreases, Ko is negative. It can be seen that the method for determining the positive and negative effects of the regulator of the single-loop control system is as follows: first determine the sign of the object amplification factor Ko, and then determine the positive and negative of the adjustment valve amplification factor Kv according to the selection of the control valve for air opening or air closing No., the final product of Kc, Kv, and Ko should be positive to determine the mode of action of the regulator. In short, the choice of gas opening and gas closing is considered from the perspective of safety in process production. When the air supply is cut off, is the regulating valve safe in the closed or open position? For example, in the combustion control of a heating furnace, a regulating valve is installed on the fuel gas pipe, and the fuel supply is controlled according to the temperature of the furnace chamber or the temperature of the heated material at the outlet of the heating furnace. At this time, it is better to choose a gas-open valve, because once the supply of gas is stopped, the valve is more closed than the valve is fully open. If the air supply is interrupted and the fuel valve is fully opened, excessive heating may be dangerous. Another example is a heat exchange device that is cooled by cooling water. The heat material is cooled by heat exchange with the cooling water in the heat exchanger. The regulating valve is installed on the cooling water pipe. The temperature of the heat exchanged material is used to control the amount of cooling water. When the air source is interrupted, the regulating valve should be safer in the open position, and the gas-closed (ie FO) regulating valve should be used. 4. Maintenance of pneumatic regulating valve Pneumatic control valve is of great significance to ensure the normal operation and safe production of process equipment. Therefore, it is necessary to strengthen the maintenance of the pneumatic control valve. A. Key inspection parts during maintenance a. Check the inner wall of the valve body: in the case of high pressure difference and corrosive medium, the inner wall of the valve body and the diaphragm of the diaphragm valve are often subject to the impact and corrosion of the medium, and it is necessary to focus on checking the pressure and corrosion resistance; b. Check the valve seat: the inner surface of the thread used to fix the valve seat is susceptible to corrosion due to the medium infiltration during work and the valve seat is loosened; c. Check the spool: The spool is one of the movable parts of the regulating valve. The erosion by the medium is more serious. During the maintenance, you should carefully check whether the spool is corroded or worn. Especially in the case of high pressure, the valve The cavitation caused by cavitation is more serious due to core wear. Spools with severe damage should be replaced; check the packing. B. Daily maintenance of pneumatic control valve When the control valve uses graphite-asbestos as filler, lubricating oil should be added to the filler once every three months to ensure that the control valve is flexible and easy to use. If the packing pressure cap is found to be very low, the packing should be supplemented. If the PTFE dry packing is hardened, it should be replaced in time; the operation of the regulating valve should be paid attention to during the circuit inspection, and the valve position indicator and regulator should be checked. Whether the output is consistent; for the control valve with a positioner, it is necessary to check the air source frequently and find the problem and deal with it in a timely manner; the sanitation of the control valve and the integrity of the parts should be kept. Three common faults and causes (1) The failure and cause of the failure of the regulating valve 1. No signal, no gas source. the reason: ①The air source is not turned on; ② The air source is dirty, resulting in blockage of the air source pipe or clogging of the filter and pressure relief valve (particularly pay attention to the freezing of water in the air source in winter) ③ Compressor failure makes the pressure of the air source low; ④ Leakage of the air source main pipe. 2. There is gas source, no signal. the reason: ①The regulator is faulty, ② the air source pipe is leaking; ③ the valve positioner is leaking; ④ the regulator valve diaphragm is damaged. 3. The positioner has no air source. the reason: ①The filter is clogged; ② The pressure reducing valve is faulty; ③ The pipeline is leaking or clogged. 4. The positioner has no air source and no output. the reason: ①The orifice of the positioner is blocked; ②The amplifier is malfunctioning; ③The nozzle is blocked. 5. There is signal, no action. the reason: ①The valve core is off, ② The valve core is stuck; ③ The valve stem is bent; ④ The spring of the actuator is broken. (2) Failures and causes of unstable operation of the regulating valve 1. Air source pressure is unstable. the reason: ①Leakage of the gas main; ② Failure of the pressure reducing valve. 2. Signal pressure is unstable. the reason: ①The time constant (T = RC) of the control system is inappropriate; ② The output of the regulator is unstable. 3. The pressure of the air source is stable, and the signal pressure is also stable, but the action of the regulating valve is still unstable. the reason: ①The ball valve of the amplifier in the positioner is not tightly closed due to dirt wear, and the output oscillation will occur when the gas consumption is particularly increased; ②The nozzle baffle of the amplifier in the positioner is not parallel, and the baffle cannot cover the nozzle; ③ The output pipe and line are leaking; ④ The rigidity of the actuator is too small. (3) Faults and causes of vibration of the control valve 1. The regulating valve vibrates at any opening. the reason: ①The support is unstable; ②There is a vibration source nearby; ③The valve core and bushing are seriously worn. 2. The regulating valve vibrates as it approaches the fully closed position. the reason: ①The control valve is large, and it is often used in small openings; ② The flow direction of the single seat valve is opposite to the closing direction. (4) The failure and cause of the slow operation of the regulating valve 1. The valve stem is only dull when acting in one direction. the reason: ①Leakage of diaphragm in pneumatic thin film actuator; ② Leakage of “O†type seal in actuator. 2. The valve stem is slow in reciprocating motion. the reason: ①There is a blockage in the valve body; ② There is a problem with the packing, which is too tight or needs to be replaced. (5) Faults and causes of the control valve has been closed in place but the leakage is large 1. The leakage is large when the valve is fully closed. the reason: ①The valve core is worn and the internal leakage is serious, ②The valve is not adjusted properly and the valve is not tightly closed. 2. The valve cannot reach the fully closed position. the reason: ①The pressure difference of the medium is too large, the rigidity of the actuator is small, and the valve is not tightly closed; ②There is foreign matter in the valve; ③The bush is sintered. (6) The adjustable flow range becomes smaller The main reason is that the valve core is corroded and becomes smaller, so that the adjustable minimum flow rate becomes larger. Mobile Phone Dust Screen,Mobile Phone Matching Products,Computer Mobile Phone Matching Products SHAOXING HUALI ELECTRONICS CO., LTD. , https://www.cnsxhuali.com
