There is a problem with the instrument. The reason is more complicated. It is difficult to find the crux. At this time, it is necessary to calm down and analyze the segmentation. First, the reason for the analysis is in that unit. It can be roughly divided into three sections: on-site detection, intermediate transmission, and terminal display. At the same time, we must also consider the seasonal reasons, summer temperature is too high, winter antifreeze; when the parameters involved in the adjustment are abnormal, first adjust the regulator to the manual state, observe whether the adjustment system is the cause, and then check other factors one by one.
No matter what kind of instrument failure, we must first understand the production process conditions and conditions of the installation location of the instrument, understand the structural characteristics and performance of the instrument itself; combine with the process personnel before maintenance, analyze and judge the real cause of the instrument failure Also know if the meter is equipped with regulation and interlocking functions. Comprehensive consideration and careful analysis should keep the process as stable as possible during the maintenance process.
First, the field measurement instrument. Generally divided into four categories of temperature, pressure, flow, and liquid level
a) , temperature instrument system common fault analysis
(1) , sudden increase in temperature: This fault is mostly caused by thermal resistance ( thermocouple ) open circuit, loose terminal , (compensation) wire breakage, temperature failure, etc., then you need to know the location and wiring layout of the temperature. Using the multimeter's resistance (millivolt) file to measure several sets of data at different locations can quickly identify the cause.
(2) , sudden temperature decrease: This fault is mostly caused by short circuit of thermocouple or thermal resistance, short circuit of conductor and temperature failure. It is necessary to start from the weak points that are easy to malfunction, such as the wiring port and the corner of the wire, and check them one by one. The temperature of the site rises, and the total control indication does not change, mostly due to the liquid (water) with a lower boiling point at the measuring component.
(3), there was a substantial temperature shock or rapid fluctuations: should case main check process operating conditions (involved in the regulation of the inspection control system).
b) , common faults and analysis of pressure instrumentation systems
(1) , the pressure suddenly becomes smaller, becomes larger or the indication curve does not change: at this time, the transmitter pressure system should be checked to check whether the root valve is blocked, whether the pressure guiding tube is unblocked, whether there is abnormal medium inside the pressure guiding tube, and sewage discharge. Wire plugging Whether the drain valve leaks, etc. Winter media freezing is also a common phenomenon. The transmitter itself is unlikely to malfunction.
(2), pressure fluctuations: this situation must first be combined with the craftsman, usually caused by improper operation. The parameters involved in the adjustment should mainly check the adjustment system.
c), common faults and analysis of flow meter system
(1), the minimum flow indication value: generally caused by the following reasons , the detection component is damaged (zero point is too low;; there is a problem; the line is short circuited or open circuit; positive pressure chamber block or leak; system pressure is low; parameters involved in adjustment Check regulators, regulators and solenoid valves .
(2) , the maximum flow indication: the main reason is that the negative pressure chamber pressure system is blocked or leaking. It may not be possible for the transmitter to be calibrated.
(3) , flow fluctuations: flow parameters do not participate in the adjustment, generally for the process reasons; participate in the adjustment, you can check the PID parameters of the regulator; with the parameters of the isolation tank, check whether there are bubbles in the pressure tube, positive and negative pressure Whether the liquid in the pressure tube is the same.
4) , common faults and analysis of liquid level instrumentation system
(1) , the liquid level suddenly becomes larger: mainly check whether the pressure-reducing system of the negative pressure chamber of the transmitter is blocked, leaked, gas-collected, and liquid-deficient. The specific method of filling is: first stop the table according to the stop table; close the positive and negative pressure root valve; open the positive and negative pressure drain valve to release pressure; open the double chamber balance container filling plug; open the positive and negative pressure chamber to discharge the silk plug; The liquid level indication is maximum at this time. Close the drain valve; close the drain plug of the positive and negative pressure chamber; slowly fill the double chamber balance container with the same medium, and then open the drain wire to block the exhaust; until it is full, open the positive pressure chamber plug and change The transmitter indication should return to zero. The transmitter is then applied in the order in which it was posted.
(2), the level becomes abruptly smaller: mainly the plenum impulse system is blocked, the drain, the plenum, the lack of fluid, whether balancing valve shut and the like. The specific method for checking whether the pressure guiding system is unblocked is to stop the transmitter, open the drain valve, and check the sewage discharge (except for the medium that cannot be leaked).
(3), the master control room and the field indicating the level does not match: firstly judged whether or not the fault site level gauge, this time can artificially increase or decrease the liquid level, and master control according to the site where the specific indication of the problem analysis (field level The closing of the root valve, blockage, and external leakage may cause on-site indications to be inaccurate). The liquid level can be restored by checking the zero point, the range, and the filling. If it is still not normal, you can notify the technician to perform on-site monitoring and re-installation of the transmitter to adjust the pressure.
4), the frequent fluctuation level: first and personnel combined with test feed process, where the material is determined after the normal process conditions, can be stabilized by adjusting PID parameters. The specific method is: adjust the valve to the manual state, first adjust the set value and the measured value to make the liquid level fluctuation smooth, and then slowly adjust the opening of the regulating valve, so that the liquid level slowly rises or falls, meets the process requirements, and then adjusts The set value is consistent with the measured value, and the regulating valve is automatically applied after the parameter is stabilized.
In short, once the instrument parameters are found to be abnormal, first of all, combined with the process personnel, starting from the two aspects of the process operating system and the field instrument system, comprehensive consideration, careful analysis, especially considering the relationship between the measured parameters and the control valve , the fault will be divided Step segmentation judgment, it is easy to find out the problem, solve the problem with the right medicine.
Second, the field control instrument is mainly valve
Valve safety and use can be divided into the following types:
1, the exhaust valve: negative excess gas pipeline, improve the efficiency and reduce energy waste pipe.
2 , diverter valve: distribute, separate or mix the medium in the pipeline.
3 , safety valve : to prevent the pressure of the medium in the pipeline or device exceeds the specified value, so as to achieve the purpose of safety protection.
4, the check valve: the media back to prevent the pipeline.
5 , cut off valve: switch on or cut off the flow of media in the pipeline.
6 , regulating valve: adjust the pressure, flow and other parameters of the medium.
Now we mainly introduce the self-standing regulating valve and the pneumatic regulating valve .
a), self-operated pressure regulating valve
1. Working principle of self-operated pressure regulating valve (post-valve pressure control)
The pre-valve pressure P1 of the working medium passes through the throttle after the valve core and the valve seat, and becomes the post-valve pressure P2. P2 is input into the lower 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 the valve seat, and controls the pressure behind the valve. When the post-valve pressure P2 increases, the force acting on the top plate of P2 also increases. At this time, the force of the top plate 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 plate is balanced with the reaction force of the spring. At this time, the flow area of ​​the valve body and the valve seat is reduced, and the flow resistance is increased, so that P2 is lowered to a set value. Similarly, when the post-valve pressure P2 is lowered, the action direction is opposite to the above, which is the working principle of the self-operated (post-valve) pressure regulating valve.
2, the working principle of self-operated pressure regulating valve (pre-valve pressure control)
The pre-valve pressure P1 of the working medium passes through the throttle after the valve core and the valve seat, and becomes the post-valve pressure P2. At the same time, P1 is input into the upper chamber of the actuator through the control pipeline 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 the valve seat, and controls the pressure before the valve. As the pre-valve pressure P1 increases, the force acting on the top plate by P1 also increases. At this time, the force of the top plate is greater than the reaction force of the spring, so that the valve core moves away from the valve seat until the force of the top plate is balanced with the reaction force of the spring. At this time, the flow area of ​​the valve body and the valve seat is reduced, and the flow resistance is reduced, so that P1 is lowered to a set value. Similarly, when the pre-valve pressure P1 is lowered, the action direction is opposite to the above, which is the working principle of the self-operated (pre-valve) pressure regulating valve.
3. Working principle of self-operated flow regulating valve
After the controlled medium is input to the valve, the pre-valve pressure P1 is input to the lower diaphragm chamber through the control line, and the pressure Ps after the throttling of the throttle valve is input to the upper membrane chamber. The difference between P1 and Ps is ΔPs=P1-Ps, which is called effective pressure. . The thrust generated by P1 acting on the diaphragm and the thrust difference generated by Ps acting on the diaphragm are balanced with the spring reaction force to determine the relative position of the spool and the valve seat, thereby determining the flow rate through the valve. When the flow rate through the valve increases, that is, ΔPs increases, as a result, P1 and Ps act on the lower and upper membrane chambers respectively, so that the valve core moves toward the valve seat, thereby changing the flow area between the valve core and the valve seat, so that The Ps is increased, and the increased thrust of the Ps acting on the diaphragm plus the spring reaction force and the thrust of the P1 acting on the diaphragm are balanced at a new position to achieve the purpose of controlling the flow. On the contrary, the same reason.
b) Pneumatic control valve
Pneumatic control valve uses compressed air as the power source, cylinder as the actuator, and drives the valve by means of electric valve positioner , converter, solenoid valve, limit valve and other accessories to realize switching or proportional adjustment, receiving industry The control signals of the automation control system are used to complete various process parameters such as flow, pressure, temperature and liquid level for adjusting the pipeline medium.
1, pneumatic control valve classification.
The pneumatic regulating valve operates in two types: air opening type and air closing type. Air to Open is when the air pressure on the diaphragm head increases, the valve moves in the direction of increasing opening. When the upper limit of the input air pressure is reached, the valve is fully open. Conversely, when the air pressure is reduced, the valve moves in the closing direction, and when there is no input air, the valve is fully closed. Therefore, sometimes the gas-open valve is also called Fail to Close FC. The air to close action direction is exactly the opposite of the air open type. When the air pressure increases, the valve acts in the closing direction; when the air pressure is reduced or not, the valve is in the opening direction or fully open. Therefore, it is sometimes called Fail to Open FO. The pneumatic opening or closing of the pneumatic regulating valve is usually achieved by the positive and negative action of the actuator and the different assembly of the valve state structure.
2 , several common technical terms
The regulating valve has two parts: an actuator and a valve body part. The regulating valve generally adopts a pneumatic film actuator, and its action mode is positive or negative. When the signal pressure increases, the push-down is the positive acting actuator. When the signal pressure increases, the push-up moves up to the reaction actuator. The valve body parts are divided into two types: positive and reverse. When the valve stem is moved down, the flow area of ​​the valve core and the valve seat is reduced to be a positive-loading type, and vice versa. The mode of action of the regulating valve is divided into two types: gas opening and gas closing. The gas opening and gas closing are combined by the positive and negative action of the actuator and the positive and negative assembly of the valve body parts.
Whether the gas opening or the gas shut-off of the regulating valve is comprehensively considered in the first place is the process safety consideration. After determining the gas shut-off or the gas opening, the effect of the actuator is determined, and finally the positive and negative assembly of the valve body is determined. The way is as described above.
The positive acting actuator means that when the gas pressure on the diaphragm increases, the actuator push rod moves toward the valve body; the reaction actuator refers to the movement of the actuator stem away from the valve body when the gas pressure on the diaphragm increases; and the air opening (air to open), air to close type valves are completely different concepts. The positive acting actuator and the positively mounted (reversely mounted) valve are pneumatically closed (air open); conversely, the reaction actuator and the reverse mounted (positively mounted) valve can be pneumatically closed (air open).
The positive and negative action of the positioner corresponds to the air opening and air closing of the control valve you have purchased. That is to say, in order to achieve 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 automatically activated, the positive and negative effects of the regulator can be embodied.
The positive and negative action of the valve positioner is determined according to the gas 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 circuit, and the control circuit must be guaranteed to meet the control requirements.
For the regulator, according to the uniform regulation, 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; the measured value increases, the regulator output decreases, When Kc is regular, the regulator is called a reaction regulator.
3 , the choice of pneumatic control 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 action is correct. Otherwise, instead of negative feedback but positive feedback in the closed loop, it will continuously increase the deviation and eventually The controlled variable will be directed to the high or lowest limit.
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 regulating valve, and the amplification factor Ko of the controlled object is positive, negative feedback control can be realized. Regulators, regulating valves and object amplification factors are specified as follows:
(1) The sign of the amplification factor of the regulator; for the regulator, according to the uniform regulation, the measured value increases, the output increases, and the regulator amplification factor Kc is negative, which is called positive action. The measured value increases, the output decreases, and Kc is positive, which is called a reaction.
(2) The sign of the amplification factor of the regulating valve; the amplification factor Kv of the regulating valve is defined as the positive opening valve Kv and the closing valve Kv being negative.
(3) The sign of the magnification factor of the object; the magnification factor Ko of the object is defined as: if the manipulated variable increases, the controlled variable also increases, Ko is positive; the manipulated variable increases, the controlled variable decreases, and Ko is negative. It can be seen that the method for determining the positive and negative effects of the single-loop control system regulator is as follows: firstly determine the sign of the object magnification coefficient Ko, and then determine the positive and negative of the amplification factor Kv of the regulating valve according to the selection of the regulating valve for air opening or gas closing. No. Finally, the 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 is based on the safety perspective of process production. When the air supply is cut off, is the control valve safe in the closed position or in the open position? For example, in the combustion control of a furnace , a regulating valve is installed on the fuel gas pipe to control the supply of fuel according to the temperature of the furnace or the temperature of the heated material at the outlet of the furnace. At this time, it is more safe to use the gas-opening valve, because once the gas source is stopped, the valve is closed more than the valve is fully open. If the air supply is interrupted and the fuel valve is fully open, there is a danger of excessive heating. Another example is a heat exchange device cooled by cooling water. The hot material is cooled by heat exchange with the cooling water in the heat exchanger. The regulating valve is installed on the cooling water pipe , and the temperature of the heat exchanged material is used to control the amount of cooling water. When the air supply is interrupted, it is safer to make the regulating valve in the open position. The gas-closed (ie FO) regulating valve should be used.
4 , the maintenance of pneumatic control valve: pneumatic control valve is very important to ensure the normal operation and safe production of the process equipment. Therefore, it is necessary to strengthen the maintenance of the pneumatic control valve.
A, the key inspection site during maintenance
a. Check the inner wall of the valve: In the case of high pressure difference and corrosive medium, the diaphragm of the inner wall of the valve and the diaphragm valve are often subjected to the impact and corrosion of the medium, and the pressure and corrosion resistance must be checked.
b. Check the valve seat: the inner surface of the thread for fixing the valve seat is susceptible to corrosion and the valve seat is slack due to the penetration of the medium during operation;
c. Check the valve core: the valve core is one of the movable parts of the regulating valve, which is seriously damaged by the medium. During the inspection, it is necessary to carefully check whether the various parts of the valve core are corroded and worn, especially in the case of high pressure difference, the valve The cavitation caused by cavitation of the core is more serious. Damaged spools should be replaced; check seal packing.
B, routine maintenance of pneumatic control valve
When the regulating valve is made of graphite-asbestos, the lubricating oil should be added to the packing for about three months to ensure the flexibility of the regulating valve. If it is found that the packing pressure cap is pressed very low, the filler should be replenished. If the polytetrafluoroethylene dry packing is found to be hardened, it should be replaced in time; the operation of the regulating valve should be paid attention during the inspection, and the valve position indicator and regulator should be checked. Whether the output is consistent; check the gas source for the regulating valve with the positioner, and find the problem in time; always keep the health of the regulating valve and the complete and easy use of all components.
c), common faults and causes
(1) The failure and cause of the regulating valve not working
1. No signal, no air source.
Reason: 1 gas source is not open;
2 The gas source is dirty, causing the gas source pipe to clog or the filter and the pressure reducing valve to be clogged (special attention to the winter air source with water freezing);
3 compressor failure makes the air source pressure low;
4 gas source manifold leaks.
2. There is a gas source and no signal.
Reason: 1 regulator failure, 2 gas source tube leakage; 3 valve positioner leakage; 4 control valve diaphragm damage.
3. The positioner has no air supply.
Reason: 1 filter is blocked; 2 pressure reducing valve is faulty; 3 pipe is leaking or blocked.
4. The positioner has a gas source and no output.
Reason: 1 The orifice of the positioner is blocked; 2 The amplifier is out of order; 3 The nozzle is blocked.
5. There is signal, no action.
Reason: 1 spool lost, 2 spool stuck; 3 stem bent; 4 actuator spring break.
(2) Failure and cause of unstable operation of the regulating valve
1. The air source pressure is unstable.
Reason: 1 gas source manifold leakage; 2 pressure reducing valve failure.
2. The signal pressure is unstable.
Reason: 1 The time constant of the control system (T=RC) is not appropriate; 2 The regulator output is unstable.
3. The air supply pressure is stable and the signal pressure is stable, but the action of the regulating valve is still unstable.
Reason: 1 The ball valve of the amplifier in the positioner is not tightly damaged by the dirt, and the output oscillation occurs when the gas consumption is particularly increased;
2 The nozzle flapper of the amplifier in the positioner is not parallel, and the flap cover cannot cover the nozzle;
3 output tube, line leakage; 4 actuator rigidity is too small.
(3) Faults and causes of regulating valve vibration
1. The regulator valve vibrates at any opening.
Reason: 1 support is unstable; there is vibration source near 2; 3 valve core and bushing are seriously worn.
2. The regulator valve vibrates near the fully closed position
Reason: 1 The regulating valve is large, and it is often used at a small opening; 2 The flow direction of the single seat valve is opposite to the closing direction.
(4) Faults and causes of slow operation of the regulating valve
1. The valve stem is slow to operate only in one direction.
Reason: 1 diaphragm leakage in the pneumatic diaphragm actuator; 2 "O" seal leakage in the actuator.
2. The valve stem is dull when reciprocating.
Reason: 1 There is stickiness in the valve body; 2 There is a problem with the packing, the pressure is too tight or needs to be replaced.
(5) Faults and causes of the control valve being closed but the leakage is large
1. The leakage is large when the valve is fully closed.
Reason: 1 The valve core is worn, the internal leakage is serious, and the 2 valves are not adjusted well.
2. The valve does not reach the fully closed position.
Reason: 1 The pressure difference of the medium is too large, the rigidity of the actuator is small, the valve is not tight; 2 the valve has foreign matter; 3 the bush is sintered.
(6) The flow adjustable 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.
Car Bumper Stickers,High Quality Headlight,Car Bumper Repair,Car Bumper Prototype Cnc Machining Services,Rapid Prototyping Services,Medical Device Prototype,Medical Device Rapid Prototyping
Guangdong Shunde Teamwork Model Technology Co., Ltd , https://www.gdtwmx.com