Field instrument common faults and handling

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Instrument problems, the reasons are more complex, it is difficult to find the crux of the matter, this time to calm down, step by step analysis, first analyze the reasons in that unit, can be roughly divided into three sections: on-site testing, intermediate transmission, terminal display; At the same time should also be considered seasonal reasons, anti-summer temperature is too high, winter frost; when involved in the regulation of abnormal parameters, the regulator first convert to manual mode, observing and analyzing whether the cause of the system to adjust, and then again one check other factors .

No matter which type of instrument fails, we must first understand the instrument's installation location of the production process conditions and conditions, understand the instrument's own structural characteristics and performance; before the maintenance and technology personnel must be combined to analyze and determine the real cause of the instrument failure At the same time, it is also necessary to understand whether the instrument is accompanied by adjustment and interlocking functions. Comprehensive consideration, careful analysis, to maintain the process stability as much as possible during the maintenance process.

First, the scene measuring instrument. Generally divided into four categories: temperature, pressure, flow, liquid level

a ) : Analysis of Common Faults in Temperature Instrument System

( 1 ): sudden increase in temperature: This fault is mostly caused by thermal resistance (thermocouple) disconnection, loose terminal blocks, (compensated) wire breakage, temperature failure, etc. At this time, it is necessary to understand the location and wiring layout of the temperature. The use of multimeter resistance (mV) files to measure several sets of data at different locations can quickly identify the cause.

( 2 ): Temperature suddenly decreases: This fault is caused by a short circuit in the thermocouple or RTD, a short circuit in the wire, and a temperature failure. To start with weak points that are prone to failure, such as junctions and wire bends, check each one. The temperature rises on site, and the total control indicates the same, mostly due to the lower boiling liquid (water) at the measuring element.

( 3 ): The temperature fluctuates greatly or fast oscillates: At this moment should mainly check the technological operation situation (inspects the adjustment system which participates in the adjustment).

b ) : Common faults and analysis of pressure gauge systems

( 1 ): The pressure suddenly becomes smaller, larger, or there is no change in the indicated curve: At this time, the transmitter pressure-inducing system should be checked to check whether the root valve is blocked, whether the pressure-rising tube is unblocked, whether there is abnormal medium inside the pressure-rising tube, or if the pressure is discharged. Plugging and drain valve leakage. Winter 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 craftsman, which is usually caused by improper operation. The parameters involved in the adjustment mainly check the regulation system.

C ) : 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 problem; line short circuit or open circuit; positive pressure chamber is blocked or leaked; system pressure is low; parameters involved in adjustment Check regulators, regulators, and solenoids.

( 2 ): Maximum flow indication: The main reason is that the negative pressure chamber pressure system is blocked or leaked. It may not be necessary for the transmitter to be calibrated.

( 3 ): Large flow fluctuations: flow parameters do not participate in the adjustment, generally for process reasons; participate in the adjustment, you can check the PID parameters of the regulator ; with the parameters of the isolation tank, check the pressure tube for bubbles, positive and negative pressure cited The pressure in the tube is the same as the liquid.

d ) : Common Faults and Analysis of Level Gauge System

( 1 ) : The liquid level suddenly becomes larger: The main pressure transmitter of the transmitter is checked for plugging, leakage, gas collection, and lack of liquid. Specific method of irrigation fluid is: according to the order of the stopwatch stop watch; root positive and negative pressure valve closed; opening the drain valve to vent positive and negative pressure; Double Balanced open irrigation fluid container plugs; open sewage plugs negative pressure chamber; At this point, the liquid level indication is maximum. Close the drain valve; close the positive and negative pressure chamber drain plug; use the same medium slowly poured into the double chamber balance container, then open the drain plug to open the exhaust; until it is full, open the positive pressure chamber plug, change The sender indicates that it should return to zero. Then use the transmitter in accordance with the order of the cast.

( 2 ): The liquid level suddenly becomes smaller: the main pressure check chamber pressure check system is mainly checked for whether it is blocked, leaked, gas-collected, lack of liquid, and whether the balance valve is closed. The specific method for checking whether the lead pressure system is unblocked is to stop the transmitter, open the drain valve, and check the discharge condition ( except the medium that cannot be leaked ) .

( 3 ): The total control room indication does not match the on-site liquid level: First, it is judged whether it is an on-site level gauge failure. At this time, it is possible to artificially increase or decrease the liquid level. According to the site and general control instructions, the specific analysis of the problem causes (site level Total root valve closed, clogging easily cause leakage indication field are not allowed). The fluid level can be restored by checking the zero point, volume, and filling fluid. If it is still not normal, the technicians can be notified to monitor and reset the transmitter.

4 ): Frequent fluctuations in the liquid level: First, check with the craftsman to check the feeding and discharging conditions. After determining that the process conditions are normal, the PID parameters can be adjusted to stabilize. The specific method is: adjust the valve manual state, first adjust the set value consistent with the measured value, so that the liquid level fluctuations stabilized, and then slowly adjust the opening of the regulating valve, so that the liquid level slowly rise or fall to achieve the process requirements, and then adjust The set value is consistent with the measured value. After the parameter is stabilized, the regulating valve is automatically shot.

In short, once the instrument parameters are found to be somewhat anomalous, they must first be combined with the process personnel to start with the process operating system and the field instrumentation system. Comprehensive consideration and careful analysis should be made in particular to consider the correlation between the measured parameters and the control valve. Step by step determination, it is easy to identify the problem, the right medicine to solve the problem.

Second, the scene control instrument is mainly valve

Valve safety functions and uses can be divided into the following categories :

1 : Exhaust valve: Exclude excess gas in the pipeline , improve the efficiency of the pipeline and reduce energy consumption.

2 : Diverter valve: Distributes, separates, or mixes media in the pipe.

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: Prevents reverse flow of medium in the pipe.

5 : Cut-off valve: switch on or cut off the circulation of the medium in the pipeline.

6 : Regulating valve: adjust the pressure, flow and other parameters of the medium.

Now mainly introduce self-regulating valve and pneumatic control valve.

a ) Self-operated pressure regulator

1 , the principle of self-operated pressure regulating valve (post-valve pressure control)

Working medium P1 before the pressure valve after the throttle valve body after the valve seat, the valve pressure becomes 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 balances with the reaction force of the spring , determines the relative position of the valve core and the valve seat, and controls the pressure behind the valve. When the pressure P2 increases, the force acting on the top plate by P2 also increases. At this time, the reaction force of the top plate is greater than the force of the spring, so that the closed position of the valve seat, until the spring reaction force and the top plate of the balance date. At this time, the flow area of ​​the valve plug and the valve seat is reduced, and the flow resistance becomes larger, so that P2 is reduced to a set value. Similarly, when the post- valve pressure P2 decreases, the direction of action is opposite to that described above. This is the principle of the self-operating (after-valve) pressure regulating valve.

2 , the principle of self-operated pressure regulating valve (pre-valve pressure control)

Working medium P1 before the pressure valve after the throttle valve body after the valve seat, the valve pressure becomes P2. Meanwhile P1 via the control line input to the actuator chamber acting on the film on the top plate, the biasing force of the spring reaction force equilibrium produced, determines the relative position of the spool, the seat, the control inlet pressure. When the pre-valve pressure P1 increases, the force that P1 exerts on the top plate also increases. At this time, the spring force is greater than the top plate of the reaction force, the valve body moves away from the valve seat, until the spring reaction force and the top plate of the balance date. At this time, the flow area of ​​the valve plug and the valve seat is reduced, and the flow resistance becomes smaller, so that P1 is reduced to a set value. Same reason, when the inlet pressure P1 decreases, the opposite direction of the action, which is self-operated (before the valve) the pressure regulating valve works.

3 , the principle of self-operated flow control valve

After the medium was controlled inlet valve inlet pressure P1 by the control line input of the next film forming chamber, the pressure Ps is input via the throttle throttling membrane chamber, i.e., the difference between P1 and Ps is â–³ Ps = P1-Ps is called the effective pressure . The thrust generated by P1 acting on the diaphragm and the thrust produced by Ps acting on the diaphragm balance with the spring reaction force to determine the relative position of the valve plug and the valve seat, thereby determining the flow through the valve. When the flow through the valve increases, that is, â–³ Ps increases. As a result, P1 and Ps respectively act on the lower and upper membrane chambers to move the spool toward the valve seat, thereby changing the flow area between the spool and the valve seat. As Ps increases, the thrust of the increased Ps acting on the diaphragm plus the spring reaction force and the thrust of P1 acting on the diaphragm are balanced at the new position to achieve the purpose of controlling the flow. On the contrary, the same reason.

b ) Pneumatic regulating valve

Pneumatic regulating valve uses compressed air as power source, cylinder as actuator, and uses electric valve positioner, converter, solenoid valve, limit valve and other accessories to drive the valve, realizes switch or proportional adjustment, and receives industry. The control signals of the automation control system complete the adjustment of the pipeline media: flow, pressure, temperature and liquid level and other process parameters.

1 : classification of pneumatic control valve.

Pneumatic air control valve-open action points type two gas off. 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 pressure is reached, the valve is in the fully open state. Conversely, when the air pressure decreases, the valve closing direction, when there is no input into the air, the valve is fully closed. Therefore, sometimes open-valve type valves are also known as Fail to Close FC . The air to close ( Air to Close ) direction is exactly the opposite of the air to open type. When the air pressure increases, the valve moves in the closing direction; when the air pressure decreases or not, the valve opens in the opening direction or fully opens. It is sometimes called fail-open type (Fail to Open FO). The air opening or closing of the pneumatic control valve is usually achieved through the positive and negative action of the actuator and the different assembly methods of the valve structure.

2 : Several common terminology

There are two parts regulating valve actuator and valve components. Pneumatic diaphragm actuators are generally used for regulating valves. Their working modes are positive and negative. When the signal pressure increases, the actuator that pushes down is a positive acting actuator. When the signal pressure increases, the actuator that pushes up moves is the reaction actuator. Valve body parts are divided into two types, positive and negative. When the valve stem is moved down, the flow area of ​​the valve plug and the valve seat is reduced to be installed, and vice versa. The function of the regulating valve is divided into two kinds: air opening and air closing. The air opening and closing are formed by the positive and negative action of the actuator and the positive and negative components of the valve body.

The air-opening or air-closing of the regulating valve is a comprehensive consideration in many aspects. The first consideration is the safety of the process. After determining whether the air or air is on, the function of the actuator is determined. 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 rod moves toward the valve body; the counteracting actuator means that when the gas pressure on the diaphragm increases , the actuator rod moves away from the valve body; (air to open), closed gas (air to close) two concepts are entirely different type valve. Positive-acting actuators and positive-fit (reverse-loaded) valves are air-closed (air-opened); conversely, reactive-actuated actuators and reverse-loaded (positive-mounted) valves are gas-closed (air-opened).

The positive and negative effects of the positioner correspond to the air opening and closing of the control valve you purchase. That is to say in order to achieve the negative feedback of the entire valve itself. The positive and negative action of the regulator is used to set the negative feedback of the entire control loop. When the regulator is automatically operated, the positive and negative actions of the regulator can be embodied.

The positive and negative action of the valve positioner is determined according to the air-opening of the control valve. The positive and negative actions of the regulator are determined according to the characteristics of each link of the control circuit, and it is necessary to ensure that the control circuit meets the control requirements. For example, to implement negative feedback control, in the automatic control system, the adjusted parameter often deviates from the set value due to the influence of interference, that is, the adjusted parameter generates a deviation:

For the regulator, in accordance with the unified regulations, if the measured value increases, the regulator output increases and the regulator amplification coefficient Kc is negative, then the regulator is called a positive-acting regulator; the measured value increases, the regulator output decreases, Kc is a positive regulator that is called a reaction regulator.

3 : Selection of pneumatic control valve.

Before any control system is put into operation, the positive and negative actions of the regulator must be properly selected so that the direction of control action is correct. Otherwise, in the closed loop it is not negative feedback but positive feedback, which will continuously increase the deviation, and ultimately must The controlled variable will be directed to the highest or lowest limit.

In a single-loop control system, negative feedback control can be achieved as long as the product of the gain factor Kc of the regulator, the gain factor Kv of the regulating valve , and the gain factor Ko of the controlled object is positive. The sign of the regulator, the valve, and the target amplification factor are specified as follows:

( 1 ) The sign of the controller amplification factor; for the regulator, according to the unified regulations, the measured value increases, the output increases, and the controller amplification factor Kc is negative, which is called positive action. The measured value increases, the output decreases, and Kc is positive, which is called 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 value of the air-opening valve Kv and the negative value of the air-closing valve Kv .

( 3 ) The sign of the object magnification factor; the object's magnification 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 determination method of the positive and negative action 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 values ​​of the control valve amplification factor Kv according to the selection of the control valve as air-open or air-off . No. The final product of Kc , Kv , Ko should be positive, and the mode of action of the regulator can be determined.

In short, the open air off gas is selected according to a security point of departure to consider the production process. When the air supply is shut off, is the regulator valve in a closed position safe or open position safe? For example, a furnace combustion control, regulating valve installed in the fuel pipe track, a heating temperature of the material at the outlet of the furnace is controlled based on the temperature of the fuel supply or furnace. At this time, it is better to use the air opening valve to be safer, because once the air source is stopped, the valve is closed more properly than the valve is fully open. If the air supply is interrupted and the fuel valve is fully opened, it is dangerous to overheat. Another example is a heat exchange device that is 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 cooling water quantity is controlled by the material temperature after heat exchange. When the air supply is interrupted, the regulator valve should be in the open position to be safer, and the air-closed ( FO ) control valve should be selected .

4 : Maintenance of pneumatic control valve : Pneumatic control valve is very important for ensuring the normal operation and safe production of process equipment. Therefore, it is necessary to strengthen the maintenance of pneumatic control valve.

A. Check points during maintenance

Check the internal wall of the valve: In the occasion of high pressure difference and corrosive medium, the diaphragm of the valve body wall and diaphragm valve is often subjected to the impact and corrosion of the medium, and the pressure resistance and corrosion resistance must be checked;

b. Check valve seat: Because the medium penetrates during work, the inner surface of the thread used to fix the valve seat is susceptible to corrosion and causes the valve seat to loosen;

c. Checking the valve plug: The valve plug is one of the movable parts of the regulating valve. It is seriously eroded by the medium. During the inspection, it is necessary to carefully check whether the various parts of the valve plug are corroded and worn. Especially in the case of high pressure difference, the valve The wear of the core due to cavitation is even more serious due to cavitation. Damaged spools should be replaced; check the seal packings.

B. Routine maintenance of pneumatic control valve

When the regulating valve adopts graphite-asbestos as the filling material, a lubricant should be added to the packing for about three months to ensure that the regulating valve is flexible and easy to use. Found filling pressure cap is very low, should be added a filler, such as polytetrafluoroethylene found filler dry hardening, should be replaced; should be noted that in the operation of control valve circuit inspection, adjustment and check valve position indicator output is consistent; regulating valve with a positioner should always check the gas source, identify problems and timely treatment; control valve should be kept hygienic and easy to complete the components.

c ) Common Faults and Causes

(1) Failure of the regulating valve and its causes;

1 . No signal, no air source.

Reason: 1 The air source is not open;

2 Dirty air source, resulting in blockage of the gas source pipe or blockage of the filter and the pressure relief valve (special attention is paid to the fact that the winter air source is icing with water);

3 compressor failure so that the pressure of the air source is low;

4 Air source leakage.

2 . There is a source of gas, no signal.

Reasons: 1 regulator failure, 2 gas source pipe leakage; 3 valve positioner leakage; 4 control valve diaphragm damage.

3 . Locator without air source.

Reasons: 1 filter clogging; 2 pressure relief valve failure; 3 pipeline leakage or clogging.

4 . The positioner has a gas source without output.

Causes: 1 Positioner orifice block; 2 amplifier failure; 3 nozzle blocking.

5 . There is a signal, no action.

Reasons: 1 spool off, 2 spool stuck; 3 stem bent; 4 spring breaking actuator.

(2) Failures and causes of unstable operation of the regulating valve;

1 . The source pressure is not stable.

Causes : 1Gas pipe leakage; 2Fault valve failure.

2 . The signal pressure is not stable.

Causes : 1 The time constant of the control system ( T = RC ) is not appropriate; 2 The regulator output is not stable.

3 . The source pressure is stable and the signal pressure is also stable, but the operation of the regulating valve is still unstable.

Reasons : ( 1) The ball valve of the amplifier in the positioner is loosely shut off due to the abrasion of the dirt. When the air consumption increases, the output oscillation will occur.

2 The nozzle baffle of the amplifier in the positioner is not parallel, and the baffle cannot cover the nozzle;

3 output pipe, line leak; 4 actuator rigidity is too small.

(c) malfunctions and causes of valve vibration;

1 . The regulator valve vibrates at any opening.

Reasons : 1 The support is unstable; 2 There is a vibration source nearby; 3 The valve plug and the bush wear badly.

2 . The regulator valve vibrates near the fully closed position.

Reasons: 1 The control valve is selected large, often used in a small opening; 2 single-seat valve medium flow and closed in the opposite direction.

(4) malfunctions and causes of slow motion of the regulating valve

1 . The stem is only dull when operated in one direction.

Causes: 1 Diaphragm leakage in the pneumatic diaphragm actuator; 2 " O " seal leakage in the actuator .

2 . The valve stem is dull when it is reciprocated.

Reasons: 1 There is sticky material in the valve body; 2 Filler problems, pressure too tight or need to be replaced.

(5) Faults and causes of large leaks in which the regulating valve has been closed;

1 . The amount of leakage is large when the valve is fully closed.

Reasons: 1 The valve core is worn, the internal leakage is serious, and the 2 valve is not adjusted well.

2 . The valve does not reach the fully closed position.

Reasons: 1 medium pressure difference is too large, the actuator stiffness is small, the valve is not strict; 2 foreign objects in the valve; 3 bushing sintering.

(6) The flow adjustable range becomes smaller.

The main reason is that the spool is corroded and the adjustable minimum flow rate is increased.

Field instrument temperature liquid level temperature pressure flow meter common faults and processing Guangzhou Dichuan Instruments Co., Ltd.

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