At present, with the continuous improvement of the automation level of petrochemical, steel, paper, food, and pharmaceutical enterprises, higher requirements are placed on the technical level of field instrument maintenance personnel. In order to shorten the processing time of the instrument and ensure the safe production and improve the economic benefits, this paper publishes a little on-site maintenance experience of the instrument for the reference of the instrument maintenance personnel.
First, the basic analysis steps of the field instrument system failure
The field instrument measurement parameters are generally divided into four parameters: temperature, pressure, flow, and liquid level.
According to the different measurement parameters, different field instrument faults are analyzed.
1. First of all, before analyzing the field instrument failure, it is necessary to thoroughly understand the production process, production process conditions and conditions of the relevant instrument system, understand the design scheme, design intent of the instrument system, the structure, characteristics, performance and parameter requirements of the instrument system.
2. Before analyzing and inspecting the faults of the on-site instrument system, it is necessary to understand the changes in the load and raw material parameters of the production operators, check the record curve of the faulty instruments, and conduct a comprehensive analysis to determine the cause of the instrument failure.
3. If the meter records a dead line (a line that has no change at all), or if the recorded curve is originally fluctuating, it suddenly becomes a straight line; the fault is likely to be in the instrumentation system. Because most of the current recording instruments are DCS computer systems, the sensitivity is very high, and the change of parameters can be very sensitive. At this point, you can artificially change the process parameters to see the curve changes. If it does not change, the basic conclusion is that the instrument system has a problem; if there is a normal change, it is basically determined that the instrument system has no major problems.
4. When the process parameters are changed, it is found that the recording curve is abrupt or jumps to zui or ziu is small, and the fault at this time is also often in the instrument system.
5. Before the fault occurs, the meter recording curve has been normal. After the fluctuation, the recording curve becomes irregular or the system is difficult to control, and even the manual operation cannot be controlled. At this time, the fault may be caused by the process operating system.
6. When the DCS display meter is not normal, you can go to the site to check the indication value of the same intuitive meter. If they are very different, it is likely that the instrument system has failed.
In short, when analyzing the cause of the field instrument failure, pay special attention to the changes in the characteristics of the controlled object and the control valve, which may be the cause of the failure of the field instrument system. Therefore, we must comprehensively consider and carefully analyze the two aspects of the field instrument system and the process operating system to check the reasons.
Second, four measurement parameters instrument control system failure analysis steps
1
Temperature control instrument system failure analysis steps
When analyzing the temperature control instrument system failure, we must first pay attention to two points: the system instrument uses electric instrument measurement, indication and control; the measurement of the system instrument often lags behind.
(1) The indication value of the temperature meter system suddenly changes to zui or small, usually the instrument system failure. Because the temperature meter system measures a large lag, no sudden changes will occur. The cause of the fault at this time is mostly caused by thermocouple, thermal resistance, compensation wire breakage or transmitter amplifier failure.
(2) The temperature control instrument system indicates that there is a rapid oscillation phenomenon, which is mostly caused by improper adjustment of the control parameter PID.
(3) The temperature control instrument system indicates that there is a large and slow fluctuation, which is probably caused by a change in process operation. If the process operation does not change at that time, it is likely to be a malfunction of the instrument control system itself.
(4) Failure analysis step of the temperature control system itself: check whether the input signal of the regulating valve changes, the input signal does not change, the regulating valve acts, the diaphragm of the regulating valve head is leaked; check whether the input signal of the regulating valve positioner changes, the input signal No change, the output signal changes, the positioner is faulty; check the positioner input signal changes, and then check whether the regulator output changes, if the regulator input does not change, the output changes, this is the regulator itself failure.
2
Pressure control instrument system failure analysis steps
(1) When the pressure control system indicates that there is rapid oscillation fluctuation, first check whether the process operation has changed. This change is mostly caused by poor process setting and PID parameter setting of the regulator.
(2) The pressure control system indicates that the dead line is present, the process operation changes the pressure indication or does not change, the general fault occurs in the pressure measurement system, first check whether the measurement of the pressure guiding conduit system is blocked, not blocked, check the pressure change The transmitter output system has no change, there is a change, and the fault is in the controller measurement indication system.
3
Flow control instrumentation system failure analysis steps
(1) The flow control instrument system indicates that the value reaches zoi hours. First, check the on-site inspection instrument. If it is normal, the fault is displayed on the instrument. When the on-site inspection instrument indication is also small, check the opening of the regulating valve. If the opening degree of the regulating valve is zero, it is often a fault between the regulating valve and the regulator. When the on-site inspection instrument indicates that the Zui is small and the opening of the regulating valve is normal, the cause of the failure may be caused by insufficient system pressure, blockage of the system pipeline, volume of the pump, crystallization of the medium, improper operation, and the like. If it is a malfunction of the instrument, the reasons are: the orifice differential pressure flowmeter may be a positive pressure pressure conduit plug; the differential pressure transmitter positive pressure chamber leak; the mechanical flowmeter is a gear stuck or a filter plug.
(2) When the flow control instrument system indicates that the value reaches zui, the test instrument will often indicate that the zui is large. At this time, the manual remote control valve can be opened or closed. If the flow rate can be lowered, it is generally caused by the process operation. If the flow value does not fall, it is caused by the instrument system. Check whether the regulating valve of the flow control instrument system is operating; check whether the measuring system is normal; check whether the signal transmission system of the instrument is normal.
(3) The flow control instrument system indicates that the value fluctuates frequently, and the control can be changed to manual. If the fluctuation is reduced, the instrument is the cause or the instrument control parameter PID is not suitable. If the fluctuation is still frequent, it is the process operation. Caused by.
4
Liquid level control instrument system failure analysis steps
(1) If the indicator value of the liquid level control instrument system changes to ziu or ziu, you can check whether the instrument is normal. If the indication is normal, change the liquid level control to the manual remote control level to see the liquid level change. If the liquid level can be stabilized within a certain range, the fault is in the liquid level control system; if the liquid level is stable, it is generally the fault caused by the process system, and the cause should be found from the process.
(2) When the differential pressure type liquid level control instrument indication and the on-site direct reading type indicating instrument are not in control, first check whether the on-site direct reading type indicating instrument is normal. If the indication is normal, check the negative pressure guiding of the differential pressure type liquid level meter. Whether there is leakage in the pressure tube sealing liquid; if there is leakage, refilling liquid, zero adjustment point; no leakage, it may be that the negative migration amount of the meter is wrong, and the migration amount is readjusted to make the meter indication normal.
(3) When the liquid level control instrument system indicates that the value changes frequently, the capacity of the liquid level control object should be analyzed first to analyze the cause of the fault. The large capacity is generally caused by the instrument fault. When the capacity is small, it is necessary to analyze whether the process operation has changed. If there is a change, it is likely that the process causes frequent fluctuations. If there is no change, it may be caused by instrument failure.
to sum up:
The above is only the on-site fault analysis of the four parameters of the field to control the instrument separately. There are some complicated control loops in the actual site, such as cascade control, split control, program control, interlock control and so on. The analysis of these faults is more complicated and needs to be analyzed.
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 accompanied by regulation and interlocking functions. Comprehensive consideration and careful analysis should keep the process as stable as possible during the maintenance process.
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