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Field Instrument Failure Analysis and Solutions

This site2017-04-07 14:42:43

When analyzing the cause of field instrument failure, special attention should be paid to the characteristics of the control object and the control valve, which may be the cause of the field instrument system failure. Therefore, we must comprehensively consider and analyze carefully from the two aspects of the field instrument system and process operating system to check the reason. The field instrument measurement parameters are generally divided into four major parameters: temperature, pressure, flow rate, and liquid level.

1. Before analyzing and checking the failure of the on-site instrument system, it is necessary to ask the site operator to understand the production load and the changes in the parameters of the raw materials, view the record curve of the faulty instrument, and conduct a comprehensive analysis to determine the cause of the instrument failure.

2. Before analyzing field instrument failures, you should thoroughly understand the design scheme and design intent of the instrument system, the structure, characteristics, performance, and parameter requirements of the instrument system, the production process, production process conditions, and conditions of the relevant instrument system.

3. If the instrument recording curve is a dead line (a line that has not changed at all is called a dead line), or the recording curve originally fluctuated, it now suddenly becomes a straight line; the fault is likely to be in the instrument system. Because the current recording instruments are mostly DCS computer systems, the sensitivity is very high, and changes in the parameters of the digital display instrument can be reflected very sensitively. At this time, you can artificially change the process parameters to see the curve changes. If it does not change, it is basically determined that there is a problem with the instrument system; if there is a normal change, it is basically determined that there is no major problem with the instrument system.

4. When changing the process parameters, it is found that the recording curve suddenly changes or jumps to the maximum or minimum, and the fault at this time is often in the instrument system.

5. When the DCS display instrument is found to be abnormal, you can go to the site to check the indication value of the same intuitive instrument. If they are very different, it is likely that the instrument system is malfunctioning.

6. Before the fault occurred, the instrument recording curve always behaved normally. After the fluctuation occurred, the recording curve became irregular or made the system difficult to control, even the manual operation could not be controlled. At this time, the fault may be caused by the process operating system.

How to analyze field instrument faults with different parameters
1. Failure analysis of flow control instrument system:

(1) When the indication value of the flow control instrument system reaches the minimum, first check the on-site detection instrument. If it is normal, the fault is displayed on the instrument. When the indication of the on-site testing instrument is also the smallest, check the opening degree of the regulating valve. If the opening degree of the regulating valve is zero, it is often a failure between the regulating valve and the regulator. When the on-site testing instrument indicates the smallest and the opening degree of the regulating valve is normal, the failure may be caused by insufficient system pressure, blockage of the digital display controller system, pump failure, medium crystallization, and improper operation. If it is a malfunction of the instrument, the reasons may be: the orifice differential pressure flowmeter may be blocked by a positive pressure induced pressure conduit; the positive pressure chamber of the differential pressure transmitter is leaking; the mechanical flowmeter is blocked by gears or blocked by a filter.

(2) When the indication value of the flow control instrument system reaches the maximum value, the detection instrument often indicates the maximum value. At this time, the remote control valve can be manually opened or closed. If the flow can be reduced, it is generally caused by process operation. If the flow value cannot be lowered, it is caused by the instrument system. Check whether the regulating valve of the flow control instrument system is operating; check whether the instrument measurement pressure system is normal; check whether the instrument signal transmission system is normal.

(3) The indicator value of the flow control instrument system fluctuates more frequently, and the control can be changed to manual. If the fluctuation is reduced, it is caused by the instrument or the PID of the instrument control parameter is inappropriate. If the fluctuation is still frequent, it is the process operation Caused by.

2. Failure analysis of liquid level control instrument system
(1) When the indication value of the liquid level control instrument system changes to the maximum or minimum, you can first check the detection instrument to see if it is normal. If the indication is normal, change the liquid level control to manual remote liquid level to see the liquid level change. If the liquid level can be stabilized in a certain range, the fault is in the liquid level control system; if the liquid level cannot be stabilized, it is generally a fault caused by the process system, and the cause must be found from the process aspect.

(2) When the differential pressure level control instrument indication and the on-site direct-reading indication instrument are not compatible, first check whether the on-site direct-reading indication instrument is normal. If the indication is normal, check the negative pressure guide of the differential pressure level instrument. Check whether there is leakage in the pressure sealing fluid; if there is leakage, refill the sealing fluid and adjust the zero point; if there is no leakage, the negative migration of the meter may be wrong. Readjust the migration to make the meter indication normal.

(3) When the indication value of the liquid level control instrument system changes and fluctuates frequently, the capacity of the liquid level control object must be analyzed first to analyze the cause of the failure. The large capacity is generally caused by the failure of the instrument. For small capacity, we must first analyze whether the process operation has changed. If there is a change, it is likely that the process caused frequent fluctuations. If there is no change, it may be caused by a malfunction of the instrument.

3. Failure analysis steps of temperature control instrument system:

When analyzing the temperature control instrument system failure, we must first pay attention to two points: the system instrument mostly adopts electric instrument measurement, indication, and control; the system instrument measurement often has a large lag.

(1) The indication value of the temperature instrument system suddenly changes to the maximum or minimum value, which is generally a malfunction of the instrument system. Because the temperature measuring system has a large measurement lag, no sudden change will occur. The faults at this time are mostly caused by broken thermocouples, thermal resistors, compensation wires or transmitter amplifiers.

(2) The temperature control instrument system indicates a rapid oscillation phenomenon, which is mostly caused by improper adjustment of the control parameter PID.

(3) The temperature control instrumentation system indicates that large and slow fluctuations are likely to be caused by changes in process operation. If the process operation does not change at that time, it is likely that the instrumentation control system itself is malfunctioning.
(4) The failure analysis steps of the temperature control system itself: check whether the input signal of the control valve changes, the input signal does not change, the control valve operates, the control valve diaphragm leaks; check whether the input signal of the control valve positioner changes, input signal Does not change, the output signal changes, and the positioner is faulty; check the input signal of the positioner is changed, and then check whether the output of the regulator is changed. If the input of the regulator is not changed, the output is changed. At this time, the regulator itself is faulty.
4. Failure analysis of pressure control instrument system:
(1) When a rapid oscillation fluctuation is indicated by the pressure control system instrument, first check whether there is a change in the process operation. This change is mostly caused by the poor operation of the process operation and regulator PID parameters.
(2) The dead line of the pressure control system indicator indicates that the pressure indication remains unchanged when the process operation changes. Generally, the fault occurs in the pressure measurement system. First, check whether the measurement impulse conduit system is blocked, and check whether the pressure changes. Whether the output system of the transmitter changes or not, the fault lies in the controller's measurement and indication system.
The above is just the on-site failure analysis of the four common parameters of the on-site control instrument. There are also some complicated control loops on the actual site, such as cascade control, split-range control, program control, interlock control, and so on. The analysis of these failures is more complicated, and it needs to be analyzed in detail.
Solution: exclusion
1. When the system is under control, the judgment should also be the same. It is easy to find the problem by determining the failure after each part is independent.
2. The meter is only displayed on site. If you suspect that the displayed data is abnormal, you can estimate the data according to the process environment. If it is too far away, just check and adjust the meter.
3. On-site control instruments, if the displayed data is suspected to be abnormal, you can automatically disconnect, observe, and fix the data of process conditions to determine whether the collected data is normal. If the control result is unstable, first determine whether the data collection is normal. The method is the same as above. Because the data collection is abnormal, the control result is definitely not good.
The above problems are judged. When checking the soft manual output, whether the actuator is operating normally. Finally, adjust the PID parameters.
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