Field instrument fault analysis and solution

When analyzing the causes of field instrument failures, special attention must be paid to changes in the characteristics of the controlled object under test and the control valve, which may be the cause of field instrument system failures. Therefore, we must consider and carefully analyze the field instrument system and the process operating system in two aspects, and check the causes. Field instrument measurement parameters are generally divided into four parameters: temperature, pressure, flow, and liquid level.

1. Before the analysis and inspection of the failure of the field instrument system, the operating personnel at the site should be informed of the changes in the parameters of the production load and raw materials, and the record curves of the failure meters should be viewed and comprehensive analysis should be conducted to determine the cause of the instrument failure.

2. Before analyzing the failure of the field instrument, it is necessary to thoroughly understand the design scheme and design intent of the instrument system, the structure, characteristics, performance and parameter requirements of the instrument system, and the production process, production process conditions and conditions of the related instrument system.

3. If the meter records a dead line (a line with no changes in the line is called a dead line), or the record curve originally fluctuates, it 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 the changes of the parameters of the digital display instrument can be reflected very sensitively. At this point 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 changing the process parameters, it is found that the recorded curve has abruptly changed or jumped to the maximum or minimum. At this time, the fault is often in the instrument system.

5. When it is found that the DCS display meter is not normal, you can go to the site to check the indicator value of the same intuitive meter. If they are very different, it is likely that the meter system has failed.

6. Before the failure occurs, the instrument record curve has been performing normally. After the fluctuation, the recording curve becomes irregular or the system is difficult to control. Even the manual operation cannot be controlled. At this time, the failure may be caused by the process operating system.

How to analyze the parameters of different parameters in the field
1. Flow control instrument system failure analysis:

(1) When the indicated value of the flow control instrument system reaches the minimum, the on-site inspection instrument is first checked, and if it is normal, the failure is displayed on the instrument. When the on-site detection instrument indicator 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 fault between the regulating valve and the regulator. When the on-site detection instrument indicates the minimum and the opening of the regulating valve is normal, the cause of the failure is likely to be caused by insufficient system pressure, digital display controller system pipeline blockage, pump failure, medium crystallization, and improper operation. If the instrument is faulty, the reasons are: the orifice differential pressure flowmeter may be a positive pressure induction conduit plug; the differential pressure transmitter positive pressure chamber leakage; the mechanical flowmeter is a gear stuck or a filter plug.

(2) When the indicated value of the flow control instrument system reaches the maximum, the detection instrument will often indicate the maximum value. At this point you can manually control the valve to open large or small, if the flow can be reduced, it is generally caused by the process operation. If the flow rate does not fall, it is the cause of the instrument system. Check whether the regulating valve of the flow control instrumentation system is active. Check whether the instrument measures the pressure eliciting system. Check whether the instrument signal transmission system is normal.

(3) The flow control instrumentation system indicates that the value fluctuates more frequently. The control can be changed to manual. If the fluctuation is reduced, it is the reason of the instrument or the instrument control parameter PID is not suitable. If the fluctuation is still frequent, it is the process operation. Caused by.

2 level control instrument system failure analysis:

(1) When the indicating 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 the manual remote liquid level to see the change of the liquid level. 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 looked up from the process.

(2) When the differential pressure type liquid level control instrument indicator and on-site direct-reading indicator instrument indicator are not on, first check whether the direct-reading indicator instrument is normal. If the indicator is normal, check the negative pressure guide of the differential pressure level instrument. If there is leakage, refill the liquid and adjust the zero point; if there is no leakage, the negative migration amount of the instrument may be wrong. Adjust the migration amount to make the indicator normal.

(3) When the indicating value of the liquid level control instrumentation system changes frequently and fluctuates frequently, it is necessary to first analyze the capacity of the liquid level control object to analyze the cause of the failure. The large capacity is generally caused by the instrument failure. The first thing to do is to analyze whether there is any change in the process operation. If there is any change, it is likely that the process will cause frequent fluctuations. If no change may be caused by instrument failure.

3 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 electrical instrumentation to measure, indicate, and control; the measurement of the system instrument often lags behind.

(1) The indicator value of the temperature meter system suddenly changes to the maximum or minimum, which is generally a failure of the instrument system. Because the temperature meter system measures large lags, sudden changes do not occur. Most of the faults at this time are due to thermocouple, thermal resistance, loss of compensation wire, or failure of the transmitter amplifier.

(2) Temperature control instrumentation system indicates rapid oscillation, which is caused by improper adjustment of control parameter PID.

(3) The temperature control instrumentation system indicates the occurrence of large and slow fluctuations, which may be due to changes in process operations. If the process operation does not change at that time, it is likely that the instrument control system itself has failed.

(4) Failure analysis procedure of the temperature control system itself: check if the input signal of the regulator valve changes, the input signal does not change, the valve action is adjusted, the diaphragm of the valve diaphragm is leaked; check whether the input signal of the regulator valve positioner changes, or the input signal No change, the output signal changes, the positioner has a fault; check the positioner input signal changes, and then check the regulator output if there is no change, if the regulator input does not change, the output changes, this time is the fault of the regulator itself.

4 pressure control instrument system failure analysis:

(1) When the pressure control system instrument indicates rapid oscillation fluctuations, first check whether there is any change in the process operation. This change is mostly caused by process operation and regulator PID parameter setting is not good.

(2) The pressure control system instrument indicates dead line, the process operation changes the pressure indicator or does not change, the general failure occurs in the pressure measurement system, first check whether the measurement pressure piping system has a plugging phenomenon, no plugging, check the pressure change The transmitter output system has no change, there are changes, the fault is in the controller measurement indicator system.

The above is only the site of the four common parameters of the instrument to control the field failure analysis, the actual site there are some complex control loops, such as cascade control, split control, program control, interlock control and so on. The analysis of these failures is more complex and requires detailed analysis.

Solution: elimination method 1. System control, the determination should also be the case, determine the fault after each part is independent, it is easy to identify the problem 2. The instrument is only displayed on the spot, doubt display data is not normal, the data can be estimated according to the process environment , Too far away, just check the adjustment of the instrument on it. 3. On-site control of the instrument, suspect that the display data is not normal, can be automatically disconnected, observed, fixed data of the process conditions, can determine whether the collection of data is normal. If the control result is not stable, it is also first determined whether the data acquisition is normal. The method is the same as above, because the data acquisition is abnormal, and the control result is definitely not acceptable.

After the above problem is judged, the actuator is started normally when checking the soft manual output. Finally adjust the PID parameters.