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Normally, the automatic control instrument is composed of three parts: sensor, transmitter, and display. The sensor is responsible for detecting the analog signal of the tested object. The transmitter is responsible for the sensor output signal into the standard current signal (4~20 mA), and the signal transmission to the PLC controller; The display is responsible for the visual display of the measured data.

Instrument failure is usually manifested as an abnormal indication, such as low indicator, high indicator, stationary indicator, abnormal fluctuation, etc., which can be caused by two kinds of reasons:

1. Abnormal process parameters themselves;

2. A fault occurs in one link of the measurement system, resulting in inaccurate data display.

To Correctly Diagnose The Cause Of The Fault:

1. Have a certain understanding of the measuring principle, physical structure, and operating characteristics of the instrument;

2. Be familiar with the entire working process of the measurement system;

3. Have a deep understanding of the industrial production process, material characteristics, equipment properties, etc.

The Following Four Common Difficulties Of The Instrument For Specific Explanation.

1. Flowmeter failure

1) if the flow meter value reaches the highest, the general field testing instrument will also display the highest, then manually adjust the size of the remote regulating valve, if the flow value decreases, it is a process problem; If the flow value is unchanged, it should be the fault of the instrument system. It is necessary to detect whether the instrument signal transmission system and the pressure lead system are abnormal.

2) If the flow index fluctuates abnormally, the system can be transferred from automatic control to manual control. If there is still fluctuation, it is due to process reasons; If the fluctuation decreases, it indicates that the PID parameter or instrument problem.

3) if the instrument flow reaches the lowest, first check the field test instrument, if the field instrument also shows the lowest, check the opening of the regulating valve, the opening is zero that the fault occurs in the flow regulation device if the opening is normal, it is very likely to be caused by material crystallization, pipeline obstruction or too low pressure. If the field instrument is normal, the indication indicates that the instrument has problems, the reason is usually the mechanical instrument gear stuck, the positive pressure chamber leakage of the differential pressure transmitter, etc.

2. Level instrument failure

1) When the liquid level instrument value reaches the highest or lowest, it shall be judged according to the field test instrument. If the field instrument is normal, the system shall be changed to manual control to check whether the liquid level changes. If the liquid level can remain stable within a certain range, it indicates that there is a problem in the liquid level control system; otherwise, it is due to technological reasons.

2) For the differential pressure level instrument, when the display data of the control instrument is inconsistent with that of the field test instrument, and there is no obvious abnormality in the field instrument, check whether the liquid seal of the pressure guide pipe is normal. If there is leakage, add the sealing liquid, and the instrument returns to zero; If there is no leakage, it is preliminarily inferred that the negative migration of the instrument is wrong, which needs to be corrected.

3) When the data of the liquid level control instrument fluctuates abnormally, it should be judged according to the equipment capacity. If the equipment capacity is large, it is usually the instrument that has problems; If the equipment capacity is small, it is necessary to check the process operation first. If the process operation changes, it is most likely to be the fluctuation caused by process reasons, and otherwise, it is the problem of the instrument.

3. Pressure gauge failure

When the pressure instrument data is abnormal, targeted detection and diagnosis should be carried out according to the physical state of the measured medium — solid, liquid and gas state.

1) When the pressure control instrument has abnormal fluctuations, it is necessary to confirm the changes of process operation first, because such changes are mostly caused by abnormal process operation and PID parameters.

2) When the control instrument stagnates, that is, when the instrument data remains constant despite changes in process operation, it is usually due to a failure of the pressure measuring system. At this time, it should first confirm whether the pressure guide is blocked. If the pipeline is smooth, then confirm whether the pressure transformer output device is in a normal state. If abnormal changes are found, it can be confirmed that the problem is in the measuring indication system.

4. Temperature instrument failure

Temperature instrument failure is usually shown as a high, low, or slow response, when the temperature instrument failure, we should pay attention to the following two points: first, the temperature instrument is mostly an electric instrument; Second, the system has obvious hysteresis when the instrument is tested.

1) The sudden change of temperature instrument data to the highest or lowest level usually belongs to the problem of the instrument system. This is because the instrument system itself has a certain lag, and sudden changes rarely occur. If there is a sudden change, it is generally caused by thermal resistance, thermocouple, or transformer amplifier abnormalities.

2) When the temperature control instrument has high-frequency abnormal fluctuation, it is usually caused by improper PID parameter setting.

3) when the temperature control instrument has obvious slow fluctuations, it is generally caused by changes in process operation. If the effect of process operation can be ruled out, it is most likely that the instrument control system is faulty.