Application of automatic control instrument in petrochemical industry - part 2

There are two reasons for the abnormality of the instrument: one is the abnormality of the process parameters itself; the other is the occurrence of a certain link in the measurement system failure, resulting in inaccurate data display.

To correctly diagnose the cause of the failure, first, have a certain understanding of the measurement principle, physical structure, and use characteristics of the instrument;

The second is to be familiar with the entire workflow of the measurement system;

The third is to have a relatively in-depth understanding of the process flow, material characteristics and equipment properties of chemical production. The following is a detailed explanation of the common difficult faults of chemical instruments:

01 Flow meter failure

1) If the value of the flow meter reaches the highest, the general on-site detection instrument will also display the highest. At this time, manually adjust the size of the remote control valve. If the flow value decreases, it means a process problem; if the flow value does not change, it should be a fault of the instrument system. , it is necessary to detect whether there is any abnormality in the instrument signal transmission system, measurement pressure system, etc.

2) If the flow index fluctuates abnormally, the system can be switched from automatic control to manual. If there is still fluctuation, it means that it is caused by the process; if the fluctuation is reduced, it means that it is a PID parameter problem or an instrument problem.

3) If the flow rate of the instrument reaches the minimum, first check the on-site detection instrument. If the on-site instrument also shows the lowest value, check the opening degree of the regulating valve. The opening degree is zero, indicating that the fault occurs on the flow adjustment device. If the opening degree is normal, it is very likely that Material crystallization, pipeline blockage or low pressure. If the on-site instrument is normal, it means that there is a problem with the display instrument. The reason is usually that the mechanical instrument gear is stuck, and the positive pressure chamber of the differential pressure transmitter is leaking.

02 Level meter failure

1) When the value of the liquid level instrument reaches the highest or lowest value, judge according to the on-site detection instrument. If the on-site instrument is normal, change the system to manual control to check whether the liquid level changes. If the liquid level can remain stable within a certain range, The explanation is that there is a problem with the liquid level control system, and vice versa is the reason for the process.

2) For the differential pressure liquid level instrument, when the display data of the control instrument and the field detection instrument do not match, and there is no obvious abnormality in the field instrument, check whether the liquid seal of the pressure guiding pipe is normal. Return to zero; if there is no leakage, it is preliminarily inferred that the negative migration of the instrument is wrong and 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, there is usually a problem with the instrument; if the equipment capacity is small, the process operation should be checked first. If the process operation changes, It is most likely the fluctuation caused by the process, otherwise it is the problem of the instrument.

03 Pressure gauge failure

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

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

2) When the control instrument is stagnant, that is, the instrument data remains constant even when the process operation changes, it is usually caused by the failure of the pressure measurement system. At this time, it is necessary to first confirm whether the pressure guiding catheter is blocked. , and then confirm whether the pressure transmission output device is in a normal state. If abnormal changes are found, it can be confirmed that the problem occurs in the measurement indicating system.

04 Temperature meter malfunction

The failure of the temperature instrument is usually manifested as high, low or slow response. When the temperature instrument fails, pay attention to the following two points: First, most of the temperature instruments use electric instruments; hysteresis.

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

2) When high-frequency abnormal fluctuation occurs in the temperature control instrument, 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 operations. If the influence of the process operation can be ruled out, it is very likely that the instrument control system has failed.