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Industrial Automation Control System

The causes and solutions of the four major instrument failures

 

In order to realize the production of industrial automation control, it is necessary to comprehensively monitor the temperature, pressure, flow, level, and other data in the production process. These functions are realized through the corresponding detection instrument. Once the instrument fails, it will have a serious impact on the normal operation of industrial production. 

Therefore, the industrial control personnel must be familiar with the physical structure of the four instruments, measuring principles and performance indicators, can accurately diagnose and deal with the instrument fault, so as to ensure normal industrial production.

 

 

What are the four major industrial automation control instruments?

 

 

1 Temperature meter

 

The chemical reactions and changes in petrochemical production need to be carried out smoothly under specific temperature and pressure environments. In order to monitor the temperature changes in real-time and control the temperature range precisely, a certain number of temperature meters must be set in the production. At present, the production temperature is mainly measured by contact, and the temperature data is measured by thermocouple, thermal resistance and other temperature measuring components, and the automatic temperature control effect is achieved with the help of Fieldbus technology.

Identification of thermocouple and thermal resistance

The appearance of industrial thermocouple and thermal resistance protection bushing is almost the same, and some temperature measuring elements are very small, such as armored type, and the appearance of both is basically the same.

 

A.In the case of the nameplate, know the model, can be identified through the nameplate.

 

Thermocouple: the principle is the thermoelectric effect, and its grading number is S, B, E, K, R, J, T, seven kinds of standardized models.

Thermal resistance: the principle is the thermal effect of resistance (the resistance value of a conductor or semiconductor changes with the temperature of this property), and its grading signs are Pt10, Pt100, Pt1000, Cu50 and Cu100. PT100 and CU50 are the most widely used.

 

B.In the case of no nameplate, and do not know the model, the following methods can be used to identify.

 

(1) look at the lead line of the temperature measuring element, usually, there are only two leads of the thermocouple, if there are three leads is the thermal resistance.

However, for those with four lead lines, it is necessary to measure the resistance value to determine whether it is a two-branch thermocouple or a four-wire thermal resistance. First, find two pairs of leads with almost zero resistance from the four leads, and then measure the resistance value between the two leads. If it is infinite, it is a double thermocouple. A pair of leads with almost zero resistance is a thermocouple. If the resistance of the two pairs of lead wires is between 10-110, it is a single four-wire thermal resistance, and its resistance value is closest to the thermal resistance of what index number, it is the thermal resistance of the index number.

(2) if there are only two leads, you can use the digital multimeter to measure the resistance value to judge, because the resistance value of the thermocouple is very small, the thermal resistance is almost zero; If the resistance value is very small, it may be a thermocouple. Thermal resistance at room temperature, its minimum resistance value will be greater than 10.

 

 

2 Pressure meter

 

Pressure gauges are of various types, such as pressure sensors, pressure transmitters, special pressure gauges, etc. Pressure gauges can be used for pressure measurement in extreme environments such as high temperature and corrosion, and can also be used for pressure measurement in easy-to-crystallize and powder-granular media.

Under normal circumstances, the pressure regulation system will transmit the collected signal to the distributed control system with the help of the pressure transmitter to realize the automatic pressure measurement and control effect.

 

 

3 level instrument

 

According to the different measuring methods, the level instrument can be divided into buoyancy type, direct reading type, differential pressure type, radiation type, radar type, etc. In petrochemical production, radar level instrument is gradually favored by the industry because of its high measuring accuracy and strong applicability to petrochemical materials.

 

 

4 Flowmeter

 

Flowmeter is basically based on two measuring principles, one is volume flow measurement, the other is mass flow measurement. Orifice differential pressure flowmeter, electromagnetic flowmeter, ultrasonic flowmeter, vortex street flowmeter, etc., almost all flowmeters are volume detection flowmeters now. Because now is the use of intelligence, as long as the selection of flow meter internal parameters change, can also detect the mass flow rate.

 

 

Cause analysis of instrument failure

 

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 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 the 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.

 

 

Conclusion

 

The application of industrial automation instrument control systems has brought great convenience to the industrial control industry and repositioned the interactive relationship between humans and industrial control systems.

When analyzing and dealing with actual industrial control instrument faults, it is necessary not only to master a solid instrument theory and knowledge, but also to have a detailed understanding of process practice, so as to quickly diagnose the instrument faults, and then take appropriate means to deal with the faults, to ensure the smooth progress of industrial production.

 

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