Common fault analysis of the hottest field instrum

At present, with the continuous improvement of the automation level of petrochemical, steel, paper, food and pharmaceutical enterprises, higher requirements are put forward for the technical level of field instrument maintenance personnel. In order to shorten the time of dealing with instrument failure, ensure safe production and improve economic benefits, this paper publishes some experience of instrument on-site maintenance for the reference of instrument maintenance personnel

I. Basic Analysis steps of on-site instrument system fault

the measurement parameters of on-site instruments are generally divided into four parameters: temperature, pressure, flow and liquid level

now, according to the different measurement parameters, analyze the faults of different field instruments

1. First of all, before analyzing the faults of field instruments, it is necessary to thoroughly understand the production process, production process and conditions of relevant instrument systems, and understand the design scheme, design intention, structure, characteristics, performance and parameter requirements of instrument systems

2. Before analyzing and checking the failure of the on-site instrument system, it is necessary to understand the production load and the parameter changes of raw materials from the on-site operating workers, check the recording curve of the faulty instrument, and conduct a comprehensive analysis to determine the cause of the instrument failure

3. if the recording curve of the instrument is a dead line (the line without any change is called a dead line), or the recording curve was originally a fluctuation, but now it suddenly becomes a straight line; The fault is likely to be in the instrument system. At present, most of the recording instruments are DCS computer systems, which have very high sensitivity, and the changes of parameters can be very sensitive. At this time, you can artificially change the process parameters to see the curve changes. If there is no change, it is basically concluded that there is a problem with the instrument system; If there are normal changes, it is basically concluded that there is no major problem with the instrument system

4. When changing process parameters, it is found that the recorded curve has a sudden change or jumps to the maximum or minimum. At this time, the fault is often in the instrument system

5. Before the fault occurs, the instrument recording curve has been normal. After the fluctuation, the recording curve becomes irregular or makes the system difficult to control, even manual operation can not be controlled. At this time, the fault may be caused by the process operation system

6. when the DCS display instrument is found to be abnormal, you can go to the site to check the indicated values of the same intuitive instrument. If they are very different, it is likely that the instrument system has failed

in short, when analyzing the cause of field instrument failure, special attention should be paid to the characteristic changes of the tested control object and control valve, which may be the cause of field instrument system failure. Therefore, we should comprehensively consider and carefully analyze the on-site instrument system and process operation systems such as flow marks and fusion wiring, and check the cause

II. Steps of failure analysis of instrument control system with four measuring parameters

1. Steps of failure analysis of temperature control instrument system

when analyzing the failure of temperature control instrument system, first of all, to reduce friction, we should pay attention to two points: the instruments of this system mostly adopt electric instruments for measurement, indication and control; The measurement of instruments in this system often lags behind

(1) the indication value of the temperature instrument system suddenly changes to the maximum or minimum, which is generally the instrument system fault. Because the measurement lag of the temperature instrument system is large, sudden changes will not occur. At this time, the fault causes are mostly caused by thermocouple, thermal resistance, broken compensation wire or transmitter amplifier failure

(2) the indication of temperature control instrument system shows rapid oscillation, which is mostly caused by improper PID adjustment of control parameters

(3) the indication of temperature control instrument system fluctuates greatly and slowly, which is likely to be caused by the change of process operation. If the process operation does not change at that time, it is likely to be the fault of instrument control system itself

(4) fault analysis steps of the temperature control system itself: check whether the input signal of the regulating valve changes, the input signal does not change, the regulating valve acts, and the diaphragm of the regulating valve diaphragm head leaks; Check whether the input signal of the regulating valve positioner changes, the input signal does not change, the output signal changes, and the positioner is faulty; Check whether the input signal of the positioner changes, and then check whether the output of the regulator changes. If the input of the regulator does not change, the output changes. At this time, it is the fault of the regulator itself. 2. Failure analysis steps of pressure control instrument system

(1) when the instrument indication of pressure control system shows rapid oscillation fluctuation, first check whether there is any change in process operation, which is mostly caused by poor process operation and PID parameter setting of regulator

(2) dead line in the instrument indication of the pressure control system, process 2 - the pressing depth under the total experimental force f0+f1; If the operation changes or the pressure indication does not change, the general fault occurs in the pressure measurement system. First, check whether the measuring impulse conduit system is blocked. If not, check whether the output system of the pressure transmitter is changed. If there is any change, the fault lies in the measurement indication system of the controller

3. Fault analysis steps of flow control instrument system

(1) when the indicated value of flow control instrument system reaches the minimum, check the on-site detection instrument first. If it is normal, the fault is in the display instrument. When the indication of the on-site detection instrument is also minimal, check the opening of the regulating valve. If the opening of the regulating valve is zero, it is often a fault between the regulating valve and the regulator. When the indication of the on-site detection instrument is minimum and the opening of the regulating valve is normal, the fault reason is likely to be caused by insufficient system pressure, blocked system pipeline, insufficient pump capacity, medium crystallization, improper operation and other reasons. If the instrument fails, the reasons are as follows: the orifice differential pressure flowmeter may be blocked by the positive pressure conduit; Differential pressure transmitter positive pressure chamber leakage; Mechanical flowmeter is gear stuck or filter blocked, etc

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

(3) the indicated value of the flow control instrument system fluctuates frequently, so the control can be changed to manual. If the fluctuation decreases, it is due to the instrument or the improper PID of the instrument control parameter. If the fluctuation is still frequent, it is due to the process operation

4. fault analysis steps of liquid level control instrument system

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

(2) if the indication of the differential pressure liquid level control instrument and the field direct reading indicating instrument cannot match, first check whether the field direct reading indicating instrument is normal. If the indication is normal, check whether the sealing liquid of the negative pressure impulse pipe of the differential pressure liquid level instrument has leakage; If there is leakage, refill the sealing liquid and adjust the zero point; There is no leakage. It may be that the negative migration of the instrument is wrong. Readjust the migration to make the indication of the instrument normal

(3) when the indicated value of the liquid level control instrument system changes and fluctuates frequently, first analyze the capacity of the liquid level control object to analyze the cause of the fault. The large capacity is generally caused by the instrument fault. If the capacity is small, first 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 there is no change, it may be caused by instrument failure

the above is only the on-site fault analysis of the on-site four parameter separate control instruments. There are also some complex control circuits on the actual site, such as cascade control, split range control, program control, interlock control, etc. The analysis of these faults is more complex and needs to be analyzed in detail