Fault Analysis of Field Instrumentation System

1. Basic analysis steps for field instrument system failure

Field instrument measurement parameters are generally divided into four major parameters: temperature, pressure, flow, and liquid level.

Now, according to the different measurement parameters, we will analyze the faults of different field instruments.

1. First of all, before analyzing the field instrument failure, it is necessary to have a relatively thorough understanding of the production process, production process and conditions of the relevant instrument system, understand the design plan and design intent of the instrument system, and the structure, characteristics, performance and parameter requirements of the instrument system.

2. Before analyzing and inspecting the on-site instrument system failure, it is necessary to ask the on-site operators about the production load and changes in raw material parameters, check the record curve of the faulty instrument, and conduct a comprehensive analysis to determine the cause of the instrument failure.

3. If the instrument recording curve is a dead line (a line without any change is called a dead line), or the recording curve was originally fluctuating and now suddenly becomes a straight line; the fault is likely to be in the instrument system. Because most of the current recording instruments are DCS computer systems, they are very sensitive and can react to parameter changes very sensitively. At this time, you can artificially change the process parameters to see how the curve changes. If there is no change, it is basically determined that there is a problem with the instrument system; if there is a normal change, it is basically determined that there is no major problem with the instrument system.

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

5. Before the fault occurred, the instrument recording curve had been normal. After the fluctuation, the recording curve became irregular or made the system difficult to control, and even manual operation could not control it. At this time, the fault may be caused by the process operating system.

6. When it is found that the DCS display instrument is abnormal, you can go to the site to check the indication values ​​of the same intuitive instrument. If they are very different, it is very likely that the instrument system is faulty.

In short, when analyzing the cause of field instrument failure, special attention should be paid to the characteristic changes of the measured control object and the control valve, which may be the cause of the field instrument system failure. Therefore, we should comprehensively consider and carefully analyze the two aspects of the field instrument system and the process operation system to check the cause.

2. Fault analysis steps of the instrument control system of the four major measurement parameters

1. Temperature control instrument system fault analysis steps

When analyzing the failure of the temperature control instrument system, we must first pay attention to two points: the system instruments mostly use electric instruments for measurement, indication, and control; the measurements of the system instruments often have a large lag.

(1) If the indicated value of the temperature instrument system suddenly changes to the maximum or minimum, it is generally due to a fault in the instrument system. Because the temperature instrument system has a large measurement lag, it will not change suddenly. The cause of the fault is mostly caused by the disconnection of the thermocouple, thermal resistor, compensation wire or the failure of the transmitter amplifier.

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

(3) If the temperature control instrument system indicates large and slow fluctuations, it is likely caused by changes in process operations. If the process operations have not changed at the time, it is likely that the instrument control system itself has a fault.

(4) Fault analysis steps for the temperature control system itself: Check whether the input signal of the control valve changes. If the input signal does not change, the control valve operates and the diaphragm of the control valve diaphragm head is leaking; check whether the input signal of the control valve positioner changes. If the input signal does not change, the output signal changes and the positioner is faulty; check whether the positioner input signal changes and then check whether the regulator output changes. If the regulator input does not change but the output changes, then the regulator itself is faulty.

2. Pressure control instrument system fault analysis steps

(1) When the pressure control system instrument indicates rapid oscillations, first check whether the process operation has changed. This change is mostly caused by poor process operation and regulator PID parameter adjustment.

(2) The pressure control system instrument indicates a dead line. The pressure indication does not change even after the process operation changes. Generally, the fault occurs in the pressure measurement system. First, check whether the pressure measuring pipe system is blocked. If not, check whether the pressure transmitter output system has changed. If there is a change, the fault is in the controller measurement indication system.

3. Flow control instrument system fault analysis steps

(1) When the flow control instrument system indication value reaches the minimum, first check the on-site detection instrument. If it is normal, the fault is in the display instrument. When the on-site detection instrument also indicates the minimum, 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 on-site detection instrument indicates the minimum and the regulating valve opening is normal, the cause of the fault is likely to be insufficient system pressure, system pipeline blockage, pump failure, medium crystallization, improper operation, etc. If it is an instrument failure, the reasons are: the orifice differential pressure flowmeter may be blocked by the positive pressure guide tube; the differential pressure transmitter positive pressure chamber leaks; the mechanical flowmeter is stuck in the gear or the filter is blocked, etc.

(2) When the flow control instrument system indicates the maximum value, the detection instrument will often indicate the maximum value. At this time, the control valve can be manually opened or closed remotely. If the flow rate can be reduced, it is generally caused by process operation. If the flow value cannot be reduced, it is caused by the instrument system. Check whether the control valve of the flow control instrument system is working; check whether the instrument measurement pressure system is normal; check whether the instrument signal transmission system is normal.

(3) If the indication value of the flow control instrument system fluctuates frequently, the control can be changed to manual. If the fluctuation is reduced, it is due to the instrument or the instrument control parameter PID is not appropriate. If the fluctuation is still frequent, it is caused by the process operation.

4. Liquid level control instrument system fault analysis steps

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

(2) When the indication of the differential pressure liquid level control instrument and the on-site direct reading indicating instrument do not match, first check whether the on-site direct reading indicating instrument is normal. If the indication is normal, check whether the sealing liquid of the negative pressure pressure pipe of the differential pressure liquid level instrument is leaking; if there is leakage, refill the sealing liquid and adjust the zero point; if there is no leakage, it may be that the negative migration amount of the instrument is incorrect, and readjust the migration amount to make the instrument indicate normal.

(3) When the indication value of the liquid level control instrument system changes and fluctuates frequently, we must first analyze the capacity of the liquid level control object to analyze the cause of the failure. Large capacity is generally caused by instrument failure. Small capacity should first analyze whether the process operation conditions have changed. If there are changes, it is likely that the process has caused frequent fluctuations. If there are no changes, it may be caused by instrument failure.

The above is only the on-site fault analysis of the four major parameter independent control instruments. In the actual field, there are some complex control loops, such as cascade control, split-range control, program control, interlocking control, etc. The analysis of these faults is more complicated and requires specific analysis.