Core logic and practical deconstruction of fault analysis for temperature control instrument systems
To troubleshoot faults in the temperature control instrument system, it is necessary to first understand two underlying characteristics – this is the starting point for all analyses:
I. Electrification architecture: The system consists of "measuring element (thermocouple/thermal resistance) → signal transmission (compensating wire/cable) → transmitter → controller (PLC/DCS) → actuator (control valve)". The entire link relies on the stable transmission and processing of electrical signals.
Secondly, measurement lag: Temperature is an intuitive manifestation of "thermal inertia" - heat needs to be conducted to the measuring element through a medium (such as materials or air). For example, when using a thermocouple to measure the temperature of a reaction kettle, the heat has to penetrate from the liquid inside the kettle to the probe and then be conducted to the thermocouple wire. This process usually takes several minutes. If measuring the temperature of solid materials (such as sinter), the thermal inertia is even greater, and the temperature change will be slower. This "lag" determines the gradual change characteristic of the temperature signal - under normal process fluctuations, the temperature will not suddenly jump or oscillate at a high frequency.
I. The indication suddenly jumps to an extreme value: The "irrefutable evidence" of instrument hardware failure
If the temperature indication suddenly jumps to the maximum value (e.g., 1200°C) or minimum value (e.g., 0°C) of the range, it is 100% an instrument failure—because the lag of temperature does not allow instantaneous changes caused by process fluctuations. The causes of the failure are concentrated in the breakage of the signal chain or the failure of electronic components:
Measurement element disconnection: A thermocouple forms a closed circuit (generating electricity based on temperature difference). If the wire breaks, the circuit resistance suddenly increases to infinity, and the transmitter will output an out-of-range signal (e.g., above 20 mA for a 4 - 20 mA range), corresponding to the indication of "maximum". After a thermal resistor (e.g., Pt100) is disconnected, the resistance value far exceeds the upper limit of the range (e.g., the normal range is -200~800°C, and the resistance → ∞ after disconnection), and the transmitter will interpret it as "temperature exceeds the upper limit".
Compensation cable fault: The compensation cable is the "extension" of the thermocouple (matching the thermoelectric characteristics). If it is broken or connected reversely, it is equivalent to cutting off the thermocouple circuit, and the result is the same as that of the "element disconnection" - the indication directly jumps to the extreme value.
Transmitter amplifier failure: When the electronic amplification module (such as operational amplifier, ADC chip) inside the transmitter malfunctions, it will directly output a fixed extreme value (for example, it continuously outputs 20mA after the chip is burned out), resulting in a sudden change in the indication.
In short, the essence of this type of fault is "signal chain interruption", which has nothing to do with the process. After all, "thermal inertia" cannot cause the temperature to jump from room temperature to 1000°C within a few seconds.
II. Rapid oscillation: A typical manifestation of "over - reaction" of PID parameters
If the temperature indication shows high-frequency, small-amplitude cyclic fluctuations (e.g., repeated rising and falling within 5 - 15 minutes), it is highly likely that the PID parameters of the controller are mismatched. The three parameters of PID have a direct impact on the control stability:
Excessive proportion (P): The system is overly sensitive to the "temperature deviation". For example, when the temperature is only 1°C higher, the controller outputs a signal to "close the regulating valve by 50%", causing the temperature to drop rapidly. When the temperature is 1°C lower than the set value, it outputs a signal to "open it by 50%", resulting in a vicious cycle of "over - adjustment - reverse deviation - over - adjustment again".
Excessive integral (I): The function of the integral is to "accumulate deviations to eliminate static errors". However, if the value of I is too large, it will lead to "overshoot". For example, as soon as the temperature reaches the standard, the integral is still accumulating and continues to output adjustment signals, causing the temperature to change excessively, which in turn triggers reverse adjustment.
Too small differential (D): The differential is to "predict the changing trend of the deviation". If the value of D is too small, the system has no prediction for the "future changes" of the temperature and can only make adjustments after the deviation appears, resulting in oscillations of "chasing the deviation".
The core feature of rapid oscillation is "high frequency" - due to the imbalance of PID parameters, the system enters an "endless loop of over - adjustment", which is completely different from the "slow change" of process fluctuations.
III. Large and slow fluctuations: First identify the process, then check the instrument
If the temperature indication shows low-frequency and large-amplitude fluctuations (e.g., dropping from 800°C to 700°C and then rising to 850°C within 30 - 60 minutes), it is necessary to first confirm whether there are changes in the process operation.
Process reasons: For example, if the feed rate suddenly increases by 50% but the heating steam volume is not increased synchronously, the temperature will gradually drop due to "insufficient heat"; or if the steam pressure suddenly rises from 0.8 MPa to 1.2 MPa, the heating volume will surge and the temperature will gradually rise. The core of this type of fluctuation is "changes in process input", and due to the large temperature lag, the change process is slow.
Instrument reasons (when the process remains unchanged): If the process parameters such as the feed rate and steam pressure are all stable, slow fluctuations mostly come from the chronic faults of the instruments.
Regulating valve leakage: The valve core is worn or the sealing parts are aged, resulting in the valve not being able to close tightly. For example, the regulating valve needs to be closed to an opening of 20%, but in fact, it starts to leak when closed to 30%. The heating capacity remains excessively high, and the temperature rises slowly. The controller tries to close the valve further, but the leakage is not eliminated. Then the temperature drops slowly, creating fluctuations.
Transmitter drift: The zero point or range of the transmitter gradually shifts due to environmental temperature and component aging. For example, when the actual temperature is stable at 800°C, the output of the transmitter gradually increases from 16 mA (corresponding to 800°C) to 18 mA (corresponding to 850°C). The controller makes adjustments based on this, resulting in temperature fluctuations.
The troubleshooting logic for this type of fault is "from external factors to internal factors" – the process is the "external cause", and the instrument is the "internal cause". First, eliminate the external causes and then check the internal cause.
IV. System fault troubleshooting: The "reverse traceability method" from the actuator to the controller
If you need to locate deep - seated faults, it is recommended to conduct troubleshooting in the order from the back to the front, following the sequence of "actuator → positioner → regulator". The reason is very simple: the actuator (control valve) is in direct contact with the process medium, so it has the highest probability of failure; the positioner comes next; and the regulator (such as DCS card) has the lowest probability of failure. Specific steps:
1. Check the control valve: Observe whether the input signal of the control valve (such as the 4 - 20mA output from the controller) is stable. If the input signal is stable but the valve core of the control valve still moves back and forth, it indicates a control valve failure. The most common problems are diaphragm leakage in the diaphragm actuator (the air pressure cannot be maintained and the valve core moves by itself due to the spring force) or valve core jamming (the valve core is stuck by medium crystallization).
2. Check the positioner: If the input signal of the control valve is stable, but the output signal of the positioner (the air pressure supplied to the diaphragm actuator) fluctuates, it indicates a positioner malfunction. For example, there may be dust accumulation on the nozzle baffle (resulting in unstable air pressure output) or a circuit board failure in the electronic positioner.
3. Check the regulator: If the input signal of the positioner fluctuates, then check whether the input signal of the regulator (the temperature signal transmitted by the transmitter) is stable. If the input signal is stable but the output signal of the regulator fluctuates, it indicates a regulator fault - for example, the PID module is damaged (outputting changing signals without reason) or the communication card is faulty (receiving incorrect instructions).
This "from the end to the source" method can quickly narrow down the scope of the fault and avoid the inefficiency of "checking from start to finish". After all, the control valve is the component most susceptible to wear and tear from the process medium. Checking it first can always save time.
The underlying logic of fault analysis
The core laws of temperature system failures all revolve around "lag" and "electrified architecture":
- Sudden jump → Signal chain breakage (hardware failure);
- Rapid oscillation → Imbalance of PID parameters (control logic issue);
- Slow amplitude fluctuations → Focus on the process first and then the instrument (distinguish internal and external causes);
- System troubleshooting → Reverse tracing (from the actuator to the controller).
Once you thoroughly understand these logics, you can shift from "empirical judgment" to "logical derivation" and truly achieve rapid fault location and resolution.