Abstract : This paper analyzes common commissioning faults of the CFB coal-fired boiler, gasification unit, and BQG air separation unit in the Qilu Petrochemical resource optimization project before start-up. The probability of fault occurrence and the characteristics and causes of faults in each loop are described. The aim is to summarize experience and learn lessons, providing a reference for improving the quality and efficiency of instrument commissioning in the future. Keywords : Instrument commissioning; common faults; signal flow; signal classification; typical links I. Introduction Computer Control System Structure Introduction: For ease of analysis, a general description is given in the order of control system signal and data flow. (See Figure 1) On-site, process parameters (pressure, flow rate, temperature, liquid level, composition, etc.) are converted into electrical signals by measuring instruments and sent to the DCS card. The card processes the signals and sends them to the controller. The controller sends the signals to the operator station or field control valves and other equipment according to the control scheme (configuration requirements). Operators send parameters to the controller through the control station according to the control intention. The controller sends control signals to the field control equipment, which then changes the parameters of the controlled object to achieve the control objective. The two signal flow lines (input circuit and output circuit) mentioned above vary greatly in different systems, devices, and designs, and must be thoroughly understood when diagnosing and troubleshooting. II. Signal Classification and Typical Components To facilitate detailed analysis and troubleshooting, signal types are classified into four main categories: 1. AI Point – Analog Signal Input Point: Primarily field transmitters, temperature points, valve stroke feedback signals, and special instrument indications (shaft displacement, shaft vibration, and speed), etc. (See Figure 2 for each component) 2. DI Point – Digital Input Point (Switching Input Point): Primarily various field pressure, flow, level, and temperature switches, or electrical relay contacts, or limit switches and push-button signals for control valves. (See Figure 3 for each component) 3. AO Point – Analog Signal Output Point: Primarily electrical signals destined for field control valves and speed controllers. (See Figure 4 for each step) 4 DO Points --- Digital (Switch) Output Points: Mostly control signals to field solenoid valves, electrical relay contacts, and alarms (See Figure 5 for each step) III. Fault Analysis and Handling 1. Common Faults of Analog Input Due to the rapid development of field measuring instruments and the complexity of measurement objects, the number of points is relatively large. This type of fault occurs most frequently during debugging, has the most complex causes, and is difficult to handle. My incomplete statistics indicate it accounts for approximately 40% of line faults. The faults encountered so far have the following causes: 1. Improper power supply configuration or faulty fuse 2. Unclear distinction between four-wire/three-wire/two-wire/RTD common terminals 3. Reversed polarity of field signal lines 4. Reversed polarity at the terminal block of the equipment room junction box 5. Incorrect power supply/input/output polarity at the safety barrier 6. Incorrect active/passive configuration of AI cards 7. Incorrect instrument parameters/set switches in each step of the input circuit 8. Incorrect DCS control point configuration 9. Incorrect indoor/outdoor common line configuration. 10. Incorrect connection wires or terminals. 11. Safety barrier malfunction. 12. Instrument malfunction (minor). The following is a detailed analysis of several typical fault phenomena: 1.1 Field transmitter cannot receive a signal. This type of fault often occurs in the field or equipment room, where a 275 or 375 handheld terminal cannot be used to check the loop connection. Field checks often reveal no power supply, and the DCS often displays bad values ​​or an open circuit. My complete and detailed troubleshooting approach (see Figure 6) follows the principle of proceeding from indoors to outdoors, and from simple to complex. In actual commissioning, the above process is not always strictly followed; often, some technical adjustments are made based on system characteristics, personal experience, and construction conditions. This type of fault is prone to occur in terminal cabinets, terminal boxes, AI cards, safety barriers, fuses, etc. Incorrect cable connections also occur frequently. Additionally, overlapping faults and faults that were overlooked during inspection but were intentionally created are also common. The highest average efficiency is achieved by processing this type of fault sequentially from indoors to outdoors and from outdoors to indoors (two groups). Segmented and focused inspections are relatively efficient, often depending on the operator's skills and experience. 1.2 Faults where RTDs/thermocouples (or resistance boxes) fail to indicate correctly after connection are relatively simple, involve fewer steps, and are relatively easy to handle. Fault locations are often at junction boxes, junction cabinets, AI cards, etc. {Analysis and handling are the same as 1.1}. Causes are often due to improper design and construction coordination, resulting in mismatched common terminals of RTDs, mismatched thermocouple compensation wires, incorrect AI card channel connections, incorrect selection of thermocouple/RTD type parameters during configuration, and incorrect wiring and settings of temperature transmitters, leading to abnormal DCS indication. This type of fault is characterized by: batch errors, easy detection (judged by whether the flowchart indicates room temperature). Ensuring the physical location of this point corresponds to the control scheme tag number is crucial (mixed points are difficult to detect), requiring point-by-point verification. Common methods include: open circuit method and short circuit method. 1.3 Field signals have been sent normally, but the operator station has no indication. This type of fault occurs more sporadically, often due to incorrect wiring, incorrect cables, incorrect channels, or incorrect tag numbers. Configuration errors account for a significant proportion. Troubleshooting this type of fault is often unpredictable, sometimes easy and sometimes difficult, requiring comparison with multiple drawings and a step-by-step check. 1.4 Mismatch between field signals and operator station indications: This type of fault is mostly due to soft faults such as instrument settings and DCS configuration issues. Specifically, it manifests as: zero point and range, migration issues, unit conversion, accumulation issues, flow square root issues, small signal cutoff, etc. {Temperature points are listed separately in 1.2} This type of fault is easy to detect and handle. Its frequency of occurrence is generally low. 2 Common faults in digital inputs: These signals are mostly from field valve position switches, pump status, level switches, flow switches, pressure switches, temperature switches, and various buttons. The number varies considerably depending on the device. These points are often important interlocking and alarm points, playing a crucial role in system safety. However, the faults are not particularly complex, and the probability of occurrence is not high, approximately 10% of all fault points. The causes encountered include: incorrect cable routing, incorrect wiring, incorrect selection of normally open and normally closed circuits, design errors, configuration errors, relay configuration errors and relay malfunctions, incorrect power supply connection, and incorrect card channels. Among these, design errors and power supply errors are often accompanied by a large number of abnormal DI points, which are key points to pay attention to during commissioning. The following is a brief analysis of several common fault phenomena. 2.1 The field signaling component has activated, but the operator station display remains unchanged. 2.1.1. Check the power supply section, using the same method as described in 1.1. 2.1.2. Check if the relays are activated, if the contacts are actually activated, and if the wiring is correct, and address any issues. 2.1.3. Check if the signaling component contacts are actually activated, and if the wiring is correct, and address any issues. 2.1.4. Check if the DI card and channel configuration are correct and address any issues. 2.1.5. Check if the configuration is correct and address any issues. 2.1.6. Check for incorrect cable routing and address any issues. 2.2 The field signaling component has activated, and the operator station displays multiple activations. This type of fault is mostly due to incorrect connection of the common line. After one relay is energized, other relays are malfunctioning due to the in series current. Configuration errors, caused by omissions in the copy/paste configuration, can also cause this. This is a localized and occasional phenomenon. Care and attention can prevent it. 2.3 The field signaling component does not activate, but the operator station displays that it has activated. This type of fault is mostly due to incorrect selection of normally open/normally closed or incorrect selection of the "NOT" operation in the configuration. After ruling out incorrect wiring and cables, these two reasons should be considered the main causes. 2.4 The field signaling component activates again, but the operator station displays that it has not activated accordingly. Cause analysis: 2.4.1 The signaling component is stuck, or the relay is stuck and cannot be effectively disconnected. 2.4.2 There is an RS trigger or time relay in the configuration with memory function, and the state does not have the conditions for change. Solution: 2.4.3 It is necessary to know the function of the DI point in the system and the circuit links it forms, and check and handle accordingly. 3 Common faults of digital output: These signals are mostly control results: alarm lights, solenoid valves, de-energized signals, or field start/stop pump signals, etc. The number of such signals varies greatly depending on the device and is related to the safe operation of the device. Therefore, more meticulous attention is required during debugging, not only to check whether the signal is correct, but also to pay attention to the timing and speed of the action. During debugging, the number of fault points similar to DI is about 10%. Common faults are mostly: wrong wiring, wrong cable, wrong configuration, and relay corresponding faults. Faults of the driven equipment itself are also relatively common. Common fault phenomena include: when the condition is met, the alarm light does not light up, the solenoid valve does not move, and the electrical equipment does not receive a contact signal. When the condition is not met, the alarm light lights up falsely, the solenoid valve moves falsely, and the electrical equipment receives an incorrect contact signal. When the condition is met, the action speed of the solenoid valve or the started equipment does not meet the requirements, etc. The troubleshooting ideas and inspection methods for circuit faults are the same as those described in 1.1. The cause analysis is the same as that of DI points, only the signal flow is reversed. Pay attention to the characteristics of each link and the time-related settings. 4 Common faults of analog outputs These signals are the direct link of the control system to control the process and are also variables that process personnel frequently manipulate and change. Their quality directly affects the process indicators and is related to the product quality. Field equipment controlled by this type of signal generally includes control valves and speed controllers. Because there are many types of valves with significant performance differences, valve selection has a particularly important impact on the control system. The faults encountered during commissioning are similar to those encountered with analog inputs, and their difficulty is similar. The handling methods are also basically the same (refer to 1.1 for handling; the signal flow is reversed). The difference is that the probability of a fault occurring in the field control valve itself is greater than that of a fault in the transmitter. Therefore, individual unit commissioning before system commissioning is an indispensable step. The number of fault points is approximately 40% (slightly less than with analog inputs). The fault phenomena encountered include: output is given, but the valve does not move; the output value does not correspond to the valve position; the direction of the output value's action is opposite to the required direction of the valve's action. In addition to the various links and configurations mentioned earlier for analog inputs, valve positioner malfunctions are also a cause. These will not be discussed in detail here. 5. Other Common Faults Two other common faults encountered during commissioning are power system faults and grounding system faults. The following analysis and discussion will focus on faults related to these two systems encountered during commissioning. 5.1 Power System Failure: During the CFB boiler instrumentation function test, after a normal power-on check, the 24V power supply experienced a shutdown self-protection event a few days later. Three possible causes were initially identified: a ground fault in the circuit, fluctuations in the 220V power supply, and harmonic coupling into the power system. However, these were subsequently ruled out. Later, the power supply manufacturer's engineers conducted on-site inspection and determined that the power supply capacity was too large, the load was too small, and the power supply was operating at a low load. The power supply modules were reduced from six to three. After one month of operation, the power supply section again experienced a power-off self-protection event, causing the control system to malfunction and affecting the normal operation of the boiler. After replacing the power supply modules and re-energizing, normal operation was restored. As the application manufacturer, we have limited knowledge of the internal technology of power supplies and hope to discuss this with experts. 5.2 Grounding System Failure: Due to different grounding requirements from various DCS and ESD manufacturers, grounding issues often prevent the system from functioning properly during commissioning. Many systems require separate grounding, but during implementation, various reasons (trampling on grounding bars, incorrect grounding wire connections, etc.) lead to chaotic grounding that fails to meet system requirements. If the design is well-considered, these issues can be completely avoided during construction. Another common grounding fault is the shield grounding of signal lines, which is generally achieved using a single-point grounding indoors. However, during construction, this is often overlooked, or indoor and outdoor units mistakenly believe the other is grounded, resulting in the shield grounding not being connected. This type of fault is difficult to detect during signal calibration and requires a special inspection. Otherwise, it will affect the normal operation of the circuit. Experienced construction teams rarely make this error. IV. Fault Count Statistics and Explanation The fault count is based on the number of faults discovered, resolved, and known to have been discovered and resolved during the functional testing of the three major device control systems (this is the number of points where the fault occurred, not the number of circuit points). Since this is a statistical summary from personal debugging, the data is not entirely accurate and is only listed for qualitative explanation: (Total number of points: 1470) Fault Point Type | Number | Percentage AI | 600 | 40.8% AO | 580 | 39.4% DI | 130 | 8.84% DO | 120 | 8.16% Grounding Fault | 20 | 1.36% Other Faults | 10 | 0.68% V. Conclusion This article summarizes experience gained during instrument debugging. It systematically categorizes common faults, analyzes the causes of faults in detail, and provides detailed ideas and methods for fault handling and inspection. It is hoped that this will provide a reference for future control system construction and debugging.