There are many unpredictable reasons in the process of engineering design, construction and instrument installation. Therefore, many problems will surely arise in the process of automatic instrument debugging. To troubleshoot and solve the problem, the commissioning worker must understand the performance of the meter. This article analyzes some common problems in the debugging process of the instrument and puts forward some exclusion methods for reference of similar projects.
First, the mutual interference between the regulating units When installing the pressure regulating unit and the flow regulating unit on the same pipeline of the industrial installation, there are two valves: valve 1 and valve 2, both of which will inevitably interfere with each other.
When the pipeline pressure is lower than the given value, the pressure regulator closes the valve 1, which will cause the pipeline flow to drop, and the flow regulator will then open the valve 2, but this will cause the pressure to drop, and so repeatedly, resulting in two regulating systems Nothing works.
To solve this problem, it must be corrected according to Party A's process requirements and adjustment plans. In general, the pressure regulation system is more important. The ratio of the flow regulator and the integral time can be appropriately increased by adjusting the parameters of the regulator. When the disturbance is affected, the pressure regulation system immediately acts and the pressure is adjusted back to the given value. The value, while the flow regulation system works slowly, returns to the given value over time. This weakens the influence of the flow regulation system on the pressure regulation system and guarantees the stability of the main regulated parameter, pressure.
Second, the cold end temperature changes on the measurement of the thermocouple is a common instrument, often encountered in many factory instrument debugging. In the thermocouple debugging process, especially in areas where the thermocouples are installed relatively concentrated, such as in the large steel furnace project of Shangsan Iron and Steel Co., Ltd., there are 16 temperature points at the bottom of an electric furnace showing unbalanced temperature, including one thermocouple. Deviations are particularly large. Thermocouples with large deviations after inspection are not only installed properly, but also the instrument body is normal.
The temperature scale of the temperature display instrument matched with the thermocouple is determined according to the index table, and the actual cold junction temperature is higher than 0°C, and is not constant. At this time, the thermal resistance generated by the thermocouple is necessarily biased. Small, and the measured value also changes with the temperature of the cold junction. To eliminate this phenomenon, you can use two methods:
1 Insulate the unheated process piping near the thermocouple;
2 Lay a new compensation wire.
Third, the communicator to verify the smart transmitter In the smart transmitter single debugging, HART communicator is more and more widely used. The HART Communicator can establish a good communication interface with the transmitter and set and correct parameters for the transmitter. But for different types of smart transmitters, according to this loop connection, the HART communicator does not play the same role. For example, during the process of commissioning the chlor-alkali polymerization pilot plant, the owner adopted the SIEMENS and ROSEMOUNT smart transmitters according to the system design requirements. With the same loop connection method, the HART Communicator can establish a good communication interface with the SIEMENS intelligent transmitter, set and correct parameters, but it cannot establish the communication mode with the ROSEMOUNT intelligent transmitter.
The principle analysis of various types of data is performed on the entire debug circuit. According to Ohm's law R=U/I, when using a DC 1-5V power supply, a current of 4-20 mA is required and the resistance is 250 Ω. However, the internal resistance of different types of smart transmitters is different. It is necessary to consider different internal resistance values. Therefore, it is not reasonable to use standard resistors in the circuit. The standard resistors can be replaced with precision resistors, according to different The smart transmitter regulates. After practical operation, when the resistance is 50Ω, the HART Communicator can establish communication mode with ROSEMOUNT intelligent transmitter for debugging.
IV. Problems in the Direct Digital Control System (DCS) Analog to Digital (A/D) Conversion During the commissioning of the DCS system in the new project of PetroChina Jinfei Plastics Co., Ltd., we found that there was a wrong route for the pneumatic actuator. However, the body of the inspected instrument is in good condition, the single-unit debugging record is normal, and the installation position is no problem. Why can this happen? The test analysis is that the 4-20mA current signal output by the DCS control system is incorrect.
From the principle of the DCS control system, the DCS system is used to measure various physical quantities (such as temperature, pressure, flow rate, liquid level, etc.) in the production process by a single instrument measurement and convert it into a 4-20mA current signal as a DCS system. The input, and the current signal is converted into a corresponding voltage signal through a fixed resistance, amplified, converted to digital by modulus (A / D), and then sent to the computer, the computer according to the predetermined degree of control of its analysis, comparison And then through the step controller, from the corner-current converter (D / A) into a 4-20mA current signal sent to the scene.
It is not difficult to see that the deviation of the fixed resistance will cause the output signal of the DCS control system to be incorrect. If it is adjusted in time, this problem can be solved.
First, the mutual interference between the regulating units When installing the pressure regulating unit and the flow regulating unit on the same pipeline of the industrial installation, there are two valves: valve 1 and valve 2, both of which will inevitably interfere with each other.
When the pipeline pressure is lower than the given value, the pressure regulator closes the valve 1, which will cause the pipeline flow to drop, and the flow regulator will then open the valve 2, but this will cause the pressure to drop, and so repeatedly, resulting in two regulating systems Nothing works.
To solve this problem, it must be corrected according to Party A's process requirements and adjustment plans. In general, the pressure regulation system is more important. The ratio of the flow regulator and the integral time can be appropriately increased by adjusting the parameters of the regulator. When the disturbance is affected, the pressure regulation system immediately acts and the pressure is adjusted back to the given value. The value, while the flow regulation system works slowly, returns to the given value over time. This weakens the influence of the flow regulation system on the pressure regulation system and guarantees the stability of the main regulated parameter, pressure.
Second, the cold end temperature changes on the measurement of the thermocouple is a common instrument, often encountered in many factory instrument debugging. In the thermocouple debugging process, especially in areas where the thermocouples are installed relatively concentrated, such as in the large steel furnace project of Shangsan Iron and Steel Co., Ltd., there are 16 temperature points at the bottom of an electric furnace showing unbalanced temperature, including one thermocouple. Deviations are particularly large. Thermocouples with large deviations after inspection are not only installed properly, but also the instrument body is normal.
The temperature scale of the temperature display instrument matched with the thermocouple is determined according to the index table, and the actual cold junction temperature is higher than 0°C, and is not constant. At this time, the thermal resistance generated by the thermocouple is necessarily biased. Small, and the measured value also changes with the temperature of the cold junction. To eliminate this phenomenon, you can use two methods:
1 Insulate the unheated process piping near the thermocouple;
2 Lay a new compensation wire.
Third, the communicator to verify the smart transmitter In the smart transmitter single debugging, HART communicator is more and more widely used. The HART Communicator can establish a good communication interface with the transmitter and set and correct parameters for the transmitter. But for different types of smart transmitters, according to this loop connection, the HART communicator does not play the same role. For example, during the process of commissioning the chlor-alkali polymerization pilot plant, the owner adopted the SIEMENS and ROSEMOUNT smart transmitters according to the system design requirements. With the same loop connection method, the HART Communicator can establish a good communication interface with the SIEMENS intelligent transmitter, set and correct parameters, but it cannot establish the communication mode with the ROSEMOUNT intelligent transmitter.
The principle analysis of various types of data is performed on the entire debug circuit. According to Ohm's law R=U/I, when using a DC 1-5V power supply, a current of 4-20 mA is required and the resistance is 250 Ω. However, the internal resistance of different types of smart transmitters is different. It is necessary to consider different internal resistance values. Therefore, it is not reasonable to use standard resistors in the circuit. The standard resistors can be replaced with precision resistors, according to different The smart transmitter regulates. After practical operation, when the resistance is 50Ω, the HART Communicator can establish communication mode with ROSEMOUNT intelligent transmitter for debugging.
IV. Problems in the Direct Digital Control System (DCS) Analog to Digital (A/D) Conversion During the commissioning of the DCS system in the new project of PetroChina Jinfei Plastics Co., Ltd., we found that there was a wrong route for the pneumatic actuator. However, the body of the inspected instrument is in good condition, the single-unit debugging record is normal, and the installation position is no problem. Why can this happen? The test analysis is that the 4-20mA current signal output by the DCS control system is incorrect.
From the principle of the DCS control system, the DCS system is used to measure various physical quantities (such as temperature, pressure, flow rate, liquid level, etc.) in the production process by a single instrument measurement and convert it into a 4-20mA current signal as a DCS system. The input, and the current signal is converted into a corresponding voltage signal through a fixed resistance, amplified, converted to digital by modulus (A / D), and then sent to the computer, the computer according to the predetermined degree of control of its analysis, comparison And then through the step controller, from the corner-current converter (D / A) into a 4-20mA current signal sent to the scene.
It is not difficult to see that the deviation of the fixed resistance will cause the output signal of the DCS control system to be incorrect. If it is adjusted in time, this problem can be solved.
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