There are many issues with incorrect process parameters reflected by pressure and differential pressure transmitters during on-site installation, such as blocked pre is malfunctioningssure pipes, improper installation of throttling devices, condensing tanks, and pressure pipes, electromagnetic interference, and maintenance issues. Below, I will analyze some of the main problems and causes encountered in the use of pressure transmitters!
1、 The transmitter has no output
Inspection and Testing
1. Check if the power supply of the transmitter is reversed;
2. Measure the power supply of the transmitter to see if there is a 24V DC voltage;
3. If it has a header, check if the header is damaged (you can first short circuit the two wires of the header, if they are normal after the short circuit, it means that the header is damaged);
4. Connect the ammeter in series to the 24V power circuit and check if the current is normal;
5. Is the power supply connected to the transmitter power input terminal;
Terms of settlement
1. Connect the power polarity correctly.
2. It is necessary to ensure that the power supply voltage to the transmitter is ≥ 12V (i.e. the input voltage of the transmitter power supply is ≥ 12V).
3. If there is no power supply, check whether the circuit is disconnected and whether the detection instrument is selected incorrectly (input impedance should be ≤ 250 Ω);
4. If the meter head is damaged, it needs to be replaced. If it is normal, it means that the transmitter is normal. At this time, check whether other instruments in the circuit are normal.
5. Connect the power cord to the power terminal block.
2、 Transmitter output ≥ 20mA
Inspection and Testing
1. Is the transmitter power supply normal
2. Whether the actual pressure exceeds the selected range of the pressure transmitter;
3. Is the pressure sensor damaged? Severe overload can sometimes damage the isolation diaphragm.
4. Is the wiring loose;
5. Is the power cord wired correctly
Terms of settlement
1. If the power supply of the transmitter is less than 12VDC, it should be checked whether there is a large load in the circuit. The input impedance of the transmitter load should comply with RL ≤ (transmitter supply voltage -12V)/(0.02A) Ω
2. Re select a pressure transmitter with an appropriate range.
3. If the pressure sensor is damaged, it needs to be sent back to the manufacturer for repair.
4. Connect the wires and tighten them.
5. The power cord should be connected to the corresponding terminal block.
3、 Transmitter output ≤ 4mA
Inspection and Testing
1. Is the transmitter power supply normal
2. Whether the actual pressure exceeds the selected range of the pressure transmitter;
3. Is the pressure sensor damaged? Severe overload can sometimes damage the isolation diaphragm.
Terms of settlement
1. If the power supply of the transmitter is less than 12VDC, it should be checked whether there is a large load in the circuit. The input impedance of the transmitter load should comply with RL ≤ (transmitter supply voltage -12V)/(0.02A) Ω.
2. Re select a pressure transmitter with an appropriate range.
3. If the pressure sensor is damaged, it needs to be sent back to the manufacturer for repair.
4、 Incorrect pressure indication
Inspection and Testing
1. Is the transmitter power supply normal
2. Is the reference pressure value necessarily correct
3. Is the range of the pressure indicating instrument consistent with the range of the pressure transmitter
4. Is the input and corresponding wiring of the pressure indicator instrument correct
5. The input impedance of the transmitter load should comply with RL ≤ (transmitter supply voltage -12V)/(0.02A) Ω
6. Is the input terminal open when the multi-point paper recorder is not recording;
7. Is the corresponding equipment casing grounded
8. Is it wired separately from AC power and other power sources
9. Is the pressure sensor damaged? Severe overload can sometimes damage the isolation diaphragm.
10. Is there sand, impurities, or other blockages in the pipeline? Impurities can affect measurement accuracy;
11. Is the temperature of the pipeline too high? The operating temperature of the pressure sensor is -25~85 ℃, but in actual use, it is best to use it within -20~70 ℃.
Terms of settlement
1. If the power supply of the transmitter is less than 12VDC, it should be checked whether there is a large load in the circuit. The input impedance of the transmitter load should comply with RL ≤ (transmitter supply voltage -12V)/(0.02A) Ω.
2. If the accuracy of the reference pressure gauge is low, it is necessary to replace it with a higher accuracy pressure gauge.
3. The range of the pressure indicating instrument must be consistent with the range of the pressure transmitter.
4. If the input of the pressure indicator is 4-20mA, the output signal of the transmitter can be directly connected; If the input of the pressure gauge is 1-5V, a resistor with an accuracy of one thousandth or more and a resistance of 250 Ω must be connected to the input of the pressure gauge, and then connected to the input of the transmitter.
5. The input impedance of the transmitter load should comply with RL ≤. If it does not comply, corresponding measures can be taken according to its different characteristics, such as increasing the supply voltage (but must be lower than 36VDC), reducing the load, etc.
6. If the input terminal of the multi-point paper recorder is open when there is no recording: 1. No other load can be carried; 2. Use other recorders with input impedance ≤ 250 Ω when no records are available.
7. The corresponding equipment casing is grounded.
8. Separate wiring from AC power and other power sources.
9. If the pressure sensor is damaged, it needs to be sent back to the manufacturer for repair.
10. When there is sand, impurities or other blockages in the pipeline, it is necessary to clean the impurities and add a filter screen in front of the pressure interface.
11. The temperature of the pipeline is too high. Add a buffer tube to dissipate heat. It is best to add some cold water in the buffer tube before use to prevent overheated steam from directly impacting the sensor, which may damage the sensor or reduce its service life.
In addition to the above-mentioned faults, pressure transmitters are also prone to the following faults:
5、 Installation issues
In steam flow measurement, steam mainly involves two types, one is externally supplied steam and the other is superheated steam from the boiler. External steam is steam that has undergone temperature and pressure reduction, with a low temperature and a large amount of water added. It is used when needed and the steam flow rate is constantly adjusted according to the user's requirements. In the actual flow measurement process, sometimes the flow rate is too high and sometimes too low, which is very unstable and often requires discharge. After each discharge, the measurement of the transmitter is accurate again, but the number of discharges in the steam pipeline is too high, which can easily lead to steam leakage at various contacts on the pressure pipe.
The biggest problem found in measuring the use of superheated steam is that sometimes the machine stops and the flow rate deviates after restarting, resulting in inaccurate readings, and sometimes there is still a small amount of flow displayed after stopping. The installation position of the transmitter is generally lower than the measuring pipeline. However, in actual installation, the condensing tank and transmitter for external steam flow are both higher than the measuring pipeline, and the pressure guiding pipeline laid at least 1 meter downwards from the throttling device is also too short.
There is also a problem of inconsistent height between the condensing tank and the measuring pipeline in the superheated steam flow of the boiler, which leads to an imbalance in the height of the condensed water and causes a static pressure difference.
6、 Blockage problem of pressure pipe
In pressure measurement, sometimes the indicated pressure does not vary with the operating conditions. After opening the drain valve, there is only a small amount of sewage and no water flows out. This is because there is a small amount of floating dust in the water quality or compressed air, which enters the pressure pipe and settles with the water flow. Over time, the wall of the pressure pipe will corrode and accumulate scale, leading to blockage.
7、 The transmitter equipment itself
In the measurement of lubricating oil pressure, the lubricating oil pressure signal is involved in the shutdown interlock control. The signal measured by the lubricating oil pressure transmitter is transmitted to the computer for display on one hand, and on the other hand, this signal is compared through the program. When the pressure is lower than 0.06MPa, a low oil shutdown signal is issued to stop the machine.
From the trend chart of the lubricating oil pressure signal, it can be seen that the pressure drops instantaneously in a straight line, causing the machine to trip. After inspecting the transmitter, it was found that the internal module of the transmitter was damaged. Although the transmitter is regularly calibrated every year and used for production after passing the calibration. After several years of operation, the accuracy, sensitivity, stability and other performance indicators of the transmitter will gradually decrease, and the internal diaphragm and integrated block will also be damaged and malfunction.
8、 There is interference issue
In the measurement of exhaust pressure in air compressors, there is a significant fluctuation in the exhaust pressure signal. After verifying the transmitter, it meets the accuracy requirements and eliminates any faults with the transmitter itself; Check that the pressure conduit and joints are not damaged or leaking, and that the connection of the signal cable is in good contact. But the cable route is introduced into the control room through the cable tray next to the high-end room. There is a lot of electromagnetic interference around. Source: Chemical 707.
Reprinted from official account: Yi Qibang
Mega-tek Instrument Classroom