When the pressure transmitter is installed on site, there are many phenomena that the relevant pressure and differential pressure transmitters reflect incorrect process parameters: there are problems of blockage of impulse pipe, non-standard installation of throttling device, condensate tank and impulse pipe, electromagnetic interference, maintenance, etc. The following is an analysis of some main problems and causes encountered in the use of pressure transmitter!

1、 Transmitter no output

Inspection and testing

1. Check whether the transmitter power supply is connected reversely;

2. Measure whether the power supply of the transmitter has 24V DC voltage;

3. If it is with a meter, check whether the meter is damaged (the two wires of the meter can be short circuited first. If the short circuit is normal, it indicates that the meter is damaged);

4. Connect the ammeter in series into the 24V power supply circuit and check whether the current is normal;

5. Whether the power supply is connected to the power input terminal of the transmitter;

terms of settlement

1. Connect the power polarity correctly.

2. Ensure that the power supply voltage supplied to the transmitter is ≥ 12V (i.e. the voltage at the power input of the transmitter is ≥ 12V).

3. If there is no power supply, check whether the circuit is disconnected and whether the detection instrument is selected incorrectly (the input impedance shall be ≤ 250 Ω);

4. If the meter head is damaged, it shall be replaced. If it is normal, it indicates that the transmitter is normal. At this time, check whether other instruments in the loop are normal.

5. Connect the power cord to the power terminal.

2、 Transmitter output ≥ 20mA

Inspection and testing

1. Whether the transmitter power supply is normal

2. Whether the actual pressure exceeds the selected range of the pressure transmitter;

3. Whether the pressure sensor is damaged. Severe overload may sometimes damage the isolation diaphragm.

4. Whether the wiring is loose;

5. Check whether the power cord wiring is correct

terms of settlement

1. If the transmitter power supply is less than 12VDC, check whether there is a large load in the circuit, and the input impedance of the transmitter load shall meet RL ≤ (transmitter power supply voltage - 12V) / (0.02A) Ω

2. Re select the pressure transmitter with appropriate range.

3. If the pressure sensor is damaged, it shall be sent back to the manufacturer for repair.

4. Connect the wire and tighten it.

5. The power cord shall be connected to the corresponding terminal.

3、 Transmitter output ≤ 4mA

Inspection and testing

1. Whether the transmitter power supply is normal

2. Whether the actual pressure exceeds the selected range of the pressure transmitter;

3. Whether the pressure sensor is damaged. Severe overload may sometimes damage the isolation diaphragm.

terms of settlement

1. If the transmitter power supply is less than 12VDC, check whether there is a large load in the circuit, and the input impedance of the transmitter load shall meet RL ≤ (transmitter power supply voltage - 12V) / (0.02A) Ω.

2. Re select the pressure transmitter with appropriate range.

3. If the pressure sensor is damaged, it shall be sent back to the manufacturer for repair.

4、 Incorrect pressure indication

Inspection and testing

1. Whether the transmitter power supply is normal

2. Is the reference pressure value correct

3. Is the range of the pressure indicating instrument consistent with that of the pressure transmitter

4. Whether the input of pressure indicating instrument and corresponding wiring are correct

5. The input impedance of the transmitter load shall comply with RL ≤ (transmitter supply voltage - 12V) / (0.02A) Ω

6. Whether the input terminal is open when the multipoint paper recorder does not record;

7. Is the corresponding equipment enclosure grounded

8. Is it wired separately from AC power supply and other power supplies

9. Whether the pressure sensor is damaged. Severe overload may sometimes damage the isolation diaphragm.

10. Whether the pipeline is blocked by sand and impurities, which will affect the measurement accuracy;

11. Whether the temperature of the pipeline is too high. The service temperature of the pressure sensor is - 25 ~ 85 ℃, but it is better to be within - 20 ~ 70 ℃ in actual use.

terms of settlement

1. If the transmitter power supply is less than 12VDC, check whether there is a large load in the circuit, and the input impedance of the transmitter load shall meet RL ≤ (transmitter power supply voltage - 12V) / (0.02A) Ω.

2. If the accuracy of the reference pressure gauge is low, it is necessary to replace the pressure gauge with a higher accuracy.

3. The range of the pressure indicating instrument must be consistent with that of the pressure transmitter.

4. If the input of the pressure indicating instrument is 4 ~ 20mA, the output signal of the transmitter can be directly connected; If the input of the pressure indicating instrument is 1 ~ 5V, a resistance with an accuracy of one thousandth or more and a resistance value of 250 Ω must be connected to the input end of the pressure indicating instrument, and then connected to the input of the transmitter.

5. The input impedance of the transmitter load shall conform to RL ≤. If not, corresponding measures can be taken according to different conditions, such as increasing the power supply voltage (but it must be lower than 36VDC), reducing the load, etc.

6. If the input terminal of the multipoint paper recorder is open when there is no record, then: 1. No other load can be carried; 2. Use other recorders with input impedance ≤ 250 Ω when there is no record.

7. The corresponding equipment enclosure is grounded.

8. It shall be wired separately from AC power supply and other power supplies.

9. If the pressure sensor is damaged, it shall be sent back to the manufacturer for repair.

10. When the pipeline is blocked by sand and impurities in the pipeline, the impurities shall be cleaned and a filter screen shall be added in front of the pressure interface.

11. If the temperature of the pipeline is too high, add a buffer pipe to dissipate heat. Before use, it is best to add some cold water in the buffer pipe to prevent the overheated steam from directly impacting the sensor, thus damaging the sensor or reducing the service life.

In addition to the above faults, the pressure transmitter is prone to the following faults:

5、 Installation problems  

In steam flow measurement, there are two kinds of steam, one is external steam and the other is superheated steam of boiler. The externally supplied steam is the steam after temperature and pressure reduction. The temperature is not high, and it is mixed with a large amount of water. It is not used when needed. The steam flow is always changed according to the requirements of users. In the actual flow measurement process, sometimes the flow is too large, sometimes the flow is too small, which is very unstable. It is often necessary to carry out blowdown. After each blowdown, the measurement of the transmitter is accurate, but the blowdown times of the steam pipeline are too many, which is easy to lead to steam leakage at each contact on the impulse pipe.

In measuring the use of superheated steam, it is found that the biggest problem is sometimes shutdown. After restart, the flow will deviate, resulting in inaccuracy, and sometimes there is still a little flow display after stopping. Generally, the installation position of the transmitter is lower than the measuring pipe. However, in the actual installation, the condensate tank and transmitter for external steam flow are higher than the measuring pipeline, and the pressure conducting pipeline laid down at least 1m from the throttling device is too short.

The boiler superheated steam flow also has the problem of inconsistent height between the condensate tank and the measuring pipeline, resulting in the height imbalance of condensate and the static pressure difference.

6、 Blockage of impulse pipe  

In pressure measurement, sometimes the indicated pressure does not change with working conditions. After opening the blowdown valve, only a small amount of sewage will not flow out, because there will be a small amount of floating dust in the water quality or compressed air, which will enter the impulse pipe for sedimentation with the water flow. Over time, the pipe wall of the impulse pipe will corrode and accumulate scale, resulting in blockage.

7、 Transmitter equipment failure  

In the measurement of lubricating oil pressure, the lubricating oil pressure signal participates in shutdown interlock control. The signal measured by the lubricating oil pressure transmitter is transmitted to the computer for display. On the other hand, the signal is also compared through the program. When the pressure is lower than 0.06mpa, an oil shortage shutdown signal is sent to stop the machine.

It can be seen from the trend chart of lubricating oil pressure signal that the pressure drops instantaneously and straightly, resulting in tripping. After checking the transmitter, it is found that the internal module of the transmitter is damaged. Although the transmitter is calibrated regularly every year, it can be used for production after passing the verification. After several years of operation, the accuracy, sensitivity, stability and other performance indexes of the transmitter will gradually decrease, and the internal diaphragm and integrated block will also be damaged and failed.

8、 Interference problem  

In the measurement of air compressor exhaust pressure, the exhaust pressure signal fluctuates greatly. After verifying the transmitter, it meets the accuracy requirements and eliminates the fault of the transmitter itself; Check that the impulse pipe and joint are not damaged or leaked, and the connection of signal cable is in good contact. However, the cable route is introduced into the control room through the cable bridge next to the high-level distribution room. There is a lot of electromagnetic interference around.