On rare occasions, differential (DP) transmitters can malfunction while in service. The ability to troubleshoot and locate the possible cause of malfunction of the transmitter is crucial for easy start up of the process that the malfunctioning DP transmitter may have upset. This post will discuss how to troubleshoot issues with a fluctuation reading on Honeywell SmartLine ST700 and ST800 DP transmitters, as well as an overview of common transmitter issues and potential remedies.
What is the issue?
DP transmitters from the Honeywell SmartLine ST700 and ST800 family may present with issues with the fluctuation reading. These issues may stem from faults in the cable, calibration, or I/O of the instrument, problems in communication, or errors with the hardware or configuration.
What is the solution?
Follow the steps below to verify that the transmitter hardware and process
connections are in good working condition. For common problems with DP transmitters, it is recommended to begin with the most likely or easiest problems first. Please follow the precautions below before and during troubleshooting of your malfunctioning DP transmitter:
- Isolate the failed DP transmitter from its pressure source as soon as possible. Pressure that may be present could serious injury if the transmitter is disassembled or ruptures under pressure.
- Do not use a voltage higher than the specified voltage to check the transmitter loop. This may damage the transmitter electronics.
- If there is need to open your DP transmitter troubleshooting, please follow the Honeywell troubleshoot guidelines stated in the manual for the correct steps to disassembly a transmitter.
The table below lists some problems that are common to most DP transmitters, as well as recommended solutions for these problems.
| Problem | Potential Cause | Corrective Action |
|---|---|---|
| Low output or no output | Primary elements | Check the insulation and condition of the primary element. |
| Loop wiring | Check for adequate voltage to the transmitter. Check the mA rating of the power supply against the total current being drawn for all transmitters being powered. Check for shorts and multiple grounds. Check for proper polarity at the signal terminal. Check the loop impedance (should not exceed the specification for your plant). Check the wire insulation to detect possible shorts to ground. |
|
| Impulse piping | Ensure that the pressure connection is correct. Check for leaks or blockage. Check for entrapped gas in liquid service. Check for sediment or debris in the DP transmitter process flange. Ensure that blocking valves are fully open and that bypass valves are tightly closed. Ensure that the density of fluid in impulse piping is unchanged. |
|
| Sensing element | The sensing element is not field repairable and must be replaced if found to be defective. Check for any obvious defects. At this point you may need to contact your manufacturer if there are any defects in the sensing element. |
|
| DP transmitter does not calibrate properly | Pressure source/correction | Check for restrictions or leaks. Check for proper leveling or zeroing of the pressure source. Check the weights/gauge to ensure proper pressure setting. Determine if your pressure source has sufficient accuracy. |
| DP transmitter electronics | Make sure the transmitter connectors are clean. If the electronics are still suspect, substitute with new electronics. |
|
| Power supply | Check the power supply output voltage at the transmitter. | |
| Meter | Determine if the meter is functioning properly. | |
| Erratic output | Loop wiring | Check for adequate voltage to the transmitter. Check for intermittent shorts, open circuits, and multiple grounds. |
| Process pulsation | Adjust damping. | |
| DP transmitter electronics | Make sure the transmitter connectors are clean. If the electronics are still suspect, substitute with new electronics. |
|
| Impulse piping | Check for entrapped gas in liquid lines and for liquid in gas lines. | |
| High output | Impulse piping | Check for leaks or blockage. Check for entrapped gas in liquid service. Check for sediment or debris in the DP transmitter process flange. Ensure that blocking valves are fully open and that bypass valves are tightly closed. Ensure that the density of fluid in impulse piping is unchanged. |
| DP transmitter electronics | Make sure the transmitter connectors are clean. If the electronics are still suspect, substitute with new electronics. |
|
| Loop wiring | Check for adequate voltage to the transmitter. Check for intermittent shorts, open circuits, and multiple grounds. |
Source: Honeywell SmartLine Transmitters
