【Technical Post】Measurement Errors and Fault Handling of Absolute Pressure Transmitters

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Absolute pressure transmitters are used in industrial production for the measurement of vacuum or absolute pressure. This article focuses on the application of absolute pressure transmitters in steam turbine units, and presents experience in troubleshooting such transmitters as well as their on-site installation from five different aspects. http://yunrun.com.cn/upload/201807/09/201807090111483978.png Based on the collection and analysis of numerous field applications of absolute pressure transmitters, the following five types of failures in such transmitters are considered representative. Changhui Instruments shares its experience in the installation of absolute pressure transmitters as well as in dealing with related failures, drawing on insights gained from field applications and fault diagnosis processes. Measurement errors and troubleshooting of absolute pressure transmitters yunrun.com.cn/tech/2070.html 1. Abnormalities caused by leaks in the measurement pipeline Since the sampling point of the absolute pressure transmitter is the same as that of the pressure gauge, and both the absolute pressure transmitter and the pressure gauge share the same measurement pipeline and valves, if we find that the turbine and condenser systems are operating normally during maintenance, then we should suspect that there is a fault in the vacuum measurement pipeline. For example, during a routine inspection of the field measuring instruments by our instrumentation staff, it was found that there were abnormalities in both the DCS display value for the turbine exhaust vacuum and the reading on the vacuum gauge. After inspection by the instrumentation technicians, it was ultimately determined that the cause of the abnormality in the measuring instruments was a crack at the welding point of the measurement pipeline. The on-site instrument maintenance personnel stated that the rupture was caused by vibration. An absolute pressure transmitter measures the absolute pressure in the production process. Places such as welding points, transmitter connectors, and valve connections are all areas where failures occur quite frequently. Due to the limited measurement range of absolute pressure transmitters, we believe that they are often overlooked during installation and maintenance. Therefore, it is difficult for field instrument technicians to detect leaks in the vacuum system during inspections; as a result, such faults are often discovered by process operators during normal production, only then is it known where the problem lies. From the above, we can draw a conclusion: when installing gauge pressure transmitters, installers must pay close attention to ensuring that the transmitter is placed in the correct position, and they must also make sure that the pressure guide tubes are properly connected to the threaded interfaces of the transmitter and welded securely. 2. Improper installation led to fluid accumulation in the measurement pipeline of the absolute pressure transmitter, which affected the vacuum reading. The waste heat recovery steam turbine units of the company experienced malfunctions during normal operation; when maintenance technicians inspected the turbines, they found that the vacuum level in the condenser increased gradually as the units were in operation. Since the sampling point of the high-pressure condenser is very close to the sampling point for the condenser vacuum, the field instrument maintenance personnel inspected the high-pressure condenser as well and found that the values from it varied significantly. Meanwhile, no significant abnormalities were detected when checking the unit load. After repeated tests on the low-pressure condenser, the condenser vacuum, and the high-pressure condenser by the instrument maintenance personnel, it was found that the readings for the first two were similar, while there was a large discrepancy between the readings of the latter two. This confirmed that the absolute pressure transmitter was functioning properly, and that this fault was not caused by a pipeline leak. The instrument maintenance personnel believed that the turbine unit was equipped with five sampling branches, arranged using an independent sampling method. It was therefore quickly concluded that the malfunction of this turbine unit was caused by the installation crew failing to install them properly, which led to blockages during the operation of the turbine. However, no abnormalities occurred during the initial operation and purging tests of the turbine unit, so the instrument maintenance staff decided to check the measuring tubes gradually. The inspection results also confirmed that their initial assumption was correct – the mechanical failure was indeed caused by improper installation of the sampling tube. The instrument maintenance personnel stated later that, following this malfunction in instrument installation during the measurement process, they inspected all the pressure tapping tubes used for measuring primary instruments, and analyzed the issues present in the installation of each such pressure tapping tube. Since the absolute pressure transmitter measures vacuum level, it must be installed at a position higher than the equipment in which the pressure guide tube is located. To ensure that condensate water can flow out of the pressure guide tube, field instrument installers generally use an upward tilt when installing the horizontal sections. If condensation water cannot be drained from the equipment, it is necessary to check whether there are pressure guide tubes installed horizontally inside the equipment. If such horizontally installed pressure guide tubes are found, they simply need to be reinstalled at an angle of 45° upward relative to the equipment, after which the guide tubes can be purged. Therefore, after the issue arose with the installation location of the measuring instruments this time, the on-site instrument maintenance staff redid the wiring for all the measuring tubes to ensure that each pressure transfer tube was installed in accordance with the standards for measuring absolute pressure, thereby preventing the accumulation of condensate from occurring during normal operation of the equipment. 3. Measurement errors occur due to a mismatch between the range of the gauge pressure transmitter and the range set in the DCS. As we know, there is a common issue that leads to measurement errors in instruments, namely failing to adjust the range of the gauge pressure transmitter to match that of the DCS. Since an absolute pressure transmitter can only reflect the actual operating values of the equipment for process control when it is connected to a DCS, it must be set according to the range required by the process; moreover, this range must match the one specified in the DCS configuration, in order to avoid measurement errors. 4. Zero drift of the absolute pressure transmitter. For example, during normal operation of Unit A of the steam turbine set in the company’s air separation plant, process operators noticed that the reading of the vacuum transmitter for the high-pressure condenser was increasing continuously. Moreover, when field instrument technicians inspected the equipment, they found that there was also a deviation in the measurement values of the condenser vacuum. Upon discovering this, the instrument maintenance personnel, as usual, performed a simple purging of the pipelines inside the equipment, but this purging did not resolve the measurement issue this time. Ultimately, the instrument maintenance technician decided to remove the absolute pressure transmitter for further inspection. Upon a thorough inspection of the absolute transmitter, it was found that each measurement point of the transmitter exhibited an error value of 0.19 mA in its DC output, whereas the normal error value should be within 0.08 mA; there is thus a significant difference between the two. The instrument repair technician then compared this value with previous test reports for that transmitter and found that the previous measurement values of that absolute pressure transmitter were all very close to the normal measurement error range. Based on this, the technician concluded that the current measurement error was caused by a zero offset during the operation of the gauge. When reviewing previous calibration records, we discovered something very important: compared to differential pressure transmitters and gauge pressure transmitters, absolute pressure transmitters are actually more prone to drift. The reason for this can be determined by analyzing both the structure and the measurement principle of the absolute pressure transmitter. The reason for zero drift in differential pressure transmitters is that the two ends of the sensor element in such transmitters are symmetrical, which leads to zero drift. The cause of zero drift in absolute pressure transmitters is as follows: unlike the zero drift phenomenon in differential pressure transmitters, the sensing elements of absolute pressure transmitters are asymmetric at both ends. Moreover, since the reference pressure on the low-pressure side is very stable, factors such as leakage from the reference pressure, cavity leaks, or deformation or damage to the materials can all contribute to zero drift in absolute pressure transmitters. Moreover, the zero-point drift of absolute pressure transmitters cannot be detected through simple testing methods; maintenance technicians can identify this issue with zero-point drift only when a value below 4 mA is observed. Therefore, we believe that when inspecting absolute pressure transmitters, instruments that allow for direct comparison are generally used. If no such instrument is available at the time to check for deviations, a sampling approach can be employed to address this issue. Therefore, when choosing an absolute pressure transmitter, one should opt for ones with high performance and high stability, such as imported brands like Rosemount, Yokogawa, and Honeywell, as well as domestic options like the single-crystalline silicon absolute pressure transmitter YR-ER103. On Changhui Instrument Network, detailed information on absolute pressure transmitters is available. 5. Measurement errors during maintenance processes: Instrument repair technicians often encounter measurement errors during maintenance. The reasons for this usually lie in the following aspects: ① Inaccurate standard instruments for calibration. In fact, investigations show that inaccuracies in standard instruments used for calibration occur very rarely. However, since the use of absolute pressure standard instruments differs significantly from that of other standard instruments, they need to be checked more frequently than those others. Therefore, the standard instruments used for calibration must be sent to a legally authorized metrological testing institution for calibration at regular intervals as required. Furthermore, the zero drift of absolute pressure transmitters is difficult to detect using simple methods; therefore, it is necessary to use instruments that allow for direct comparison, and the pumps must be calibrated using vacuum gauges. ②Adjustment errors: Absolute pressure transmitters are prone to zero drift, and there is a high likelihood of adjustment errors occurring during maintenance by field instrument technicians. Field instrument maintenance technicians in factories often make two mistakes: adjusting the zero point of a non-absolute vacuum to the zero position of an absolute pressure transmitter, and adjusting the local atmospheric pressure to the full scale. Therefore, in future maintenance work, we need to pay more attention to correcting errors in order to reduce discrepancies. ③Effect of ambient temperature: Since the measurement range of absolute pressure is very narrow, changes in ambient temperature have a significant impact on it. When calibrating and tuning an absolute pressure transmitter, if the room temperature is changed from 20°C to 8°C, its measured value will increase by 3 kPa compared to the previous value. Therefore, we should strictly adhere to the environmental condition specifications outlined in JJG 882-2015 \"Calibration Regulations for Pressure Transmitters.\" During maintenance, it is essential to minimize the impact of ambient temperature in order to reduce the error values of absolute pressure transmitters as much as possible.