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Absolute pressure transmitter measurement deviation and fault treatment
Release time:2018-10-30   Clicks:904
Absolute pressure transmitter is used for vacuum or absolute pressure measurement in industrial production. This post, combined with the application of absolute pressure transmitter in the steam turbine unit, introduces the fault treatment of absolute pressure transmitter and application experience in field installation from five aspects.
Based on the collection and arrangement of a large number of field applications of absolute pressure transmitter, the following five absolute pressure transmitter faults are representative. Combined with field applications and fault inspection process, changhui instrument shares experience in the installation and fault treatment of absolute pressure transmitter.
Absolute pressure transducer measurement deviation and troubleshooting yunrun.com.cn/tech/2070.html
1. Abnormality caused by leakage of measurement pipeline
Since the sampling point of the absolute pressure transmitter is the same as the sampling point of the pressure gauge, and the absolute pressure transmitter and the pressure gauge share the measuring pipe and valve, if we find that the operation of the steam turbine and the condenser system is normal during the maintenance, then we should suspect that the vacuum measurement pipe is out of order.
For example, during the regular inspection of the field measuring instruments by our instrument staff, the abnormal DCS of steam discharge vacuum and the vacuum gauge vacuum were found. After the instrument technician's inspection, it was finally found that the reason for the abnormal situation of the measuring instrument was caused by the fracture of the measuring pipe welding point. Field instrumentation maintenance personnel said the rupture was caused by vibration.
The absolute pressure transmitter measures the absolute pressure of the production process. However, welding spot, transmitter connector, valve joint and other places are very vulnerable to failure. Due to the small measurement range of absolute pressure transmitter, we believe that it is often omitted in the process of installation and maintenance. Therefore, it is very difficult for field instrument maintenance personnel to find the leakage of vacuum system during inspection, so such faults will often be found by process operators in normal production process before they know what the problem is.
It can be concluded from the above that the instrument installer must pay attention to the correct location of the pressure transmitter when installing the pressure transmitter, and also pay attention to the connection and welding of the pressure pipe with the screw interface of the transmitter.
2. Improper installation leads to the measurement of fluid accumulation in the pipeline by absolute pressure transmitter and affects the vacuum display
The company's waste heat generating steam turbine unit had a fault in normal operation. When field instrument maintenance personnel repaired the steam turbine unit, they found that the absolute pressure of the condenser vacuum gradually increased during operation. Because the sampling point of the high pressure condenser is very close to the sampling point of the vacuum of the condenser, the maintenance personnel of the field instrumentation checked the high pressure condenser and found that the numerical variation deviation of the high pressure condenser is large. And at the time of the inspection unit load at the same time found no obvious abnormal changes, instrument maintenance personnel repeatedly for low pressure condenser, the condenser vacuum and high pressure condenser for testing and found that the former two display value is close to, and then the deviation is larger, the absolute pressure transmitter is normal, the fault is not caused by pipeline leakage.
The instrument maintenance personnel believed that the turbine unit was installed with five sampling sub-tubes and independent sampling method. Therefore, it was quickly concluded that the fault of the turbine unit was caused by the failure of the installer to properly install the turbine during installation, which resulted in the blockage problem during the operation of the turbine. However, there were no abnormalities in the initial operation and purge test of the steam turbine unit, so the instrument maintenance personnel decided to inspect the measuring tube step by step. And the inspection result also confirmed their original guess is correct, this mechanical breakdown is really caused by the sampling tube installation is not in place.
After the installation failure of the measuring instrument, the instrument maintenance personnel said that after the installation failure of the measuring instrument, they checked all the measuring pressure pipes, and analyzed the problems existing in each measuring pressure pipe installation. As the absolute pressure transmitter measures the vacuum degree, the position of the transmitter must be higher than the equipment where the pipe is installed. In order to ensure the discharge of condensate water into the pipe, the field instrument installer generally adopts the upward tilt method when installing the horizontal section. If the device, the phenomenon of the condensate line not to go out, you should go to check whether there is horizontal afferent pressure tube, if found to have a horizontal installation guide tube, only need to press the guide tube to install, and relative equipment after 45 ° inclined upwards, the guide of pressure pipe purging.
Therefore, after the problems occurred in the installation position of the measuring instrument, the field instrument maintenance personnel rewired all the measuring pipes to ensure that each pipe is installed in accordance with the standard of measuring absolute pressure, so as to avoid the occurrence of the phenomenon that the condensate cannot be discharged in the normal operation of the equipment in the future.
3. Inconsistency between the measuring range of the absolute pressure transmitter and that set by DCS results in measurement error
As we know, there is a problem that often causes the measurement error of the instrument. For example, the absolute pressure transmitter is not adjusted to the same range as DCS. Since absolute pressure transmitter can reflect the actual value of equipment operation only when it is combined with DCS, absolute pressure transmitter can be set according to the required range of the process, and then the range of DCS configuration should be consistent, so as to avoid measurement error.
4. Zero drift of absolute pressure transmitter
For another example, during the normal operation of A steam turbine unit in the air separation unit of the company, the process operator found that the vacuum transmitter value of the high pressure condenser was increasing, and when the field instrument maintenance personnel checked the equipment, they also found that the measurement value of the condensate vacuum also produced deviation. When this was discovered, the instrument maintenance personnel performed simple purging on the pipelines in the equipment as before, but the problem of measurement failure was not solved after the purging. Finally, the instrument maintenance personnel decided to remove the absolute pressure transmitter for further inspection. On the in-depth inspection of the absolute transmitter, it was found that the error value of 0.19mA dc output appeared at each detection point of the transmitter, while the normal error value should be within 0.08mA, with a large gap between the two. Therefore, the instrument maintenance personnel compared this value with the previous test report of the transmitter, and found that the test value before the absolute pressure transmitter was very close to the normal measurement error value. Therefore, the instrument maintenance personnel believed that the measurement error was caused by the zero deviation of a table during operation.
When we look at the previous check and verification records, we find that it is very important that the absolute pressure transmitter is more prone to drift than the difference pressure transmitter and the table pressure transmitter. The reason can be determined by analyzing the structure and measuring principle of the absolute pressure transmitter.
The zero-point drift of the differential pressure transmitter is caused by the fact that both ends of the differential pressure transmitter sensor are symmetrical.
Absolute zero drift of the cause of pressure transmitter is: unlike differential pressure transmitter drift phenomenon, the ends of the absolute pressure transducer sensor chip is asymmetric, and because of the low voltage side is very stable reference pressure, so whether the reference pressure is deflated or cavity leakage, or deformation of the material or damage, all become the influence factors of absolute pressure transmitter drift phenomena appear. Moreover, the zero-shift phenomenon of absolute pressure transmitter cannot be detected by simple detection method. The zero-shift problem of absolute pressure transmitter can only be found by instrument maintenance personnel when the value less than 4mA appears. Therefore, we think that when the absolute pressure transmitter is checked, the instrument that can be directly compared is generally used. If the deviation cannot be checked without the instrument around at that time, the material type of sampling inspection can be adopted to solve this problem. Therefore, when choosing the absolute pressure transmitter, we should choose the absolute pressure transmitter with high performance and high stability, such as the imported rosemount, yokogawa, honeywell, etc., and the homemade absolute pressure transmitter yr-er103, etc. In the changhui instrument network, we can have a comprehensive understanding of the absolute pressure transmitter.
5. Measurement deviation in the maintenance process
The problem of measurement error is often encountered by the instrument maintenance personnel during maintenance. The reasons for this problem are as follows:
The calibration standard instrument is out of alignment
Actually after investigation, check with standard instrument controlled by the emergence of this phenomenon, there is very little, but because of the absolute pressure standard instrument used is very different compared with other standard instrument, absolute pressure standard instrument checks frequency than other standard instrument, so check instrument used in the standards, in accordance with the requirements of the time limit, to send a statutory metrological verification institutions for calibration. In addition, the zero-drift phenomenon of absolute pressure transmitter is difficult to be checked in a simple way, so it is necessary to use instruments that can be directly compared and calibrate through vacuum instrument calibration pump.
Error in fault adjustment
Absolute pressure transmitter is extremely prone to zero drift, and the possibility of error in adjustment during the overhaul of field instrument maintenance personnel is great. The factory's on-site instrument maintenance personnel often make the mistake of "adjusting the zero of the non-absolute vacuum to the zero of the absolute pressure transmitter and the local atmospheric pressure to the full range". Therefore, we should pay more attention to adjust errors and reduce errors in future maintenance.
Effects of ambient temperature
Absolute pressure measuring range is very small, so the change of environmental temperature on its influence is very big, we are checking and debugging of pressure transmitter, if the indoor temperature from 20 ℃ to 8 ℃, its measuring value will be before the numerical partial 3 kpa. Therefore, we should strictly follow the environmental conditions stipulated in JJG 882-2015 pressure transmitter verification regulations. In the process of maintenance, we must pay attention to reducing the impact of environmental temperature on the pressure transmitter and try to minimize the error value of absolute pressure transmitter.
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