On-site instrument failure analysis (one question per week), Part 2 (answers released)!
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This post was last edited by Yiding on 2009-9-25 09:07. Fault phenomenon: When a certain device’s boiler is started up, the readings from the instruments (differential pressure transmitters) are always higher than those shown on the glass gauge. Please analyze the reason for this. (Note: The glass plate level gauge is at the same height as the pressure tapping points for differential pressure measurement.) Answer: Analysis and judgment: When using a differential pressure transmitter to measure the level in a closed container, the pressure conduit is filled with condensate. A 100% negative drift is applied to remove the excess liquid column in the negative-pressure side compared to that in the positive-pressure side, so that the differential pressure ΔP of the transmitter becomes ΔP = ρgh, where h is the height of the liquid surface, ρ is the density of water, and g is the acceleration due to gravity. The range of the differential pressure transmitter is Hρg, where H is the distance between the pressure-taking valves at the top and bottom of the steam drum. During calibration, the density of water is taken as the value corresponding to the boiler’s normal operating conditions, namely ρ=0.76 g/cm3. When the boiler just starts up, the temperature and pressure have not yet reached their designed values; at this time the density of water is ρ=0.98 g/cm3. Although h remains unchanged, the value of ρgh increases, resulting in an increase in output. The glass plate level gauge is only related to h, so it shows a normal reading; however, the reading of the differential pressure transmitter is higher than that of the glass plate gauge. Solution: This situation is temporary; once the boiler is operating normally after some time, the readings on the two instruments will match. No action is required, but it is necessary to explain this clearly to the process engineers. One thing that needs to be avoided is the situation where, because the instrument technician cannot explain the cause of this phenomenon, while the process engineers insist that the readings of the two instruments should match. In such a case, the instrument technician adjusts the zero point of the transmitter so that the readings of both instruments are the same. If the zero point of the transmitter is not adjusted back after the boiler has been operating for some time, the reading of the differential pressure transmitter will be too low
Is it because the pressure sampling point on the positive pressure tube of the transmitter is too low, or is the transmitter not installed in the correct position?
1. Negative pressure side leakage: Treatment – eliminate the leak point； 2. The negative migration amount is too small; solution: recalibrate the zero point ； 3. Blockage in the glass plate level gauge – Solution: Drain the glass plate level gauge.
Since the measurement principles are different, the measurement results cannot be identical; it is sufficient that the trends are similar. 1. The installation position cannot be exactly on a horizontal line; perhaps the pressure sampling point of the transmitter is below point 0 on the glass plate. 2. Is the density set by the transmitter the actual density of the liquid?
Is there no negative transfer? The negative migration amount is still too small
Possible reasons include: 1. If it is equipped with an isolation tank or a balance vessel, the error may be caused by an imbalance in the condensate. 2. Errors resulting from the use of insufficiently accurate density values when calculating the range.
At the beginning of operation, the water temperature and pressure in the boiler have not yet reached normal levels; the density of the water is different from the value used in the calculations, which leads to errors. Once operation is running normally, everything returns to normal. If the deviation is too large, it is possible to adjust the range – make such adjustments after the system is operating properly. .
Check the amount of migration, and verify whether there is any blockage in the glass spots. Check if the condensate on the negative pressure side is fully filled.
Please check whether there is any issue with the zero-point setting of your level gauges: if you set the zero point at the bottom of the tank, the readings from the level gauges may be higher than the actual level in the tank； You can try setting the zero point at a level equal to that of the liquid level on site
Check whether the transmitter is installed and calibrated correctly? Check whether the glass plate level gauge is clogged
This raises an issue: negative migration was not performed or the amount of negative migration was insufficient; re-performing the migration should resolve the problem.