Why do false values appear after the instruments are serviced?
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This post was last edited by donfly on 2015-11-27 09:23. Today I read an accident analysis; “Let’s talk about an accident that occurred in the United States, during driving after maintenance work.” The process setup there consists of one distillation tower; what’s special is that the outlet of the safety valve leads to an open tank at atmospheric pressure! Shift A began feeding material in the evening, but there was a problem with the level gauge of that tower, which kept showing a low liquid level. Shift A did not pay attention to this issue, and Shift B, which took over in the morning, also failed to pay attention until noon when they realized something was wrong. They opened the valve for feeding material into the bottom of the tower in order to feed material backward, but the valve upstream of that valve was closed during maintenance, and the shift supervisor did not ask the operators to check and confirm this. The feed continues to flow in, pushing aside the safety valve and entering the atmospheric open tank, from where it overflows directly. And this open tank was located near the office area; on that day, a group of terrorists were inside the building, and then an explosion occurred, resulting in 15 deaths and 138 injuries. The biggest design flaw is that the outlet of the safety valve leads to an open tank at atmospheric pressure, allowing the material to come into direct contact with the atmosphere. The biggest operational mistake is that right after maintenance, many instruments may show false readings, so it is necessary to remain vigilant at all times. ” I haven’t had much hands-on experience on site, so I’m not quite sure what that last sentence means. Why do the instruments show false values after maintenance? Have you all encountered this situation?
It depends on the type of instrument; for example, the isolation fluid used after maintenance on certain orifice plates is different from that used during normal operation, and variations in the process conditions also come into play, so it’s necessary to analyze each situation individually
Differences between the instrument readings and the actual conditions are caused by unstable (fluctuating) processes.
After the instruments are calibrated, they need to be monitored.
Isn’t there a on-site level gauge? Why isn’t it working? Inspections are essentially meaningless. . . . . . . . . . .
If you have worked in a petrochemical company for some time, you will hear the phrase \"Start up the instruments.\" During the startup phase, the instruments have to operate at full capacity, and instrument maintenance technicians are busiest at this time; therefore, it is normal for instruments to malfunction right after maintenance has been carried out.
The answer above is correct; for the instruments to function properly, certain conditions must be met. During the startup of the process equipment after maintenance, its operation is not stable, and many units fail to meet the conditions required for the instruments to function properly; therefore, instrument failures are quite common. As mentioned above, this stage involves the most work and is the most exhausting for instrument maintenance workers.
It must have been an accident that happened N years ago; these days, such phenomena are rare in instruments. Moreover, for measuring the liquid level in a tank, at least two different methods of measurement are required. Even if the density of the process fluid differs between the initial stage of operation and normal production, there should still be a trend. This situation will not occur.
Under normal circumstances, the tank used for storing alkaline solution is used to hold water; is inaccurate liquid level measurement also due to the instrument?