Accident discussion: What consequences can a leak in the differential pressure level gauge of a stripping tower cause?

It’s really awkward to be the only one sitting on the sofa. The liquid level control valve shutting off automatically inevitably leads to a sharp rise in the liquid level, until the tower is full. The shutdown is carried out gradually, but production continues during this time; dozens of tons of process condensate per hour quickly fill up most of the interior space of the stripping tower. At this point, the controller should refer to the historical flow rates of the process condensate at the bottom of the tower and make manual adjustments in order to maintain a safe liquid level for a certain period of time, until the stripping tower is shut down. Otherwise, the process steam at the top of the tower will contain liquid, and the consequences… are easy to imagine.

This post was last edited by Small and on 2017-9-13 at 17:15. Unfortunately, the shift leader lacks the necessary skills; none of the personnel on duty have the ability to grasp the overall situation, let alone the basic skills to handle ordinary accidents. The level control valve remains in automatic mode, with a set point of 50%; it stays tightly closed. As production continues, the liquid level in the stripping tower keeps rising. When a change was observed in the temperature of the process steam at the top of the tower, it indicated that the liquid at the tower top was boiling violently, resulting in flooding; the steam contained water, which flowed into Section 1 of the furnace. As can be seen from the historical data from the DCS, the temperatures of the process gas at the inlet and outlet of Section 1 of the furnace dropped suddenly before rising sharply again, in such a repeating cycle. The liquid from all the separators downstream was sent to the stripping tower. This vicious cycle continued for a while until the vibration level of the synthesis compressor reached the interlock threshold, causing the unit to shut down and triggering a full-system shutdown – almost bringing the air separation unit to a stop as well. The syngas compressor was converted into a turbine for a while; fortunately, the quality of that imported equipment was very good, and no problems arose later on, which was really lucky. It was inevitable that the catalyst life of one section of the furnace would be damaged; subsequently, the conversion rate in that section decreased, and the weld cracks and explosions at the outlet of that furnace were all related to this accident. A lesson to take as a warning.

Hehe. . . What is the purpose of what the operator sets? Important controls are equipped with alarms both in the control loops and in the monitoring flowcharts. For example, for liquid level, it is possible to set alarm functions for upper limits, lower limits, upper-upper limits, and lower-lower limits within the DCS control strategy configuration. Once the liquid level goes out of range and returns to zero, an alarm sound will be emitted immediately in the control room; is this sound not sufficient to prompt the operator to check for the fault? Is it not sufficient for the operator to switch the control loop to manual mode, and not enough to maintain a constant flow rate of the liquid phase extracted from the stripping tower through manual adjustment? Not sufficient to ensure the safe operation of the stripping tower? ? How then did the tower flooding accident occur? ? What if these are not enough to complete it? Can’t the instruments on site break? If one of the instruments fails, this production equipment has to be stopped. Well, with that situation, it’s likely the device won’t be used for more than a few hours a year.

That’s why it’s said that one should treat senior employees and experienced workers well; when there’s nothing much to do, they can at least drink tea and chat. True character is revealed at critical moments.

The level gauge was broken, and the alarm kept sounding; the stripper tower had to be disconnected from the system in order to resolve the issue. The operator did hear the alarm; she knew it was a problem with the instrument, but she ignored the control valve. When problems arose later in the plant, she became panicked and confused, making mistake after mistake. In fact, she only knew how to read the instruments without using her judgment – or rather, she wasn’t qualified to be in charge of operations. The alarm was caused by a fault in the instrument, not by a problem in the process system, yet the operator turned what was essentially just an instrument failure into a shutdown incident.

This was a major accident that occurred due to issues with minor details, and it happened on a large-scale installation. Although it was a new factory, its technical capabilities were definitely top-notch, ranking among the best in the country. But accidents can happen at any time, and that’s exactly what happened here. This large-scale installation operated for long periods of time, often running for over 200 days each year; I remember once it ran for more than 270 days. If it weren’t for the shortage of natural gas, it would have run for 300 days as well. Even so, accidents that seem impossible often occur. There is an old saying in my hometown: \"The river kills the plumber.\" Sometimes, overconfidence leads to carelessness and blindness to one’s own shortcomings; by the time something goes wrong, one is completely at a loss and terrified.

The process lacks HACCP analysis

This post was last edited by Small and on 2017-9-11 at 17:26. Even with such a liquid level indicator, it doesn’t necessarily represent a key point; a location that isn’t considered critical can still be the source of accidents. Sometimes it’s not about controlling the vital points or addressing critical issues, but rather whether proper technical training is actually carried out on a regular basis. Regarding the process principles and why the workflow is set up in this way, more training and practice are needed. Operators should be aware of the consequences of each action they take, especially the shift supervisors. If the supervisor gives unclear instructions, it leads to excessive workload for those on the job site, and they may also end up in danger for no apparent reason.

Attach a photo of the flowchart for the stripping tower

No alarm should be taken lightly! Prolonged alarms can cause people to become numb! Over time, an accident happened!