Share your experiences/incidents with technical accidents 004: The neglected bypass valve

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This post was last edited by altpage on 2016-12-25 at 17:26. It contains the replies given to forum members in the production section some time ago, and it’s being shared with them again； What I mainly want to point out is this: details determine success or failure. A small bypass valve that wasn’t fully closed caused the dry point of the crude gasoline in the catalytic fractionation tower to be below the required standard, and it also led to the top circulation reflux pump drawing in water and stopping its operation intermittently; as a result, stable operation couldn’t be maintained for three days. How can routine inspections uncover more details and reach every nook and cranny? Catalytic fractionation results in a high dry point for the crude gasoline; reducing the top of tower temperature to 105 degrees resolves this issue and makes the dry point acceptable. However, the reflux stream from the top of the tower contains water, and the pump used for this reflux stream is prone to cavitation, which can lead to tower flooding ； It kept happening for several days without any solution. At that time, there was no pressure indicator on the top of the tower; only on-site gauges were available, and they were installed at a high position. Routine inspections are hardly carried out, and no one pays attention to the pressure at the top of the tower; everyone assumes that it remains stable. Since we don’t have any means to control it, we simply ignore it. I’ve failed consecutive tests for several days now; the assessment forms from the production department keep coming in every day, and I’m really frustrated ； That day, the poster, being in a bad mood, went up to the reflux tank at the top of the distillation tower to take a look. He checked the pressure there – wow, 64 kPa ； It used to be a steady 50 kPa; why is it so high now? Nothing has changed again. But the reason for the failure was finally identified: excessive pressure at the top of the tower! The pressure at the top of the tower is too high. Why? It has been working normally all along, and there haven’t been any major changes recently either! Feed rate, reflux rate, top temperature, temperature after cooling... each of these is checked individually; even the composition of the rich gas and the boiling range of the crude gasoline remain relatively stable ； I was completely puzzled and stuck in frustration... That day I was in a bad mood, so I went to the top of the distillation tower and the reflux tank. The pipes at the top of those tanks were quite complex. Suddenly, my hand touched the pressure-supplementing line that carried gas back to the top of the distillation tower; it felt slightly hot to the touch. It was strange – the control valve for that line was already closed, as were the valves at both ends, so how could there still be flow of material in that pipeline? Going down to the ground level and checking the control valve assembly, I found that the bypass was warm, indicating clearly that material was flowing through it. A \"F\" wrench was used to tighten the bypass valve a couple of times; the sound of fluid flowing in the pipeline stopped immediately. The control room was promptly informed to adjust the operation of the distillation tower, raising its top temperature to 110 degrees. Two hours later, the reflux stream from the tower no longer contained water, and its flow rate also stabilized ； Sampling again after one hour: The dry point of the crude gasoline is satisfactory! The problem that had been plaguing the device for three days was accidentally solved.