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In our Vacuum Unit we have 2 Identical Trains, Train -1 constructed in 1984 and 2 constructed in 1989. Our Vacuum system is Wet Vacuum with 2 Pre- Condensers in parallel and 3 Stage Ejector system, each having 3 no. of Vacuum Ejectors, with Condenser, with the cooling medium being Sea Water and the Sea Water entry is to the Pre-Condensers and it's return going to the other 3 Stage condensers simultaneously and also provided with a By-Pass to increase the flow to those condensers if and when required. Actually in the middle of May 2023, we experienced sudden break in vacuum in only Train - 1 and this phenomenon was continuing on & off, with this phenomenon occurring every 7-10 days and would resolve by switching the Ejectors and /or hammering the Pre-Condensers leg lines, till our Scheduled Biennial Turnaround in October 2023. And after Start-up of the unit on October 31, 2023, till December 30/31 2023 for about a couple of months the unit was running very smoothly, before the vacuum issue started cropping up again and this time the frequency of the drop/break in Vacuum was about 10-15 days, continuing upto March 2024. But, our Operation personnel thought this is due to the Pre-condensers leg line getting blocked due to Metal rust & corrosion and instructed to externally Hammer those leg lines twice in every shift from February upto Middle of March 2024., and by that that issue also resolved. And for a period of more than 6 months upto 2024 October first week, we did not face any issue except little increase in Vacuum twice or thrice when sea water flow was getting dropped, or more lighter in feed from Tankfarm. Also, as compared to the other Train, we were having 2 ejectors more online i.e.., 6 Ejectors with 2 each in all the 3 stages. But again we faced sudden break in Vacuum itself on October 5th at around 04:00 hrs, even with unit running on extremely low Throughput. So, I kindly request you to help with this issue. Note:- As per my observation, I have noticed 2 separate phenomenon happening with none being related to each other or happening together even once. 1. Pre-Condensers leg line temperatures getting low 2. 3rd Stage Condenser outlet temperature getting high which in turn increases the pressure of the non-condensible from the Hotwell increasing the back-pressure of the system irrespective of burning in our Heater or flaring it.

Nov-2024

Answers

Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

This is a very interesting practice case study. The use of sea water for the cooling system put under suspicion the fouling formation in the cooling water system. Despite the issues being observed only in one of the two crude distillation trains, it's possible to occur preferential deposition according to the alignment arrangement of the cooling water tubulations that feed the heat exchangers. Under this context it's important to check through external ultrasonic meters, if the cooling water is adequately distributed to the heat exchangers. Another key question to be verified is the quality of the steam applied in the ejectors. Again, according to the supply tubulation arrangement the steam can be fed under a saturated condition to the ejectors, leading to subsonic flow in the ejectors and consequently poor performance. Another question which can be considered is the feed quality to the crude oil distillation train. Some paraffinic crudes can suffer thermal cracking in the fired heaters and overload the top of the vacuum tower leading to high operating pressure, this can occur in some time intervals considering the changes in the crude oil slate processed by the refinery. Another question is related with the performance and gasoil and vacuum residue yields in the problematic distillation train during the vacuum break. If there is a corresponding change in the yields (lower gas oil production and higher vacuum residue production), the pressure transmitter of the vacuum tower is calibrated? The intermittent characteristic of the phenomena is intriguing, but a possibility is the occurence of self-sealing failures in the heat exchangers tubes, especially considering the use of sea water as cooling water (high fouling rates). This possibility is reinforced by the information of material deposition in the barometric leg lines. Following through a root cause failures tree, it's important to verify the performance of side withdrawal pumps. Poor performance of these machines can lead to high pressure drop in the fractionating sections and impact the tower pressure. The same verification it's necessary to sour water draining pumps of the top vessel. How is the temperature of the cooling water system? Is it possible to identify the coincidence of vacuum broken events with the moments where the cooling water temperature is high? As previously quoted, the cooling water circulation rate to the problematic train is in compliance with the design? The performance of the atmospheric tower is another verification point, its occurring light degradation (like heavy diesel) to the atmospheric residue overloading the condensation system of the vacuum tower due to the dragging of light compounds, this can occur according to the characteristics of the processed crude slate. This effect can also occur due to a failure in the pre-heating exchanger battery of the crude oil distillation unit, mainly those that exchange heat between vacuum gas oil with crude oil. It's also important to verify the possibility of air entry in the cooling system and the vacuum system, I believe that this verification was already carried out, but it's an important verification in a root cause analysis. Considering the case description, the operating issue seems to be related to poor performance of the condensation system of the vacuum tower top section. A good strategy is to make a frequent flushing and backwash of the cooling water supply alignments to the condensers aiming to help to remove fouling under operation. If there are no necessary alignments to promote this operation, it can be a good change to be installed in both crude oil distillation trains in the next maintenance shutdown.

Nov-2024