Week 5: Potential Cause of Fouling Identified
Week 5: Potential Cause of Fouling Identified
Hello everyone! Welcome to week 5 of my Senior Project. This week, my advisor and I uncovered a potential cause of accelerated fouling that we believe is likely the cause of the step-shifts we saw the week prior.
Step-Shifts
As I discussed last week, we identified step-shifts within the fouling rate within the coker heater. Basically, not only did the average tube skin temperatures rise drastically, but we also noticed the rate of fouling increased. We dived deeper into what was being fed into the heaters at the times and how, and we identified something that we think could be a potential cause of accelerated fouling.
Boiler Feed Water Injection
Boiler feed water (BFW) is injected in the form of velocity steam to prevent the crude from cooking and forming deposits on the tubes and to continue the flow of the crude unit through the tubes. However, the tubes can only withstand a maximum pressure from the crude oil and the BFW. We noticed that when the feed of crude is increased, the BFW injection remains the same on a manual setting, which, due to increased flow of crude, causes the crude to coke onto the tubes. An easy solution would be to create a ratio of BFW injection to the crude rate. However, that can cause troubles as if the maximum pressure of the tube is reached, catastrophe can strike. Unfortunately, at Valero, most process engineers are “fire fighting”, which essentially means they only have time to work on the present and continue operations. Because of this, they don’t have time to look back and identify the causes of their problems.
As seen in the graphs below, the ratio of BFW injection to crude feed drops after a certain point, which is where we see the tube skin temperatures begin to rise. We noticed this trend in all areas where our step shifts occurred. The decreased ratio of BFW to crude simply doesn’t propel the crude quickly enough through the tubes while being heated, causing our fouling.
Our Idea
To reduce fouling within heaters, our idea is to set a ratio for BFW Injection to Crude Feed Rate while still maintaining pressures below the maximum. While output may decrease, this will allow for heaters to run for longer periods of time without requiring downtime for cleaning, as well as potentially decrease the numbers of cleaning.
Summary
Next week, my advisor will be visiting the UK, so I will likely be running the same exercise on the crude and coker furnaces to look at periods of fouling and attempt to determine the cost breakdowns for spalling vs. pigging.
Thank you for reading! I can’t wait to share what next week has in store!