Open Discussion Forum Question 3
3. We have 192 selas vortex K-9206 burners installed on the vertical walls of our ethylene cracking furnace. The walls are I.F. brick lined. Furnace tubes are vertically hung along the centre of the furnace. The burners are fired with natural gas. Coil outlet temperature (COT) of the furnace tube controls the gas flow to the burners. If the COT goes low, gas flow to burners would increase to increase the heat duty and vice versa. Over the years, COT has been increased to increase the ethylene margins. Due to this, the original tip opening of the burners has been increased from 3.3mm to 3.73 mm. this modification in the burner tip was carried out about one year ago. We are now observing high tube failures with indications of high surface temperatures on failed tubes. What is the groups experience with similar modification? Asian Ethylene Producer (12 Dec 2003)
Comment from Texas Ethylene Producer -
By increasing burner capacity you have now the ability to put more heat into the furnace. This helped you to process more feed. But by this you have also increased the heat flux on the process tubes and the firebox operating temperature. Increased heat flux (and the peak to average heat flux) increases the TMT. You are now operating at relatively higher TMT proportional to the increased firebox temperature. This increased heat flux (and TMT) increases the coking rate and faster carburization of the tubes. This is the reason for the tube failures.
You can obviate failures by one or two methods. Besides continuously monitoring that the burners are performing well (uniform firing) you may come down to decoke at a shorter run time than you had before. Also you may check the tube metallurgy going to higher metallurgy ( 35:45 Ni:Cr) with rare metals to minimize carburization in the next turn around. That is if you don't have them now. Hope this information sheds some light on your problems.