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Recently it was observed that the some of the radiation tubes of our atmospheric distillation heater were deformed. The tubes have been in operation for almost 30 years. Some of the tubes (specially at the middle section) deformed to the center of the furnace. Some deformed laterally to the adjacent tube. I want to know the possible reasons behind the phenomenon. Also please advise me what is the standard of replacement of the tubes in this mentioned condition.

 
Answers
25/06/2011 A: Sudhakara Babu Marpudi, Oman Refineries and Petroleum Industries Company, m_sudhakarababu@yahoo.com
To answer this question, it is important to know if the deformed radiant tubes are vertical or horizontal. Most probable reason for the Deformation towards the centre of the furnace is the coking in side the tubes due to flame impingements caused by in correct flame patterns. Check the burner alignments to ensure that the flames are vertical and the flame lengths are within the recommended limits. Lateral Deformation could be due to the blockages in the expansion path of the tubes. Furnace tubes will get deformed when the floating guide supports are choked with debris. Vertical tubes will be provided with a top support leaving the bottom sections hanging free but are provided with guides that will guide the expansion vertically and will not allow lateral movement of tubes. In case of horizontal tubes, the header boxes housing the tube bends facilitate the expansion while the sliding supports in the pipe sleeves guide the expansion into the header boxes. It is necessary to know if the lateral movement is actually towards the bottom tube (sagging of the tubes) or towards the upper tube (upward bowing) inicase the tubes are horizontal.
23/06/2011 A: Raghavendra Sangam, Larsen and Toubro Ltd, rsangam0504@yahoo.co.in
(1) It may not be possible for readers sitting remotely to comment on the reasons of tube failure. Experts need to physically examine the fired-heater, tubes, operating conditions before the incident and many other factors to find out the causes.
(2) Fired-heater or furnace tubes are normally designed for a life of 10 years. If the tubes have served for 30 years without a major incident, it is really surprising but worrysome as they have not been inspected/replaced. Please check the design specs just to verify if the tubes have been designed for more than 20 years or so.
(3) This is an excellent opportunity to re-consider the changed feed or plant operating conditions to re-design and revamp of the radiant as well as convection section tubes, refractory and internal supports etc. You may introduce tube skin temperature sensors which probably were not there 30 years ago. Consult experts in fired-heater design and revamp.
22/06/2011 A: Mike Watson, Tube Tech International Ltd, mike.watson@tubetech.com
Heat maldistribution. Convection / radiant sections typically get severely fouled between fin and are inaccessible and therefore uncleanable. Speak to Selas-Linde.com +49 89 7447 47 51 Robert Tikovski who are working with Tube Tech International UK to modify existing furnaces to be cleanable off and on-line.