Open Discussion Forum Question 7

7.      Asian Ethylene Producer  (May 2005)

We have five ethylene cracking furnaces in a Gas cracker plant where we have a problem of tube leaks in convection bank, dilution steam section.  What is the best way to repair the furnace tube leaks?

Answer from Furnace Consultant

Convection section designs are normally robust.  However, convection section failures are not very uncommon. They are normally either due to convection section flue gas maldistributions or due the debris build-up on the top of the lower convection section tubes. This brings in higher temperature flue gas into the dilution steam bank than for which the banks have been designed.

The normal procedure for repair is as follows:

1.    From the original design details locate exactly the dilution steam bank position. Various banks are normally prefabricated and stacked one over the other.  Cut open the outside casing (normally 3/16"  carbon steel) on both sides, where the dilution steam enters and leaves, and just remove the window area of the dilution steam bank removing the header box and  two layers of insulation bricks (depending on the location of the dilution steam bank you may have 6" LHV castable and a 3"diatomaceous earth block , good for ~1000degC, layers).  Removing them carefully will fully expose the tube sheet.

2.     Locate the leaking tubes and they can be cut and pulled out and replaced with the new tubes.

3.    Weld tubes and pressure test.

4.    Convection section will have to be air blown before resealing.

If the dilution steam bank leak is occurring on all the five furnaces then you may have to review the operation and find out the reason for such leaks.