When the Fire Heater Furnace was stopped running for the maintenance, the surface temperature and intemal pressure of the tube were reduced at a lower rate(equivalent ambient). The heated tube which was once enlarged would be decreased in size and volume. Some tubes, which have coke associated, cannot be decreased in to the original size. Therefore, the tube rupture was occurred. The reason of cracking phenomenon will be discussed in the following section as shown in Figure 3-2.