Page 62 - Handbook of Materials Failure Analysis

56 CHAPTER 3 Boiler tube failures: Some case studies

temperature above the safe operating limit for a long duration. As a result, the tube
metal had lost its strength, suffered creep deformation, and failed to support the
working stress resulting in final rupture. The adherent scale, particularly, on internal
surface formed due to exposure at high temperature. It had acted as an insulating
blanket in view of its much lower thermal conductivity than that of the tube metal.
The progressive build-up of scale had greatly affected heat transfer and raised the
tube metal temperature above A1 before bursting of the tube.
Technical data relating to the problem are as follows:

(i) Material specification: BS 3059/622-50 S2
(ii) Dimension of original tube: 38 mm OD 5.9 mm thickness
(iii) Working temperature: 480 °C
(iv) Working pressure: 158.2 kg/cm 2
(v) Running hours: 63,177

2.1.1 Examination details
2.1.1.1 Visual inspection
The tube sample was visually inspected and following physical evidences were
noted.
(i) The failure is open burst rupture. The general view of failed tube sample is
shown below (Figure 3.7).
(ii) Appreciable bulging/swelling at and adjacent to failure area.
(iii) Reduction in wall thickness at the vicinity of rupture.
(iv) Presence of thick black scale on the inner surface of the tube.
(v) No significant evidence of corrosion on the outer surface of the tube.
(vi) Presence of adherent black scale on the outer surface of the tube.

2.1.1.2 Dimensional measurement
Wall thickness measurement using thickness meter (type: DM-3, Krautkramer,
Germany) and outside diameter measurements using vernier caliper were carried
out on the tube sample. The locations of wall thickness and outside diameter mea-
surement are shown in Figure 3.8 and the measured values are shown in Table 3.1.

FIGURE 3.7
General view of burst of platen super heater tube.