Failure Investigation of Rear Water Wall Boiler Tube of a Coal Fired Power Plant

Abstract

This paper presents the failure investigation of rear water wall boiler tube in Unit 3, Kapar
Eenergy Venture, which failed on the 6th of March 2012. The methods applied in this study
are (i) visual inspection, (ii) non destructive testing (NDT), (iii) microscopic analysis, (iv)
hardness and bending testing, and (v) Larson-Miller Parameter analysis. Visual inspection
revealed that a rupture was developed at the failed tube which is usually caused by
overheating. The inner surface of the tube was observed for the presence of oxide scale or
some other deposits but it was found to be clean. Dimensional measurement showed that
the wall has suffered thinning where the thickness reduced about 8 percent of its original
thickness. The microstructure of the failed tube was compared with that of a virgin rear
water wall tube of same material and no significant changes were observed. Hardness was
measured and found that it exceeded the maximum allowable hardness. From the bending
test, plastic deformation was found at 741 MPa while the maximum stress applied on the
specimen was 1120 MPa with a deflection of 25.1 mm. Larson-Miller Parameter analysis
shows that the estimated metal temperature at the time of the failure was 446.38°C which is
higher than the operating temperature. Based on the results obtained, the mechanism of
failure was found to be overheating and erosion, however, the root cause of failure
mechanisms was not identified. By revisiting and inspecting further away from the tubing
area, a malfunctioned soot blower was found located near the failed tube. The steam from
the malfunctioned soot blower was hitting the failed tube for a long period causing the
surface of the tube to erode. The eroded part became thin causing the metal temperature to
rise making the metal turn into brittle due to short-term overheating. When the surface of
the tube became thin and brittle, the internal pressure forced the tube to rupture and burst.