PSI - Issue 13

Reza Khadem Hosseini / Procedia Structural Integrity 13 (2018) 232–237

233

Reza Khadem Hosseini / Structural Integrity Procedia 00 (2018) 000 – 000

2

Overheating failures are often classified as either short-term or long-term. The former frequently exhibits a thin lipped longitudinal rupture with tube bulging. While, the later can lead to tube bulging or thick-lipped rupture failures. Although, short-term overheating can occur when tubes exposed to high operating temperatures, typically above the eutectoid transformation temperature of 727 °C, long-term overheating is related to operating temperatures above 454 °C. In order to recognize and determine the kind of overheating failure, microstructural examination is used as the most efficient method (Lobley et al., 2008; The Nalco Guide, 1991). The aim of the current study was to investigate the causes of tube failure in a flue gas boiler working parallel with other two boilers in the waste heat recovery unit with a shared steam drum that produce High Pressure steam of ammonia production plant by 220 ton per hour. The design temperature and pressure of tubes in the boiler were 325 °C and 11.7 MPa, respectively. The failed tube was located 4 m. under the roof of combustion chamber. The studies were performed on a failed steam tube that had been in service for more than 30 years. Technical inspection histories had not been showed noticeable damage during these years. Recently, due to the problems with the instrumentation of controlling the level of the steam drum, boiler tubes have been exposed to water shortage, and has caused tube failure. Figure 1 shows the cross section of the failed tube. Firstly, the examination of this surface indicates that the thickness of the cross section differs in different sections. The thickness of 13 points was measured on circumference of tube at 30° intervals given in Tab. 1, from "A" to "M". The results revealed that the least thickness was seen in the adjacent areas to the failure opening with 3.56 mm, and the most one was in areas away from the failure opening with 5.11 mm. Secondly, the tube was oval where the maximum and minimum outside diameters are 75.5 and 68.5 mm, respectively, with a 10% deviation from the circularity of the original size. Finally, the rust and black thin deposit was seen on the internal surface of the tube. 2. Experimental Procedures 2.1. Visual observations

Table 1. The wall thickness of cross section of tube according to Fig. 1.

Thickness ( mm )

Thickness ( mm )

Point

Point

A B C D E

3.56 3.79 4.20 4.51 4.70 4.95 5.11

H

4.99 4.90 4.72 3.85 3.76 3.62

I J

K L

F

M

G

Fig. 1. The cross section of the failed tube.