Boiler Tube Failure Report

Supplied by FTS QA/QC Dept  

Company: ABC Chemical Company
Contact: QA/QC Mgr
Project: BTF-012
Date: November 21, 2000


Work Performed

A section from the water wall tubes from a boiler, ID number 4C-8819 manufactured by Nebraska Boiler Company was submitted for analysis. The tube sections had failed in service. The analysis was performed to determine the cause of the failure. 

The analysis consisted of a visual examination, a chemical analysis, metallography, hardness tests and a deposit analysis. 

Results

 

Visual Examination

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Figure 1. A photomacrograph of the tube sections submitted for analysis.

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Figure 2. A photomacrograph of one of the bulged zones.

A section of water wall tubes 34 inches long and 20 inches wide containing five tubes was submitted for analysis along with two other tube sections. Figure 1 shows the tube sections submitted for analysis. 

The water wall tubes all had bulges along one side of the tube sections. Most of the bulges contained cracks that penetrated the tube wall. The two separate tube sections had a white deposit on the OD of the tube sections. Figures 2 thru 4 show some of the bulges and the deposit on one of the separate tube sections.

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Figure 5. A photomacrograph of a cross section thru a bulged zone showing the knife edge at the failure.

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Figure 4. A photomacrograph of the white deposit on the OD of one of the tube sections.

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Figure 6. A photomacrograph of a portion of the deposit on the ID of a tube section 


Some of the bulges were cross-sectioned to examine the ID of the tube. These sections revealed a thick deposit on the ID. The wall of the tube section in the bulge had thinned to a knife-edge. This is a characteristic of thin lip stress rupture. Figures 5 and 6 show the wall thinning and the deposit buildup on the ID of the tube section. 
 
Thin lip stress ruptures are caused by rapid overheating of the material. Thin lip stress rupture is characterized by a thinning of the tube wall at the failure often to a knife-edge. 

Rapid overheating of tubes made of carbon and low alloy steels result in failure because of a decrease in yield strength and tensile strength with increased temperature. The tube plastically deforms (bulges), the tube wall thickness thins due to the bulging and the tube eventually fails due to tensile overload. 

Deposits can cause overheating by changing the heat transfer characteristics of a tube. Sudden or severe restrictions in flow also cause rapid overheating failures.

 Chemical Analysis

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 Figure 7. A photomicrograph of a portion of a fracture edge. (50X, Nital Etch)

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Figure 9. A photomicrograph of a portion of the ID deposit at a failed zone. (50X, Nital Etch)

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Figure 10. A photomicrograph of a portion of the ID deposit at a failed zone. (50X, Nital Etch)

A chemical analysis was performed on a sample prepared from a tube section. The quantitative chemical analysis was performed using a spark emission spectrometer. The sample met the chemical composition requirements for an ASTM A-178 Grade A material. The test results in weight percent are as follows:

   

Specified For ASTM

Element

Tube section

A-178 Grade A
     

Carbon

0.11

0.06 - 0.18

Phosphorus

0.014

0.035 max.

Sulfur

0.034

0.035 max.

Silicon

0.01

----

Manganese

0.35

0.27 - 0.63

Iron

Matrix

Matrix

 

Metallography

Microstructural Analysis. Samples were cut from the tube sections and prepared for metallographic examination by grinding, polishing and etching. The prepared samples were examined using a metallurgical microscope to assess microstructural conditions. 

A transverse cross section of the tube sections confirmed the findings in the visual examination. The tube failed due to thin lip stress rupture. The fracture edge revealed plastic deformation and grain elongation. Figures 7 and 8 show a portion of the fracture edge examined. 
 
A tenacious deposit adhered to the ID of the tube sections examined. Figures 9 and 10 show a portion of the deposit on the ID fracture edge. 
 

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Figure 11. A photomicrograph of the typical microstructure observed away from the failed zones. (200X, Nital Etch)

No defects in the material were observed in the samples examined that would have contributed to the failures. Figure 11 shows the typical microstructure away from the failures. 

Mechanical Testing

Hardness tests were performed on samples prepared from the tube section. The tests were performed using a Rockwell hardness tester and the B scale. The tests were performed at the failure, 90° away from the failure and 180° away from the failure. The average Rockwell B hardness numbers are as follows: 

    Location

    Average Rockwell B Hardness
       

    At Failure

    21

    90° Away

    39

    180° Away

    48

 
Deposit Analysis

SEM/EDS. Deposits from the ID and OD of the tube sections were analyzed using a scanning electron microscope and an energy dispersive x-ray fluorescence spectrometer. This test method will qualitatively detect elements present with an atomic number greater than oxygen. 

Sodium, magnesium, silicon, phosphorus, sulfur, calcium, iron and copper were identified in the deposit from the ID. Sodium, silicon, sulfur, chlorine, potassium, calcium and iron were identified in the deposit from the OD of the tube sections. Spectra generated during the analysis are appended to the report. 

X-ray Diffraction. The deposits from the ID and OD of the tube sections were analyzed using an x-ray diffractometer. This test method will detect crystalline compounds present in the samples. 

The major crystalline compound identified in the ID deposit was calcium carbonate (CaCO3). A moderate amount of calcium phosphate was also identified. 

The major crystalline compound identified in the OD deposit was sodium chloride (NaCl). Calcium phosphate was also present in the OD deposit. 

Spectra generated during the analysis are appended to the report. 
 

 Conclusions
 

    The following conclusions are the Author's opinions based upon the analysis. 

  1. The water wall tubes failed due to a crack that propagated completely thru the wall of the tubes. 
  1. The cracks were caused by thin lip stress rupture due to rapid overheating of the tubes. 
  1. Thick ID deposits were observed on the tube surfaces. 
  1. Two common causes of thin lip stress rupture failures are restricted heat transfer due to deposits and restricted flow thru the tube. 
  1. The material met the chemical composition requirements for ASTM A-178 Grade A material. 
  1. The hardness values at the failures indicate a reduced tensile and yield strength due to high temperatures experienced at the failed zones.
 

Performed By: ______________________________________

Lab Technician 

Reviewed By: _____________________________________
                           Metalurgist  

NOTE: Test specimen(s) and material remnants from this project will be discarded after thirty (30) days from the date of this report. Any requests for alternative handling must be submitted in writing and received prior to that deadline. 

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