Boiler Tube Failure Analysis

The service environment of fossil-fired boilers and HRSGs, along with human errors during engineering, fabrication, construction, operation, and maintenance will always result in occasional boiler tube failures (BTF). The industry goal is for a forced outage rate less than 1.5% with zero chemistry related failures. The frequency of these failures depends on the corrective actions taken to prevent or reduce boiler tube damage. Repeat BTF usually result in frequent forced outages, and ultimately in costly extended outages for major tubing replacement. It is estimated that the cost of water/steam-side boiler tube corrosion is over $1.1 billion per year.

Primary factors influencing repeat tube failures are:

• Not following state-of-the-art operation maintenance or engineering practices

• Lack of proper boiler tube failure root cause analysis

• Wrong choice of corrective/preventive action

• Lack of definitive boiler tube failure reporting and monitoring

Failure Mechanisms

There are many different types of boiler tube failure mechanisms, which can be sorted into six general categories:

• Stress rupture (short-term overheating, high temperature creep, dissimilar metal welds)

• Water-side corrosion (caustic corrosion, hydrogen damage, pitting, stress corrosion cracking)

• Fire-side corrosion (low temperature, waterwall, ash)

• Erosion (fly ash, falling slag, soot blower, coal particle)

• Fatigue (vibration, thermal expansion)

• Lack of quality control (damage during chemical cleaning, poor water chemistry control, material defects, welding defects)

Boiler Tube Sampling

Prior to removing a failed tube, mark and photo-document the tube (gas flow direction, fluid flow direction, row number, elevation, boiler section, etc.). The visual condition of nearby tubes and the proximity of the damaged tubing to attachments, sootblowers, etc. should also be documented.

Samples should be a minimum of 18” long and cuts should be made well away of any visible damage. Dry cuts should be made with an electric or air powered reciprocating saw or grinder with a thin cut-off wheel to obtain high quality cuts. Tube ends should be sealed with tape to prevent contamination or loss of deposits or scale. The tube samples should be carefully packed to prevent damage during shipment.

The following information should be provided with the tube sample(s):

• Boiler operating pressure, temperature, steaming rate, and unit MW

• Drawing of boiler showing the location of each tube sample

• Specified tube material, dimensions, etc.

• Operating hours since commercial operation date or tube replacement

• Tube failure history of the boiler

• Boiler maintenance records (i.e., replacements or modifications) for the boiler section of concern

• Boiler water chemistry (typical chemistry and frequency, extent, and duration of excursions)

• Layup procedures (short-term, long-term)

• Any additional pertinent information on the unit

Analysis

E2P (Formerly Jonas, Inc.) will perform the following (as applicable):

• Visual inspection

• Determination of chemical composition and morphology of deposits

• Deposit Weight Density (DWD) determination

• Scale thickness measurement

• Pit depth measurements

• Wall loss determination

• Metallurgical analysis - material composition and microstructure

• Determination of failure mechanism

• Root cause analysis

• Recommend corrective actions

• Determination of time for tube replacement

Periodic Tube Sampling

Boiler waterwall, superheater, and reheater tubes should be sampled and analyzed every few years to track the progression of corrosion and deposit/scale buildup so that corrective actions can be taken before a failure occurs. The samples should be removed from the area of highest heat flux, which is usually where the most severe damage occurs. Based on the results of this periodic sampling and analysis, estimates can be made for scheduling of chemical cleaning and boiler tube replacement.