Predicting and Preventing HP Evaporator Tube Failures

Two of the most common issues that power plant owners and operators face in the HP Evaporator section areUnder-deposit Corrosion (UDC) and Corrosion-Fatigue. In this short article we will cover UDC and what key actions should be taken to predict and prevent costly forced outages.

Heat flux (kW/m²) drives the rate of boiling at the tube surface and thus influences the amount of insoluble material left behind.  Nucleate boiling tends to be dominant at low vapor fractions and high heat fluxes, while convection tends to dominate at high vapor qualities and mass velocities and low heat fluxes. For intermediate conditions, both nucleate boiling heat transfer and convective heat transfer are important. How do you use this knowledge to determine if you are at risk and where to look?

Summarizing the key factors for deposition in boiling tubes:

• Heat Flux at the boiling surface and boiling regime (such as nucleate boiling)

• Local Fluid Flow disturbances

• Electrochemistry of Boiler Water

• Local Steam Mass Fraction (Quality)/Fluid Velocity

• Concentration of Contaminants (scale-forming species)

• Initial Surface Cleanliness

• Operating Pressure/Temperature

What can you do to prevent or treat UDC?

• Good water chemistry and contaminant control is essential to preventing or minimizing evaporator deposition.   

• Experience has shown that heat fluxes above 130 kW/m^² are very likely to result in rapid and damaging deposit growth.   Avoiding localized high temperature conditions (such as caused by uneven flame distributions when duct firing) will help keep peak heat fluxes low.

• Monitoring deposit buildup by borescope inspection and tube samples can help determine if in-service chemical cleaning is required to remove tube deposits.

• Deposit levels are typically measured by the Deposit Weight Density (DWD) using standard laboratory tests  and reported as grams//ft² or mg/cm²
• Boiler manufacturer and industry recommendations show some variation, yet in general good practice would dictate that cleaning be done when tube DWD reaches 30-40 g/ft² (32-43 mg/cm²).  Tetra's experience is that tube damage and leaks are highly likely when tube DWD exceeds 50 g/ft². (54 mg/cm²)

• Deposits accrete faster in "dirty" boilers, therefore the progression to when cleaning is required can be very quick.

If chemical cleaning is undertaken it is critical to use the right process to assure that the types of deposits in your boiler/HRSG are effectively removed.