Cost-Effective Condition Assessment of Steam-Cooled Boiler Tubes

Tools for Preventing Boiler Tube Failure: Part 1

17 January 2017

This blog is the first in a series discussing boiler tube condition assessments.  While these basics might be familiar to many, this first installment will serve as a refresher to the experienced and an introduction to novices.  Have you faced one of these scenarios regarding your steam-cooled ferritic (superheater and reheater) tubing?

• Your boiler has had a long operating life, and you need to know the tubing condition.
• Tubing has failed, and you know or suspect that the tubing failed due to creep.
• If tube failures have occurred with increased frequency, and you would like to predict the future failure rate.
• Tubing has operated at higher than design temperatures, and you would like to know the effect of these high temperatures.
• A visual inspection and wall thickness survey shows severe erosion and wall thinning, but tubes have failed and you want to know how long before a tube ruptures.

Boiler tube failures are the number one cause of forced outages in fossil fuel power plants.  Long term creep is a failure mechanism which affects steam-cooled tubes.  These usually occur at temperatures > ~900°F in conjunction with operational sustained stresses, and causes deformation and eventually failure.  Long term creep is typically characterized by a longitudinal split with minimal swelling.

The creep remaining useful life (RUL) prediction is based on temperature and stress over the operating life.  The operating stresses are due primarily to steam pressure.  Over time, the tube inner metal surface interacts with the steam to form a scale layer.  As the scale thickness grows and the tube wall is reduced, the scale acts as a thermal barrier insulating the tube.  To maintain outlet steam temperature, the tube heat input must increase which also increases the tube temperature.  Wall thinning will also occur over time due to erosion, corrosion or other wastage mechanisms.  This increases the operating stresses as well.  The interaction of scale growth and wall thinning results in a RUL reduction.

As the scale grows thicker, the tube life is shortened, but how do you determine the RUL?  Tube samples are an impractical method of providing an inexpensive and easy survey of an entire tube bank.  The answer is Intertek's TubeAlert system.  The system is a combined hardware and software package which allows for rapid non-destructive examination (NDE) to determine the effective metal temperatures, operating stresses, wall thinning rates and RUL.  The hardware measures wall thickness and scale thickness down to 0.002", with minimal surface preparation (i.e., no tube surface polishing).  The software is a proprietary methodology which utilizes the NDE measurements and determines the temperatures, stresses and RUL.

A typical TubeAlert examination will be performed at select elevations in the boiler furnace and penthouse.  Surveys are performed across and through tube banks.  The TubeAlert report provides tabular and graphical result presentations.  Intertek can provide additional recommendations including:

• Tube samples to confirm NDE results 
• Additional survey locations 
• Monitor of existing/install penthouse thermocouples on tubes which have experienced high temperatures
• Provision of reexamination intervals, based on operating hours  

TubeAlert is a cost-effective and efficient method of determining the condition assessment of your tubing.

For additional information on TubeAlert, contact the author or Kevin Hara at kevin.hara@intertek.com (408) 636-5333.

Clayton Q. Lee, Director, Thermal Engineering, Intertek Asset Integrity Management (AIM). The author has over thirty years of experience consulting to the power industry in the areas of heat transfer, fluid mechanics, stress analysis and fracture mechanics. He has authored several sponsored applied research reports used by the power industry.

Tags: 2017 | Asset Integrity Management | Clayton Lee | Energy