High Temperature Strain Survey of Power Boilers Case Study

Power station boilers and their internal components suffer from periodic cracking.  These failures can be due to various reasons such as poor design, ageing with increased susceptibility to metal fatigue and creep damage, changed operating conditions (i.e. cycling / two shifting), support restraints limiting thermal expansion, uneven support loading, irregular loads, flow issues, soot blower issues, clinker build-up, ash fouling and many more. 

Once a boiler component is cracked, what do you do?  The asset manager has the arduous responsibility to investigate the cause while also organising the repair in such a manner as to minimise lost production.  Depending upon experience and knowledge the cause may be easily identified and the repair may be simple, but this is not always the case.  Frequently, the cause may be suspected, but can not be proven as limited or even no data is available.

We believe that every failure is an opportunity to learn, where experimental stress analysis becomes the method to providing quantified data to support hypotheses or “Gut feel” suspicions.  Installing strain gauges to boiler components can be done quickly (a few days at most).  Upon return to service of the boiler the strain gauge readings quickly show what is happening to the boiler at the location of concern.  High temperature strain gauges (useful to 750°C) are installed with thermocouples to measure metal surface temperatures.  Transient events such as boiler startup, shutdown and load changes often show large thermally induced strains until the steady state condition is reached.  Repetitive temperature differences cause differing thermal expansion and depending upon boundary condition restraints may induce high localised stresses resulting in cracking (i.e. thermal fatigue).  Associated strains are measured by the high temperature strain gauges from which stresses are calculated.  The data obtained provides quantified evidence as to what the metal surface of the component is experiencing.  This data can be used to make informed decisions about future repairs, life expectations, shutdown times and maintenance budgeting.  Often spare components can not be sourced quickly and strain gauging can determine how long the failing component will continue to operate safely.

HRL Technology has installed hundreds of strain gauges on boiler components to gain insight into causes of failures. The results are conclusive and often facilitate a design repair solution that can be implemented to the asset manager’s timeframe, not a forced plant shutdown.  Occasionally the results obtained show a completely unexpected event such as quenching due to passing valves.  Though we suspect what is happening, we are still often surprised by data results obtained, yet satisfied that the root cause was determined.  In this instance a valve repair corrected the situation.

The intent of this case study is to have asset managers take the opportunity to install strain gauges to obtain actual quantified data.  The data can then be used to make informed decisions about the assets future requirements.