Engineering guide
This module supports the in-service monitoring of steam boiler components to TRD 508 Annex 1 ("Additional inspections of components"), part of the German Technical Rules for Steam Boilers (TRD). For components operated in the creep range or under cyclic loading, the accumulated exhaustion (damage) is updated: from the operating hours and load cycles accrued since the last evaluation, the creep and fatigue fractions are calculated and added to the component's total exhaustion.
The procedure is the basis for setting inspection intervals and the scope of recurring inspections: when the total exhaustion reaches defined thresholds, additional examinations such as surface crack testing or microstructure (replica) examinations are required at the highly stressed locations. To this end, the module manages the boiler plant, component, material and the evaluation data of several reference dates, and reports the cumulative operating hours, the cumulative load cycles and the total exhaustion before and after the current evaluation.
The TRD rules were withdrawn at the end of 2012. For existing boilers whose service life records are kept to TRD 508, the procedure remains in use; the European counterpart for water-tube boilers is the in-service calculation of the expected life to EN 12952-4 in conjunction with the fatigue analysis of EN 12952-3.
Scope
- Additional tests of components
Typical workflow
Define the component and the monitoring period
The boiler plant, component and material are identified; the evaluation date delimits the period since the last exhaustion assessment. The total exhaustion from the previous evaluation serves as the starting value.
Collect the operating data
For the period, the operating hours per temperature/pressure duty class and the load cycles that occurred (cold and warm starts, pressure cycles) are compiled — nowadays typically from process data, historically from boiler logbooks.
Calculate creep and fatigue exhaustion
The creep fraction results from the ratio of the accrued hours to the allowable creep rupture life at the respective operating conditions; the fatigue fraction from the ratio of the counted cycles to the allowable number of load cycles. Both fractions are accumulated linearly.
Update and assess the total exhaustion
The new total exhaustion is the sum of the previous value and the increment of the period; in parallel, the cumulative operating hours and load cycles are updated. Based on the exhaustion thresholds of TRD 508, it is decided whether and to what extent additional inspections of the component are due.
Frequently asked questions
At what exhaustion level do additional inspections become necessary?
TRD 508 ties the scope of recurring inspections to exhaustion thresholds: extended examinations at the most highly stressed locations are already required from a total exhaustion of around 50 %; at higher values, the intervals shorten and the inspection scope grows (e.g. microstructure and surface crack examinations). The specific thresholds and measures must be taken from TRD 508 with Annex 1 and agreed with the authorized inspection body.
Why are creep and fatigue exhaustion added together?
Components in the high-temperature range consume service life simultaneously through creep (time-dependent) and through load cycles (cycle-dependent). The code uses linear damage accumulation, in which both fractions are calculated separately and summed to the total exhaustion. This is an engineering approximation — it covers creep-fatigue interaction only in a global way, which is why the inspection thresholds are deliberately conservative.
What to do when historical operating data are incomplete?
Missing periods must be estimated conservatively, for instance from typical operating patterns, start-up counts from shift logs or comparable plants. A documented, rather unfavourable assumption is better than a gap, because the exhaustion calculation is only as reliable as its input data. Once digital process data acquisition is introduced, the counting should be continued automatically.
Does 100 % exhaustion mean the end of the component's life?
No, not automatically. The calculated exhaustion carries safety margins relative to the actual damage. When high exhaustion levels are reached, the findings on the real component — microstructural condition, indications from crack testing, dimensional stability — decide on continued operation, shortened inspection intervals or replacement.
Technical content source: LV-Solved-Problems-Content 628d313247e7