Engineering guide
Module 301A performs the fatigue (cyclic loading) assessment for cylindrical shells with and without branches to TRD 301, Annex 1 (edition 1996-08) — the German TRD steam boiler code. It calculates the allowable number of load cycles for pulsating internal pressure with superimposed temperature transients — in other words, the fatigue life of boiler drums, headers and manifolds with nozzles and branches, including oblique and Y-shaped branches. The TRD codes have been withdrawn; for new plant, DIN EN 12952-3 with its fatigue assessment applies (or EN 12952-4 for in-service monitoring). For existing boilers whose remaining life is to be assessed against the original design code, Annex 1 of TRD 301 remains the governing procedure.
The assessment captures the stress peaks at the opening edge via stress concentration factors for the membrane and thermal stress components, and takes into account material properties such as yield strength at cycle temperature, tensile strength, modulus of elasticity, coefficient of thermal expansion and thermal diffusivity, as well as the influence of out-of-roundness and water contact (corrosion influence on the crack initiation curve). The results are the crack initiation cycle number with safety factors and the allowable number of load cycles.
In practice, the module is used for remaining-life assessments of older boilers in increasingly flexible power plant operation: more frequent start-stop cycles consume fatigue life much faster than the original design, based on a few hundred cold starts, anticipated.
Scope
- Cylindrical shells with / without branches
Typical workflow
Define the loading cycles
The pressure and temperature cycles of the component are recorded: cold, warm and hot starts with their pressure ranges, rates of temperature change and cycle temperatures. Each cycle type is assessed separately and later combined via damage accumulation.
Provide material and geometry data
Required are the yield strength at mean cycle temperature, tensile strength, modulus of elasticity, differential coefficient of thermal expansion and thermal diffusivity, together with the wall thicknesses including allowances. The out-of-roundness of the shell enters as an additional source of bending stress.
Determine stress concentration factors at the opening edge
For the undisturbed shell and for branches (perpendicular, oblique, Y-shaped with angle), the stress concentration factors for the membrane component (pressure stress) and the thermal stress component are determined — they describe the stress amplification at the bore edge relative to the plain shell.
Calculate the stress range
From the pressure range and the temperature gradient, the governing equivalent stress range at the critical location is formed; mean stress influence and elastic-plastic correction when the yield strength is exceeded are accounted for using the procedure of Annex 1.
Crack initiation cycle number and allowable number of cycles
From the material's crack initiation curve follows the crack initiation cycle number; safety factors on cycle number and stress range, plus the penalty for water-wetted surfaces, yield the allowable number of load cycles. Multiple cycle types are summed to the total fatigue usage via linear damage accumulation.
Frequently asked questions
Why is the branch (nozzle edge) the fatigue-critical location?
At the bore edge, two stress amplifications superimpose: the notch effect of the opening on the pressure membrane stress (membrane stress concentration factor) and the concentration of thermal stresses during start-up and shutdown (thermal stress concentration factor). In addition, the inner edge is often water-wetted, which further reduces the endurable number of cycles due to corrosion influence. That is why fatigue cracks on boiler drums almost always appear in a radial pattern at nozzle and water-level bores, not in the undisturbed shell.
What does the distinction between water-wetted and non-wetted surfaces mean in the assessment?
The TRD crack initiation curves are based on tests in air; in areas in contact with boiler water, corrosion fatigue acts in addition and significantly lowers the crack initiation cycle number. Annex 1 accounts for this via a reduction factor for water-wetted surfaces. When entering the data it is therefore essential to identify correctly whether the critical location is in contact with the medium — a common source of error that can substantially overestimate the fatigue life.
Crack initiation is not yet failure — why is the design still based on crack initiation?
The allowable number of load cycles in the TRD refers to the technical crack initiation, because from that point onward crack growth depends on factors that are hard to quantify (medium, residual stresses, inspection intervals). Continued operation of cracked components requires fracture mechanics assessments and shortened inspection intervals. The safety factors on cycle number and stress range cover the scatter of the crack initiation curve, not crack growth.
How do I deal with today's flexible operation, which the original design did not anticipate?
The fatigue usage is tracked cycle type by cycle type: for each cycle type actually operated (cold, warm, hot start, load change), the usage fraction is formed from the number of cycles operated and the number allowed, and the fractions are summed — analogous to in-service monitoring per TRD 508 or, today, EN 12952-4. If the sum exceeds defined thresholds, the usual consequences are intensified recurring inspections at the critical nozzle edges (surface crack testing) and, where necessary, limiting the start-up gradients.
Technical content source: LV-Solved-Problems-Content 628d313247e7