As published in Energy-Tech Magazine:
Dispatching requirements for many combined cycle plants (CCP) have pushed design limits for fast starts, load and on-off cycling operations. Gas turbine technology incorporated in CCP arrangements typically has the design capability to cycle multiple times a day. But where does this leave the steam turbine? This article will focus on the impact of flexible operations on high temperature casings.
Cracking of high pressure, high temperature steam turbine inner and outer casings can result as a unit accumulates operating hours and cycles. Cracking is typically due to low cycle fatigue or fatigue in combination with high temperature creep. Other factors such as design, casting quality, poor material properties, manufacturing issues and occurrence of operational excursions can be aggravating issues. In severe cases and if not addressed, cracks may propagate until they reach a critical size at which time rapid propagation and failure may occur. To prevent such a scenario, diligent inspection and maintenance is required.
Cracking will most often occur in and around inlet penetrations. These locations are exposed to high temperature differentials during both steady state and transient conditions. The casing around the inlet areas is usually thick walled with complex geometry. Other areas where cracking can occur are at transitions from the inlet to adjacent zones in the shell.
During transients, casing surfaces are exposed to rapid changes in steam temperature. Due to thermal capacitance, the inner core of the casing lags these changes. The result can be thermal gradients which cause high thermally induced surface stresses. In combination with geometric discontinuities, the local peak stress can exceed the material yield at temperature.
Steady state conditions can also result in high thermal stresses. This is particularly true for inner casings since they are often subject to significant differences in steam temperature between inner and outer surfaces.
A further cause of cracking are operational excursions such as water induction. An introduction of water while casing surfaces are at a high temperature results in rapid surface chilling. The rapid chilling results in extremely high thermal stresses. A single and severe water induction event can result in casing distortion and greatly reduce the fatigue life of exposed surfaces.
Once cracks develop they may propagate with additional time and cycles. In extreme cases, cracks will propagate until they reach a critical crack length, at which time the crack propagates rapidly through the structure. Although catastrophic failures are rare, such crack propagation can result in high temperature steam leakage and unacceptably large deformations affecting alignment and hard part clearances.
Diligent inspection and maintenance reduces the likelihood and impact of casing cracking. If cracking is significant, analysis provides insight into the risk of continued operation as well as to repair or replace options.
Metals expand when heated based on the material coefficient of thermal expansion. If a free casing of cylindrical geometry is heated gradually and uniformly, it will grow in all dimensions in a stress-free state. However, if the casing is heated such that radial thermal and/or non-linear axial thermal gradients develop in the casing, the resulting thermal expansion and internal constraint of the cylinder geometry causes development of thermal stress. This stress is “self-equilibrating” in that the overall net stress is zero and any stresses that exceed yield are relieved due to plastic deformation. During transient and steady state operations of HP and IP units, the turbine casings are subject to steam conditions which develop thermal gradients as described above.
In addition to thermally induced stresses, the casings are subject to stresses arising from primary loads such as pressure and loads transmitted from mating parts. The stresses from these loads are not relieved due to plastic deformation.
The combination of the above stresses in areas of stress concentration (e.g. sharp corners or edges) can result in very high localized stress. If the localized stress exceeds the yield, plastic deformation and residual stress result. Given enough cycles, cracks can initiate in such locations. This is typically due to low cycle fatigue given the relatively low number of cycles and high stresses involved.
Since these locations are subject to high temperatures at load, the combination of high temperatures and non-relieving stresses due to primary loads results in high temperature creep. The change of the material due to creep effectively reduces fatigue strength. Units that have operated over long periods of time (> 30 years) may experience cracking driven as much or more by creep as due to cyclic operation.
Once initiated, cracks may propagate with additional load cycles. The degree to which propagation occurs is driven by the stress field into which the crack propagates. In the case of purely low cycle fatigue driven by surface stresses, cracks may not propagate or propagate for a period of time and stop. This happens when cracks are propagating away from a high surface stress environment into a lower stress field below the surface.
However, it is also possible a crack propagates into a stress field dominated by pressure or other primary loads. In such a case, the crack may continue to propagate. In worse case scenarios, the crack may reach a length that equals the critical crack length based on material properties and geometry. In such a case, the crack will propagate rapidly through the structure in the direction where the critical crack length is limiting. In this event, there may be severe consequences of steam leakage, gross deformation in the area of the crack, or a combination potentially leading to significant consequential damage and risk to personnel.
Several factors can accelerate the effects of cracking: severe transients that lead to sudden and rapid decreases in inlet temperature. Operation at temperatures above the normal operating limit (even for short durations). Cold start or shutdown transients more severe than predictable. Reduced material properties due to manufacturing process (poor castings with high porosity) or machining (unintended stress risers). Poor design practices that do not properly or fully account for thermal effects have also been identified as a cause of cracking.
It is important to non-destructively examine (NDE) casings for the presence of cracks whenever the unit is down for a major inspection or when the casings are available otherwise. If inspection confirms the presence of cracking, depths should be evaluated. Shallow cracks often can be removed by excavation, grinding or polishing as appropriate. For deeper cracks, further evaluation is required to determine severity and risk. In many cases, it is sufficient to note the crack depth for tracking and comparison at future outages. However, deep cracks should be analyzed to determine risk of continued operation.
To determine when analysis is merited, the critical criteria is the critical crack length and understanding of the nature of the underlying stresses. While under the critical crack length, crack propagation can be predicted and monitored. Once it exceeds the critical length it propagates quickly and in an unpredictable fashion. The other factor to consider is the stress field in which the cracks exists. Finite element analysis may be required depending on the situation.
If a crack is determined to be larger than what can be safely removed and likely to continue to propagate towards the critical crack size, weld repair may be an option. Such repairs should only be performed by a well-qualified shop with demonstrated competency with complex repairs and stress relieving techniques. In extreme cases, component replacement may be the best option.
TG Advisers (TGA), was recently engaged to develop recommendations for a combined cycle unit that relatively early in its design life revealed a number of cracks in the IP inner casing in and adjacent to the IP inlet. The largest individual crack depth was approximately half of the local wall thickness. Due to the complexity of the geometry and the associate flow in the IP inlet, TGA utilized 3D finite element analysis for both a thermal and stress analysis. The analyses included a steady state case and thermal transient cases for a cold start and shut down. Since the analysis indicated the maximum stresses in the area where occurred exceeded the elastic limit of the material, additional finite element analysis for multiple start and stop cycles and accumulated creep damage were completed to evaluate inelastic effects and cumulative operating time. The results were incorporated into a crack growth model to evaluate the risk of continued operation.
In this case, the crack was predicted to continue to growth towards critical crack size. TGA provided a risk-based assessment of how many additional cycles and time the unit could operate before major repair or replacement was required. This provided the client sufficient time to evaluate weld repair options as well as determine cost and schedule for a replacement cylinder.