By Pascal Fontaine and Jean- François Galopin, CMI Energy
Today, most combined-cycle power plants are expected to be available 24-seven. Deregulation and consequent merchant power have made it important for combined-cycle plants to supply electrical power to the grid as and when needed with minimum notice. Combined cycles are also often forced to run on partial loads. Even units originally designed for baseload are eventually forced to cycle like the new, more efficient power plants that have been built recently. As generally recognized nowadays, the cycling criterion is an integral part of the heat recovery steam generator (HRSG) design.
This article discusses HRSG fatigue analysis using the European Norm EN 12952-3, how the European Norm can be used to assess HRSG cumulative damage and how to optimize the HRSG cyclic lifetime. The article focuses on fatigue and will not discuss the interaction between fatigue and creep on hot superheater.
There are two main types of stress inside the component wall: mechanical stress that originates from internal pressure and thermal stress that originates from thermal expansion. During start-up, these stresses are in opposite directions, while they are in the same direction during shutdown. Eventually, these stresses combine into Von Mises stresses (Figure 1).
Cold Start-up Stress Cycle
A cold start-up (from sub-cooled water) is special compared to a hot start-up because the inner shell heats up while the internal pressure is atmospheric and constant. This induces compression thermal stress that is not yet counterbalanced by any tensile stress from pressurization. Once pressure begins to increase, mechanical tension stress will release thermal stress as the two move in opposite directions. Therefore, the largest amplitude of this stress cycle will occur during a cold start-up, which is the most severe condition. In addition, the internal protective magnetite layer - ferrous oxide - could crack during the early start-up stages. This ferrous oxide, Fe3O4, is created naturally under pressure at the drum’s normal operating condition. The Euro Norm allows an empirical stress range of -600 N/mm and +200 N/mm. Because this magnetite layer forms under pressure, it is already under compression strain at cold conditions and, therefore, subject to surpass the lower limit during a cold start-up (Figure 2).
During subsequent pressure increase, boiling water heats the drum shell from the inside and the heat is transferred from inside to outside. This creates a temperature profile, which becomes significant between inner and outer shell surfaces. The outer shell surface temperature lags behind the inner one. The temperature profile over the drum thickness is constant and slides up (Figure 3). This shifting temperature profile has roughly a quadratic shape vs. wall thickness; so does the induced thermal stress profile. Once pressure is stabilized at the normal operating condition, this temperature profile will vanish. So, too, will thermal stress leaving only mechanical stress in the equipment wall. During shutdown, the opposite phenomenon occurs with temperature profile between external and internal surfaces on drum shells. This complete cycling stress (including start-up and shutdown) should be considered during component fatigue analysis. Regardless of the stress transients, the ASME code considers the permanent operating conditions on creep for thickness calculation.
Temperature difference between inner and outer walls is roughly proportional to the square of the drum thickness, as is the thermal stress. This means thin walls are beneficial in terms of cycling fatigue. For example, the thickness of a superheater outlet header made of P91 material generally can be reduced by a factor of two when compared to the same header made of P22 material. Consequently, the induced thermal stress would be reduced by a factor of four for the P91 header compared to the P22 header (Figure 4).
The start-up/shutdown cycle represents a simplified stress range. The allowable number of cycles is calculated with the Euro Norm EN 12952-3 (Figure 5). In practice, cold, warm and hot cycle stress numbers are determined based on each plant’s specified cyclic service. Other stress cycles can also be considered, such as the partial cycle when the first unit is started on a 2-2-1 arrangement, or even low cycle fatigue (LCF) due to attemperation in operation. Then, the Miner’s rule is used to determine the fraction of cumulative fatigue damage for each. When applied, the Euro Norm shows that a cold start-up is 20 times more damaging than a warm start and that the stress range resulting from a hot start is typically below the fatigue limit and does not contribute to the total fatigue damage (except for the damaging quenching issue, discussed in the Superheater Quenching and Euro Norm section that follows).
Fatigue damage is sensitive to the stress range because of its logarithmic nature (Figure 5). A small variation in stress amplitude greatly affects the corresponding number of cycles, even more so for small amplitude 2fa on warm start-ups. Fatigue calculation does not fix exactly the line between a crack and a no-crack initiation, but is rather a statistical probability of crack occurrence under Nafa conditions, eventually representing the risk of failure percentage. Fatigue’s sensitivity and probabilistic nature result in an uncertainty in fatigue lifetime analysis. Robust modeling by a finite element can reduce the other uncertainties that come from simplified stress ranges and boundaries.
ASME I considers continuous operation at design conditions, but does not mandate assessment for fatigue analysis. European boiler manufacturers perform this analysis using the European Norm 12952-3 to calculate the acceptable pressure/time gradients. The European Norm 12952-3 was derived largely from the German code TRD 301, which many manufacturers used previously for cyclic assessment. The HRSG is designed according to ASME, but is also checked for fatigue analysis using Euro Norm. Input data includes the component’s diameter, material, thickness and operating pressure and temperature. Two methods can then be used for analysis: either the number of cycles is calculated for given start-up and shutdown rates; or, those allowable rates are calculated for a given number of cycles. An interesting feature is the ramp rate variation vs. operating pressure (Figure 6). As the pressure increases, the allowable pressure ramp rate also increases. In practice, Euro Norm fatigue analysis is applied to the thick HP drum wall as well as to outlet headers of the reheater and HP superheater.
As explained earlier, a cold start is the most stressful event in terms of fatigue effect, inasmuch as it includes the largest cycle range. Unless required by a specification, CMI designs for 2,000 cold start-ups during a 20-year lifetime. By applying the norm for an extended number of cold start-ups, CMI designers calculate negative temperature gradients (Figure 7). This is, of course, impossible and should be interpreted as a statistical fact that the material would have no more strength reserve to perform this extended number of start-ups, meaning that such cumulative cycling damage has exhausted the component fatigue lifetime.
TRD 301 (German code) was generally looked upon as the reference for pressure vessel cycling analysis. However, when compared to finite element (FE) analysis, this code proved rather conservative. For instance, TRD considered the empirical stress concentration factor around nozzles, which were proven to be high by FE, as cautions are taken in construction to avoid sharp edges on those openings. Also, the drum feedwater nozzle is always equipped with an internal thermal sleeve to accommodate induced thermal shock without causing undue stress when cold feedwater is first admitted into a warm drum. The main improvement of the Euro Norm compared to the former TRD 301 for cyclic analysis comes from being able to use finite element analysis to determine stress concentration factors (Figures 8a and 8b).
To fully understand all changes between EN 12952-3 and TRD 301, CMI conducted an overall survey of the two codes on a specific boiler under engineering. TRD 301 and EN 12952-3 calculation methods are similar, but there are some differences, mainly related to the empirical coefficients used within the formula on stress evaluations:
The results of the comparison can be found in Table 1.
As mentioned before, the critical components are the HP drum and high temperature superheater/reheater headers and/or manifolds. Table 2 contains the results based on detailed connections between branches (nozzles) and the main body on an HP drum and a HP superheater.
Knowing that the maximum cumulated damage differs from one code to the other, TRD 301 results should be multiplied by a factor of two to compare percentages of fatigue exhaustion. Table 3 includes the results from the cumulative damage comparison.
This analysis confirms that the stress induced factors are the most important improvement. Euro Norm appears to be less conservative than TRD 301, and more representative of physics.
Superheater Quenching and Euro Norm
Mechanical stress is high for the HP drum, while thermal stress is high for the superheater and reheater, especially in the case of water quenching. This phenomenon occurs because condensate water remains inside the still-hot superheater during initial gas purge. This quickly chills some component parts compared to others and it induces a low cycling fatigue cycle in the overall warm start-up cycle. Although the phenomenon can cause a lot of damage, it is difficult to determine just how much due to its empirical nature. The quenching issue is not yet addressed by Euro Norm and a good engineering practice for a large and multiple drainage system is necessary.
Four materials are often used for drum design: SA 302 Gr B, SA 299, SA 516 Gr 60, SA 516 Gr 70. There is a trade off between (mainly) SA 302 Gr B and SA 299 around 100 bar, where a drum made from SA 299 becomes much thicker than a drum made from SA 302 Gr B. This material is generally used for HP drums to reduce wall thickness (Figure 4). The two other materials are mainly used for lower pressure drums, where fatigue is not an issue (low pressure, low thickness). While performing the Euro Norm calculation with the same inputs, but with the drum thickness as a free parameter, it is interesting to note that an optimum thickness falls into the allowable temperature gradient, which can be explained as follows:
Considering the temperature profile generated over the drum thickness, thermal stress on the inner shell increases roughly as the square of the wall thickness, while mechanical stress decreases proportionally according to this thickness. Therefore, during cold start-up, the inner thermal stress quickly becomes predominant, and consequently it reduces the allowable temperature gradient. Coincidentally, the calculated wall thickness according to ASME code is typically close to this optimum wall thickness calculated according to Euro Norm for material fatigue.
If the main body diameter is important, the nozzle diameter is also important. Table 4 shows results of comparing two downcomer drums with a three or four downcomer equivalent drum (meaning that circulation/speed criteria are conserved). The results show that the drum equipped with thicker downcomers has a better fatigue resistance.
However, this statement should not be generalized. When performing the same kind of calculation with header and tube connection, the opposite could be noticed.
Three designs for attaching the main body to the branch on the drum or superheater are typically used in the boiler industry. Figure 9 illustrates the three designs incorporating stress induced factors (SIF) as specified by the Euro Norm:
It is important to design the welding connections in the most appropriate way. CMI standard design is:
- Drums: set-through nozzle, implemented for years (with/without internal extension)
- Superheater headers: set-on nozzles/branches without root weld reappropriation whenever fatigue analysis allows it, and with ground over (i.e. stubs) when required by fatigue analysis.
Note that attachment connections that use reinforcing pads cannot be calculated using Euro Norm formulas. Euro Norm requires a finite element analysis.
For HP superheaters and reheaters made of alloyed steel and subject to water quenching, full strength penetration welding (Figure 10) is CMI’s standard for tube-to-header welding attachments. This design surpasses ASME minimum requirements. For economizers and evaporators, however, partial penetration welds (Figure 11) meet ASME requirements and are proven under cycling.
Optimization of Boiler Start-up Time
Figure 6 illustrates that as pressure builds, the allowable temperature gradient increases. These calculated temperature/time gradients are converted into pressure/time gradients because these parameters are more accessible and controllable during transients. This feature is used to optimize start-up by applying progressive pressure ramp rates (Figure 12), which allow the overall boiler start-up time to be optimized without consuming any extra boiler lifetime. CMI implements these progressive pressure gradients into the plant DCS, either via a curve of variable controlled set points applied on the HP steam turbine by-pass valve (Figure 13), or via various HP pressure control set points during start-up. As long as the condenser is not yet ready, the boiler relies only on superheater start-up vents sized ad hoc. Only the HP drum pressure needs to be controlled, and no limitations are imposed on IP and LP circuits because those are less critical fatigue items. However, the bottleneck of the overall cold start-up time is typically on the steam turbine side, rather than on the boiler side.
It is good to be reminded that each new component avails a fatigue life time, and fatigue damage is always cumulative and cannot be reversed. Therefore the main conclusions are:
- Compared to TRD 301, the Euro Norm 12952-3 improves methodology, but it is still conservative. To stick to reality, stress concentration factors could be determined by finite element analysis and used with the Euro Norm, as is allowed by the code.
- Branch-to-main components should be engineered to minimize fatigue. This means that set-through arrangements are preferred for drums, whereas set-on are preferred for tube-to-header arrangements.
- Euro Norm shows that allowable pressure gradient varies with pressure. This allows boiler start-up time to be optimized without consuming fatigue lifetime by using progressive pressure gradients.
- If possible, exact alternate stress values should be obtained either by finite element or finite difference calculations, to have more representative results.
- The sensitivity on allowable number of starts, due to the logarithmic and probabilistic natures of fatigue, results in an inherent uncertainty in fatigue lifetime analysis.
- The superheater water quenching, which remains the big cycling issue for hot start-up, especially for horizontal HRSGs, is not adequately addressed by the Euro Norm. Therefore, good engineering practice - efficient HP superheater and reheater drainage - remains the key design point for overcoming this quenching problem.
Pascal Fontaine is marketing manager for CMI Energy and Jean-François Galopin is CMI Engery’s design manager. CMI Energy, headquartered in Belgium, engineers, supplies and installs heat recovery steam generators for combined cycle power plants and cogeneration applications.