HRSG CYCLING

Comparing ASME, European boiler codes and their impacts on design, operation

By S Hampson, D G Robertson, and S Simandjuntak, European Technology Development Ltd (UK)

In today’s competitive electric generation marketplace more and more plants are cycling, which brings with it the likelihood of increased damage to powerplant equipment. For heat-recovery steam generators (HRSGs) serving in combined-cycle systems, low-cycle fatigue damage to thick-section components will accumulate as a result of large through-wall temperature gradients that develop during thermal transients.

The peak thermal stresses induced by high-temperature gradients in superheater-header and steam-drum walls are strongly influenced by the thickness of the components and the methods of operation during startup and shutdown. It seems obvious that design of the HRSG should include fatigue analysis of the critical components in order to determine the allowable number of operational cycles and acceptable limits for startup and shutdown ramp rates.

However, Section I of the ASME Boiler and Pressure Vessel Code,  which provides rules governing the construction of power boilers, has no mandatory requirement for the designer to perform a fatigue analysis.

Regardless of the stress transients during startup and shutdown, ASME Section I only considers the steady-state operating conditions (specifically, the creep regime for superheaters) for the purpose of component thickness calculations. Some consideration of fatigue and creep-fatigue interaction is given in other sections of the ASME Code, but many boilers designed to ASME Section I may not have had an assessment of fatigue life.

By contrast, the European standard Euro Norm (EN) 12952-3 requires that a fatigue analysis be conducted when a boiler will be subject to cyclic operating conditions. Clearly, there is a fundamental difference between the requirements of ASME Section I and the European standard with regard to design of boilers for cyclic duty.

The authors recently completed cyclic-life capability studies for new HRSGs installed in two combined cycles. The studies included a review of HRSG component design features to identify any aspects that might constitute areas of weakness under cyclic operating conditions, as well as a fatigue-life assessment of key HRSG components to determine their capability for meeting the operational requirements specified by the plant operator.

The allowable numbers of load cycles to crack initiation were calculated for different temperature ramp rates using the EN 12952-3 methodology. Fig 1 shows some results for the connection of the saturated steam pipe to the HP drum.

Plant operators typically identify startups as cold, warm, and hot. They often are defined by the length of time the HRSG has been shut down. However, fatigue damage is known to be directly proportional to temperature changes. Thus another way of defining startups is by referring to temperature and/or pressure of the internal surface of the metal/component.

For the purposes of this discussion, a cold startup is characterized by a zero-pressure condition when the temperature of the steam-drum inside wall is 175F or less. A warm start is defined by an HP drum pressure of 45 to 75 psig when metal temperature at the surface of the inside wall is at least 285F (in HP drum). To be classified as a hot start, HP drum pressure must be at least 500 psig and metal temperature 465F.

When the metal temperature is at ambient (for example, in cold climates, the temperature could be around 40F and in warm climates around 100F) and the HRSG is starting with zero steam pressure, the startup event is called a cold ambient startup. This can be quite damaging in thick pressure parts like steam drums and superheater headers because of the large temperature gradients generated during the initial stages of startup when condensate heating occurs.

The fatigue analysis described by EN 12952-3 takes account of the thermal stresses arising from through-wall temperature gradients during cyclic operations. Note that thermal shocks caused by condensate quenching, which can be a significant issue for HRSGs with horizontal hot-gas paths, are not addressed by the Euro Norm.

Superheater and reheater drainage should be a key design point, if the risk of damage from condensate quenching is to be minimized.  However, service experience has often shown that insufficient attention was paid to drainage at the design stage, and modifications to drain systems have been implemented following component cracking and failures caused by undrained condensate.

Condensate is formed in superheater tubes during cooling on shutdown, GT trip, and purge prior to warm/hot start. Condensate quenching is the thermal shock caused by condensate entering hot components, such as superheater headers. This phenomenon cannot be avoided by draining because the quench has already occurred by the time the condensate reaches the superheater drain.

Once formed, however, condensate should be removed to avoid the consequences of condensate migration through the system. The passage of condensate slugs, also known as condensate migration, is of particular concern because it can produce random transient cooling or quenching of tubes in particular tube bundles.

For a new plant, it requires the identification of the sources of such condensate and their elimination. This may dictate modifications to the plant, especially the number and location of condensate drains, and/or operating procedures. Studies have shown how temperature monitoring can be used to correlate condensate migration and quenching events with plant operational cycles.

In a recent study, cracking was found in the HP superheater outlet drain pot of an HRSG. The plant had operated mainly in base-load mode with occasional cycling. The damage had developed over a period of about three years, during which the plant had performed 50 two-shift cycles—that is, hot restarts.

The pattern of the cracking on the internal surface of the damaged component was indicative of thermal quenching (Fig 2), which supported the view that crack initiation was caused by condensate moving from the superheater module to the drain pot. Metallographic examination of the damage location showed transgranular cracks growing through the microstructure, indicating crack propagation by fatigue.

Findings. After completing fatigue analyses of critical HRSG components according to guidelines presented in EN 12952-3, the authors were able to establish acceptable limits for startup and shutdown rates in order to achieve specified numbers for cold, warm, hot operational cycles over the design life of the plant.

Where the desired numbers of different cycle types could not be achieved, then changes to operating procedures or modifications to component design were necessary. At the design stage, this frequently means that either the specified temperature ramp rates should be reduced or a stronger material should be selected in order to reduce the required component thickness and, hence, reduce the magnitude of thermal stresses during operational transients.

Also conducted was the application of fatigue analysis as part of root-cause failure analysis of a HRSG superheater drain pot, which had been damaged by thermal quench cracking. Fatigue calculations based on monitored temperature data showed that cracking could occur as a result of multiple quenching events that occurred during the two-shift cycles performed by the unit. Use of stronger material would improve the cyclic life of the component, but it was also necessary to identify measures that would reduce the severity of the thermal transients experienced during operational cycles. CCJ

References

  • European standard EN 12952-3, “Water tube boilers and auxiliary installations—Part 3: Design and calculation for pressure parts”
  • P Fontaine and J-F Galopin,  “HRSG optimization for cycling duty based on EN 12952-3,” Proceedings of the conference on cyclic operation of powerplants, European Technology Development Ltd, September 2007, London
  • M Pearson and R W Anderson, “Measurement of damaging thermal transients in F-class horizontal HRSGs,” Proceedings of the international seminar on cyclic operation of heat-recovery steam generators, European Technology Development Ltd, September 2005, London
  • ASME Boiler and Pressure Vessel Code, Section II, Part D, “Materials properties”

 

Obtain, read latest version of ASME Code

The foregoing comparison of ASME Section I and European EN 12952-3 is instructive, but let’s dig deeper to understand why things are the way they are regarding the ASME Code and what has been done to update Section I.

Robert W Anderson, the chairman of the HRSG User’s Group, had the following to say in the “HRSG Users Handbook,” edited by Robert C Swanekamp, PE, and published for the HRSG User’s Group (go to www.hrsgusers.org for details) by PSI Media Inc, publisher of the COMBINED CYCLE Journal:

“Many purchasers of combined-cycle and cogeneration plants are under the impression that their specifications will be sufficient as long as they require compliance with the ASME Code. Unfortunately, they couldn’t be more wrong—at least if they plan on cycling their HRSGs.

“The sole purpose of the ASME Code is to ensure safety—to prevent catastrophic failures and the resultant loss of life, injury, and property. It does an exceptional job of this. . . . But the ASME Code was never intended to be a ‘design and construction handbook’ that guarantees reliable and durable equipment fit for cycling HRSGs that will operate behind high-temperature GTs.

“These activities include the modeling of thermal transients, determination of component fatigue life, consideration of creep/fatigue interaction, location and configuration of dissimilar metal welds, effective management of condensate, implementation of effective desuperheater protective logic, and effective integration of major plant equipment.

“In addition to not mandating their execution, the Code gives little guidance regarding how to effectively perform these activities for the conditions experienced by today’s HRSGs. The ASME is well aware of the shortcomings in its current Code and is working diligently to correct them.”

That passage was written in 2006 and a couple of ETD’s references have 2005 and 2007 dates. Progress is slow in the world of engineering standards and work on the ASME Code was ongoing while the references cited were written.

Note that Section I of the 2007 edition of the Code includes Part PHRSG, “Requirements for Heat Recovery Steam Generators.” It cites requirements for superheater and reheater condensate removal systems designed to minimize thermal shock and prolong the lifetime of pressure parts. It also specifies rules for desuperheater drain pots and provides sketches of drain arrangements. Owner/operators should review the latest version of the Code before preparing specifications for new HRSGs.