Designing and modifying HRSGs for cycling operation

June 24, 2008 at 7:04 am | Posted in Combined Cycle, Gas Turbine, Power Plant, Steam Turbine | Leave a comment

Designing and operating HRSGs for cycling duty requires close cooperation between the manufacturer and the user. Once HRSGs are in service, a monitoring system should provide a thorough picture of the mechanisms affecting the life and integrity of the boiler.

Today’s heat-recovery steam generators (HRSGs), a vital component in the popular combined-cycle plant, are exposed to more severe duty than merely running at baseload. Deregulation has spurred the development of merchant plants, which are required to start up, load follow, and shut down with minimal notice in response to market conditions. Unless properly designed and operated to withstand this cycling duty, the integrity of the HRSG will be compromised.

Industry professionals need to understand the various damage mechanisms that can result, as well as how to control them. For new projects, damage control begins with the specification of certain design features. Operation and maintenance practices also play an essential role in managing HRSG life, both for new and existing plants. Existing plants also should explore modifications and monitoring programs that can boost the reliability and extend the service life of HRSGs in cycling duty.

Many components

Existing HRSGs that are already in service pose another problem. Most of these were designed for baseload operation but now are being cycled. Users should explore the HRSG modifications that are discussed below to determine the applicability to their specific plants. At a minimum, users of existing HRSGs should implement a monitoring program, outlined here, to gauge the effects of cycling.

A typical combined-cycle HRSG produces “main steam,” at three pressure levels, plus “reheat steam.” Supplemental firing through a duct burner often is installed, as well, to increase steam-production capacity. Most combined-cycle plants also have a feedwater heater to increase the heat recovery and reduce the deaerating load of the integral deaerator.

The heat-transfer sections of the HRSG are located in series along the exhaust-gas path to optimize heat recovery. It is not uncommon to have 15 to 20 different heat-transfer sections—superheaters, reheaters, evaporators, and economizers—at various locations along the gas path. Emissions-control regulations often require the addition of a selective catalytic reduction system (SCR) for NOx control and, in some cases, a separate catalytic converter for CO control. Some HRSGs also are equipped with an exhaust-gas bypass damper to enable simple-cycle gas-turbine operation, though these dampers have not found wide usage in the U.S.

Many mechanisms

All of these components are exposed to different temperatures and pressures at different times while the HRSG is started, operated, and shut down. How well the components react to the transients and steady-state conditions determines the integrity of the entire HRSG. The basic mechanisms affecting the life of an HRSG are:

  • Low-cycle fatigue (LCF). Damage occurs at a low number of stress cycles when the strain is high. LCF is the most dominant damage mechanism in today’s HRSGs.
  • Creep. Damage is caused by the material being at high temperature and stress for a considerable period of time.
  • Thermal shock. The impingement of cold water or steam on hot surfaces can damage the base material.
  • Oxidation. Oxidation and exfoliation can occur due to exposure to high temperatures.
  • Differential expansion. If adjacent tubes or pipes are at different temperatures, the resulting uneven expansion or contraction can stress both components. Piping supports also play a part in this.
  • Corrosion fatigue. A crack is initiated because of corrosion, and progressive damage occurs because of fatigue and corrosion. This typically occurs at a temperature range of 300F to 500F.
  • Corrosion in tubes. Improper water chemistry is the cause.
  • Flow-accelerated corrosion (FAC). The combination of high flow and poor chemistry control can cause FAC in specific HRSG locations.
  • Corrosion-product migration. The migration of corrosion products to other HRSG components may cause further corrosion or other damage.
  • Deposits. Temperature or water-chemistry fluctuations may result in deposits on heat-transfer surfaces.
  • Erosion. Transient high velocities may initiate or perpetuate erosion.

All components in an HRSG can be affected by these damage mechanisms (see Table). However, some components are more vulnerable because of their location, construction, or exposure. These critical components need to be designed and monitored more closely for potential failures. The critical components typically are:

  • Superheater and reheater outlets.
  • Tube-to-header joints in hot sections.
  • Drum-to-downcomer nozzles in the high-pressure drum.
  • Bent portions of heat-transfer tubes.
  • Attemperators.
  • Bypass valves.

To a plant operator, “cycling” refers to the quick start-up, load following, and shutdown of generating units. But to a materials engineer, cycling is the reversal of stresses in a component. Stress cycling can happen by the imposition and relaxation of loads or because of a reversal of temperatures. When a component is exposed to rapidly changing temperatures, all parts of the component may not heat up or cool down uniformly. The subsequent differential expansion creates high stresses and stress reversals.

The main causes of stress cycling are a change of plant load and emergencies. Plant load changes include short-notice start-ups, gas-turbine load changes, part-load operations, and varying duct-burner firing.

The way a boiler is shut down and laid up also can cause cycling. In an emergency—such as a steam-turbine trip—the boiler has to shut down very rapidly, causing undue, or reversing, stresses. Sometimes a boiler is force-cooled to make repairs during a short outage window. This force cooling can create temperature differentials. And the failure or abnormal operation of other plant equipment—such as pumps and valves—also can impose stress-cycling conditions on the HRSG.

Cycling relates to mechanisms

Most of the time, cycling either creates or exacerbates the effect of damage mechanisms on HRSG components. Creep damage, by definition, is caused by a prolonged exposure to high temperature and stress. Though creep is not caused by cycling, it is exacerbated by it. Fatigue and fatigue damage are the most prevalent mechanisms affecting the boiler life, and they are a direct consequence of cycling.

Water-chemistry upsets that result in corrosion may be due to cycling or to a failure of water-chemistry controls. For example, for a rapidly starting HRSG, the superheater is exposed to high temperature on the outside of the tubes and headers, while the inside surfaces of tubes and headers may still be cool. This creates high thermal stress. Some specific examples of how cycling creates or exacerbates the damage mechanisms in critical components are discussed below.

Superheaters and reheaters:

  • Fatigue. The impingement of hot gasses on cold surfaces at start-up or of cold gasses on hot surfaces at shutdown creates thermal gradients. Similarly, the condensed steam in the tubes after shutdown impinge on hot surfaces if the condensate remains in the tubes. The high-pressure components are more vulnerable to fatigue effects due to their increased wall thicknesses.
  • Thermal shock. Condensate in a superheater section or colder reheat steam in the hotter and dry reheater section would result in thermal shock to the inner surfaces of the tubes and headers.
  • Creep. Only high-temperature components are prone to creep damage. High-temperature transients and continuous high-temperature operation may increase the creep rate. However, if the creep is coupled with fatigue due to cycling, the damage will be much higher than what can occur if the same fatigue or creep is working alone.
  • Oxidation. Exposure of the metal to higher temperature than what it was designed for, particularly during start-ups, can result in oxidation. Oxidation and exfoliation can happen both inside and outside the tubes and piping, caused by exhaust gas on one side or steam/water on the other. Dry reheater designs are particularly vulnerable.
  • Differential expansion. Uneven heating of tubes—caused by uneven distribution of (1) exhaust-gas or steam/water flows or (2) exhaust-gas temperatures—can cause adjacent tubes to expand or contract differently. Both compressive and tensile loads are imposed.
  • Deposits. Uneven or excessively fast ramp rates may result in the accumulation of condensate in the superheaters and, consequently, the formation of deposits.

Evaporators:

  • Low-cycle fatigue. This often occurs in natural-circulation evaporators during start-up, because circulation has not been fully established.
  • Differential expansion. Just as with superheaters and reheaters, the uneven heating of evaporator tubes—caused by uneven distribution of (1) exhaust-gas or steam/water flows or (2) exhaust-gas temperatures—can cause adjacent tubes to expand or contract differently. Both compressive and tensile loads are imposed.
  • Deposits. Uneven heating or heat-flux anomalies may result in local dry-outs and deposition of salts in evaporator tubes. These deposits further distort the flow and heat-flux distribution, so the problem can snowball.
  • Flow-accelerated corrosion. Two-phase flows in evaporator sections result in FAC, particularly in the low-pressure sections.
  • Corrosion-product migration. Corrosion products formed at one location may migrate to another, and, under the right conditions, may form deposits there. These deposits then result in uneven heat fluxes.
  • Erosion. Solids in the water, and water in two-phase flow systems, can cause erosion at higher velocities.

Economizers and feedwater heaters:

  • Low-cycle fatigue. The impingement of cold water on hot surfaces, particularly during a quick shutdown and restart, sets up LCF.
  • Differential expansion. Uneven heating of tubes due to flow- or temperature-distribution problems can cause adjacent tubes to expand differently. Both compressive and tensile loads are imposed.
  • Deposits. Uneven or excessively fast ramp rates can result in solids precipitation and deposition, causing uneven heating.
  • Flow-accelerated corrosion. Single-phase FAC in economizers is being recognized as one cause of failures. Often it occurs because, during startups and at transition time, the water chemistry may be out of control.
  • Corrosion fatigue. Chemical imbalances can create corrosion, and cyclic loading can exacerbate the effect due to fatigue.
  • Erosion. Solids in the water can cause erosion at higher velocities.

Design needs dynamic analysis

In the design phase, the most important requirement is to make sure that all the factors affecting the life of the HRSG have been accounted for. This means that either the design features must be modified to mitigate the effect of the damaging factor, or the damaging factor must be eliminated. The most common way to ensure this is to do a life-cycle analysis, which typically consists of the following steps:

  • Define the basic operating conditions.
  • Establish the lifetime operating details.
  • Determine the most critical components.
  • Conduct a dynamic-response analysis of the critical components. This is perhaps the most critical factor in the life-cycle analysis. There are various software programs available that indicate the dynamic behavior of a component. However, these programs need to be custom-fitted to a given HRSG. To do this, the model has to be very well-defined. In addition, the expected operating conditions—such as pressure and temperature ramps that cause changes in the components—need to be defined thoroughly. Most of the life-cycle analyses currently on the commercial market do not give good results because of conservatism employed in the model. This often happens because the details of the component or the operating conditions are not worked out completely, and the model is not refined.
  • Calculate the cumulative damage factor (CDF). Various methods of calculating the CDF are available, including methods suggested by ASME, Germany’s TRD, and British Standards. It is important to use a consistent method rather than indiscriminately mixing various methods with different assumptions and data used in deriving the formulas.
  • Revise design or operating conditions if the CDF is higher than allowable.

Design features

Some of the specific design features that help in minimizing or monitoring the damage due to cycling are listed below. It is possible to design an HRSG without these features and still maintain the integrity of the boiler over the intended life of the unit. The key is to do a life-cycle analysis. If the life-cycle requirements are met with only some of these features, then the design should still be acceptable. Features that improve the cycling ability include these:

  • Smaller-diameter tubes, headers, and steam drums
  • Automated vents and drains
  • High-pressure superheater bypass to reheater
  • Wet reheaters
  • On-line water-chemistry monitors
  • Stack dampers
  • Leak-proof valves
  • Metal thermocouples
  • Minimization of piping and tube bends
  • Flexible connections
  • Motorized blowdowns
  • Economizer recirculation
  • No cascading blowdowns
  • External “kettle” boilers
  • Ability to clean
  • Spray coatings
  • Higher-grade metal alloys
  • Bypass dampers
  • Multiple attemperators
  • Good steam-/water-side flow distribution at all loads.

Operating practices

How the boiler is started up is a key factor affecting HRSG life. Rapid starting exacerbates fatigue, so it should be minimized, if it can’t be avoided altogether. The manufacturer-supplied start-up procedure may have some contingency built in, which can be analyzed and possibly removed. However, this is possible only if all the operating conditions, including the condition of the HRSG when starting and at shutdown, are well-defined.

After a “soak” period, which allows temperature differentials to be minimized, an HRSG can be ramped up or ramped down quickly. Ramping a running boiler is much faster than starting a new boiler. So it is advisable to ramp up the running unit and ramp down as the cold unit comes on-line.

Similar to ramping, load changes should be done gradually to minimize cycling effects. Often, HRSG manufacturers suggest making load changes in incremental steps rather than continuously. For example, instead of a continuous 10% change, the load may be changed in five 2% load changes with a dwell (or hold period) after each change.

Shutdowns by themselves may not be a big problem, unless purge requirements for the subsequent restart cause cold air to blow into a hot HRSG. Limitations on air flow should help minimize any damage.

The chemistry upsets and oxygen intrusion that frequently occur during HRSG layup substantially affect boiler life. When the boiler is put on-line, the heat will increase the damage.

Modifying existing units

Many of the HRSGs in service today were designed for baseload conditions but now are required to run as cycling units. Plant owners need to determine the impact that this will have on reliability and life expectancy. To do so, an engineering firm—preferably an HRSG manufacturer—should be retained to conduct a condition-assessment study, suggest equipment modifications that can extend service life, and implement a monitoring program to track damage mechanisms.

The condition assessment includes:

  • Identification of critical components and determination of their condition
  • Checking of instrumentation and controls
  • Review of operational history
  • Establishment of a baseline benchmark

Equipment modifications can be suggested after the proposed operations are defined by the owner and a complete life-cycle analysis of the critical components is performed. Sometimes a modification is feasible and cost-effective, but it may not be permitted because of code requirements.

A monitoring program should be implemented to track such variables as:

  • Temperatures, gas, water, and steam
  • Drum levels
  • Attemperator flows
  • Steam and water flows and velocities
  • Water chemistry
  • Steam purity and constituents
  • Gas-turbine and burner fuel
  • Exhaust-gas conditions

Data from the monitoring program should be reviewed and checked against the assumptions used in the equipment modifications. If there are any changes between the actual and initially specified operating conditions, these are noted and their effect on the life expectancy is assessed. Based on this assessment, some further adjustments may be necessary.

After at least a year of operation following equipment modifications, a condition assessment should be performed again, including measurement of wall thicknesses and concentrations of corrosion products and deposits. The assessment should include both destructive and nondestructive testing of suspect components, and all data should be checked against the baseline benchmark. Based on this assessment, additional equipment modifications or changes to operating conditions may be suggested.

Keep talkin’

For long service life of HRSGs, it is essential to have long-term cooperation and open exchange of information between the operators and the designers (see box). Both new and existing HRSGs can meet the rigors of cycling duty with the right design features, modifications to equipment, and adjustments in operating practices.

By Akber Pasha and Robert Allen, Vogt-NEM Inc

Edited by R.C. Swanekamp, PE

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