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Monitoring—and mitigating—  combustion combustio n dynamics By Phil Karwowski, Siemens Power Generation Inc

D

ry low-NOx  (DLN) combustors have dramatically lowered pollutant emissions and reduced the consumption of high-purity water at gasturbine-based powerplants. But they are prone to flame instability under certain operating conditions. Resulting pressure pulsations can damage both combustors and downstream hot-gas-path components. To mitigate these so-called “combustion dynamics,” GT manufacturers have developed sophisticated monitoring and control equipment to warn of impending instability and ensure stable combustion. What follows is a brief primer on combustion dynamics, their potential effects on gas-turbine components, and action items that users should consider.

components, dynamics of these frequencies can range from benign to highly destructive. Trouble begins only when the vibrations are excessive in amplitude, or when they occur at frequencies corresponding to natural resonances in that particular system. Trouble can culminate with fatigue failure of combustor components, which when broken free are launched downstream to inflict secondary damage on other hot-gas-path components. GT combustor dynamics can increase with variations in the fuel/air ratio, changes in fuel composition, or modifications to the combustor hardware. They also can decrease with

ultra-lean fuel/air ratios, on the very edge of flame instability. This yields the low emissions demanded today, but inherently makes them more susceptible to pressure pulsations.

audible ofcan-annular f requencies.combusfrequencies. In therange typical tor of a GT, combustion dynamics can range in frequency from below 50 Hz to around 5000 Hz (Fig 1). In terms of their impact on turbine

andcombustion by injectingzone water steam into the or or adding tuned resonators to the combustion components. The reason you hear so much about combustion dynamics today is that DLN combustors operate at

high-cycle fatigue. Remember, it’s not just large amplitudes that the designer must evaluate, but also resonant frequencies. Each pilot-nozzle assembly, for example, may have a different

Design solutions

The turbine designer has several options to minimize the impact of combustion dynamics. These include varying combustor geometries, changing fuel-injector manufacturing tolerances, and beefing up the components known to be vulnerable. However, the design and fabrication of combustion systems involves multiple tradeoffs, including: n P h y s i c a l constraints, principally C-stage fuel injection axial length of the Basket with thick combustor and area thermal barrier coating within the chamber. Dual-fuel n Cooling and co atpilot nozzle Transition piece ing requirements that Always been must be balanced there with the overall air Combustion dynamics flow. are not unique to DLN n Thermal expansion combustors, or even to considerations. ConGTs. Whenever comsider that the metal bustion is present—in a temperatures expewatercoal-fired furnace, a die- Pilot rienced by an F-class injection skid sel-engine cylinder, a transition piece range trash incinerator, etc—  from ambient temperDynamic monitoring sensor there are combustion ature while the unit dynamics. They are the is on turning gear to combustor of pressure waves of defined amplitudes 1. In the can-annular combustor approximately 1500F when the and frequencies that are an inherent a typical GT—such as the 15-ppm unit is at full-fire. result of the combustion process. The DLN combustor for the W501F shown For resistance to intermediate-frehere—combustion dynamics can dynamics are caused by small presquency dynamics, the likely design sure oscillations in the flame zone. range in frequency from 50 to 5000 solution is to make affected com Anyone who has heard the popping Hz. These may be benign, or highly ponents more robust, in order to destructive, depending depending on their ampliof a gaseous welding torch when the withstand low-cycle fatigue mechagas flow rate is adjusted has expe- tudes and on the resonant frequencies nisms. To combat higher frequency rienced combustion dynamics. The of GT components dynamics, the likely design solution popping noise is a common example requires keeping the amplitudes of of pressure oscillations created in the appropriate changes in these factors, the dynamics low enough to avoid

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2 0 0 7 O UT A G E H A N DB O O K natural frequencies. For example, the panels of a transition piece may exhibit relatively low natural frequencies (100 to 200 Hz), while the end-rail assemblies of that transition piece may have higher natural frequencies (typically greater than 500 Hz).

Dynamics in the intermediatefrequency range (defined here as 2. Intermediate-fr Intermediate-frequency equency combus-

tion dynamics caused damage at the lower panel of this transition piece natural frequency because of slight differences in their diameters, wall thicknesses, and lengths. Think of a large pipe organ: The pitch (or natural frequency) of each individual pipe depends on its length and inner diameter. Short pipes with small diameters produce high notes; larger, longer pipes produce the bass tones. Complicating the designer’s challenge is the fact that any one combustion component may have several

100 to 1500 Hz) are what damaged the lower panel of a transition piece in one gas turbine (Fig 2). Investigation revealed that the root cause was an incorrect valve position on the combustor bypass system. The incorrect valve position substantially increased the amplitudes of these intermediate-frequency combustor dynamics, leading to a fatigue failure and propagation of the crack. Potential design solutions included elimination of the combustor-bypass system, and a more robust design of the transition piece.

Dynamics in the high-frequency range (above 1500 Hz) caused damage to combustor baskets at

three different powerplants (Fig 3). In these cases, equipment changes apparently had reduced the damping in the system as originally designed. Potential solutions here included equipping the baskets with resonators, adjusting the inlet-guide-vane position schedule, and increasing the amount of steam injection (on the turbines that use steam for power augmentation). Full-scale tests using various combinations of combustor baskets and transition pieces identified configurations with improved damping capabilities. The accompanying table is helpful for identifying the potential causes of combustion dynamics in the canannular systems common to many late-model GTs.  

At the plant level While GT manufacturers are working continually to improve combustor designs, users can take steps at the plant level to monitor and detect

3. High-frequency dynamics were the culprit at three different powerplants, where combustor baskets were damaged

Matrix assists in identifying cause of combustion dynamics     Description

Lowfrequency dynamics      

Frequency range, Hz

 Amplitude alarm setpoint, psig

0 to 25

0.5

25 to 100

1.0

Component risks

• Swirler damage • Basket damage • Nozzle damage

Potential causes

• Flashback indications • Lean blowout • Damaged swirler(s) • Air-flow restriction • High injection flow rates • Pilot-nozzle distress

Mitigation strategies

• Increase pilot-stage fuel fraction • Increase C-stage fuel fraction • Repair/replace the basket • Remove air-side obstructions • Reduce the injection flow rate

Intermediate- 100 to 500 frequency dynamics  

2.0

• Transition panels • Fuel composition • Transition seals • Fuel splits • Fretting • Bypass-valve distress • Wear

• Combustion tuning • Active tuning

Intermediate- 500 to 1500 frequency dynamics  

1.0

• Downstream components • Fretting • Wear

• Equipment distress

• Inspect and repair combustor components

Highfrequency dynamics  

0.5

•• Baskets Cross-flame tubes • Flashback thermocouples

•• Over-firing IGV position error • Fuel composition • System damping • Basket distress

•• Install resonators Adjust Helmholtz IGV position • Increase steam injection • Preheat the fuel

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500 to 5000

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  n 200   e   )    h  s   a   g     w   s   l180   n  a    %  ,   o  r   u160   s   i   s   t   n   i   a   s   o   n    i   s  m  e140   s  e   i    i   n   o   l   m    t   e120   e   p     d   i   e   p    O  r   g100    N  a   p   i   n   r    i   m    f   o   80   c    ( 1340 1360 1380 1400 1420 1440 1460 1480 Wobbe Index, Btu/scf (based on higher heating value, HHV)       x

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4. Variability in fuel composition—tracked using the Wobbe Index—can change the NOx emissions, and correspondingly the amplitudes of combustor dynamics. Using the same engine and combustor settings, emissions were driven higher by increasing the content of the non-methane components of natural gas—ethane, propane, and butane 5. Installation of tuned resonators is one way to dampen high-frequency combustion dynamics. Use of thick thermal barrier coatings on combustor baskets

also reduces the potential for HFD Resonators to reduce dynamics

dynamics before they cause extensive damage. In fact, a GT’s flame-stability margin is, to a large extent, a function of site-specific, dynamic parameters—including fuel composition, the amount of wear on combustion-liner seals, and ambient conditions. For example, a sudden decrease in the content of higher hydrocarbons in your fuel will increase combustion dynamics. Similarly, a change in fuel composition that leans out the fuel-air ratio will increase combustion dynamics—though your regulatory-compliance manager may be pleased because NOx emissions would likely drop. Caution: Just because you’re getting pipeline natural gas from the same supplier you’ve always used doesn’t mean your fuel composition isn’t changing. In recent years, many pipeline suppliers have cut back on the processing of their product, allowing more ethane (C2), propane (C3), and butanes (the C4s) to remain in the pipeline gas that users once thought of as only methane (CH 4). The result is greater variability in heating values and an increased tendency to form damaging liquids. (An article specifically addressing fuelquality concerns appears elsewhere in the 2007 Outage Handbook.) Because of the possible variability in fuel composition, and other sitespecific factors, each DLN combustor should be tuned during plant commissioning, and periodically thereafter. Many plants follow a regimen of semi-annual tuning as the seasons change, typically bringing in the OEM’s specially trained engineer to perform the sensitive adjustments using portable pressure-monitoring equipment. But an increasingly popular alternative is to permanently install an online monitoring system that continuously measures the dynamic pressure pulsations and provides early warning Robust that the combustor is out-of-tune. software and skilled analysts can interpret the data collected by this combustion dynamics monitoring (CDM) system. The latest CDM systems even proCOMBINED CYCLE JOURNAL,

Thick TBC on baskets reduces cooling flow

vide protection logic to automatically unload and protect the GTs during excursions of combustion dynamics. These “active systems” provide the best level of protection against damaging amplitudes and frequencies. The CDM system can be designed to monitor the Wobbe Index (WI, sometimes referred to as the Gas Index). This parameter indicates the relationship between the fuel’s volumetric flow and its energy content. Specifically, the WI is the ratio of the higher heating value to the square root of gas specific gravity. Even more instructive is the Modified Wobbe Index, which takes into account the temperature of the fuel. The MWI is the ratio of the lower heating value to the square root of the product of the specific gravity and the absolute gas temperature. Using this real-time data as input, an effective method of reducing combustion dynamics is to increase the fuel temperature—which temperature—wh ich reduces its density, hence increases its velocity for a given mass flow rate (Fig 4). To learn more about the WI and MWI and the fuel variability being experienced by others, access the following articles from the COMBINED CYCLE Journal at www.psimedia. info/ccjarchives: n  CTOTF Spring Turbine Forum report, 4Q/2005 (refer to section “Gas quality concerns most GT owner/operators”). n “Improve GT operating

flexibility, reliability with fuel-system mods,” 2006 Outage Handbook supplement to the 3Q/2005 issue. Effective control of combustor

Third Quarter 2006

dynamics also has been achieved with the installation of Helmholtz resonators, which attenuate specific frequencies in the combustion system (Fig 5). These resonators are passive devices, and are tuned for specific frequencies where dynamics are known to occur. Helmholtz resonators typically are effective for combustor dynamics at frequencies above 1000 Hz. CCJ OH

 The authoritative informatio infor mation n reso resource urce for owner/operators of gasturbine-based peaking, cogen, and combined-cycle plants. Subscribe via the Internet, access: www.psimedia.info/subscriptions.htm OH-3