FluiddynamicsoftheHRSGgassideDesigners of heat-recovery steam generators are using computational fluid

dynamics software as one tool to reveal the invisible forces affectingthe flow over, under, around, and through structures such as inletducts, distribution grids, and guide vanes.

ByJeffOaiber,VogtPowerInternationalInc.

For years, original equipment manu-facturers (OEMs) of heat-recoverysteam generators (HRSGs) have

supplied distribution grids (DGs) as ameans for controlling the undesirablevelocities, temperatures, spin angles,and pulsations generated by certain gasturbines. In the past, the combustionturbine OEMs readily provided informa-tion about the velocity profiles of theseunits, but in recent years that informa-tion has been increasingly more difficultto obtain, let alone substantiate.

The fact that some turbine installations

produced large disturbances led HRSGOEMs to take the strong-arm approach ofadding a distribution grid. The presence ofa DG can produce a pressure drop of up to2 inches (water column), adversely affect-ing the turbine's performance and/ornecessitating an increase in the size of the

You:mightwant to use CFD i.f .

HRSG. Although the use of guide vaneshas proved successful, the technique alsoproved to be more black art than science.The number of failures that have resultedhas made many HRSG users averse tousing guide vanes.

The challenge for designers of HRSGsis to develop analysis tools that allowthem to thoroughly understand the fluiddynamic storm raging at the entrance tothe HRSG and use those tools to developdesigns that. reduce the flow disturbancesand induced vibrations. As a result, manymanufacturers of burners and HRSGseither have purchased software programsor contracted for computational fluiddynamics (CFD) analysis or cold-flowmodeling of the problems (see box).

This replacementof intuitionby science isa positive development;eliminatingflow andvibrationproblemsrequires a comprehensive

---. .

.Vou have a new gas turbine---be it a modification, conversion, upgrade, or series#1/ttnit #1.

.. You are retrofitting/adding a duct burner.

. An emissions-control system is being retrofitted or added.

.A.nemissions-control system or a burner is not performing to spec, or plantemissions need to be lowered.

. Vou want to reduce ammonia slip (legislation is just around the corner).

. You want to troubleshoot flow instabilities, excessive pressure drop, or poorsteam performance.

. You'reexperiencing any circulation problems, especially in low-pressureevaporators.If not corrected, they can contribute to flow-acceleratedcorrosion.

. When multiple and identical units will be built; CFDanalysis can save you millionsof dollars in operating costs overthe life of the units.

II Yourturningvanesend up in yoursuperheater.II. You hear Loud noises from the HRSGinlet duct and later find pieces of the distribution

grid in the stack.

. You look through the burner view port and see 3D-foot flames dancing thoroughyour superheater/reheater.

.Vouhear rumbling and see your inlet ductwork "breathing."

58 www.powermag.platts.com

understanding of the interaction betweenfluid dynamics and installed hardware.Although the level of that understandinghasincreased significantlyover the past decade,some key design infonnation still remains aclosely guarded secret today, and someOEMs are even attempting to patent certaindevices for changing flowdistribution.

Optimizing design velocityThe amplitude and uniformity of gas veloc-ity through sections of the HRSG affect notonly the unit's heat-transfer and burneroperation, but the performance of down-stream emissions control systems as well.The desirability of reducing an HRSG'sfootprint and materials costs has led allOEMs to continue pushing the design enve-lope. But some may have gone too far;there are at least two OEMs whose success

with aggressive ductwork configurationshave raised questions about what is propergas flow perfonnance.

Together, proper design and carefulanalysis can overcome most duct designchallengesby eveningout the distributionof gas flow across the heating surface ofa burner. Every burner has an idealapproach velocity and flow distribution.Louisville-based Vogt Power International,Inc. prefers not to use increased bafflingto provide them because doing so cancause reliability problems. For example,some HRSG OEMs use long runners toguide flow to burners located in largecross-sectional areas of the inlet duct. Butsuch runners are susceptible to failuredue to gaps in support along their length,turbulence, uneven thermal growth, etc.

AnotherdesignchallengethatCFD analysisis particularly good at meeting is a commonone: minimizing the footprint and intercon-necting ductwork of a gas turbine-HRSGcombination.For one particularinstallationofa General Electric 7FA turbine, Vogt usedCFD to minimizelocallyhighnear-wallveloc-itiesand backflow,reducingductworkby 30%

POWER I March 2006

(Figure 1). On another project with the samegoal, another OEM reportedly designed a ductthat left the top 20% of heating surface open,

reducing the effectiveness of the design andfailing to meet performance goals.

A final bit of prectica1 advice in the field ofHRSG burner design: Don't be tempted toincrease the burner firing rete to increase steam

production. Doing so either will worsen the dis-tribution of gas flow or cause premature failureof pressure parts or liners by overheating them.ill some jurisdictions, increasing the firing retealso will increase the ammonia slip of the selec-tive catalytic reduction (SCR) system down-stream of the HRSG above permissible limits.

Following are some of the parameters that

Vogt takes into account when configuringHRSG inlet duct geometry:

. The gas turbine's exhaust geometry anddirection.

. The dimensions of the heating sur-face module.

. The location of duct burners.

. The mass flow rates and average veloci-ties of the gas turbine's exhaust.

. The mass flow distribution at theentrance to the heating surface.

. Local velocities within the inlet duct:Near duct walls, and at the entrance to the

first heating surface.

Fluttering tubesField reports suggest that flow- or pressure-induced vibrations from a gas turbine mayhave caused or contributedto prematurefail-ures of relatively new interconnecting duct-work. As a result, OEMs of HRSGs havebegun to offer ductwork designs tailored to aparticular gas turbine. Vogt's experience hasbeen that using certain ductwork designswith certain turbines whose inlet blades are

water-washedcan also cause problems.As mentioned, many turbine users have

experienced premature failures of guidevanes. They have responded by removingthem. But because some users say thatremoving the vanes did not adversely affectHRSG performance, vanes' effectiveness insome applicationscould be considered ques-tionable. Vogt's contribution to this debateis to suggest using smaller vanes (if theductwork is amenable) because they seem tolast longer than larger ones. Optimizing thedesign and size of any vane requires a thor-ough understanding of three variables: itsanchoring mechanism, the forces on it, andthe delta pressure across the surface of itsblades. Figures 2 and 3 illustrate the contri-butions to dp of backflow, swirl, and thedirection of gas turbine discharge.

Vogt has learned from its CFD analysesthat these relationshipsand contributionsare

60

1. Go with the flow.-Flow velocitydistributionin the entry duct can vary significantlyand can seriously impact HRSG performance. Computational fluid dynamics (CFD) analysiscan determine the most effective design and placement and design of.a distribution grid.Courtesy: Vogt Power International

2. Blowin' in the wind. Pressure distributionson the windward and leeward side ofinner turningvanes. Courtesy:Vogt Power International

POWER IMarch 2006

2.23e+022.11e+021.99e+021.88e+021.76e+021.64e+021.53e+021.41e+021.29e+02 /' I I I / I'TI BA II 1//1 rlbw '.1.18e+021.06e+029.45e+018.2ge+017.12e+015.96e+014.7ge+013.63e+012.46e+011.30e+011.34e+00

-1.03e+01-2.20e+01-3.36e+01-4.53e+01-5.6ge+01-6.86e+01

1.38e+021.32e+021.27e+021.21e+021.16e+021.10e+021.05e+029.91e+019.36e+018.81e+018.26e+017.71e+017.16e+016.61e+016.06e+015.51e+014.96e+014.40e+013.85e+013.30e+OO2.75e+012.20e+011.65e+011.1Oe+015.51e+010.00e+01

3. Turning point. Turning vanedesigns can significantly affect flow distribu-

tions in downstream ducts. Courtesy: VogtPower International

easily misunderstood and miscalculated.Sometimes the solution is closer attention to

the constructionof vanes.Changesin industrypractice-today's use of stitch welds ratherthan sealwelds for transitions, for example-confirm the benefits of this attentiveness.

Similarly, in bridge construction, rivets werecommonly used in lieu of welds;becauseriv-ets lack a fixed point, they cannot transmit a

moment. These changes in attachment prac-tice have proven capable of reducing theamount of cracking in inelastic structures.

Figures 4 and 5, respectively, depictthe flow contours and velocity distribu-

tions of a Vogt inlet-duct design. As partof the design, we generally attempt tominimize vibration and damage to linersby predicting near-wall velocities, limit-ing high local velocities, and minimizinglarge regions of backflow-particularlybackflow with large velocities.

Losing pressure-and NH3Among the steps that Vogt takes to mini-mize HRSG gas side pressure drop (GSPD)are the following:

. Design inlet ducts for maximumpres-sure recovery.

. Design distribution grid(s) and placethem where they will be the most effec-tive over the largest operational rangeand with the smallest GSPD penalty.. Achieve the best velocity distributionacross burners (both up-front andinters tag e) with minimum GSPDand/or distribution equipment.

. Use and/or modify upstream modules toimprove gas distribution.

. Improvedistributionby making use ofcatalyst layers' "built-in" GSPD. Here,it's important to realize that plate andhoneycomb catalysts of the same effec-tiveness exhibit different pressure drops.

. Model and design stack breaching tominimize GSPD. Often, overlooking thisdesign issue can result in the unwanted

March2006IPOWER

HRSGDESIGN'.

4. Allshook up. CFD results illustrate the contours and flow path lines (colored by tem-perature) in an HRSG entry duct. Courtesy: Vogt Power International

5. Fast and furious. Velocity distributions confirm the final duct design. Courtesy: VogtPower International

addition of as much as 1/3 of an inch ofpressure drop.

. Maximize the "stack effect."

believe is more harmful to the environ-ment than NO..

The final word on this subject has to dowith emissions guarantees.A few years ago,a customer requested guarantees of 1 ppmfor both NO. and ammonia slip from anHRSG. Although the company met them,the customer questioned both the accuracyand placement of the measurement device.In yet another similar case, an outside labwas required to "prove" that emission guar-

In this area, one of Vogt's most notablesuccesses to date has been our ability to"shoehorn" an emissions system into anexisting HRSG without a spool piece. Inother cases, the CFD analysis and solutionresulted in a dramatic reduction of ammo-

nia slip - which, incidentally, many

61

antees were met. Be forewarned: the loca-

tion of a probe relative to its fluid dynamicscan make or break a guarantee. This isanother reason why a clear understanding

of HRSG gas side fluid dynamics is manda-tory today. Variations in inlet duct designcan make a dramatic difference in reportedresults (Figure 6).

6. Terminal velocity. Comparison of axial velocity distribution on a cross-sectionbetween the distribution grid and burner. Courtesy: Vogt Power International

Axialvelocity(ft/see)

100.0

~95.090.085.080.075.070.065.060.055.050.045.040.0 Full35.0 cross- 59.8%@+/- 25% 80.5%@+/- 25% 85.2%@+/- 25% 86.0%@+/- 25%30.0 section RMSvelocity RMSvelocity RMSvelocity RMSvelocity25.0

20.0 1ft off 84.6%@+/- 25% 94.9%@+/- 25% 95.1%@+/- 25% 98.9%@+/- 25%15.0 walls RMSvelocity RMSvelocity RMSvelocity RMSvelocity10.05.00.0

Basedesign Modular ductfloor & burner

arrangement

Modular designwith roof &floor baffle

Modular designwith roof &floor baffle

Lessons learnedCFD analysis of the exhaust gas flow dis-tribution in the inlet ductwork of com-

bined-cycle power plants has contributedboth to lowering their operating costs andimproving their emissions performance.Here's an example.

At one site where a gas turbine wasupgraded, the plant owner simultaneouslyreplaced both the CO and SCR catalyststo withstand its higher exhaust tempera-tures. After the retrofits, both NO. andCO emissions were within permitted lim-its, but still above expected values. Oper-ators found that overspraying wasrequired to reduce NO. to the desiredvalue of 3.0 ppm at 15% O2, but thatdoing so increased ammonia slip at thestack to an unacceptably high level. Vogtwas asked to find a long-term solution tothe problem.

The company's investigation andanalysis confirmed that the poor emis-sions performance was caused by highlybiased gas flow distributions at theentrance to the CO catalyst modules andthe ammonia injection grid. Using multi-ple models to evaluate alternate configu-

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POWER IMarch2006

--- -- -- - --

D-

HRSGDESIGN

7. Off the grid. Constant value surface velocity before (top) and after (bottom) CFD analy-sis and design of a distribution grid. Courtesy: Vogt Power International

Axial velocity(ft/see)

~~~:~26.024.022.020.018.016.014.012.010.08.06.04.02.00.0

-2.0-4.0--£.0--£.0

-10.0-12.0-14.0-16.0-18.0-20.0-22.0-24.0-26.0-28.0-30.0

Axial velocity(ft/see)

~30.028.0. 26.024.022.020.018.016.014.012.010.08.06.04.02.00.0

-2.0-4.0--£.0--£.0

-10.0-12.0-14.0-16.0-18.0-20.0-22.0-24.0-26.0-28.0-30.0

rations of flow control devices, Vogtdesigned a single distribution grid andconcluded that locating it upstream of theCO catalyst modules would improve flowdistributions at both the CO and SCR cat-alysts and the ammonia injection grid.Field tests confirmed that the new gridsignificantly improved emissions per-formance and reduced both requiredammonia injection flows and ammoniaslip values eight-fold. Figure 7 shows the"before" and "after" analyses.

The CFD analysis program at Vogt

Power International Inc. is managed byTony Thompson. A similar program is man-aged at Riley Power Inc. Both Vogt Powerand Riley Power are subsidiaries of Bab-cock Power International. Their assistance

in the preparation of this article is appreci-ated. All of the CFD figures referenced inthis article were completed using Fluent 6flow modeling software from Fluent Inc.(Lebanon,NH.).

-Jeff Daibercan be reachedatjdaiber@vogtpower.comor502-899-4500,ext4515.

March 2006 IPOWER

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