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89-GT-173L

Gas Turbine Exhaust Expansion Joints, Diverter andDamper Designs for the New Generation of Higher

Temperature, High Efficiency Gas Turbines

LOTHAR BACHMANNPresident

Bachmann Industries, Inc.Lewiston. ME

W. FRED KOCHManager of Engineering

Bachmann Industries, Inc.Lewiston, ME

ABSTRACT

The purpose of this paper is to update the industry on the evolutionarysteps that have been taken to address higher requirements imposed on the newgeneration combined cycle gas turbine exhaust ducting expansion joints, di-verter and damper systems. Since the more challenging applications are in thelarger systems, we shall concentrate on sizes from nine (9) square meters up toforty (40) square meters in ducting cross sections. (Reference: General ElectricFrame 5 through Frame 9 sizes.)

Severe problems encountered in gas turbine applications for the subjectequipment are mostly traceable to stress buckling caused by differential expan-sion of components, improper insulation, unsuitable or incompatible mechan-ical design of features, components or materials, or poor workmanship.

Conventional power plant expansion joints or dampers are designed forentirely different operating conditions and should not be applied in gas turbineapplications. The sharp transients during gas turbine start-up as well as thevery high temperature and high mass-flow operation conditions require spe-cific designs for gas turbine application.

Background:

For purposes of stand-by generation traditional gas turbine plants weremostly of the simple cycle type, comprised of a turbine with a single exhauststack and one expansion joint on the turbine neck. Temperatures on thesesystems were between 400 and 450 degrees C. The ducting was of either carbonsteel or at best, low alloy CR MO steel. Insulation was usually attached exter-nally.

In order to increase the efficiency of gas turbine power generation, theexhaust gas is passed through a heat recovery steam generator. Thus, a steamturbine is fed to generate power in addition to the gas turbine simple cycle.This concept is known as "Combined Cycle Generation".

The duct system for this concept is more complex as it usually contains aby-pass stack to atmosphere prior to the boiler or HRSG (Heat RecoverySteam Generator). Dampers or flow diverters are needed to control bypassand HRSG flow volume. There are a number of expansion joints and theremay also be transition pieces and a silencer.

The original damper choice for gas turbine service was a direct derivativelouver and guillotine damper combination previously designed for conven-tional power plant equipment. The temperatures in these gas turbines were 400to 450 degrees C thus similar to the Economizer Outlet temperatures on coalfired boilers of 300 to 375 degrees C. Very limited success in the smaller sizeswas achieved with this approach as the high transients in Gas Turbine start-up have a major impact on components. The larger the frame sizes, the morefrequent failures were experienced.

Examples are louver damper blades, which are usually airfoil shaped andwith hollow sections where one skin faces the heat blast and one face down-stream. As conductivity through the section is slow, immediate distorsion dur-ing start-up transients is unavoidable. Guillotine blades made of the traditionalplate and chain drive concept face a similar problem. Heat transfer throughthe plate is gradual and thus the guillotine blade binds immediately in Turbinestart-up due to buckling, often permanently.

Expansion joints mounted on vertical structural channel members andcovered by flow liners also see distorsions in the hardware causing tearing ofthe expansion joint membrane, and often immediate failure in start-up..

In recent times, sizes and efficiencies of combined cycle gas turbine plantshave increased and temperatures have now risen up to 600 degrees C in someunits with mass flow velocities up to 30 to 40 meters per second. Duct crosssections are now between 8 and 40 square meters depending on turbine framesizes.

Design Changes Addressing Gas Turbine Applications:

Serious redesign of damper and expansion jont concepts were necessitatedby the upward evolution in order to make the combined cycle concept reliableand workable.

The initial change was to insulate the duct, damper and expansion jointcomponents internally in order to avoid stresses in the external componentwalls. By better maintaining shape and rigidity, the physical accuracy of thesystem was vastly improved.

In addition further development of gas turbine expansion joints was re-quired in order to handle both thermal expansion movements as well as thetransition from externally insulated turbine connections to internally insulatedducts and dampers.

Finally, two damper concepts evolved which will be discussed in detail asfollows with a discussion of gas turbine expansion joints to follow.

The two damper concepts:1) Multi-Damper Flow Diverter Dystems2) Single Blade Gas Flow Diverter Systems

The Multi-Damper Flow Diverter System (MDS) is comprised of a by-pass"Tee" fitting, onto which a double louver or by-pass flap is mounted on theby-pass side, and a single louver plus a guillotine damper on the boiler inletside.

The Single Blade Gas Flow Diverter System (GFD) is comprised of thesame by-pass "Tee" fitting. However a single large blade, capable of modulat-ing as well as alternately isolating the boiler inlet for turbine start-up and thenthe by-pass for normal base load operation, is pivoted within that "Tee".

Before going into the comparative advantages and drawbacks of the twoabove approaches, what must be undertaken is an examination of the mechan-

Presented at the Gas Turbine and Aeroengine Congress and Exposition—June 4-8, 1989 —Toronto, Ontario, Canada

Copyright © 1989 by ASME

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ical design steps that resulted finally in damper components suitable for hightemperature Gas Turbine Service.

Louver Damper Blades: The traditional or conventional air-foil waschanged to a flat-blade design, reinforced by a perforated, aerodynamicallyshaped bent plate member. The reinforcing member is directed against the heatsource of the turbine exhaust blast. The perforations allow gas to flow throughthe interior of the blade. Hence the flat-plate "cold side" of the blade heatsup simultaneous with the reinforcing member avoiding undesirable buckling.

GASTURBINE SIDE

FIG,

Seats: Traditional jamb-seals have been replaced with leaf-spring sealsmounted on the blades, seated on thermal expansion compensated body-mounted seats, thus allowing for minor transient distorsion, large thermalblade growths while still maintaining full seating throughout all conditions.

r/iHI

FIG. 2

Body: The body is internally insulated with layered ceramic blankets onstainless studs. An internal lining of 3 mm thick stainless steel tiles avoidserosion of the insulation due to turbulent mass flow and further adds to theinsulating effectiveness. Frame members remain at cold or nearly cold condi-tions throughout all operating conditions.

Drives and Components: Electro-mechanical actuation with self-lockingworm gear drive is preferred over pneumatic actuation as back-lash and vibra-tion can be better controlled throughout all operating angles. Drive compo-nents are made up of back-lash inhibiting components, such as automotivetype ball joints, locking devices, pin secured blade operator arm to blade shaftconnections. etc.

Leak-tightness of Louver Dampers

With warpage and buckling kept to transient minimums and the sealingsystem designed to accommodate residual inaccuracies as well as expansiondifferentials occurring throughout operating cycles, Louver Damper leak-tightness of approximately 0.25 to 0.50% of normal mass flow volume can beachieved and maintained over a five year time period. Taken into consider-ation in the design life were stress relief and creep related changes in the var-ious components as well as wear and tear, all very considerable.

Because of the relative leakage in the Louver damper design, Double Lou-ver Dampers with a seal air curtain at 50mm WG above operating pressuresupplied between the two sets of louvers are placed in the by-pass stack inorder to avoid energy loss to atmosphere.

Guillotine Dampers have been re-designed as follows:

INSULATION LINERINSULATION LINER

SEAL AIR CAVITYFRAME INSULATION

BLADE INSULATION

FRAME BLADE STIFFENER

BLADE FRAME BLADE MEMBRANE

BODY SEAT AND FLEXIBLE SEAL

FIG. 5

Blade: The blade is made up of a main frame and a matrix of stiffenerswith a recessed, stress absorbing membrane which is bolted into the mainframe. The stiffener matrix is located on the upstream side of the blade andheat shielded with a ceramic blanket, and stainless tile system similar to theinside surfaces of the ducting walls.

Body: The body is internally insulated to maintain a constant, low stresscondition in order to preserve accuracy in the frame members throughout allcyclic conditions and stability over years of operating time.

Seal: The guillotine seal system consists of a novel concave blade peripheryriding on a convex body seat capped with a flexible seal element. This arrange-ment allows the blade to grow thermally across the seat without compromisingsealing quality and at the same time provides a double seal, resulting in aconfiguration similar to having a double blade guillotine.

BLADE FRAME

H SEAL AIR CAVITY

BODY SEAT III II IMP FLEXIBLE SEAL

SEAL DETAILFIG. 6

Drive: The drive system is a rack and pinion combination which is de-signed to positively actuate the blade open and close without binding or buck-ling.

FIG. 4

PINION RACK

on I ono 000

NI

DC FIG. '

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FIG. 11

Actuation: is by electro-mechanical actuator with a self-locking worm gearsystem and with integrated limit and torque switches.

The space provided inside the seal engagement is pressurized by a seal airblower integrated into the guillotine unit, thus assuring absolute cross-bladeleak tightness.

This Multi-Damper Flow Diverter combination offers

Operation By-Pass: 0% leakage into the by-pass stack via the double louverdamper, preventing any energy loss.

HRSG Inlet: Modulation into the HRSG by means of a single modu-lation Louver Damper followed by a 0% leakage Isola-tion Guillotine damper, open-shut service, to isolate theHRSG for simple cycle plant operation and or HRSGrepair and maintenance.

SEAL AIR

7 f T

FULL HRSG FULL BY-PASSFIG. 9

The advantages of the MDS, or multi-damper system approach, are:the units are economical to transport, as the general face to face dimensionof the individual dampers are 18" for Guillotines and Single LouverDampers and 36" for Double Louver Dampers. Like the ExpansionJoints, they represent just slices of duct systems.

I SEAL AIR

FIG. 10

The disadvantages of the MDS approach are:

As the name infers, the dampers are a number of units which have toperform separate functions in simultaneous operation, which means that allcontrols must work perfectly. It has happened many times, that due to controlmalfunction or equipment failure, both outlet ports (by-pass and HRSG) havebeen accidentally blocked by closed dampers causing major system damageunder turbine operation.

One other disadvantage is economy. As the system is comprised of a"Tee" fitting onto which the Multi-Damper arrangment has to be mounted tomake up a flow diverter system, all costs of the fitting and dampers have tobe evaluated against the cost of the alternative single blade GFD, Gas FlowDiverter.

The above two (2) stated disadvantages of the Multi Damper System haveinspired an alternative type of diverter approach. The Single Blade Flow Di-verter System, GFD Gas Flow Diverter.

GTFD GAS TURBINE FLOW DIVERTER

A li

Early attempts, mainly from upscaling small pivot actuated single bladeflow diverters, demonstrated the limitations in size capacity due to blade insta-bility in operation at various blade operating positions. The large moments inthe pivot driven blades simply could not be managed. One way was to changeaspect ratios by going to rectangular cross-sections thus reducing moments.This approach nevertheless could still not cover the very large turbine framesizes of today. Multiple blades have been used and resulted in duplicateddrives, operational complexity and increased costs.

A novel drive system was developed for large frame single blade Gas FlowDiverter Systems which attaches to the center of the blade and operates thesingle blade via a system of toggle arms. The blade center attachment locationof the drive arms largely reduces moments and thus results in smooth opera-tion and modulation throughout the entire actuation stroke. The toggle armattachments are expansion compensated to allow for length changes in differ-ent operational positions.

The "Tee" body forms the major component and all other damper partsare incorporated into the "Tee" body to form the total Flow Diverter Con-cept. To ensure low stress in the major, controlling components, the "Tee"body is internally insulated with a high quality ceramic insulation and stainlesssteel tile system. The blade is heat shielded to prevent energy loss to the atmo-sphere and to avoid radiation into the HRSG for worker safety.

Both by-pass and HRSG openings are equipped with double metallic leafspring seals to ensure 0% leakage when the blade is in either position and theseal air is engaged.

The advantages of the GFD concept area) operational safety, as it is impossible to block both outlets with only

one blade, throughout all size rangesb) economy as the "Tee" body concept incorporates all the damper com-

ponents in one assembly, making this approach more economical as comparedto Multi-Damper Systems

AIR

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c) flexibility, as most large combined cycle power plants will preliminarilyoperate on simple cycle until the HRSG island is installed (including steamturbine generators, etc). This means that the Diverter "Tee" body can be in-stalled and used during the simple cycle phase, then retrofitted with the flowcontrol components prior to the combined cycle operation. The "Tee" bodyincorporates all the provisions for this retrofit and the change over can beachieved in about 2-3 days.

d) The "Tee" body and all other damper parts are made of modulizedand shippable components and are designed for easy field assembly by boltingand a mimimum amount of welding.

Based on the summarized advantages , the efficient internally insulatedSingle Blade Gas Flow Diverter System approach is destined to be the moreand more adopted approach in combined cycle plants by major users in thenear future.

Gas Turbine Expansion Joints

At this point a brief discussion of gas turbine expansion joints will beundertaken to illustrate the design complexity required to meet the demandsof today's high efficiency turbines.

The fact that gas turbine exhaust systems are made as compact as possibleleads to minimum installation gaps for expansion joints. Also, major systemcomponents usually have varying metal temperatures plus different anchorand mounting elevations, which requires that expansion joints in most caseshave to absorb not only axial but also lateral movement in vertical and hori-zontal directions.

Due to lateral movements and short face to face installation requirements,non-metallic, high temperature fabric expansion joints are selected for thisduty. With today's advanced fabric expansion joint material technology, theflexible element material technology is not the prime concern. The metalliccomponents and insulation configurations present the greatest challenge.

As actual operating temperatures experienced in gas turbine exhaust sys-tems are usually between 850°-1200°F, (450 0 -650 0 )

The more difficult problems encountered are in the metal components ofthe fabric expansion joints, such as flow liners and mounting flanges, whichsuffer from thermal stress due mostly to the following:

a.) Rapid temperature rise at turbine start up causes thermal shock and mayconsequently crack the frames.

b.) Poor temperature distribution between liners and frames may cause bind-ing, warping and cracking of liners and frames.

c.) Temperature differentials between externally and internally insulatedduct interphases, where the expansion joint becomes a transition element.

d.) Attachment of internal insulation is important to the free expansion ofall components to avoid binding and extensive stress.

e.) Poor expansion joint fabric mounting methods to metal mountingflanges may cause early failure of the expansion fabric in the attachmentarea.

Three models are typically available:

Model GTEJ-1: Inlet and outlet side of expansion joint with internal insula-tion.

Model GTEJ-2: Inlet side of expansion joint with external insulation and out-let side with internal insulation for transition applications.

Model GTEJ-3: Inlet and outlet side of expansion joint with external insula-tion, mostly for small systems.

FRAME FRAMECARBON STEEL CARBON STEEL

LINER GUIDES STAINLESS STEEL LINER

MODEL GTEJ-1 COLD FRAME TO COLD FRAME(Internally Insulated Ducting)

FRAME FRAMESTAINLESS STEEL CARBON STEEL

,' j 7 ^ _) TRANSITION,T' EXPANSION JOINT j

THERMALMODEL GTEJ-2 DISTRIBUT. HOT FRAME TO COLD FRAME

CAVITY (External To Internally Insulated Ducting)

MODEL GTEJ-3 HOT FRAME TO HOT FRAME(Externally Insulated Ducting)

Most expansion joint applications in gas turbine ducting on large ma-chines would be accommodated by Model GTEJ-1, where the gas turbine ex-haust ducting is internally insulated, therefore the ducting itself runs cool.

Where the turbine connection runs hot, the expansion joint would act asa transition element from hot externally insulated to cool internally insulatedducting in most cases. Here Model GTEJ-2 applies.

On small systems, it is more practical and common to insulate the outsideof all components, therefore the expansion joint frames run hot and provisionshave to be made for external lagging attachment, hence Model GTEJ-3 ap-plies.

CONCLUSION:

The continuing trend to larger and more efficient or hotter gas turbinesalong with increased use worldwide of the gas turbine plants will continue todemand that expansion joints, gas flow diverters and damper systems success-fully operate in one of the most challenging applications for this equipmentworldwide. Therefore, only the equipment which is specifically gas turbineapplication engineered, designed and manufactured to the most finite detailwill be successful in the service required.

Gas Turbine Systems supplied without key consideration to the criticaldiverter damper system and expansion joint equipment will continue to experi-ence major plant shutdowns, operational disruptions and system inefficien-cies.

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