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EXHAUST SYSTEMS
How to inspect, replace GT
exhaust system components
By David Clarida, Integrity Power Solutions LLC
Y
ou don’t have to visit many
generating facilities powered by gas turbines (GTs)
or attend many user-group
meetings to realize that exhaust systems get little or no respect. These
relatively simple systems essentially
consist of metal components and
insulation arranged to direct nominal 1000F gas exiting an engine
to a heat-recovery steam generator
(HRSG), or stack, while protecting
personnel.
Exhaust systems usually are
ignored on rounds, rarely getting
more than a passing glance unless an
operator hears gas whistling out of a
hole, crack, or tear in the ductwork,
expansion joint, or between mating
faces of adjacent flanges. What’s
there to check, anyway?
However, exhaust systems are not
something to forget when an outage
approaches. There are inspections
you should make before and during
an outage to evaluate the condition
of system components. While most
issues identified can be corrected at
least temporarily with weld material
and insulation, the wear and tear on
these systems—particularly those at
plants in cycling service—is considerable and their lifetimes are limited.
Consider yourself lucky to get 15
years out of an exhaust system.
Purpose of this article is threefold:
Help you assess the condition of your
plant’s exhaust system, catch up on
the latest designs which can help
eliminate many of your recurring
repair jobs, and gain from one plant’s
experience in replacing the exhaust
systems on its GTs.
A good exhaust system is
defined here as one that can be used
on any OEM’s gas turbine and has
a quality insulating system. Equipment interfaces and site and owner
requirements may impact the physical design, but most important is the
2
Stack
Diffuser
Exhaust frame
Cowl
Exhaust
silencer
Expansion joint
Wing
Exhaust plenum
1. Exhaust systems for simple-cycle GE frames are arranged as illustrated. For
cogeneration and combined-cycle plants, a heat-recovery steam generator sits
between the exhaust plenum and the stack
thermal design, which depends on
the insulating system.
Arrangement of the insulating system has evolved over time. Today’s
offerings are much improved over
those available only a few years ago,
assuring users of longer operating
lifetimes and a higher degree of personnel safety.
Exhaust systems are comprised of
several components and vary depending on the turbine OEM. To illustrate:
Frame engines manufactured by GE
Energy for simple-cycle service typically are arranged as illustrated in
Fig 1. Hot gas exits the GT via the
exhaust frame and is distributed in
the plenum by the diffuser. Exhaust
then flows to the stack via ductwork,
which is connected to the plenum
and any associated ductwork by an
expansion joint.
For GE frames serving in cogeneration and combined-cycle systems,
the expansion joint at the back end of
the plenum connects to the transition
piece for the HRSG. GTs made by
Siemens Energy are characterized by
an exhaust cylinder bolted to the outlet flange of the engine. A so-called
exhaust manifold bolts to the cylinder on one end and to the round (GT
exit configuration)-to-square (HRSG
inlet configuration) transition on the
other. An expansion joint is located
just upstream of the HRSG’s transition piece.
Focus here is on exhaust systems for GE frames 5, 6, and 7, which
are similar in design. The profile of a
recent exhaust-system upgrade for
an F-class Siemens engine is available at www.combinedcyclejournal.
com, click Klamath Cogen on the
3Q/2009 cover.
For GE machines, the exhaust
frame is a structural part of the turbine and supports the aft bearing.
It is in direct contact with turbine
exhaust and not protected by insulation like the downstream exhaust
plenum and ductwork. Years of
demanding duty, frequent cycles,
and/or overheating contribute to degradation conducive to early failures
and costly forced outages.
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EXHAUST SYSTEMS
Overheating also is known to
cause turbine bearing issues, high
lube-oil temperatures and resultant
varnishing, and instrumentation
failures. Important to maintaining
your exhaust system in the condition
required to achieve safety and dispatch goals are periodic inspections
and follow-up repairs or replacement
of components as necessary.
Knowing where to survey for damage and what type of wear and tear to
look for are critical to success. Sidebars provide checklists for inspection
of ductwork, frames, and diffusers.
While the guidelines provided are
specific to GE frames, they apply, in
general, to exhaust systems offered
by all frame OEMs.
Over the last several years, design
of the frame and diffuser for 90-deg
exhausts have improved significantly
and these components are better able
to accommodate the thermal stresses
inherent in the original systems. Thus
replacement of an exhaust frame
and/or diffuser with one of upgraded
design can both eliminate maintenance issues and significantly reduce
the probability of premature failure.
Inspection of your system may
suggest repairs or replacement of
one or more exhaust-system components. Each situation is different and
the decision to fix or replace must
include a sound financial evaluation. In some instances, repairs have
turned out more costly than a full
replacement would have been. Annual repair costs add up and control of
expenditures over the long term may
suggest upgrade through full replacement earlier rather than later.
Design. Externally insulated
exhaust duct systems still are in service on some legacy engines. For the
purpose of personnel protection, this
type of system may keep high temperatures from reaching the external
cladding. However, the structural
shell plant is prone to degradation
and cracking because of direct exposure to exhaust temperature. Units
that start daily usually are at greatest
risk because thermal cycling accelerates the degradation process.
Internal insulation of exhaust
ducts improves reliability and longevity by preventing direct exposure
of the structural shell to exhaust
gas temperatures. The challenge
presented by this type of system
is designing the insulation barrier
to maintain its integrity and effectiveness during both base-load and
cycling operation.
An internal “floating” liner design
meets these requirements. Use of
insulation pans (steel plans filled
with insulation) does not provide
4
Inspection checklist for exhaust ductwork
Before outage
1. Take skin temperatures at several
locations with a thermal imagery
camera or a single-point temperature gun. Examples: flanges, areas
where paint is burned, locations
where excessive heat would pose a
personnel hazard, etc. Skin temperatures above 250F-300F may indicate
a problem with the insulation system. Temperatures above 650F are
of particular concern because they
can initiate degradation of the carbon steel shell and structure.
2. Listen for any internal noise. It can
indicate a failure in the internal insulation system.
3. Inspect all expansion joints for
uniform compression during operation. Look for burning or holes in the
fabric belt or for holes in a metal-bellows expansion joint. Non-uniform
compression can indicate improper
thermally induced movement of
exhaust ductwork.
1
3
6
1
7
8
1
the same level of protection and may
actually allow direct exposure of the
shell plate to the hot gases.
The floating-liner design consists
of metal sheets that are secured,
but not fastened, atop insulation
compressed to an optimum density
between the liner plates and the
outer shell plate. Thus the liner is
free to expand and contract throughout the various stages of operation.
The compressed insulation used
today can withstand very high temperatures and is tight-fitting to minimize
the potential for open spots where the
hot gas can contact the outer walls of
the ductwork. This is a great improvement over the mineral wool formerly
used, which would break down and
lose its insulating quality over time
because of heat and turbulence.
Liner sheets are installed with a
“scaled” effect that allows any liquids
present within the exhaust to exit via
4. Check turbine and load compartment temperatures. Are you able to
enter compartments and get close to
ductwork? Verify correct temperature
ratings for vibration monitors in the
load tunnel, which can run between
300F and 400F at base load for GE
frames.
5. Verify lube-oil temperature is
within manufacturer’s recommended
range. High temperature is conducive to varnishing, which should be
avoided.
During outage
6. Inspect the liners or insulation
pans inside the exhaust plenum and
ductwork. Verify that the insulation
is in good condition and has not
moved, creating “open” areas. Make
repairs/add insulation as necessary.
7. Walk through the exhaust plenum
and ductwork during the daylight
hours, looking for holes/cracks in
the shell plate and/or gaps between
flanges.
8. Check for cracks in the external shell, structural members, and
welds.
9. Check for wear marks on parts
expected to move because of thermal expansion/contraction during
heat-up and cool-down—including
internal liner, duct-structure interfaces, etc—to verify that components
are moving as designed.
10. Verify instrumentation (exhaust
thermocouples, vibration detectors,
etc) is in working order.
a drain. This reduces the potential for
water damage to the insulation and
“slumping” of the material, which can
cause the insulation to rot and gases
to leak by and damage the shell.
In sum, modern exhaust systems
are characterized by internally insulated ductwork, which minimizes
stresses from high thermal gradients. The external gas-tight casing
plate is maintained at temperatures
only slightly above ambient, virtually
eliminating problems caused by thermal expansion/contraction. The floating liner eliminates stresses inside
the ductwork.
Case history: Optim
Energy Altura Cogen
Altura Cogen’s staff, headed by Plant
Manager Randy Cormier, relishes the
challenges of major plant improvement projects. You feel the confidence
COMBINED CYCLE JOURNAL, Fourth Quarter 2009
EXHAUST SYSTEMS
as key personnel
explain what they
are doing and why.
It seems as if no
one ever “sits” at
this facility: as
one major project
ends, the next one
begins.
One reason is
that Altura—forCormier
merly known as
CoGen Lyondell—is an ageing and
vital, must-run facility. The nominal 600-MW plant, owned by a subsidiary of Optim Energy LLC, is a
7E-powered 6 × 1 cogeneration facility which provides steam to a petrochemical complex and electricity both
to that plant and the grid.
Readers may recall Altura Cogen’s
recent success in reducing NOx emissions by more than 80%. This mega
project included conversion of five
1985-vintage 7Es from diffusionflame combustion systems to PSM’s
(Jupiter, Fla) LEC III, installation
of an inlet bleed heat system on each
engine, and replacement of the original Mark IV GT control systems with
Ovation® (Emerson Process Manage-
Exhaust frame
Diffuser
Insulation pan
Floating liner on exhaust plenum floor
2. Floor liners (floating at left, insulation pan at right) illustrate a significant difference between first- and third-generation exhaust-plenum designs
3. Hot flanges (left) are a characteristic of the original insulation pan design;
floating liners have cold flanges (right)
ment’s Power & Water Solutions division, Pittsburgh).
For more information on the lowNOx retrofit project, which earned
the Texas facility the COMBINED
CYCLE Journal’s 2008 Pacesetter
Plant Award, access www.combined-
Inspection checklist for frames, diffusers
Before outage
1. Check turbine and load compartment temperatures. Are you able to
enter compartments and get close
to ductwork? Verify correct temperature ratings for vibration monitors
in the load tunnel, which can run
between 300F and 400F at base
load for GE frames.
“L” seals for cracking and/or leakage. Telltale clue generally is high
compartment temperature. Maintain
a historical record of these data.
Increasing temperature over time
typically indicates leakage rate is
increasing.
5. Check exhaust-frame strut airfoils
for cracking and leakage.
2. Verify lube-oil temperature is
within manufacturer’s recommended
range. High temperature is conducive to varnishing, which should be
avoided.
6. Inspect diffuser turning vanes and
struts for cracks.
3. Inspect the frame assembly to see
if the latest design is installed.
8. Inspect exhaust frame for flange
deformation.
During outage
4. Inspect exhaust-frame flex and
EXHAUST FRAME
7. Check the diffuser for circumferential cracking.
9. Verify instrumentation (exhaust
thermocouples, vibration detectors,
etc) is in working order.
EXHAUST DIFFUSER
5
4
8
6
7
6
cyclejournal.com/archives.html, click
4Q/2007, click Altura Cogen on the
magazine cover.
Exhaust system retrofit. An
upgrade project that had been on
the minds of plant personnel for
years was replacement of the 7E
exhaust systems. Wear and tear of
the so-called Generation 1 systems
had reached the point where repairs
were no longer practical or economic.
Experts generally think an exhaust
system should last upwards of about
15 years; these were well beyond that
period of time.
Altura’s GT exhaust systems were
characterized by (1) load-compartment temperatures that were too
high to permit personnel access, (2)
very high skin temperatures, and (3)
the need for frequent replacement
of expansion joints. Damage to the
fabric joints was attributed to nonuniform expansion caused by a plenum so hot it didn’t expand the way
designers had intended.
With today’s tight capital and
O&M budgets, no plant manager
wants to spend more than absolutely
necessary to fix the problem at hand.
In Altura’s case, this meant replacing
the exhaust plenum, exhaust wing,
and cowl, and the expansion joint
connecting the plenum to the HRSG.
The replacement third-generation
exhaust system was provided by
Integrity Power Solutions LLC with
installation by Integrated Service Co
LLC. Both companies are based in
the Tulsa area and have personnel
with expertise in inlet and exhaust
system repair and replacement.
The components installed by
InServ under the direction of Project
Manager Gary Martin took less than
four months to fabricate and deliver
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Coatings / Repairs / Parts
300
280
260
240
220
200
180
160
140
120
100
T1
T2
T1 = 138F
T1 = 317F
T2 = 140F
T2 = 518F
BEFORE
Temperature, F
EXHAUST SYSTEMS
AFTER
4. Before/after thermal scans of the cowl revealed temperature reductions of from nearly 200 deg F to almost 400 deg
F, depending on location
Tavg = 216F
500
Temperature, F
Tavg = 494F
400
300
200
100
BEFORE
AFTER
5. Average temperature across the expansion joint between the plenum and the HRSG decreased from just under 500F
to slightly more than 200F
to the site; replacement of exhaust thermal insulation barrier conducive
frames and diffusers would have to a cooler interface.
added at least a couple of months to
Result of the upgrade was a
project start.
dramatic reduction in the external
Plus, the optimal time for change- temperature of exhaust system comout of the exhaust frame typically ponents. “Even before the thermogis during a major inspection when raphy survey was performed on the
the turbine is realigned. Recall that first unit upgraded,” Project Manthe GT’s aft bearing resides in the ager Daryl Whitfield said, “it was eviexhaust frame.
dent that its plenum was much coolTwo major benefits the
er than those for the other
third-generation exhaust
machines.”
system has over the origiA thermographic survey
nal are (1) floating floor
showed just how much coolliners described earlier
er the new third-generation
and (2) cold flanges. Fig
exhaust system was compared
2 shows the floating liner
to the original. For example,
for the exhaust plenum
before/after thermal scans of
floor at the left; photo at
the cowl revealed temperaright is of a first-generature reductions of from neartion insulation pan in the
ly 200 deg F to almost 400
Whitfield
same location.
deg F, depending on meaOne of the issues with the original surement location (Fig 4). Average
pan design was so-called “hot flang- temperature of the expansion joint
es” (Fig 3, left). Explanation: Ther- between the plenum and the HRSG
mal growth of a pan’s exposed sur- decreased from just under 500F to
faces does not match that of the outer slightly more than 200F (Fig 5).
perimeter, thereby causing distortion
Whitfield was pleased. “The end
along that perimeter. Gaps occur product we received,” he said, “perbetween pans, allowing hot exhaust formed as expected and was a value
to contact the outer shell; distortion when compared to our other options.”
and cracking result.
Cormier was cautiously optimistic,
Floating liners have “cold flanges” as you might expect of a seasoned
(Fig 3, right), which are character- plant manager. “As of now we are
ized by field-installed wrapped insu- pleased with the performance of the
lation pillows and liner plates. This new plenum,” he offered. “We expect
design creates a continuously sealed this design to perform well and look
8
forward to a positive evaluation over
the plant’s lifetime.”
How the job was done. Plenum
replacement is not difficult if you take
the time to properly plan and execute the project, and use experienced
resources (Fig 6). Here’s a summary of
the procedure used at Altura Cogen:
n Allow for adequate cooling of the
Pumpable insulation nipples
6. Existing plenum assembly before
removal of the wing, cowl, and expansion joint. Note the areas of burned
paint and the nipples installed to
inject pumpable insulation—a shortterm fix to keep the unit operating
COMBINED CYCLE JOURNAL, Fourth Quarter 2009
DeepSouth Hardware
Solutions, LLC
Expansion-joint
flange on
HRSG
transition
piece
Exhaust
diffuser
Westinghouse,
WDPF Classic,
WEStation, and Ovation
parts and service.
Exhaust Horizontal
joint
frame
Exhaust-plenum sidewall
7. Exhaust plenum floor and sidewalls are installed without removing
the diffuser and exhaust frame
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exhaust system—one to two days
after removing the gas turbine
from service. Do final parts inventory and other soft tasks during this time. Be sure to acquire
all new bolts/hardware and flex
seals. These should be included in
the field kit supplied by the plenum manufacturer.
n Unbolt the wing and cowl from the
plenum. Then unbolt the upper
walls and cut out the lower half
of the plenum. Important to note
is that the diffuser and exhaust
frame need not be removed to
change-out the plenum.
n Cut the expansion joint connecting the plenum to the HRSG. Note
that the expansion joint is bolted
to the plenum side and welded on
the other side to a flange on the
HRSG. Next step typically is to
clean, dress, and true-up the boiler
flange. This can be time-consuming and challenging because metal
moves and distorts over the years.
n Clean up the interfaces with the
gas turbine as well.
n Install the new expansion joint
and then the lower half of the plenum. Next, put the upper walls of
the plenum in place and true-up
the work before installing flex
seals. Progress to this point is
shown in Fig 7.
n Next the wing and cowl are
replaced (Fig 8).
n Finally, insulation and liner are
installed internal to the plenum at
the joints. And the floating liner is
bolted into the HRSG liner system.
The entire project was completed
over 12 one-shift days. This means
it’s possible to replace an exhaust
plenum assembly for a Frame 7
engine during an extended combustion inspection, as well as during hotgas-path and major inspections. ccj
David Clarida is
president of Integrity Power Solutions LLC (david.
clarida@ipsok.
com), a full-service
provider of GT inlet
and exhaust system repairs and
replacements.
B e f o re f o u n d i n g
IPS, he was CHROEM™ product-line leader at GE Energy
Services and aftermarket sales manager at Braden Manufacturing LLC.
Clarida began his career as a project
engineer for Western Farmers Electric
Co-op.
42367 Deluxe Plaza Ste. # 29
Hammond, Louisiana 70403
Phone: (877) 542-0095
Fax: (877) 542-0096
Email:
[email protected]
Website:
www.deepsouthhardwaresolutions.com
DISCLAIMER
Westinghouse WDPF Classic, WEStation, Ovation,
Bailey Net90 & Infi90 are Registered trade marks and
DeepSouth Hardware is not affiliated with, nor an
authorized dealer for them.
10
8. New exhaust plenum is fully assembled
COMBINED CYCLE JOURNAL, Fourth Quarter 2009