Gas Turbine Efficiency Improvements Through Shroud ... - PSM by lanyuehua

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Gas Turbine Efficiency
Improvements Through
Shroud Modifications



R. Seleski
PSM
Jupiter, FL




                          1440 West Indiantown Rd., Suite 200
                                        Jupiter, Florida 33458
                                         Phone: 561-354-1100
                                           Fax: 561-354-1199
                                                                Gas Turbine Efficiency Improvements
                                                                Through Shroud Modifications
1440 West Indiantown Rd., Suite 200
Jupiter, Florida 33458
Phone: 561-354-1100
Fax: 561-354-1199




Introduction                               The most common method to
                                           correct these difficulties is to support
A driving factor in the design of all      the bucket at both the root and tip,
power generating units is efficiency.      using a tip shroud, instead of
Higher efficiency translates to lower      cantilevering the entire bucket from
operating costs. In the case of            the root. Not only does the tip
combustion turbines, air leakage           shroud change the natural frequency
around the tips of turbine blades          of the bucket as a system, but it
reduces efficiency. This paper             provides additional friction damping.
examines the use of blade tip shrouds      Tip shrouds also restrict gas leakage
to minimize air loss. It also looks at     flow across the airfoil tip by using
tip shrouds as a technique to reduce       knife-edge seals designed to rub into
blade stress and fatigue. The paper        a honeycomb seal material that is
also examines difficulties with tip        brazed onto the shroud blocks
shrouds, including creep, curling, and     (Figure 1b).
stress, and how these can be
minimized.                                 While tip shrouding can be an
                                           excellent tool to improve combustion
Fundamentals                               turbine operation, a number of factors
                                           must be taken into account when
A gas turbine functions by allowing        designing tip shrouds, as otherwise the
passage of expanding combustion            shrouds may cause as many problems
gases through the turbine blades. To       as they solve.
accommodate gas expansion and
obtain maximum efficiency from the
unit, the turbine chamber volume and
blade length increase from inlet to
outlet. A simple blade design, such as
that shown in Figure 1a, offers two
major disadvantages. First, as blade
length increases, so do chord-to-
length aspect ratios. This can result in
an unacceptable high cycle fatigue
(HCF) margin on fundamental modes
such as 1st bending. Secondly, simple
blades allow a significant amount of
gas leakage around the blade tips.
This reduces turbine efficiency.




Page 1

T H E P R O V E N A LT E R N AT I V E www.psm.com
                                                                 Gas Turbine Efficiency Improvements
                                                                 Through Shroud Modifications
1440 West Indiantown Rd., Suite 200
Jupiter, Florida 33458
Phone: 561-354-1100
Fax: 561-354-1199




Tip Shroud Difficulties                     shown in Figure 4. Bending stress due
                                            to overhanging shroud edges also
Centrifugal forces on the rotating stages   contributes to this deflection. The
of a modern gas turbine are enormous.       deformation generally causes the
For example, an F class bucket (airfoil     concave side to lift up and the convex
only) that weighs about eight pounds        down, so that when two buckets are
pulls on the bucket platform with a         next to each other, shroud mismatch
force of over 98,000 pounds at              occurs. This can eventually result in
operating speed. That is equivalent to      loss of contact between the two as
hanging 25 full size pick-up trucks on      they shingle over the top of one
the bucket airfoil root section!            another. Minimizing this effect is
                                            critical in designing shrouds to meet
Figure 2 illustrates a classic tip shroud   life cycle requirements. Unfortunately,
design. Tip shrouds are often only          not much can be done to modify
employed to reduce HCF, as there are        airfoil shape and eliminate the
some negative impacts related to            untwisting effect. It is also difficult to
bucket design and structural                cool the thin trailing edges of the
requirements of the tip shroud itself.      buckets. The best option is to reduce
Issues include increased load (pull) on     the weight and improve the efficiency
the airfoil resulting in reduced creep      of the shroud overhang as much as
resistance of the bucket, curling and       possible. This is outlined below.
creep of the shroud overhanging             Another common problem with
edges (which causes shingling of the        existing shroud designs is the potential
shroud edges relative to one                for fatigue or overstress of thin
another), and fatigue or overstress of      unsupported edges as shown in
unsupported shroud edges. These             Figure 5.
may force premature field
replacement of components.                  Shroud Design Solutions

The shroud mass itself accounts for         Some of the pitfalls mentioned above
roughly 10% of blade weight. Figure 3       result from classic shroud design
shows the impact of the extra weight        practices that essentially made a full
on airfoil average stress. At 60% span      hoop ring out of the shrouds in
the shroud increases average stress by      roughly the same axially length as that
approximately 40%. This has a direct        of the airfoil chord. This produces a
impact on bucket creep resistance.          clean looking rotor, but is highly
                                            inefficient.
Airfoil shape and gas temperature
combine to influence shroud                 One practical method for improving
deflection (and eventually creep), as       shroud design is shown in Figure 6.

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T H E P R O V E N A LT E R N AT I V E www.psm.com
                                                               Gas Turbine Efficiency Improvements
                                                               Through Shroud Modifications
1440 West Indiantown Rd., Suite 200
Jupiter, Florida 33458
Phone: 561-354-1100
Fax: 561-354-1199




Excess material can be removed as         allows them to form part of an I-
shown. The material was traditionally     beam structure that stiffens and
used to minimize recirculation            supports the shroud overhang. The
forward and aft of the blade but it has   placement serves an additional
been found through engine                 function by increasing the potential
experience to provide little benefit      shroud contact area and reducing
and actually increases pull on the        shroud deflection and eventual
airfoil and introduces more bending       curling, but without adding any
stress on the airfoil-to-shroud fillet    additional mass.
(shroud curling). In the modified
design, the shroud center of              7EA Compatible 2nd Stage
gravity(CG) remains over the airfoil      Bucket Enhancements
minimum moment of inertia axis with
a minimal change in the forward and       The techniques outlined above have
aft CG location. The design also          served in the design of an improved
minimizes windage effects associated      7EA-compatible 2nd stage bucket.
with abrupt interruptions along the       Figure 10 is a photograph of an
leading and trailing edges of the         original shroud. Service life was
shroud (Figure 7).                        reduced by almost 66% due to
                                          shingling of the shroud contact edges.
An approach for shrouds that have         PSM engineers developed the
relatively thick cross sections is to     modified shroud shown in Figure 11.
taper that portion of the shroud          The material redistribution allowed a
which overhangs the airfoil. Figure 8     0.060” shroud thickness increase in
shows a discreet pocket that tapers       the contact area (Figure 12), but did
down to a minimum thickness before        not affect the overall mass, bucket
forming the shroud edge. This design      center of gravity, and airfoil stress. The
modification eliminates material that     redesign offers a two-fold
would otherwise hang off the edge of      improvement. It reduces the bending
the airfoil and increase the bending      stress that contributes to shroud
stress of the shroud-to-airfoil fillet.   curling, and it increases the radial
                                          height of the contact face to allow for
Modified knife-edge seal placement is     more shroud mismatch between
a technique that can be employed to       buckets.The combined upgrades have
increase the stiffness of the shroud      boosted the life of the bucket by a
without adding additional material.       factor of three, and this bucket design
Figure 9 shows a not-to-scale             has been in service for over 33,000
example of an ideal knife-edge seal       hours and 1,300 starts.
location. Placing the seals as close as
possible to the long overhung areas       In one instance, a set of modified

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T H E P R O V E N A LT E R N AT I V E www.psm.com
                                                                 Gas Turbine Efficiency Improvements
                                                                 Through Shroud Modifications
1440 West Indiantown Rd., Suite 200
Jupiter, Florida 33458
Phone: 561-354-1100
Fax: 561-354-1199




buckets was subjected to higher than         stress than the original, but reduces
normal firing temperatures. The              pull on the airfoil by 770 pounds from
buckets have been removed from               the first modification. The tapered
service and PSM engineers are                pockets reduce the amount of load
currently evaluating the components          cantilevered off the airfoil, while the
for residual life. This data will serve to   thicker contact surface serves as a
validate and update creep/failure life       stiffener to support the overhanging
as predicted above.                          shroud. As in previous designs, the
                                             leading and trailing edges of the
W501F Compatible 3rd Stage                   shroud have been contoured to
Bucket Enhancements                          minimize shroud mass. Figure 15
                                             illustrates a plot of airfoil average
A different approach has proven              section stress for the three, W501F
practical for redesign of a W501F-           3rd stage bucket shrouds. The
compatible 3rd stage bucket. Figure 5        optimized design eliminates the
shows an existing shroud that suffered       overstressed thin corners of the
a fracture. The failure occurred long        original shroud, increases the contact
before the end of expected service           surface thickness, significantly
life due to shroud curling and               minimizes shroud curling, and reduces
shingling. Figure 13 illustrates the         average airfoil stress.
upgraded shroud, where shroud
thickness was increased from 0.310”          Conclusion
to 0.375”, with a material change
from Inco 738 to MARM 247. The               Modern modeling and analysis
latter alloy offers higher creep             techniques exist to optimize turbine
resistance, which was necessary to           shroud design and ensure that service
counterbalance the increase in mass.         life matches that of the airfoil. The
                                             new designs also minimize air leakage.
The thicker shroud on the modified           Attention to the details outlined in
W501F bucket reduces shroud curling          this paper will benefit gas turbine
and allows for more mismatch                 owners and managers through more
between buckets, but the added               reliable and longer service life of the
weight increases pull on the airfoil by      equipment. In today’s power market,
over 2,500 pounds. Figure 14                 efficiency and reliable performance
illustrates two views of a more              are critical for power producers to
advanced version of the shroud.              maintain a competitive edge.
Company engineers used ANSYS                 Optimizing turbine shroud
finite element modeling to develop           performance is a realistic tool to
this “pocketed” configuration, which is      improve efficiency.
under application for patent. The
design offers the same or less bending


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T H E P R O V E N A LT E R N AT I V E www.psm.com
                                                                    Gas Turbine Efficiency Improvements
                                                                    Through Shroud Modifications
1440 West Indiantown Rd., Suite 200
Jupiter, Florida 33458
Phone: 561-354-1100
Fax: 561-354-1199




                                                                                B
                                                        A


                                      Figure 1.   Honeycomb/Tip Shroud Interaction
                                                  Improves Tip Sealing




                                        Figure 2.   Classic Tip Shroud Design




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T H E P R O V E N A LT E R N AT I V E www.psm.com
                                                                  Gas Turbine Efficiency Improvements
                                                                  Through Shroud Modifications
1440 West Indiantown Rd., Suite 200
Jupiter, Florida 33458
Phone: 561-354-1100
Fax: 561-354-1199




                               Figure 3.    Tip Shroud Increases Airfoil Average Stress




                            Figure 4.      Unsupported Edge Curling Under Centrifugal Load




Page 6

T H E P R O V E N A LT E R N AT I V E www.psm.com
                                                                  Gas Turbine Efficiency Improvements
                                                                  Through Shroud Modifications
1440 West Indiantown Rd., Suite 200
Jupiter, Florida 33458
Phone: 561-354-1100
Fax: 561-354-1199




                             Figure 5.      Overstress Failure of W501F 3rd Stage Shroud Edge




                                      Figure 6.   Edge Contouring Removes Excess Material




Page 7

T H E P R O V E N A LT E R N AT I V E www.psm.com
                                                                     Gas Turbine Efficiency Improvements
                                                                     Through Shroud Modifications
1440 West Indiantown Rd., Suite 200
Jupiter, Florida 33458
Phone: 561-354-1100
Fax: 561-354-1199




                                       Figure 7.    Contoured Tip Shroud Design Minimizes Windage Effects




                                      Figure 8.    Shroud Overhang is Treated as a Canterlivered Beam




Page 8

T H E P R O V E N A LT E R N AT I V E www.psm.com
                                                                    Gas Turbine Efficiency Improvements
                                                                    Through Shroud Modifications
1440 West Indiantown Rd., Suite 200
Jupiter, Florida 33458
Phone: 561-354-1100
Fax: 561-354-1199




                                      Figure 9.   Placement of K/E Seal over Overhang




Figure 10. Original 7EA 2nd Stage Bucket Shroud                          Figure 11. PSM 7EA Compatible 2nd Stage
                                                                                    Bucket Shroud



Page 9

T H E P R O V E N A LT E R N AT I V E www.psm.com
                                                                  Gas Turbine Efficiency Improvements
                                                                  Through Shroud Modifications
1440 West Indiantown Rd., Suite 200
Jupiter, Florida 33458
Phone: 561-354-1100
Fax: 561-354-1199




                          Figure 12. Material Redistributed to Increase Thickness in Center of Shroud




                                      Figure 13. W501F 3rd Stage Bucket Upgraded Shroud



Page 10

T H E P R O V E N A LT E R N AT I V E www.psm.com
                                                            Gas Turbine Efficiency Improvements
                                                            Through Shroud Modifications
1440 West Indiantown Rd., Suite 200
Jupiter, Florida 33458
Phone: 561-354-1100
Fax: 561-354-1199




                         Figure 14. PSM Improved W501F Compatible 3rd Stage Bucket




                      Figure 15. Airfoil Average Stress Comparisons



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