Tribol. Lubr., Vol. 37, No. 3, June 2021, pp. 99~105 ISSN 2713-8011(Print)ㆍ2713-802X(Online)Tribology and Lubricants http://Journal.tribology.kr
DOI https://doi.org/10.9725/kts.2021.37.3.99
99
Stability Characteristics of Supercritical High-Pressure Turbines
Depending on the Designs of Tilting Pad Journal Bearings
An Sung Lee1†
and Sun-Yong Jang2
1Principal Researcher, Dept. of System Dynamics Research, Division of Mechanical Systems Safety Research,
Korea Institute of Machinery & Materials, Daejeon, South Korea2Director, R&D Center, Daedong Metal Industry Co., Busan, South Korea
(Received April 14, 2021 ; Revised May 28, 2021 ; Accepted June 9, 2021)
Abstract − In this study, for a high-pressure turbine (HPT) of 800 MW class supercritical thermal-power plant,
considering aerodynamic cross-coupling, we performed a rotordynamic logarithmic decrement (LogDec) stability
analysis with various tilting pad journal bearing (TPJB) designs, which several steam turbine OEMs (original
equipment manufacturers) currently apply in their supercritical and ultra-supercritical HPTs. We considered the
following TPJB designs: 6-Pad load on pad (LOP)/load between pad (LBP), 5-Pad LOP/LBP, Hybrid 3-Pad LOP
(lower 3-Pad tilting and upper 1-Pad fixed), and 5-Pad LBPs with the design variables of offset and preload. We
used the API Level-I method for a LogDec stability analysis. Following results are summarized only in a stand-
point of LogDec stability. The Hybrid 3-Pad LOP TPJBs most excellently outperform all the other TPJBs over
nearly a full range of cross-coupled stiffness. In a high range of cross-coupled stiffness, both the 6-Pad LOP and
5-Pad LOP TPJBs may be recommended as a practical conservative bearing design approach for enhancing a
rotordynamic stability of the HPT. As expected, in a high range of cross-coupled stiffness, the 6-Pad LBP TPJBs
exhibit a better performance than the 5-Pad LBP TPJBs. However, contrary to one’s expectation, notably, the 5-
Pad LOP TPJBs exhibit a slightly better performance than the 6-Pad LOP TPJBs. Furthermore, we do not rec-
ommend any TPJB design efforts of either increasing a pad offset from 0.5 or a pad preload from 0 for the HPT
in a standpoint of stability.
Keywords − cross-coupled stiffness, LogDec stability, supercritical high-pressure turbine, tilting pad journal
bearing.
1. Introduction
In worldwide coal-fired thermal-power plants have
been progressing continuously for both efficiency and
capacity improvements from the subcritical (efficiency:
35%), the supercritical (SC, efficiency: 38%), and to
the ultra-supercritical (USC, efficiency: 42% or higher,
e.g., 49%) power plants. In South Korea, also, following
this worldwide technical trend, the subcritical (Samcheonpo
# 1~4 units: 560 MW, 538oC, and 16.5 MPa), the Korean
500 MW standard semi-SC (Samcheonpo # 5, 6 units:
500 MW, 538°C, and 24.8 MPa), the SC (Yeongheung
#1, 2 units: 800 MW, 566oC, and 24.8 MPa), and the
USC (New Boryeong # 1, 2 units: 1,000 MW, 610oC,
and 26.0 MPa) power plants have been commercialized
in turn.
Steam turbine trains of large capacity coal-fired power
plants consist of HIP (high and intermediate-pressure)
turbines for the semi-SC condition, and HP (high-pressure)
†Corresponding author: An Sung Lee
Tel: Fax: +82-42-868-7440
E-mail: [email protected]
https://orcid.org/0000-0002-8120-3314
https://orcid.org/0000-0002-2581-7899 (Sun-Yong Jang)
ⓒ Korean Tribology Society 2021. This is an open access article distributed under the terms of the Creative Commons Attribution License(CC BY, https://creativecommons.org/
licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction of the work
in any medium, provided the original authors and source are properly cited.
***-****-****
100 An Sung Lee and Sun-Yong Jang
and IP (intermediate-pressure) turbines, separately, for
the SC and USC conditions, together with LP (low-
pressure) turbines. A rotor weight of typical HIP or HP
turbine is in a range of 15 to 20 ton. Since a prevention
of rotordynamic instability due to steam whirl is a top-
priority design issue because of high steam temperature
and pressure conditions, several global OEMs have
been adopting 6-Pad LOP TPJBs as support bearings
for these rotors. This practice is more noticeable in
bearing designs for the SC and USC application HPTs.
TPJBs require a high-level of design engineering.
Number of pads, load support type (LOP, LBP), offset,
preload, and bearing clearances etc. are key design
variables[1-6]. These design variables affect not only
operating temperature characteristics of TPJBs but also
their fluid film stiffness and damping coefficients,
which are closely related to rotor vibration characteristics,
e.g., stability. Nicholas et al.[7] reported that the TPJB
designs with zero preload, center pivot (or zero offset),
and LOP provide the most stable rotordynamic charac-
teristics for high-speed compressors. Lee [8] carried out
a rotordynamic stability analysis of a light-weight (rotor
mass: 240 kg) and high-speed (9,540 rpm) process 8-
stage centrifugal compressor, where a design of decreasing
the TPJB's preload improved the stability whereas there
was no stability difference between LOP and LBP
because the rotor was of light-weight and operated in
high-speed. Ikeno et al.[9] investigated the design effect
of large (journal diameter: 320~400 mm) TPJBs for
mega ethylene plant applications. Among their reviewed
4-Pad LBP, 5-Pad LBP, and 5-Pad LOP TPJBs, 5-Pad
LOP TPJBs were the best in a standpoint of rotordy-
namic stability considering gas-induced cross-coupling,
i.e., aerodynamic cross-coupled stiffness forces by
impellers and seals etc. Zeidan[2] explained that when
a gas-induced cross-coupling is acting, the reason why
the LOP designs are better than the LBP ones in a
standpoint of the rotordynamic stability is that as in
case of the LBP TPJBs the shaft orbit is circular whilst
in case of the LOP TPJBs the shaft orbit is elliptical,
the LOP TPJBs have less instability excitation energy
than the LBP TPJBs, expressed as a product of orbit
area and cross-coupled stiffness, A (Kxy-Kyx). API STD
617[10] and 684[11] describe a method of evaluating the
dynamic stability of rotor system by calculating LogDec,
depending on applied cross-coupling.
As a groundwork Lee and Jang[12] performed a
lubrication performance analysis of TPJBs, applied to
a HPT. In this study, for the HPT of Yeongheung # 1,
2 units of 800 MW class SC power plant, considering
aerodynamic cross-coupling by high-temperature and high-
pressure steam, we performed a rotordynamic LogDec
stability analysis with not only originally applied 6-
Pad LOP TPJB designs but also other various TPJB
designs which several steam turbines OEMs currently
use in their SC or USC HPTs, and compared results
with each other. Specifically, the considered TPJB designs
were 6-Pad LOP/LBP, 5-Pad LOP/LBP, Hybrid 3-Pad
LOP (lower 3-Pad tilting and upper 1-Pad fixed), and
5-Pad LBPs with design variables of offset and preload.
2. Bearing and Rotor Analysis Models
In Table 1 are given the design and operation condition
data of HPT TPJBs in 800 MW class SC turbine train.
Figure 1 represents a lubrication analysis model of
HPT 6-Pad LOP #1 TPJB for offset = 0.5 and m = 0.0.
In difference with any usual FE rotordynamic analysis
rotor model, now, for a FE rotordynamic analysis with
a consideration of cross-coupling, Fig. 2 represents an
applied LogDec, δA, calculation-purpose rotor model,
having an aerodynamic cross-coupling element at the
station # 23 in the middle of the rotor. Figure 2 also
shows an example whose 1st eigenvalue analysis results
give a whirl natural frequency of 2,470 rpm and δA =
0.387 for a cross-coupled stiffness, δA = 1.5e + 08 N/m,
acting at 3,600 rpm.
3. Results and Discussion of Applied LogDec Stability Calculation Depending
on TPJB Designs
A LogDec stability analysis of the HPT rotor was
carried out with applying a total cross-coupled stiffness
QA, acting on all multi-stage blades by steam, at the
middle stage position. As QA increased from 0 to 2.8e
Table 1. Design and operation condition data of HPT TPJBs
#1 TPJB #2 TPJB
Journal Dia. (mm) 355.6 381.0
Pad Length (mm) 177.8 203.2
Pivot Offset 0.50 0.50
Dia. Bearing Clr. (mm) 0.48 0.51
Preload, m 0.0 0.0
Loads (N) 71,000 78,895
Rated Speed (rpm) 3,600
Oil Type ISO VG 32
Tribol. Lubr., 37(3) 2021
Stability Characteristics of Supercritical High-Pressure Turbines Depending on the Designs of Tilting Pad Journal Bearings 101
+ 08 N/m (which was wide enough to show its overall
effect on LogDec within a range of ±0.8), a change
of LogDec δA associated with a specific TPJBs design
was calculated. Here, all applied LogDec analyses were
performed only for the 1st whirl mode at 3,600 rpm,
utilizing the FE rotor model shown in Fig. 2.
3-1. LogDec Effects of LOP and LBP in 6-Pad
TPJBs
For 6-Pad TPJBs: m = 0.0 and offset = 0.5, depending
on QA, Fig. 3 shows in comparison the δA characteristics
of LOP and LBP designs. In a standpoint of stability,
in a low range of 0 < QA < 1.36e + 08 N/m the LBP
performs better than the LOP whereas in a high range
of QA > 1.36e + 08 N/m the LOP greatly outperforms
the LBP. That is, as a conservative bearing design
against a possible aerodynamic cross-coupling instability
induced by high temperature and pressure steam in the
HPT, the 6-Pad LOP TPJBs shall be recommended
over the 6-Pad LBP TPJBs.
3-2. LogDec Effects of LOP and LBP in 5-Pad
TPJBs
For 5-Pad TPJBs: m = 0.0 and offset = 0.5, depending
on QA, Fig. 4 shows in comparison the δA characteristics Fig. 1. Lubrication analysis geometry model of HPT 6-
Pad LOP #1 TPJB for offset = 0.5 and m = 0.0.
Fig. 2. A FE rotordynamic analysis model of HPT rotor
with a cross-coupling element added in the middle and
its 1st mode shapes in red, simultaneously, in hori-
zontal and vertical planes at 3,600 rpm with 6-Pad LOP
TPJBs: m = 0.0 and offset = 0.5, for QA = 1.5e + 08 N/m.
Fig. 4. Applied LogDec characteristics, at 3,600 rpm,
of the HPT rotor with 5-Pad TPJBs, depending on the
LOP and LBP: m = 0.0 and offset = 0.5.
Fig. 3. Applied LogDec characteristics, at 3,600 rpm,
of the HPT rotor with 6-Pad TPJBs, depending on the
LOP and LBP: m = 0.0 and offset = 0.5.
Vol. 37, No. 3, June 2021
102 An Sung Lee and Sun-Yong Jang
of LOP and LBP designs. In a standpoint of stability,
similarly, in a low range of 0 < QA < 1.13e + 08 N/m
the LBP performs better than the LOP whereas in a
high range of QA > 1.13e + 08 N/m the LOP greatly outper-
forms the LBP. That is, as a conservative bearing design
against a possible instability by high temperature and
pressure steam in the HPT, the 5-Pad LOP TPJBs shall
be recommended over the 5-Pad LBP TPJBs.
3-3. LogDec Effects of 6-Pad and 5-Pad in
LBP TPJBs
For LBP TPJBs: m = 0.0 and offset = 0.5, depending
on QA, Fig. 5 shows in comparison the δA characteristics
of 6-Pad and 5-Pad designs. In a standpoint of stability,
in a low range of 0 < QA < 1.13e + 08 N/m the 5-Pad
performs better than the 6-Pad whereas in a high range
of QA > 1.13e + 08 N/m the 6-Pad performs better than
the 5-Pad. That is, as a conservative bearing design
against a possible instability by high temperature and
pressure steam in the HPT, the 6-Pad LBP TPJBs shall
be recommended over the 5-Pad LBP TPJBs as expected.
3-4. LogDec Effects of 6-Pad and 5-Pad in
LOP TPJBs
For LOP TPJBs : m = 0.0 and offset = 0.5, depending
on QA, Fig. 6 shows in comparison the δA characteristics
of 6-Pad and 5-Pad designs. In a standpoint of stability,
in a full range of QA the 5-Pad performs slightly better
than the 6-Pad. Therefore, as a conservative bearing
design against a possible instability by high temperature
and pressure steam in the HPT, the 5-Pad LOP TPJBs
shall be recommended over the 6-Pad LOP TPJBs,
where this is contrary to one's expectation.
3-5. LogDec Effects of Offset and Preload in
5-Pad LBP TPJBs
For 5-Pad LBP TPJBs, the effects of offset and
preload, m, which are key design variables of TPJBs,
were analyzed.
For a fixed m = 0.0, depending on QA, Fig. 7 shows
in comparison the δA characteristics as an offset increases
from 0.5 and to 0.55 and 0.6. It is observed that over
a full range of QA, δA decreases slightly as an offset
increases from a center pivot of 0.5 to 0.55 whereas
δA decreases greatly with an offset = 0.6. Therefore, TPJBs
Fig. 5. Applied LogDec characteristics, at 3,600 rpm,
of the HPT rotor with LBP TPJBs, depending on the
6-Pad and 5-Pad: m = 0.0 and offset = 0.5.
Fig. 6. Applied LogDec characteristics, at 3,600 rpm,
of the HPT rotor with LOP TPJBs, depending on the
6-Pad and 5-Pad: m = 0.0 and offset = 0.5.
Fig. 7. Applied LogDec characteristics, at 3,600 rpm,
of the HPT rotor with 5-Pad LBP TPJBs, depending
on the offset: m = 0.0 and offset = 0.5, 0.55, 0.6.
Tribol. Lubr., 37(3) 2021
Stability Characteristics of Supercritical High-Pressure Turbines Depending on the Designs of Tilting Pad Journal Bearings 103
design with an offset = up to 0.55 may be considered
to decrease bearing temperature and increase stiffness
but TPJBs design with an offset = 0.6 shall not be
recommended for the HPT, in a standpoint of LogDec,
i.e., stability.
For a fixed offset = 0.5, depending on QA, Fig. 8 shows
in comparison the δA characteristics as m increases from
0.0 and to 0.2 and 0.4. It is observed that in a low
range of 0 < δA < 1.0e + 08 N/m δA decreases nearly equally
as m increases to 0.2 and 0.4 whereas in a high range
of QA > 1.0e + 08 N/m δA decreases in a greater magnitude
with m = 0.4 than with m= 0.2.
Showing the results of Figs. 7 and 8, together, in
superposition, depending on QA, Fig. 9 represents the
δA characteristics as an offset increases from 0.5 and
to 0.55 and 0.6, for a fixed m = 0.0 and as m increases
from 0.0 to 0.2 and 0.4, for a fixed offset = 0.5. Parti-
cularly, in a range of QA > 1.0e + 08 N/m the effects that
an offset independently increases to 0.55 and 0.6 and that
m independently increases to 0.2 and 0.4 are almost the
same on δA.
Specially, for a simultaneous application of offset =
0.6 and m = 0.4, Fig. 10 shows in comparison the δA
characteristics, depending on QA. It is observed that
for offset = 0.6 and m = 0.4, δA decreases in a greater
magnitude over a full range of QA. Therefore, it is
reviewed that a simultaneous design application of
offset = 0.6 and m = 0.4 shall be quite undesirable in a
standpoint of stability.
3-6. Comprehensive LogDec Effects of various
TPJB Designs
Depending on QA, Fig. 11 shows in a comprehensive
comparison the δA characteristics of various TPJB designs,
i.e., 6-Pad LOP, 6-Pad LBP, 5-Pad LOP, 5-Pad LBP,
Hybrid 3-Pad LOP (lower 3-Pad tilting and upper 1-
Pad fixed), and 5-Pad LBPs with design variables of
offset and preload.
It is observed that in a standpoint of stability the
Hybrid 3-Pad LOP TPJBs most excellently outperform
all the other TPJBs over nearly a full range of aerody-
namic cross-coupled stiffness, QA. Also, in a high range
of QA > 1.2e + 08 N/m, as a practical conservative bearing
design approach for enhancing a rotordynamic stability
Fig. 8. Applied LogDec characteristics, at 3,600 rpm, of
the HPT rotor with 5-Pad LBP TPJBs, depending on
the preload: m = 0.0, 0.2, 0.4 and offset = 0.5.
Fig. 9. Applied LogDec characteristics, at 3,600 rpm,
of the HPT rotor with 5-Pad LBP TPJBs, depending
on the offset and preload, independently: m = 0.0, 0.2,
0.4 and offset = 0.5, 0.55, 0.6.
Fig. 10. Applied LogDec characteristics, at 3,600 rpm,
of the HPT rotor with 5-Pad LBP TPJBs, depending
on the offset and preload, independently (m = 0.0, 0.4
and offset = 0.5, 0.55) and simultaneously (m = 0.4 and
offset = 0.6).
Vol. 37, No. 3, June 2021
104 An Sung Lee and Sun-Yong Jang
of the HPT both the 6-Pad LOP and 5-Pad LOP TPJBs
may be recommended as well.
4. Conclusions
For a HPT of 800 MW class supercritical thermal-
power plant, considering aerodynamic cross-coupling
induced by high-temperature and high-pressure steam,
we performed a rotordynamic LogDec stability analysis
with applying various TPJB designs, which several
steam turbine OEMs currently use in their supercritical
or ultra-supercritical turbines. We considered the following
TPJB designs: 6-Pad LOP, 6-Pad LBP, 5-Pad LOP, 5-
Pad LBP, Hybrid 3-Pad LOP (lower 3-Pad tilting and
upper 1-Pad fixed), and 5-Pad LBPs with design
variables of offset and preload.
Following results are summarized only in a standpoint
of LogDec stability. The Hybrid 3-Pad LOP TPJBs
most excellently outperform all the other TPJBs over
nearly a full range of cross-coupled stiffness. Besides,
in a high range of cross-coupled stiffness, both the 6-
Pad LOP and 5-Pad LOP TPJBs may be recommended
as well as a practical conservative bearing design
approach for enhancing a rotordynamic stability of the
HPT. As expected, in a high range of cross-coupled
stiffness the 6-Pad LBP TPJBs exhibit a better per-
formance than the 5-Pad LBP TPJBs. However, in
contrast to one's expectation, notably, the 5-Pad LOP
TPJBs exhibit a slightly better performance than the
6-Pad LOP TPJBs over a full range of cross-coupled
stiffness. Furthermore, we do not recommend any TPJBs
design efforts of either increasing a pad offset from
0.5 or a pad preload from 0 for the HPT, in a standpoint
of stability.
Acknowledgements
This study has been supported by the small and
medium-sized businesses technology development project
(the export enterprise technology development project:
S2460209): Development of Tilting Pad Journal Bearings
Technology for Large-Capacity Supercritical Steam
Turbines Application. The support is greatly appreciated
by the authors.
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