stability characteristics of supercritical high-pressure

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.


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


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


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


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


of the HPT rotor with LBP TPJBs, depending on the

6-Pad and 5-Pad: m = 0.0 and offset = 0.5.


of the HPT rotor with LOP TPJBs, depending on the

6-Pad and 5-Pad: m = 0.0 and offset = 0.5.


of the HPT rotor with 5-Pad LBP TPJBs, depending

on the offset: m = 0.0 and offset = 0.5, 0.55, 0.6.



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.



on the offset and preload, independently: m = 0.0, 0.2,

0.4 and offset = 0.5, 0.55, 0.6.



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


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.

References

[1] Nicholas, J., “Tilting Pad Bearing Design,” Pro-

ceedings of the 23rd Turbomachinery Symposium,

Turbolab, Texas A&M Univ., pp.179-194, 1994.

[2] Zeidan, F., “Fluid Film Bearings Fundamentals,”

Proceedings of the 1st Middle East Turbomachin-

ery Symposium, Turbolab, Texas A&M Univ., 2011.

[3] He, M., Byrne, J., Cloud, H, Vazquez, J., “Funda-

mentals of Fluid Film Journal Bearing Operating

and Modeling,” Proceedings of the Asia Turboma-

chinery & Pump Symposium, Turbolab, Texas A&M

Univ., 2016.

[4] Lee, A. S., “Design Analysis to Enhance Rotordy-

namic Stability of High-Speed Light-weight Centrif-

ugal Compressor - Part I: Effects of Bearing Designs,”

J. Korean Soc. Tribol. Lubr. Eng., Vol. 29, No.6,

pp.386-391, 2013, https://doi.org/10.9725/kstle.2013.

29.6.386

[5] Salamone, D., “Journal Bearing Design Types and

their Applications to Turbomachinery,” Proceed-

ings of the 13th Turbomachinery Symposium, Tur-

bolab, Texas A&M Univ., pp.179-188, 1984.

[6] Vance, J., Zeidan, F., Murphy, B., Machinery Vibra-

tion and Rotordynamics, Chap.5, pp.171-267, John

Wiley & Sons, 2010.

[7] Nicholas, J. C., Gunter, E. J., Barrett, L. E., “The

Influence of Tilting Pad Bearing Characteristics on

the Stability of High Speed Rotor-Bearing Sys-

tems,” Topics in Fluid Film Bearing and Rotor

Bearing System Design and Optimization, ASME,

pp.55-78, 1978.

[8] Lee, A. S., “Design Analysis for Enhancing Rotor-

dynamic Stability of High-Speed Light-weight Cen-

trifugal Compressor - Part II: Improvement to

Rotordynamic Stability,” Journal of the KSTLE,

Vol.30, No.1, pp.9-14, 2014, https://doi.org/10.9725/

kstle.2014.30.1.9

[9] Ikeno, K., Sasaki, Y., Kawabata, R., Yoshida, S.,


of the HPT rotor with various TPJBs designs in

summary.



Hata, S., “Measurement of Dynamic Performance of

Large Tilting Pad Journal Bearing and Rotor Stabil-

ity Improvement,” Proceedings of the 39th Tur-

bomachinery Symposium, Turbolab, Texas A&M

Univ., 19-29, 2010.

[10] API STD 617 8th Ed., Axial and Centrifugal Com-

pressors and Expander-compressors for Petroleum,

Chemical and Gas Industry Services, American Petro-

leum Institute, 2014.

[11] API RP STD 684 2nd Ed., API Standard Para-

graphs Rotordynamic Tutorial: Lateral Critical

Speeds, Unbalance Response, Stability, Train Torsion-

als, and Rotor Balancing, American Petroleum Insti-

tute, 2005.

[12] Lee, A. S., and Jang, S.-Y., “Design Characteristics

of HPT Bearings in 800 MW Class Supercritical

Steam Turbines Train,” Proc. Spring Conf. Korean

Tribol. Soc., Pusan, Korea, April, 2018.

Vol. 37, No. 3, June 2021

stability characteristics of supercritical high-pressure

Documents