static load p , heavily loaded s thrust b

ON THE STATIC LOAD PERFORMANCE OF A LARGE

SIZE, HEAVILY LOADED SPRING SUPPORTED

THRUST BEARING

Rasool Koosha

Graduate Research Assistant

Luis San Andrés

Mast-Childs Chair Professor

and Fellow STLE

1

STLE 74th Annual Meeting & Exhibition,

Nashville, Tennessee,

May 19-23, 2019

J. Mike Walker ’66 Department of Mechanical Engineering,

Texas A&M University

Introduction: Tilting Pad Thrust Bearings (TPTBs)

Control rotor axial placement in rotating machinery.

Offer advantages such as low power loss, simple

installation, and low-cost maintenance.

• As lubricant is sheared

• Film and pad temperatures increase.

• Load capacity of bearing depends on

lubricant viscosity,

• a function of temperature.

• Pad thermally and mechanically

induced deformations

• shape the operating fluid film

thickness and determine the bearing

forced performance.

2

Thrust Collar

Speed: Ω

Bearing Housing Orifice

Pivot

Fluid Film Pad Tilting

Static Load: W

Introduction: Spring-Supported Thrust Bearings (SSTBs)

For large size high power density applications, SSTBs are

preferred over pivoted ones. SSTBs produce

lesser elastic deformations,

self-adjustment against thrust collar misalignment,

better heat dissipation.

3

The performance of

SSTBs during machine

start-up and shut-down

processes is critical to

ensure safety and

reliability.

Prior work on SSTBs

4

Elastic deformations produce a convex pad top surface, and rotating

collar rubs on a pad to wipe out its Babbitt layer.

Bearing: SSTB in a turbine

generator unit, OD = 2.7 m

Chambers and Mikula Failure analysis

Operating Condition:

load/Pad = 2.1 MPa

Speed = 257 rpm (𝑅𝑂Ω = 37 m/s).

Time for thermal balance is proportional to pad thickness2

~ 9 h for a 0.25 m thick Babbitt & steel pad.

Bearing: SSTB OD = 2.0 m

Ettles et al. Transient response analysis


load/Pad = 3.0 MPa

Speed = 500 rpm (𝑹𝑶Ω = 53 m/s).

1987, STLE Trans., 31.

2003, J. Tribol, 125.

More work on SSTBs

5

Optimization process with objective functions to increase minimum

fluid film thickness while minimizing pad temperature raise. The

optimum design has:

Thickness for a bearing pad equal to 12% of its circumferential

arc length.

Spring bed arrangement extending edge-to-edge in the radial

direction.

Yet only supporting 55% of the pad circumferential length.

Bearing: SSTB

Distinct bearing configurations

Ettles et al. Bearing Design optimization


load/Pad = 4 MPa

Speed = 300 rpm.

2016, J. Tribol, 138.

TPTB current computational analysis

2D hydrodynamic pressure on pad surface.

Cross-film viscosity variation.

Accounts for turbulent flow effects.

3D temperature distribution in fluid film.

Heat conduction to the pads.

Accounts for turbulent flow effects.

3D temperature distribution in pad and liner.

Heat transfer boundary condition all side

of a pad.

The acting pressure & temperature gradient in a pad produce elastic deformation.

6

Elastic deformations in a pad

Advantage:

High

Accuracy.

7

Pad and Liner deformation

field from in-house FE model

Babbitt

Pivot

Peak

deformation

In-House Finite Element

model couples pad and

spring-bed stiffness.

Pressure induced

deformation

Temperature induced

deformation

3D deformation field for a

pad

Pad rigid body motion

due to spring bed

flexibility

Both pad deformation and

rigid body motion change the

film thickness bearing

performance

RESULTS AND

DISCUSSION

8

9

A SSTB holding a hydraulic turbine

Max surface speed ΩRo 23.6 m/s

Specific load per pad W/(Ap Np) 4.0 MPa

Outer/inner radius, Ro/Ri 1.43

Radial/circumferential length, L/B 1

Pad thickness/outer radius, H/L 0.2

Pad material Steel

Lubricant Thick Oil

Large size 16-pad SSTB:

Spring-beds extend

• Edge-to-edge radial length of a pad

• Partially in circumferential direction.

Pads include

• a hydrostatic pocket for hydraulic lift

• an internal multiple-pipe cooling system

Issue: SSTB shown repeated failures after accumulating a

number of startup /shutdown processes ranging from 3k to 5k.

10

Objective of Predictive Analysis

Three cases of operation:

Case 1: Nominal operating condition (4.0 MPa specific load/pad and

23.6 m/s OD surface speed) without cooling system.

To evaluate hydrodynamic load performance of bearing.

Case 2: Nominal operating condition with an active cooling system.

Bearing performance compared to Case 1 evaluates the

effectiveness of the cooling system.

Case 3: Operation with idle collar (zero speed):

To evaluate hydrostatic load performance.

Bearing performance for (a) a hot pad (keeps temperature from

nominal condition) to simulate a fast shut down, and (b) with a

cold pad that assumes a slow process of thermal equilibrium as

shaft speed decreases.

11

Case 1: Bearing operating at

nominal condition

with inactive cooling system.

OD speed = 23.6 m/s,

Specific load/pad=4 MPa

12

C1: Fluid Film Thickness

200

180

160

140

120

100

80

220

240

[µm]

Minimum

Film

Thickness

At mean diameter Fluid Film Thickness field

OD surface speed = 23.6 m/s,


Pocket

Nominal Operating condition.

Cooling system OFF

Minimum film thickness = 74 µm

Occurs within the pad surface (non at

the trailing edge)

Maximum film thickness = 246 µm at

pad OD - trailing edge.

Tra

ilin

g E

dg

e

Le

ad

ing

Ed

ge

Pressure field on pad

13

Case 1: Pressure Field

At mean diameter4.0

3.0

2.0

1.0

0.0

0.5

1.5

2.5

3.5

Hydrostatic

pressure



Low pressure zone at the pad trailing

edge inner and outer diameter


Cooling system OFF

Pressure field is extended fully along

the pad radial length but not the

circumferential length

14

C1: Film & Pad Temperatures

4035

2520

510

0

15

30

Pad Temperature riseFluid film Temperature rise

Peak Temperature

Collar Side

Pad SideTop

Side

Bottom

Side

45

50

Pocket Pocket



Both pad and fluid film show a significant temperature rise extending well into the

pad.

[⁰C]


Cooling system OFF

15

C1: Pad Elastic Deformations

50

0

-50

-75

-100

-150

-25

25

-125

Top

Side

Total deformation

Bottom

Side

Thermal deformations are larger than

mechanical deformations.

Thermal deformation warp the pad to

make a convex surface.


Cooling system OFF

[µm]



16

Case 2: Bearing operating at

nominal condition

with active cooling system.

OD speed = 23.6 m/s,


17

Case 2: Fluid Film Thickness

200

180

160

140

120

100

80

[µm]

Minimum Film Thickness

At mean diameter Film Thickness on Pad



Pocket

Nominal Operating condition &

active cooling system

Minimum film thickness of 81 µm at pad

trailing edge.

Maximum film thickness of 196 µm at

the pad OD & trailing edge.

Compared to case 1:

• Larger min film and smaller max film.

• Location of minimum film thickness moves

toward inside of pad.

Pressure Field (normalized with

respect to pad mean pressure)

18

C2: Fluid Film Pressure

4.0

3.0

2.0

1.0

0.0

0.5

1.5

2.5

3.5

At mean diameter



Hydrostatic

pressure



Pressure field covers whole pad

surface.

Compared to case 1:

Pressure field shows a slower drop

from the center to the edges along the

circumferential direction.

19

C2: Film & Pad Temperatures

40

35

25

20

10

5

0

15

30

Pad Temperature riseFilm Temperature rise

Collar Side

Pad SideTop

Side

Bottom

Side



Maximum Temperature

[⁰C]



PocketPocket

Temperature rise is moderate (max 10C) except at pad trailing

edge (40C) where cooling lines do not reach.

Compared to case 1: Pad temperature rise is 10⁰C higher.

20

C2: Pad Elastic Deformations

Max surface speed = 23.6 m/s,


40

20

0

-10

-30

-40

-50

-20

10

30Top

Side

Bottom Side

Total deformation

Mechanical (pressure induced) deformations larger

than thermal deformations. Pad warps at ID and OD.



[µm]

Mechanical deformations are larger than thermal

deformations.

Compared to Case 1: Pad deformations are smaller.

21

Case 2: Lessons Learnt

The bearing produces a sufficiently thick fluid film thickness.

Even with an active cooling system, the predicted pad temperature

rise and ensuing thermal deformations are significant at the trailing

edge where cooling lines do not reach.

The current arrangement of springs under a pad, with free leading

and trailing edges, improves hydrodynamic performance as it opens

the film at the leading edge to let flow in. [1] Ettles et al. 2016, J. Tribol, 138(4)

22

Inactive vs Active Cooling Systems: Lessons Learnt

Active cooling system effectively removes heat from bearing pad.

Bearing with an active cooling system produces

Larger fluid film thickness.

Reduced low pressure area.

Lower pad temperature and fluid film temperature rises.

Smaller pad thermal deformations.

Not the

same

23

Case 3: Bearing operating w/o

shaft speed

and with active cooling system.

Speed = 0


HOT pad: keeps same temperature as that

when operating at nominal speed.

COLD pad: reaches temperature field as if

operating at (very) low speed (steady state).

Simulate bearing performance after a

fast shut down in shat speed

24

C3: Fluid Film Hydrostatic Pressure Fields

Hydrostatic Lift

Cold Pad4.0

3.5

2.5

1.0

0.5

0

3.0

2.0

1.5

Hot Pad

Surface speed = 0 m/s


Hydrostatic Lift

Hot pad shows slightly higher peak pressure.

Compared to case 2:

Pressure drops quickly from pocket towards pad edges, leading and trailing.

Larger pocket pressure needed to carry load.

Hydraulic Pressure for Lift ACTIVE

25

C3: Fluid Film Thickness Fields

[µm]

Minimum Film Thickness

Cold Pad

Pocket

45

40

30

15

10

5

0

35

25

20

Hot Pad

Max surface speed = 0 m/s


Pocket

Minimum film: 6 µm for cold pad & 1 µm for hot pad

Compared to Case 2:

Minimum film is significantly smaller and occurs at the pad

ID and OD (as it warps).

1 mm

6 mm

26

C3: Pad Deformation (thermal+pressure)

Peak

Deformations

Top

Side

Bottom

Side

10

0

-20

-30

-50

-40

-10

20

30Cold PadHot Pad

Top

Side

Bottom

Side

surface speed = 0 m/s


Hot pad shows significantly larger pad deformations.

Unlike the cold pad, the deformations of the hot pad at the trailing edge are positive

The film thickness reduces at trailing edge

[µm]

27

Case 3 vs. Case 2: Lessons Learnt

The current bearing design, pad dimensions and spring-

bed arrangement under the pads, is not capable of

producing a sufficiently large fluid film thickness for a

hydrostatic operating condition.

Operating with a hot pad during a shutdown process

affects bearing performance since at a low speed 0

speed, the minimum fluid film thickness is critically small

(order of surface roughness) and which increases the risk

of contact.

Shutdown Process: Cold Pad vs Hot Pad

Pad minimum film thickness

Min

. F

ilm

Th

ickn

ess

Speed

Shutdown

(hot pad)

Very slow shutdown

(cold pad)0.9

0.7

0.6

0.5

0.5

0.3

0.2

0.1

0.0

1

0.8

10.90.80.70.60.50.40.30.20.10

A hot pad

produces a

smaller film

thickness.

Difference

increases at

very low

speeds.


nominal

speed

Closure

Pad pressure induced deformations are significant to

warp a pad (convex curvature) at both its leading & trailing edges to

open (enlarge) the film thickness; and

the pad OD and ID warp toward the thrust collar (concave curvature)

and then reduce the film thickness.

The pad internal cooling system effectively limits a pad temperature

rise, hence thermal elastic deformations are moderate, except at the

pad trailing edge where the cooling lines do not reach.

During a slow shut-down process with a hot pad, the model predicts a

smaller film thickness than that if the bearing pads are cooled at a

steady rate.

A fast shutdown may lead to sudden seizure of one or more pads in

the bearing.

ON THE STATIC LOAD PERFORMANCE OF A LARGE SIZE, HEAVILY LOADED

SPRING SUPPORTED THRUST BEARING

Future Work

To better design both the pad and spring bed

geometry and disposition to ensure reliable

operation

during a nominal operating condition, and

during quick start-up and shut-down processes.

Questions (?)

Learn more at http://rotorlab.tamu.edu

Thanks to the Turbomachinery Research

Consortium for a multiple year support and

continued interest.

static load p , heavily loaded s thrust b

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