1

Q. Liu & X. LuGraduate Research Assistants

2015 STLE Annual Meeting & Exhibition, May 17-21, 2015, Dallas, TX

Measurements Of Leakage and Force Coefficients in a Short Length Annular Seal Operating with a Gas in Oil Mixture

Supported by TAMU Turbomachinery Research Consortium

Luis San AndrésMast-Childs Chair ProfessorFellow STLE

May 2015

2

Bubbly Mixture Annular Pressure Seals

• As oil fields deplete compressors work off-design with liquid in gas mixtures, mostly inhomogeneous.

• Similarly, oil compression station pumps operate with gas in liquid mixtures.

• The flow condition affects compressor or pump overall efficiency and reliability.

• Little is known about seals operating under 2-phase conditions, except that the mixture affects seal leakage, power loss and rotordynamic force coefficients; perhaps inducing random vibrations that are transmitted to the whole rotor-bearing system.

JustificationSeals operate with either liquids or gases, but not both……

Subsea Application: Wet Gas Compression

Brenne, L., Bjørge, T., Bakken, L., and Hundseid, Ø., 2008, "Prospects for Sub Sea Wet Gas Compression," ASME Paper GT2008-51158.

Brenne, L., Bjørge, T., Gilarranz, J., Koch, J., and Miller, H., 2005, “Performance Evaluation of A Centrifugal Compressor Operating under Wet Gas Conditions,” Proc. 34thTurbomachinery Symposium, Houston, TX, pp. 111~120

Bertoneri, M., Duni, S., Ransom, D., Podesta, L., Camatti, M., Bigi, M., and Wilcox, M., 2012, “Measured Performance of Two-stage Centrifugal Compressor under Wet Gas Conditions,” ASME Paper GT2012-69819.

Vannini, G., Bertoneri, M., Del Vescovo, G., and Wilcox, M., 2014, “Centrifugal Compressor Rotordynamics in Wet Gas Conditions,” Proc. 43rd Turbomachinery & 30thPump Users Symposia, Houston, TX.

Bertoneri, M., Wilcox, M., Toni, L., and Beck, G., 2014, “Development of Test Stand for Measuring Aerodynamic, Erosion, and Rotordynamic Performance of a Centrifugal Compressor under Wet Gas Conditions,” ASME Paper GT2014-25349.

Announce / assess the effects of two phase flow in wet gas compression: drop in efficiency and reliability, excessive vibration and rotordynamic stability.

3

4

Background literature

Experimental & Physical Modeling

Tao, L., Diaz, S., San Andrés, L., and Rajagopal, K.R., 2000, "Analysis of Squeeze Film Dampers Operating with Bubbly Lubricants" ASME J. Tribol., 122.

Squeeze film dampers

Diaz, S., and San Andrés, L., 2002, “Pressure Measurements and Flow Visualization in a Squeeze Film Damper Operating with a Bubbly Mixture,” ASME J. Tribol., 124.

Diaz, S., and San Andrés, L., 2001, “A Model for Squeeze Film Dampers Operating with Air Entrainment and Validation with Experiments,” ASME J. Tribol., 123..

Diaz, S., and San Andrés, L., 2001, "Air Entrainment versus Lubricant Vaporization in Squeeze Film Dampers: An Experimental Assessment of their Fundamental Differences,” ASME J. Eng. Gas Turbines Power, 123.

Sponsored by National Science Foundation and TAMU Turbomachinery Research Consortium, June 1998- May 2002

5

Experimental – Seals (two phase)

Iwatsubo, T., and Nishino, T., 1993, “An Experimental Study on the Static and Dynamic Characteristics of Pump Annular Seals,“ 7th Workshop on Rotordynamic Instability Problems in High Performance Turbomachinery, Texas A&M University.

Computational – Seals (two phase)

Literature: Two Phase Flow in Annular Seals

Hendricks, R.C., 1987, "Straight Cylindrical Seals for High Performance Turbomachinery," NASA TP-1850.

Arauz, G., and San Andrés, L., 1998, “Analysis of Two Phase Flow in Cryogenic Damper Seals, I: Theoretical Model, II: Model Validation and Predictions,” ASME J. Tribol., 120.

Beatty, P.A., and Hughes, W.F., 1987, "Turbulent Two-Phase Flow in Annular Seals," ASLE Trans., 30.

Arghir, M., Zerarka, M., Pineau, G., 2009 "Rotordynamic Analysis of Textured Annular Seals with Mutiphase (Bubbly) Flow, “Workshop : “Dynamic Sealing Under Severe Working Conditions” EDF – LMS Futuroscope.

San Andrés, L., 2012, “Rotordynamic Force Coefficients of Bubbly Mixture Annular Pressure Seals,” ASME J. Eng. Gas Turbines Power, 134.

NEED EXPERIMENTAL

validation.

.

4

6

Experimental – Seals (two phase)Iwatsubo, T., and Nishino, T., 1993, “An Experimental

Study on the Static and Dynamic Characteristics of Pump Annular Seals,“ 7th Workshop on Rotordynamic Instability

Problems in High Performance Turbomachinery.

NO description of water lubricated seal (L, D, c) or gas type…..

Tests at shaft speed 1.5-3.5 krpm and supply pressure=1.2 - 4.7 bar.

Gas/liquid volume fraction =0, 0.25, 0.45, 0.70

Mxx

Cxx

Kxx

, gas volume fraction increases

Stiffness, damping and mass coefficients decrease

as gas volume fraction increases.

7

• Application–Subsea compression and pumping(wet compressors must operate with LVF up to 5%)

• Effect of two component flow on seal– Leakage rate– Dynamic force coefficients– Stability

• Status of current research: – Bulk-flow model available– Lack of experimental validation

Motivation

• Design and construct test rig.• Make oil-gas mixtures with liquid volume

fraction (LVF) at inlet plane from 0.0~1.0.• Measure flow rate thru seal for range of

LVF & pressure supply/discharge = 1 to 4. • Measure test system periodic forced

response and perform parameter identification.

8

Overview of this work

room temperature 20oC. Stationary (non-rotating) journal.

9

Wet Seal Test Rig

Main frame

Support pipe

Thermocouple

Accelerometer

Load cell

Eddy current sensor

Dynamic pressure sensor

Bearing cartridge

Rotor

Top plate

Air-Oil Mixture

seal clearance

Rotor

Bearing

Shaker

Shaker

Shaker

Stinger

10

Flows & Mixture

Air InletOil Inlet

(ISO VG 10)

Test seal section

Valve

ValveSparger(mixing) element

Oil

Air

Ps/Pa=1.5, inlet LVF= 2%

11

Seal cartridgeSeal

Diameter 127 mm (5in)Length 46 mm (1in)clearance 0.127 mm (5mil)

Supply pressure 1~3.5 barOil ISO VG 10

density 846 kg/m3

viscosity 18 cP at 20ºCAir density 1.2 kg/m3 at

1barviscosity 0.02 cP at

20ºC, 1bar

Flow

Seal

Rotor & journal

L/D=0.36

Flow rate of mixture vs. supply pressure 1.1 ~3.5bar (abs)Room temperature ~20ºC– Liquid volume fraction (at supply pressure) at inlet:

12

Flow Rate Measurement

Mass fraction is constant as there is no mass transfer between oil and air.

LVF= Qlβ =inlet PaQ +Ql g Ps

Oil

Air

0.012

0.014

0.016

0.018

0.02

0.022

0 0.2 0.4 0.6 0.8 1

Mass flow ra

te of m

ixture [k

g/s]

seal inlet LVF

Mass Flow Rate in Wet SealSupply PS=3.5 bar(abs), ambient Pa=1 bar(abs), (non-rotating) journal.

Liquid mass fraction () is large even for small LVF because of large density ratio (oil/air)~200 .Flow ranges from turbulent (pure air) to laminar (pure oil).Predicted flow ~ 10% lower than measured.

L/D=0.36 c=0.127 mm

Ps/Pa=3.5

Reynolds #

Liquid mass fraction

mass flow rate

Measurement

Prediction

Flow visualization in Wet Sealambient pressure Pa=1 bar(abs),

inlet temperature 20oC. Non-rotating journal.

Inlet LVF=0.45

Smaller bubbles with

higher supply

pressureL/D=0.36 c=0.127 mm

Seal land length= 46 mm

Top

Bottom

Top

Bottom

PS=1.5bar(abs)=0.45

PS=3.0 bar(abs)

=0.45

15

Force Coefficients in Annular SealsSeal reaction force is a

function of the fluid properties, flow regime,

operating conditions and geometry.

For small amplitudes of rotor lateral motion: forces are linearized

with stiffness, damping and inertia force

coefficients:

c

rotor

L

D

Axial pressure field (liquid)

stator

Pa

PS Pe

W Axial velocity

X

Y

Film thickness H=c+eX coseY sin

rotor

X XX XY XX XY XX XY

Y YX YY YX YY YX YY

F K K C C M Mx x xF K K C C M My y y

Model system (2-DOF): structure + sealEOM: Time Domain

EOM: Frequency Domain

Measure:

Estimate Parameters:

K C M

K, C, M

Parameter Identification

S SEAL S SEAL BC SEAL(K +K )z+(C +C )z+(M +M )z=F

2iω ω K + C - M z = F

Load F=Fo sin(t) Displacement z,

R e ( )

Im ( ) (?)

2ω

ω

H K - M

H C

1( ) ( ) 2iω ω H z = F H K + C - M = F z

0

5

10

15

0 20 40 60 80 100

Dis

plac

emen

t (um

)

Frequency (Hz)

inlet LVF 0%inlet LVF 4%

-90

-60

-30

0

30

60

90

0 0.02 0.04 0.06 0.08 0.1

Load

(N)

Time (s)

inlet LVF 0% inlet LVF 4%0

10

20

30

40

50

0 0.02 0.04 0.06 0.08 0.1

Dis

plac

emen

t (um

)

Time (s)

inlet LVF 0% inlet LVF 4%

Applied load and seal motion X direction, frequency = 30 Hz

FFT

0

5

10

15

20

0 20 40 60 80 100

Load

(N)

Frequency (Hz)

inlet LVF 0%

inlet LVF 4%

FFT

Load Displacement

Unexplained drift at low frequencies

Seal coefficients (K, C, M)SEAL = (K, C,M) – (K, C, MBC)S

SEAL Test system(lubricated)

Structure (dry)

•Instrumental Variable Filter Method (IVFM) (Fritzen, 1985, J.Vib, 108)

2Re H K M Im (?) H CComplex stiffness

Parameter Identification

-16

-12

-8

-4

0

4

0 50 100 150 200

Rea

l(H) (

MN

/m)

Frequency (Hz)

Hᵪᵪ

Hᵧᵧ

K - Mω²0

4

8

12

16

20

0 50 100 150 200

Imag

inar

y(H

) (M

N/m

)

Frequency (Hz)

Hᵪᵪ

Hᵧᵧ

Ps/Pa=2, inlet LVF 2%

Test System Dynamic Stiffness2.0 = Supply pressure PS/ambient pressure Pa

inlet temperature 20oC. (non-rotating) journal.

βinlet=0.02

βinlet=0.04

liquid volume fraction at inlet:

K-2M fits well the test

data. -16

-12

-8

-4

0

4

0 50 100 150 200

Rea

l(H) (

MN

/m)

Frequency (Hz)

Hᵪᵪ

Hᵧᵧ

K - Mω²

Uncertainty ±0.7MN/m

-16

-12

-8

-4

0

4

0 50 100 150 200

Rea

l(H) (

MN

/m)

Frequency (Hz)

Hᵪᵪ

Hᵧᵧ

K - Mω²

Test System Quadrature Stiffness2.0 = Supply pressure PS/ambient pressure Pa

inlet temperature 20oC. (non-rotating) journal.

liquid volume fraction at inlet (

Ima(H) shows damping is frequency

dependent. Very little

damping with all gas.

βinlet=0.0

0.0 (all air)

(mm=6±0.8 g/s)

βinlet=0.02

0.90

(mm=6±0.2 g/s)

βinlet=0.04

0.95

(mm=7±0.2 g/s)

20

0

0.2

0.4

0.6

0.8

1

0 50 100 150 200

Imag

inar

y(H

) (M

N/m

)

Frequency (Hz)

Hᵪᵪ

Hᵧᵧ

0

4

8

12

16

20

0 50 100 150 200

Imag

inar

y(H

) (M

N/m

)

Frequency (Hz)

Hᵪᵪ

Hᵧᵧ

Hᵪᵪ Hᵧᵧ

Inlet LVF 0%

Inlet LVF 4%

Uncertainty: ±0.7MN/m

10

100

1,000

10,000

100,000

0 50 100 150 200

Cyy

_sea

l (N

-s/m

)

Frequency (Hz)

10

100

1,000

10,000

100,000

0 50 100 150 200

Cxx

_sea

l (N

-s/m

)

Frequency (Hz)

Wet Seal Direct Damping C=Im(H)/2.0 = Supply pressure PS/ambient pressure Pa

Stationary (non-rotating) journal. βinlet=0.04

βinlet=0.02

βinlet=0 (all air)

βinlet=0.02

βinlet=0

βinlet=0.04

liquid volume fraction at inlet 0% (all gas), 2%, 4%.

Damping is frequency dependent.

Small LVF causes damping to increase 20

fold w/r to air only.

CXX & CYY differ due to uneven clearance and

inhomogenous flow

• Seal mass and stiffness coefficientsParameter Identification

LVF at seal Liquid mass KXXseal KYYseal MXXseal MYYsealinlet fraction MN/m MN/m kg kg

0 0 1.4 1.0 0 0

2% 90% 1.5 1.3 0.3 0.7

4% 95% 2.5 1.3 1.2 0.9

With mixture, CXX>CYYmixture is not evenly distributed in seal clearance. Liquid volume is larger in X direction than in Ydirection

• Assembled new test rig and trouble shoot.• Generate mixtures and measure flow rates:

– LVF = 1% to 100%– supply pressure up to 50 psig (3.5 bar abs)– Rotor speed: stationary

• Forced response – Installed shakers and performed single frequency

dynamic load measurements– Identification of force coefficients

• Visual inspection of bubbly mixture23

Completed work

24

Conclusion• Mass flow rate through seal dictated by liquid

content (density ratio ~ 200).• Damping coefficients are frequency dependent.

For operating conditions with a small volumefraction of oil in gas (4%), damping can be up totwenty times larger than that obtained for the puregas condition.

• The effect of a few droplets of liquid onaffecting the test system forced response isoverwhelming.

• Extend the range of LVF at seal inlet in tests to quantify its effect on force coefficients.

• Operate test seal rig with journal rotation (6 krpm) and with a higher supply pressure (6 bar).

• Benchmark bulk-flow model predictions1.• Extend existing predictive model1 considering

inhomogeneous flow.

25

Future Work

San Andrés, L., 2012, “Rotordynamic Force Coefficients of Bubbly Mixture Annular Pressure Seals,” ASME J. Eng. Gas Turbines Power, 134 (2), p. 022503.

Acknowledgments

Turbomachinery Laboratory Staff and Students• Turbomachinery Research Consortium

Questions (?)

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

26