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LEAKAGE AND ROTORDYNAMIC FORCE

COEFFICIENTS OF A THREE-WAVE

(AIR IN OIL) WET ANNULAR SEAL:

MEASUREMENTS AND PREDICTIONS

Xueliang LuGraduate Research Assistant

Texas A&M University

Luis San AndrésMast-Childs Chair Professor

Fellow ASME

Funded by Turbomachinery Research

Consortium

accepted for

journal publication

Proceedings of ASME Turbo Expo 2018: Turbomachinery Technical

Conference and Exposition, June 11-15, 2018, Oslo, Norway

Paper GT2018-75200

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Annular Pressure Seals

Seals in a Multistage Centrifugal Pump or Compressor

Seals (annular, labyrinth or textured) separate regions of high pressure

and low pressure to minimize leakage (secondary flow); thus

improving the overall efficiency of a machine extracting or delivering

power to a fluid.

Impeller eye or

neck ring sealBalance piston seal

Inter-stage

seal

• Wet gas compression and multiphase oil boosting can

save up to 30% CAPEX when compared with a L/G

separation station.

• Subsea facilities must handle gas in liquid mixtures with

a varying gas content:

Wet gas compressor: LVF < 5%

Multiphase pump: 0< GVF< 100%

Justification subsea facilities

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Need of concerted effort to quantify effect

of two phase flow in sealing components

The aims are to improve reliability and to

reduce operating costs.

Prior Work Apps

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Vannini et al., 2016, “Experimental Results and CFD Simulations of Labyrinth and Pocket Damper

Seals for Wet Gas Compression,” ASME J. Eng. Gas Turb. Power, 138.

0.45 X SSV increases in

magnitude with LVF

13.5 krpm, 10 bar

Balance piston:

Labyrinth sealFluids:

Air and water

Rotor lateral vibration

LVF: 0~3%

Trapped liquid in seal

rotates with great

momentum and causes

0.45X vibes.

1 X

0.45 X

Two-phase flow in a wet gas compressor

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Vannini et al., 2016, “Experimental Results and CFD Simulations of Labyrinth and Pocket Damper

Seals for Wet Gas Compression,” ASME J. Eng. Gas Turb. Power, 138.

13.5 krpm, 10 bar

Balance piston:

Pocket damper

seal (PDS)

Rotor lateral vibration

0.45 X SSV reduces from 20 μm

to a few microns.

Liquid does not accumulate

in PDS, thus mitigating SSV.

1 X

0.45 X

Two-phase flow in a wet gas compressor

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Helico-axial pump (1.5 to 4.6 krpm)

Rotor SSV appears under some

two-phase flow conditions : low

differential pressure with a

high-viscosity mixture.

Bibet, P. J., et al., 2013, "Design and Verification Testing of a New Balance Piston for High Boost

Multiphase Pumps," Proc. 29th International Pump User Symposium, Houston, TX.

Bibet et al. (2013)

Two-phase flow in a Multiple Phase pump

Pump operates stable with liquid.

(600 cPoise)

When vibrations occur, seal whirl

frequency ratio > 1.0.

high amplitude

SSVs

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Childs and students (2012-2017)

Brief Review of Research on Wet Seals at TAMU

San Andrés and students (2014-present)

Measures leakage and rotordynamic force coefficients of wet seals with

an air in silicon oil mixture. GVF: 0 10%, LVF: 0 8%. Max pressure: 70

bar, shaft speed 20 krpm (RΩ=96 m/s)

J. Eng. Gas Turb. Power, 2017, v. 139

ASME J. Eng. Gas Turb. Power, 2018

ASME GT2017-63254 (San Andrés and Lu) S&D Best Paper Award

Quantifies leakage and dynamic force coefficients of wet seals [five types]

with an air in ISO VG10 oil mixture. GVF: 0 1, Max supply pressure: 5

bar, shaft speed: 5.2 krpm (RΩ=35 m/s).

Tribol. Trans., 2016 2018 ATPS/TPS

Application: Subsea multiphase pumps and wet gas

compressors

Overview of GT2018-75200

• Make oil-gas mixtures with inlet GVF = 0.0 0.9.

• Measure flow rate for range of GVF & pressure

supply/discharge = 1 3.5.

• Measure test system periodic forced response and perform

parameter identification.

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TYPICAL OPERATION RANGE FOR ELECTRICAL SUBMERSIBLE PUMP

• Vertical pumps (with plain bushings) operate w/o a radial

load, hence are prone to show SSVs.

• A three-lobe bearing generates a centering stiffness to

stabilize a pump.

Does a three-wave seal operate stably with either a

gas or a liquid or a mixture?

• Generates centering stiffness in (unloaded)

vertical pump.

• Easy to fabricate with low cost. 10

Three-wave seal (from Dimofte in early 1990’s)

Diameter D = 2R 127 mm

Length L 46 mm

Number of waves 3

Max clearance cmax 0.274 mm

Min clearance cmin 0.108 mm

Mean Clearance

cm= ½(cmin + cmax)0.191 ± 0.004 mm

Wave amplitude

ew = cmax - cm

0.083 mm

εw = ew/cm =0.43

Benefits

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Three-wave seal

Measure wall thickness ht

c = ½ OD - ht – ½ D

cm= 0.191 mm

D: shaft diameter

Design clearance Measured clearance

Test seal

Measure OD

Clearance:

Test Rig at TAMU

Turbo Lab

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Air InletOil Inlet

(ISO VG 10)

Test seal

section

Valve

Valve

Sparger

(mixing)

element

Wet seal test rig shaft speed: 3.5 krpm (23.3 m/s)

GVF at inlet:

/

/in

g a s

l g a s

Q P P

Q +Q P P

α : Gas volume fraction

Ps: pressure at seal inlet plane

Pa: ambient pressure= 1 bar(a)

Qg: gas flow rate at Ps

Ql: liquid flow rate

Supply pressure (Ps) 1.0~3.5 bar (abs)

Oil ISO VG 10

density(ρl) 830 kg/m3

viscosity (μl) 10.6 cP at 34 ºC

Air density (ρga) 1.2 kg/m3 at 1bar

viscosity(μga) 0.02 cP at 20 ºC

Section A-A

Stinger Y

Stinger X

Ω

Seal element

Journal

housing

Support rod

(90° apart, four)

Y

X0

Shaker Y

Shaker XSeal

Top lidSeal

housingSeal element

Load cell

Accelerometer

Displacement

sensor

Journal

Shaft

Shaker

stinger

Centering

bolts

Support

rod

Wet seal test rig

Stinger

Top journal speed, Ωmax 3.5 krpm

Rotor surface speed, ½DΩmax 23.3 m/s

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A A

Centering bolts

Leakage for uniform

clearance seals and wavy-seal

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#1 c1=0.203 mm

#2 c2=0.274 mm

Paper GT2017-63254

#3 cm=0.191 mm

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.00 0.20 0.40 0.60 0.80 1.00

Three-wave seal

(4 krpm)

Three wave seal

(0 rpm)

Plain seal-1, (0 rpm)

Cseal#1 = 0.203 mm; Cseal#2 = 0.274 mm; Cwavy-seal = 0.191 mm

Mass f

low

/ l

iqu

id m

ass f

low

Plain seal-2

(0 rpm)

Gas volume fraction at seal inlet

Leakage for all seals shows

same trend as GVF

increases it drops!Three-wave seal leaks a little

more.

Predictions agree with test

data.

Leakage (Mixture) gas volume fraction increases

mixture

liquid

mm

m

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Normalized with respect to liquid

(GFV=0)

Seal Dynamic Force

Coefficients

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#3

Three-wave seal cm=0.191 mm

Dynamic load tests

Excite test system

with periodic loads

and measure test

system forced

response and

perform parameter

identification.

room temperature 20oC.

journal speed: 3.5 krpm(RΩ: 23 m/s)

Inlet GVF: 0 to 0.9

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Seal reaction force is a function of the

fluid properties, flow regime, operating

conditions and geometry.

For small amplitudes of rotor

motion, the force is represented

with stiffness, damping and inertia

force coefficients:

X

Y

F K k x C c x M 0 x

F k K y c C y 0 M y

Dynamic force coefficients

X

Y

K k C cF x x

F k K y c C y

For two-

phase flow

or a gas

frequency dependent

EOM: Time Domain

' ')SEAL SEAL SEAL S S SK z+C z+M z = F-M a-(K z C z

Relative displacement Absolute acceleration Absolute displacement

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Model system (2-DOF): structure + SEAL

Measure:

Load F=Fo sin(t)

Displacement z,

acceleration a

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Structure (pipe) stiffness,

Ks = 690 kN/m

Structure damping,

Cs= 0.2 kN.s/m

Housing mass & seal

MBC = 7 kg

Test rig structure parameters

Dry system

natural frequency,

fn= 50 Hz

& damping ratio, ξ = 4.5 %

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Model system Dynamic Complex Stiffness (H)

'S S S F-M A- K + C z H ziω

( ) ( )

( ) ( )

Re

Im

H K

H C

Parallel to rotor center displacement

Parallel to rotor center velocity

EOM: Frequency Domain

Components of complex dynamic stiffness H

functions of frequency ().

Direct dynamic complex stiffness (MN/m)Ps/ Pa = 2.5.

3.5 krpmGVF = 0

GVF = 0.9

GVF=0: K is a parabolic

function of frequency.

GVF>0: K does not reduce

with frequency.

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GVF = 0.2

Frequency (Hz)

Frequency (Hz)

Frequency (Hz)

Cross coupled complex stiffness (MN/m)Ps/ Pa = 2.5.

3.5 krpm

Re(HXY) & - Re(HYX) decrease

with GVF. Peculiar dip at

ω Ω or ω ωn

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GVF = 0

GVF = 0.2

GVF = 0.9

Frequency (Hz)

Frequency (Hz)

Frequency (Hz)

Quadrature dynamic stiffness (MN/m)Ps/ Pa = 2.5

3.5 krpmGVF = 0 GVF = 0.2

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GVF = 0.9

Im(H)= C is proportional

to frequency.

Damping C decreases as GVF

increases.

Frequency (Hz)Frequency (Hz)

Frequency (Hz)

Effective damping (kN.s/m)

Ps/ Pa = 2.5.

3.5 krpm

Ceff =(C - k/ω)

Ceff reduces with increase in GVF.

Cross frequency: 0.46X→ 0.43X

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GVF = 0

GVF = 0.2

GVF = 0.9

Frequency (Hz)

Frequency (Hz)

Frequency (Hz)

Compare test results for plain

cylindrical seals and three-wave seal

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#1 & # 2

Plain sealsc1=0.203 mm, c2=0.274 mm (worn)

#3

Three-wave seal cm=0.191 mm

Large uniform clearance seal

emulates a seal worn condition

Ps/ Pa = 2.5 & 3.5 krpm

Direct dynamic stiffness K (MN/m)

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Symbols: test results Lines: predictions

Three wave seal (#3) shows largest

K (promotes static stability).

Worn seal (#2) shows lowest K.

K : soft to hard as GVF increases

lesser added mass!

Ps/ Pa = 2.5; 3.5 krpm

GVF = 0.9

GVF = 0.0

GVF = 0.2

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Direct damping coefficient C (kN.s/m)

Worn seal (#2) shows

smallest damping.

Three wave seal (#3) shows

largest damping.

C drops with GVF

C ~ Cpl (1-GVF)

Damping is frequency

independent

Symbols: test results

Ps/ Pa = 2.5; 3.5 krpm

For stability, Ceff >0 is a must.

Increase in GVF Ceff drops.

Cross frequency drops from ~

½ X to lower magnitude.

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Effective damping (kN.s/m) Ceff =C –k /ω

Symbols: test results Lines: predictions

Ps/ Pa = 2.5; 3.5 krpm

GVF = 0.0

GVF = 0.2

GVF = 0.9

Test data and model to become a reference for the design of seals in

electrical submersible pumps.

(a) Three wave seal leaks more than a plain seal but produces largest direct

stiffness K.

(b) Mass flow continuously drops with an increase in gas volume fraction (GVF).

(c) Force coefficients are frequency dependent for operation with gas/oil

mixture.

(d) Three wave seal cross –coupled stiffness k decreases with both frequency

and GVF.

(e) Direct damping C decreases with GVF C~Cl (1-GVF)

(f) Effective damping Ceff increases with frequency and drops with GVF. Cross

over frequency is ~ ½ X.

(g) Air injection produces a hard centering stiffness.

(h) Future work will focus on non-homogenous flow models for seals supplied

with air/liquid mixtures with large GVF conditions.

Conclusion

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Paper GT2018-75200

For more test results (6 seals) please read our ATPS/TPS papers

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Six test seals

Smooth surface

plain seal x 2:Nominal c and worn (>c)

Three-wave seal:Large dynamic stiffness

Groove seal: Typ for pumps (turbulent

flow)

Step clearance

seal x 2: Often used in water

turbines/pumps.

Plain seals #1 & 2:(c1= 0.203 mm, c2 = 0.274 mm)

#3

Three-wave seal(cm=0.191 mm)

#4

Grooved seal (cr=0.211 mm, dg=0.543 mm,

lg=1.5 mm, ll=0.904 mm,

Ng=14)

#5

Upstream step clearance (cT=0.164mm, cB=0.274 mm,

LT=0.11L).

#6

Downstream step

clearance (cT=0.274 mm, cB=0.164 mm,

LT=0.82L).

San Andrés, L., Lu, X., and Zhu, J., 2018, “On the Leakage and Rotordynamic Force Coefficients of Pump

Annular Seals Operating with Air/Oil Mixtures: Measurements and Predictions,” Proc. 2nd Asia

Turbomachinery & Pump Symposium, Singapore, Mar. 13-15.

Clearance c~0.2 mm

Acknowledgments

Turbomachinery Research Consortium

Questions (?)

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

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Backup slides

Flow visualization in

upstream pipe, seal

clearance and

downstream pipe

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D = 26 mm

Mixture in upstream pipe GVF = 0.5. Ps =2.5 bara

Mixture in seal inlet GVF = 0-0.9. Ps/Pa=2.5. 0 rpm

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Mixture in downstream pipe