LEAKAGE AND ROTORDYNAMIC FORCE COEFFICIENTS OF A THREE-WAVE (AIR IN OIL) WET ANNULAR SEAL: MEASUREMENTS AND PREDICTIONS

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1 Proceedings of ASME Turbo Expo 2018: Turbomachinery Technical Conference and Exposition, June 11-15, 2018, Oslo, Norway Paper GT LEAKAGE AND ROTORDYNAMIC FORCE COEFFICIENTS OF A THREE-WAVE (AIR IN OIL) WET ANNULAR SEAL: MEASUREMENTS AND PREDICTIONS Xueliang Lu Graduate Research Assistant Texas A&M University Luis San Andrés Mast-Childs Chair Professor Fellow ASME Funded by Turbomachinery Research Consortium accepted for journal publication 1

2 Annular Pressure Seals 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. Inter-stage seal Impeller eye or neck ring seal Balance piston seal Seals in a Multistage Centrifugal Pump or Compressor 2

3 Justification subsea facilities 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% 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. 3

4 Prior Work Apps 4

5 Two-phase flow in a wet gas compressor Rotor lateral vibration Balance piston: Labyrinth seal Fluids: Air and water LVF: 0~3% 13.5 krpm, 10 bar 1 X 0.45 X SSV increases in magnitude with LVF Trapped liquid in seal rotates with great momentum and causes 0.45X vibes X 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,

6 Two-phase flow in a wet gas compressor Rotor lateral vibration Balance piston: Pocket damper seal (PDS) 13.5 krpm, 10 bar 1 X 0.45 X SSV reduces from 20 μm to a few microns X Liquid does not accumulate in PDS, thus mitigating SSV. 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,

7 Two-phase flow in a Multiple Phase pump Helico-axial pump (1.5 to 4.6 krpm) Pump operates stable with liquid. (600 cpoise) Rotor SSV appears under some two-phase flow conditions : low differential pressure with a high-viscosity mixture. high amplitude SSVs Bibet et al. (2013) When vibrations occur, seal whirl frequency ratio > 1.0. Bibet, P. J., et al., 2013, "Design and Verification Testing of a New Balance Piston for High Boost Multiphase Pumps," Proc. 29 th International Pump User Symposium, Houston, TX. 7

8 Brief Review of Research on Wet Seals at TAMU Childs and students ( ) J. Eng. Gas Turb. Power, 2017, v. 139 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) San Andrés and students (2014-present) Tribol. Trans., 2016 ASME J. Eng. Gas Turb. Power, ATPS/TPS ASME GT (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). Application: Subsea multiphase pumps and wet gas compressors 8

9 Overview of GT 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? Make oil-gas mixtures with inlet GVF = Measure flow rate for range of GVF & pressure supply/discharge = Measure test system periodic forced response and perform parameter identification. TYPICAL OPERATION RANGE FOR ELECTRICAL SUBMERSIBLE PUMP 9

10 Three-wave seal (from Dimofte in early 1990 s) Diameter D = 2R Length L Number of waves 3 Max clearance c max Min clearance c min Mean Clearance c m = ½(c min + c max ) Wave amplitude e w = c max - c m 127 mm 46 mm mm mm ± mm mm ε w = e w /c m =0.43 Benefits Generates centering stiffness in (unloaded) vertical pump. Easy to fabricate with low cost. 10

11 Three-wave seal Measure OD Measure wall thickness h t Clearance: c = ½ OD - h t ½ D D: shaft diameter Test seal c m = mm Design clearance Measured clearance 11

12 Test Rig at TAMU Turbo Lab 12

13 Wet seal test rig shaft speed: 3.5 krpm (23.3 m/s) Oil Inlet (ISO VG 10) Valve Air Inlet Valve Sparger (mixing) element Test seal section α : Gas volume fraction P s : pressure at seal inlet plane P a : ambient pressure= 1 bar(a) GVF at inlet: Q g : gas flow rate at P s Q l : liquid flow rate in Q P / P g a s Q +Q P / P l g a s Supply pressure (P s ) 1.0~3.5 bar (abs) Oil ISO VG 10 density(ρ l ) 830 kg/m 3 viscosity (μ l ) 10.6 cp at 34 ºC Air density (ρ ga ) 1.2 kg/m 3 at 1bar viscosity(μ ga ) 0.02 cp at 20 ºC13

14 Wet seal test rig Shaker Y Top journal speed, Ω max Rotor surface speed, ½DΩ max 3.5 krpm 23.3 m/s Stinger Seal Top lid housing Seal element Seal Centering bolts Stinger Y Y 0 X Journal Ω Shaker X Seal element Stinger X housing Support rod A Centering bolts Journal Shaft A Load cell Shaker stinger Accelerometer Displacement sensor Section A-A Support rod (90 apart, four) 14

15 Leakage for uniform clearance seals and wavy-seal #1 c 1 =0.203 mm Paper GT #2 c 2 =0.274 mm #3 c m =0.191 mm 12

16 Mass flow / liquid mass flow Leakage (Mixture) gas volume fraction increases Normalized with respect to liquid (GFV=0) m m m mixture liquid Leakage for all seals shows same trend as GVF increases it drops! Three-wave seal leaks a little more Three-wave seal (4 krpm) Plain seal-1, (0 rpm) Predictions agree with test data Plain seal-2 (0 rpm) 0.2 Three wave seal (0 rpm) C seal#1 = mm; C seal#2 = mm; C wavy-seal = mm Gas volume fraction at seal inlet 16

17 Seal Dynamic Force Coefficients #3 Three-wave seal c m =0.191 mm 17

18 Dynamic load tests Excite test system with periodic loads and measure test system forced response and perform parameter identification. Inlet GVF: 0 to 0.9 room temperature 20 o C. journal speed: 3.5 krpm (RΩ: 23 m/s) 18

19 Dynamic force coefficients 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: F K k x C c x M 0 x F k K y c C y 0 M y Y X F K k x C c x F k K y c C y X Y For twophase flow or a gas frequency dependent 19

20 Model system (2-DOF): structure + SEAL EOM: Time Domain K z+c z+m z = F-M a-(k z ' C z SEAL SEAL SEAL S S S ') Relative displacement Absolute acceleration Absolute displacement Measure: Load F=Fo sin( t) Displacement z, acceleration a 20

21 Test rig structure parameters Structure (pipe) stiffness, K s = 690 kn/m Structure damping, C s = 0.2 kn.s/m Housing mass & seal M BC = 7 kg Dry system natural frequency, f n = 50 Hz & damping ratio, ξ = 4.5 % 21

22 Model system Dynamic Complex Stiffness (H) EOM: Frequency Domain iω ' F-M A- K + C z H z S S S Components of complex dynamic stiffness H Re Im H H ( ) ( ) K C ( ) ( ) functions of frequency ( ). Parallel to rotor center displacement Parallel to rotor center velocity 22

23 Direct dynamic complex stiffness (MN/m) GVF = 0 GVF = 0.2 P s / P a = krpm Frequency (Hz) GVF = 0.9 Frequency (Hz) GVF=0: K is a parabolic function of frequency. GVF>0: K does not reduce with frequency. Frequency (Hz) 23

24 Cross coupled complex stiffness (MN/m) GVF = 0 GVF = 0.2 P s / P a = krpm Frequency (Hz) GVF = 0.9 Frequency (Hz) Re(H XY ) & - Re(H YX ) decrease with GVF. Peculiar dip at ω Ω or ω ω n Frequency (Hz) 24

25 Quadrature dynamic stiffness (MN/m) GVF = 0 GVF = 0.2 P s / P a = krpm GVF = 0.9 Frequency (Hz) Frequency (Hz) Im(H)= C is proportional to frequency. Damping C decreases as GVF increases. Frequency (Hz) 25

26 Effective damping (kn.s/m) GVF = 0 GVF = 0.2 C eff =(C - k/ω) P s / P a = krpm Frequency (Hz) GVF = 0.9 Frequency (Hz) C eff reduces with increase in GVF. Cross frequency: 0.46X 0.43X Frequency (Hz) 26

27 Compare test results for plain cylindrical seals and three-wave seal P s / P a = 2.5 & 3.5 krpm #1 & # 2 Plain seals c 1 =0.203 mm, c 2 =0.274 mm (worn) #3 Three-wave seal c m =0.191 mm Large uniform clearance seal emulates a seal worn condition 27

28 Direct dynamic stiffness K (MN/m) GVF = 0.0 Symbols: test results Lines: predictions GVF = 0.2 GVF = 0.9 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! P s / P a = 2.5; 3.5 krpm 28

29 Direct damping coefficient C (kn.s/m) Symbols: test results Damping is frequency independent Three wave seal (#3) shows largest damping. Worn seal (#2) shows smallest damping. C drops with GVF C ~ C pl (1-GVF) P s / P a = 2.5; 3.5 krpm 29

30 Effective damping (kn.s/m) C eff =C k /ω Symbols: test results Lines: predictions GVF = 0.0 GVF = 0.2 GVF = 0.9 For stability, C eff >0 is a must. Increase in GVF C eff drops. Cross frequency drops from ~ ½ X to lower magnitude. P s / P a = 2.5; 3.5 krpm 30

31 Conclusion Paper GT (a) (b) (c) (d) (e) (f) (g) (h) Three wave seal leaks more than a plain seal but produces largest direct stiffness K. Mass flow continuously drops with an increase in gas volume fraction (GVF). Force coefficients are frequency dependent for operation with gas/oil mixture. Three wave seal cross coupled stiffness k decreases with both frequency and GVF. Direct damping C decreases with GVF C~C l (1-GVF) Effective damping C eff increases with frequency and drops with GVF. Cross over frequency is ~ ½ X. Air injection produces a hard centering stiffness. Future work will focus on non-homogenous flow models for seals supplied with air/liquid mixtures with large GVF conditions. Test data and model to become a reference for the design of seals in electrical submersible pumps. For more test results (6 seals) please read our ATPS/TPS papers 31

32 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) Clearance c~0.2 mm Plain seals #1 & 2: (c 1 = mm, c 2 = mm) #3 Three-wave seal (c m =0.191 mm) #4 Grooved seal (c r =0.211 mm, d g =0.543 mm, l g =1.5 mm, l l =0.904 mm, N g =14) #5 Upstream step clearance (c T =0.164mm, c B =0.274 mm, L T =0.11L). #6 Downstream step clearance (c T =0.274 mm, c B =0.164 mm, L T =0.82L). Step clearance seal x 2: Often used in water turbines/pumps. 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. 2 nd Asia Turbomachinery & Pump Symposium, Singapore, Mar

33 Acknowledgments Turbomachinery Research Consortium Questions (?) Learn more at 33

34 Backup slides Flow visualization in upstream pipe, seal clearance and downstream pipe 34

35 Mixture in upstream pipe GVF = 0.5. Ps =2.5 bara D = 26 mm 35

36 Mixture in seal inlet GVF = Ps/Pa= rpm 36

37 Mixture in downstream pipe 37