NOVEL CARBON-GRAPHITE GAS BEARINGS FOR TURBOMACHINERY

Transcription

1 May 2018 NOVEL CARBON-GRAPHITE GAS BEARINGS FOR TURBOMACHINERY Luis San Andrés Mast-Childs Chair Professor

2 Porous Type Gas Bearings Porous type gas bushing pads Porous type gas bearings (PTGB) have sub-micron sized holes distributed in their matrix material, allowing for an even distribution of gas flow over the entire bearing surface. Porous type thrust bearings When compared to orifice restricted hydrostatic gas bearings, PTGBs have higher stiffness & damping coefficients. 2

3 Justification Gas film bearings are effective means to support high speed rotors w/o complicated lubrication systems. Tilting pad bearings (TPJBs) with external pressurization permit operation with minute air gaps, offer high stiffness for accurate positioning, operate with little drag, and provide enhanced stability. A porous gas bearing (PGB) produces a more uniform film distribution than orifice-fed gas bearings. 3

4 Five-pad porous type tilting-pad bearing Shaft rotation A Pressurized air supply into pad Bearing cartridge Section A-A Rotor Porous Pads Ω Ф Load Pad cartridge Thermocouples A Porous pad Five-Pad PGBs Axial Length, L b 73.1 mm Width, w 50.8 mm Radius, R b 50 mm Arc Length, θ p 60º Height, H 25.8 mm Pivot Offset 50% Pad preload 0 4

5 Porous gas bearing test rig Rotor weight=29 kg Load on bearings: 19.5 kpa (2.8 psi) In-kind support from NewWay Bearings 5

6 2017 Rotor speed coast down (RΩ 0 =94 m/s) t > 14.5 min (870 s) friction free operation Minute drag friction coefficient (f) while rotor is airborne. Estimated film thickness (h) is large. P s bar f [-] h µm

7 2017 Imbalance response tests Added imbalance (e = 54 mm) m u = m e/m m 1 = 2.4 g u 1 = 4.45 g mm/kg m 2 = 3.2 g u 2 = 5.93 m 3 = 4.0 g u 3 = 7.42 (ISO ) for rigid rotor at 18 krpm u G=2.5 = 1.3 g mm/kg As supply pressure P s increases: Critical speed krpm. Peak response amplitude decreases. Peak response ~ imbalance Large damping ratio ~ 12.5% 7

8 Funded TRC ( ) Extend gas-tpjb code model porous media bearings NOT done Prior work likely in error. No evidence of useful predictive code left. Characterize pad lift force vs supply pressure and measure opening gap. In progress but not reported. Analysis incomplete. Revamp rotor-bearing system with FLEXIBLE ROTOR NOT started. Conduct rotor speed coast down and response tests Student let go (fired). Remaining funds kept for incoming student Fall imbalance 8

9 New Proposal MEASUREMENTS OF THE STATIC AND DYNAMIC LOAD CAPACITY OF A POROUS CARBON-GRAPHITE, TILTING PAD AIR BEARING FOR OIL-FREE COMPRESSORS Objective: Characterize load capacity and force coefficients of a large size (101 mm, 4 ) tilting pad gas bearing 9

10 Test Rig (Front View) Maximum rotor speed Maximum static load Maximum torque 16,000 RPM 20 kn 100 Nm Available shaft OD size 3.5, 4, 4.5 Maximum allowable bearing length

11 Test Rig (Iso and Side Views) Shaker heads X &Y Static load Test rig revamped in 2017 with new air turbine and a digital torquemeter. Test rig has a rich history of tests performed on oil seals, oil seal rings and oil-lubricated tilting pad bearings for compressors. 11

12 Test Carbon-Graphite TPJB 4 four-pads bearing and custom housing designed to fit seamlessly in test rig (L/D=0.60, 60 o pads and 50% offset) Critical design is spring support system for pivoted pads pads lift off supply pressure and shaft speed and bearing load capacity. Expected very low drag torque will demand of an alternate drive motor. One VFD available, max. 18 krpm [95 m/s]. 12

13 Tasks (a) Design and manufacture bearing and custom housing (NewBearings collaboration) (b)in ad-hoc jig measure pad pivot stiffness. Record pad lift off pressure as function of supply pressure and spring preload. (c) Assemble bearing and measure nominal opening clearance as a function of supply pressure (100 psi max). (d)for max. air pressure, turn rotor at 6, 9 and 12 krpm (64 m/s), and apply static (max.1 klbf) [Specific load ~ 100 psi]. Apply dynamic loads and deliver bearing stiffness and damping coefficients.

14 Budget Support for graduate student (20 h/week) x $ 2,200 x 12 months $ 26,400 Fringe benefits (2.4%) and medical insurance ($422) x 12 months $ 5,698 Tuition & fees three semesters (24 ch) $ 13,275 Supplies: bearing parts and rig ancillary parts $ 4,627 Total BUDGET $ 50,000 Experimental program will deliver (independent) test results demonstrating the applicability of PGBS, tilting type, to high speed high load rotating machinery.

15 Acknowledgments Thanks NewWay Air Bearings for their continued support. Questions (?) 13