Proceedings of the 6th Asia International Conference on Tribology, pp. 1-2, September 2018

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Development of gas foil thrust bearings for high-speed cryogenic turboexpander

Suraj K Behera1,*, Ranjit Kumar Sahoo1

1) Faculty of Mechanical Engineering, National Institute of Technology, Rourkela, India, Rourkela, Sundergarh, Odisha, India-769008

*Corresponding e-mail: beherask@nitrkl.ac.in Keywords: Gas foil bearings; Cryogenic; Turboexpander

ABSTRACT – Cryogenic turboexpander is considered; as the heart of modern gas liquefier for its high thermodynamic efficiency and high reliability. The operating speed of small-sized turboexpander is usually greater than 50,000 rpm. Such high rotational speed brings constrain in the selection of the bearings. Oil-free gas bearings is a novel approach to keep the process gas contamination free. In the current research, a pair of gas foil thrust bearings (GFTB) are designed and developed for a vertical turboexpander used in nitrogen liquefier. The rotational speed of the rotor is 80,000 rpm with an axial load of 83 N in the direction of turbine wheel. The author believes the detailed design and development methodology will be helpful to the bearings designer around the globe. 1. INTRODUCTION

The pressure differential acting upon the compressor wheel and turbine wheel generate thrust load in a cryogenic turboexpander. Also due to the impulsive force generated because of axial flow on these wheels [1]. Above axial load need to be supported by oil-free gas thrust bearings to avoid contamination of the process gas. Compliant type gas bearings such as gas foil thrust bearings (Fig 1) has many attractive advantages compared to the rigid bearings such as compensation for misalignment, accommodation of thermal distortion, larger clearance, tailored damping, etc. [2]. This paper explains about design and development methodology for a pair of bump-type GFTB for a vertical turboexpander used in nitrogen liquefier. This paper presents an successful replacement of a pair of aerostatic thrust bearing for a pair of bump-type GFTB in a cryogenic turboexpander. The radial bearings for current application is tilting-pad journal bearings.

The design and development of GFTB in current research work is started with the calculation of axial load due to pressure differential [1]. The work is further extended to the determination of foil bearings parameters based on combined structural and aerodynamic analysis [2] and design of dies for forming the bumps [3]. Finally, assembly of foil bearings is tested in the turboexpander test facility at NIT Rourkela, India.

Figure 1: Schematic of bump type GFTB.

2. AERODYNAMIC ANALYSIS OF GFTB The thrust load calculated in the current research is

based on Newton’s 2nd law [1]. The resultant thrust load is 83 N (in the direction of the turbine). For the safe design, the load capacity is multiplied by a factor of 1.5 (120 N).

The GFTB is an aerodynamic or self-acting bearing and operates on the principle of aerodynamics [2]. The popular and easy method to analyse gas foil thrust bearings is by solving aerodynamic and structural equation simultaneously based on pioneered work by Heshmat et al. [2]. The dimensionless Reynolds equation is expressed in Eqn. (1)[2].

Figure 2 Dimensional nomenclature of GFTB.

( ) ( )3 32

1 1 ... 1php prh p h p

r r r r θ θ θ

∂∂ ∂ ∂ ∂ + = Λ ∂ ∂ ∂ ∂ ∂ The Finite Difference Method (FDM) is used to

solve the Eqn. (1), to determine bump material and its dimensions. The simulation is performed for the below design parameters :

(a) Load carrying capacity for various feasible foil materials such as Inconel X-750, Phosphor Bronze, SS 302 and Beryllium Bronze.

(b) Load carrying capacity for various foil thickness, bump pitch and bump length.

(c) A load analysis for the upper and lower foil bearings.

The foil bearing parameters for current application is selected based on literature studies and numerical analysis is given in Table 1.

Table1: Selected bearings parameters Bearing parameters Dimensions Inner radius ( R1) 10 mm Outer radius ( R2) 22 mm Rotational Speed ( N ) 80,000 RPM Angular extent (β) 900 Ratio of angular extent (b) 0.6 Viscosity of gas 178.4×10-7 N.s/m2 Bump Foil Young’s Modulus ( E ) 114 GPa ( Phosphor

Bronze) Bump Foil Poisson’s Ratio (ν) 0.29 Top Foil Thickness ( tt) 0.1 mm Bump Foil Thickness ( tb) 0.1 mm Bump Length ( 2l0 ) 2.5 mm Max bump Pitch ( s ) 3.17 mm

The pressure distribution over the lower bearing is shown in Fig. 3. The load carrying capacity is simulated for upper and lower bearings to determine the necessary

Bump Foil

Top Foil

Bearing Base

Attachment Mechanism

Behera et al., 2018

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spacing required between twin bearings. A minimum film thickness of 20 µm is maintained for the lower bearing. The simulation shows (Fig. 4) that the load balancing equation satisfied for total bearing clearance of 56 µm. Therefore, the film thickness for upper bearing at the designed speed of the rotor will be 46 µm.

Figure 3: Pressure profile over lower bearing surface

Figure 4: Load distribution over lower and upper thrust

bearing. 3. DIES FOR BUMP FORMATION

A cryogenic turboexpander is considered to be a precision machine. The fabrication methodology of bumps is an important part of the development process.

Figure 5 Displacement for bumps during forming

The upper and lower dies for bumps are designed based on basic die design principle [4]. The designed die is simulated in DEFORM 3D to study the process of forming operation such as (a) displacement of bump foil, (b) Stress during forming operation, (c) spring back on load removal,(d) possible damage during forming. Fig 5 shows the displacement of the bump foil during forming operation.

Figure 6 Top and bottom die for bump formation

a. b. Figure 7 a. Bump foil b. Assembly of foils with bearing base

The die dimension is modified to avoid cracks and minimize spring back during formation operation. The fabricated die is shown in Fig 6. The formed bump and assembled bump foils are shown in Figs 7a and 7b respectively. 4. VIBRATION ANALYSIS OF PROTOTYPE

TURBOEXPANDER A detailed performance analysis is carried out to study the behavior of the turboexpander at its stable operation. The signal from the accelerometer is recorded on a storage type oscilloscope during test runs.

Figure 8 Vibration of bearing housing at upper journal bearing

(near tilting pad journal bearing) The obtained signals are converted to acceleration and FFT of the vibrational spectrum at 81,000 RPM is shown in Fig. 8 near the upper journal bearing. The vibrational spectrum near upper journal bearing is compared with the results of vibration analysis using aerostatic bearings in one of the earlier studies at NIT Rourkela [4]. The comparison prevails 30% reduction of the vibration level near upper journal bearing. No comparison could be made with the vibration level near lower journal bearing because of the absence of data in the previous development program. The detailed vibration analysis shows an enhanced rotor stability. 5. CONCLUSIONS The work presented in this paper is a modest attempt to implement the gas foil thrust bearings to a high-speed cryogenic turboexpander. The design methodology for bump foil is explained with simplified steps. The FEM analysis of forming process using commercial software (DEFORM 3D) is explained, which simplifies the die design process. Finally, from the vibration signature, the level vibration is found to be reduced significantly near the upper journal bearing. The central objective of above research work is to suggest a structured design and fabrication methodology for bump type gas foil thrust bearings through the theoretical and experimental studies. REFERENCES [1] Nguyen-Schäfer, H., (2015), Rotordynamics of

automotive turbochargers, Springer. [2] Heshmat, H., Walowit, J., and Pinkus, O., (1983),

"Analysis of gas-lubricated compliant thrust bearings," ASME J. Lubr. Technol, 105(4),638-646.

[3] Dykas, B., Bruckner, R., DellaCorte, C., Edmonds, B., and Prahl, J., (2009), "Design, fabrication, and performance of foil gas thrust bearings for microturbomachinery applications," Journal of Engineering for Gas Turbines and Power, 131(1), 012301.

[4] Choudhury, B. K., (2013), "Design and construction of turboexpander based nitrogen liquefier," Ph. D. dissertation, NIT Rourkela.

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