EFFECT OF HYDRODYNAMIC THRUST BEARINGS ON

ROTORDYNAMICSISROMAC 12

Honolulu, Hawaii February 19, 2008

Joel V. MadisonC.E.O.

Ebara International CorporationReno, NV

Overview• Cause for investigation• Pump model• Thrust balancing device (TEM)• Performance testing results• Post-testing machine examination• TEM Wobble• Calculations• Results and analysis• Additional Analysis• Conclusions drawn• Questions

Cause for Investigation

• During performance testing of an Ebara International high pressure pump, the machine exhibited increased vibration levels at 50 Hz

• Examination of straightness and rotating balances checked and verified

• Concentricity of casing and close clearances checked and verified

• FFT recorded for the TEM probe• Found large synchronous vibration at operating

speed of 50 Hz

Cause for Investigation

• Large synchronous vibration at the operating speed led to examination of the TEM components

• Examination of the machine found the TEM impeller situated at an angle

• Will the tilt of the TEM affect the rotordynamic performance?

Cause for Investigation

High Pressure Pump

• EIC model # 4ECC-1513• 13 stage high pressure

pump• Rated Flow: 147 m3/hr• Rated Head: 2185 m• Operating liquid: LNG• Operating temperature:

-159oC

Thrust Balancing Device (TEM)

TEM and shaft assembly at maximum variable orifice gap

TEM and shaft assembly at minimum

variable orifice gap

Performance Testing Results• High vibration levels at synchronous

speed of 50 Hz• Velocity vs. frequency response was

recorded during testing via accelerometer mounted on motor casing

• Radial velocity traces recorded at the middle bearing

• Velocity response recorded for 5 different flow rate points

• Maximum velocity of 2.5 mm/s at 50 Hz regardless of flow rate

Velocity Response

Accelerometer vs. Frequency at a Flow Rate of 99.98 m3/s

Velocity Response

Accelerometer vs. Frequency at a Flow Rate of 126.76 m3/s

Velocity Response

Accelerometer vs. Frequency at a Flow Rate of 142.52 m3/s

Velocity Response

Accelerometer vs. Frequency at a Flow Rate of 178.20 m3/s

Velocity Response

Accelerometer vs. Frequency at a Flow Rate of 203.62 m3/s

TEM Examination Findings• Running clearances and rotating

assembly was dismantled and examined

• During inspection of the TEM impeller it was found that the back shroud was not machined within tolerance

• TEM impeller back shroud was situated at an angle to the shaft

Required Geometric Relation• TEM impeller is required to maintain

perpendicularity between hub and back shroud within 0.025 mm

• Asymmetry of 0.711 mm found along impeller back shroud and thrust plate face

TEM Impeller WobbleAn angle between the TEM impeller and the shaft

causes an asymmetric pressure distribution resulting in TEM impeller wobble during operation.

CalculationsAssumptions:• TEM impeller was situated at an angle with

respect to shaft and stationary thrust plate• Running clearance varied across radial axis

between thrust place and TEM impeller• Non-uniform pressure distribution resulted• Asymmetric pressure distribution can be

modeled as a single un-balanced force acting on the TEM impeller back shroud

• Un-balanced force causes a bending moment acting on the shaft

Calculations

• Thrust calculations indicate that the force is 3692 N and 7.94 cm from the rotating axis

• Since the impeller is a rotating part, the moment depends on the rotational direction and speed

• The pump speed is a constant 50 Hz :. the direction of the moment will be opposite every 1/100 seconds

)2sin(*)( tMtM πω=

Calculations

Pump model with applied moment

Calculations• The rotordynamic behavior was investigated

under 5%, 10%, 20%, 50%, and 100% of the thrust

• Transient analyses were performed for a duration of 0.1s with increments of 0.001s

• The system also investigated under zero moment condition

• Un-balanced mass was included• Transient response was plotted for all cases to

investigate the velocity as a function of frequency at the middle location of the hydraulic side of the pump where max displacement will occur

Applied moment as a function of time for all cases

Applied Moment ResultsApplied Moment at Station # 69 of Shaft Model

-400.00

-300.00

-200.00

-100.00

0.00

100.00

200.00

300.00

400.00

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1

Time [sec]

Mom

ent [

N.m

]

M(t) 5% [N.m]M(t) 10% [N.m]M(t) 20% [N.m]M(t) 50% [N.m]M(t) 100% [N.m]

Transient Response: Zero MomentZero Applied Moment:• Max velocity 0.63 mm/s at 50 Hz• Due to un-balanced mass

Velocity Transient Response

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 100 200 300 400 500 600

Frequency, Hz

Velo

city

, mm

/s

[No Moment]

Transient Response: 5% & 10%

5% & 10% Applied Moment:

• Slight increase of system velocity

• Excitation due to non-uniform pressure has no effect over un-balanced excitation

Velocity Transient Response

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 100 200 300 400 500 600

Frequency, Hz

Velo

city

, mm

/s

Applied Moment: %5

Velocity Transient Response

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 100 200 300 400 500 600

Frequency, Hz

Velo

city

, mm

/s

Applied Moment: 10%

Transient Response: Zero Moment20% Applied Moment:• Max velocity 0.63 mm/s at 50 Hz• Due to un-balanced mass

Velocity Transient Response

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 100 200 300 400 500 600

Frequency, Hz

Velo

city

,mm

/s

Applied Moment: 20%

Transient Response: 5% & 10%50% Applied Moment

• Velocity twice the zero moment case

• System unstable at 50 Hz• Vibration level of system

significantly increased by the moment

100% Applied Moment• Increase system velocity buy

4 times the zero moment case

• Max velocity 2.4 mm/s at 50 Hz

Velocity Transient Response

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 100 200 300 400 500 600

Frequency, Hz

Velo

city

, mm

/s

Applied Moment: 50%

Velocity Transient Response

0

0.5

1

1.5

2

2.5

0 100 200 300 400 500 600

Frequency, Hz

Velo

city

, mm

/s

Applied Moment: 100%

Transient Response ResultsVelocity Transient Response

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

0 50 100 150 200 250 300 350 400 450 500

Frequency, Hz

Velo

city

, mm

/s

Applied Moment 100%

Applied Moment 50%

Applied Moment 20%

Applied Moment 10%

Applied Moment 5%

Applied Moment 0%

Transient response for all cases

Additional Analysis

Deflected shape of the shaft at operating speed

Shaft Deflected Shape at Operating Speed of 3000 RPM

0.000

0.025

0.050

0.075

0.100

0.125

0.150

0.175

0.200

0.225

0.250

0.275

0.300

0.325

0.350

0.375

0.400

0.425

0.450

0.475

0.500

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Shaft Axial Location [in]

Dis

plac

emen

t [m

ils]

Applied Moment 0%Applied Moment 5%Applied Moment 10%Applied Moment 20%Applied Moment 50%Applied Moment 100%

Loca

tion

of th

e M

omen

t

Additional Analysis

Effect of moment on the critical speed

Rotordynamic Response Plot

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

5.50

6.00

6.50

7.00

7.50

0 1000 2000 3000 4000 5000 6000 7000 8000 9000

Rotor Speed [RPM]

Dis

plac

emen

t [m

ils]

Applied Moment 100%Applied Moment 50%Applied Moment 20%Applied Moment 10%Applied Moment 5%Applied Moment 0%

Conclusions• A small magnitude of moment due to

asymmetric clearances between the TEM impeller and thrust plate have a minimal effect on the rotordynamics

• Increased magnitude of moment results in a resonant effect due to synchronous rotational speed and asymmetric pressure distribution

• A non-uniform pressure distribution across the TEM impeller and asymmetric clearances may result in instability of rotating equipment during operation

• The critical speed is not affected by the moment

Questions?