Understanding dynamic response

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Understanding Dynamic Response of a simple structure. This applies to conventional turbo-machinery diagnostics and understanding the sychronous vibration response through the full speed range.

Understanding Dynamic Response of a simple structure. This applies to conventional turbo-machinery diagnostics and understanding the sychronous vibration response through the full speed range.

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  • 1. GE Oil & Gas Understanding Dynamic Response Extract from Bently Nevada Machinery Diagnostics Training
  • 2. Transient Vibration Data Formats • Amplitude and Phase displayed together • Slow roll runout vector • Heavy/high spot location • Rotor and structural resonances • Rotor mode shape “1st critical” Bode’ and Polar Plots
  • 3. Bode' Plot
  • 4. Polar Plot •Typical “synchronous rotor response”. Phase lag angle increases with machine speed. Amplitude increases to a max. at “critical” speed, then reduces
  • 5. Dynamic Stiffness – Simple Model K D M Dynamic Unbalance Force F(t) = m.r. W2 = U. W2 DampingSpring Mass Displacement From rest d (t) Equation of Motion: dMdDdKFt  )( ti t ed W )(let: dieid ti WW W deid ti WW W 222 then: and: dMdDidKF WW 2    DiMK d F WW 2 Simple Dynamic Stiffness: Direct displacement accelerationvelocity Quadrature
  • 6. Dynamic Stiffness at Low Speed K Synchronous Dynamic Stiffness f = 20 D.W M.W2 At low speeds, dominant factor is “Spring Stiffness”
  • 7. Dynamic Stiffness at Resonance K Synchronou s Dynamic Stiffness f = 90 D.W M.W2 At resonance, dominant factor is “Damping Stiffness” This is also known as “Quadrature Stiffness” 2 W MK At Resonance: M K res W Resonant Frequency:
  • 8. Dynamic Stiffness at High Speed K Synchronous Dynamic Stiffness f = 150 D.W M.W2 At high speeds, dominant factor is “Mass Stiffness” – i.e: Inertial effect
  • 9. Synchronous Vibration Response – Low Speed M.W2 K D.W At low speeds, displacement is in same direction as unbalance force Unbalance Unbalance Force = U.W2 Dynamic response (displacement) = Force / Dynamic Stiffness Displacement
  • 10. M.W2 K D.W At resonance, displacement has a 90 degree phase lag from unbalance force Unbalance Force = U.W2 Dynamic response (displacement) = Force / Dynamic Stiffness Displacement f = 90 Synchronous Vibration Response – Resonance Unbalance
  • 11. M.W2 K D.W At high speeds, displacement vector is almost opposite the unbalance force Unbalance Force = U.W2 Locus of displacement vectors through the whole speed range Dynamic response (displacement) = Force / Dynamic Stiffness Displacement Synchronous Vibn Response – High Speed Unbalance
  • 12. 1X (Synchronous) Response 0° 90° 180° 270° Probe Heavy Spot WR = System Resonance Frequency W WWR High Spot Angle of Heavy Spot fu f 0 90° 180° PhaseLag 1XAmplitude|A| A
  • 13. Transient Vibration Analysis • Reveals information on whether Dynamic stiffness has changed from run to run. Confirms integrity of the rotor. • Shows symptoms caused by increased system stiffness – like seal rubs • Indicates where we should add our trial balance weights • Gives clues about why the machine behaves this way – ie: might be running near a natural resonance…
  • 14. Questions?