Understanding Startup Vibration in Rotating Machinery

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Portable balancer & Vibration analyzer Balanset-1A

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Vibration sensor

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Balanset-4

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Magnetic Stand Insize-60-kgf

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Reflective tape

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Dynamic balancer “Balanset-1A” OEM

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Definition: What is Startup Vibration?

Startup vibration refers to the vibration characteristics and behavior of rotating machinery during the acceleration period from rest to normal operating speed. This encompasses both the expected transient vibration as the machine passes through critical speeds and any abnormal vibration phenomena that may occur specifically during the startup phase, such as thermal bow, bearing instabilities, or mechanical settling.

Understanding and monitoring startup vibration is critical for safe machine operation, as many vibration-related problems manifest most clearly during startups, and the startup transient can be the most mechanically stressful period of operation.

Typical Startup Vibration Characteristics

Normal Startup Vibration Progression

In a properly functioning machine, vibration during startup follows a predictable pattern:

Initial Phase (0-20% Speed)

• Very low vibration from unbalance (force proportional to speed²)
• Any significant vibration indicates mechanical problems or thermal bow
• Slow roll vibration provides baseline for mechanical condition

Acceleration Through Critical Speeds

• Vibration amplitude increases as each critical speed is approached
• Peak amplitude at the critical speed (resonance)
• Rapid decrease as speed continues past critical speed
• 180° phase shift through each critical speed
• Multiple peaks if multiple critical speeds exist below operating speed

Approach to Operating Speed

• Vibration settles to steady-state level
• Primarily 1× component from residual unbalance
• Thermal stabilization may cause gradual changes over first 30-60 minutes

Common Startup Vibration Problems

1. Thermal Bow

Thermal bow is the most common startup-specific vibration issue:

• Symptom: High vibration during initial acceleration, gradually decreasing as machine warms up
• Cause: Asymmetric heating creating temporary shaft curvature
• Frequency: 1× synchronous
• Behavior: High even at slow roll speeds; decreases as thermal equilibrium reached
• Solution: Extended warm-up procedures, turning gear operation

2. Excessive Critical Speed Vibration

• Symptom: Very high vibration peaks when passing through critical speeds
• Causes: Poor damping, high unbalance, operating too close to critical speed
• Risk: Potential damage to bearings, seals during each startup
• Solution: Improve balancing, increase acceleration rate through critical zones, add damping

3. Rub During Acceleration

• Symptom: Sudden erratic vibration and sub-synchronous components
• Cause: Insufficient clearances, excessive critical speed vibration causing contact
• Risk: Thermal damage, seal destruction
• Solution: Verify clearances, improve balance, slower acceleration

4. Bearing Instability During Startup

• Symptom: Sub-synchronous vibration developing during acceleration
• Cause: Bearing not yet at operating temperature and optimal stiffness/damping
• Behavior: May disappear as bearing warms up
• Solution: Extended warm-up at intermediate speed before full acceleration

Startup Procedure Design

Optimizing Acceleration Rate

Acceleration rate should be tailored to machine characteristics:

Slow Acceleration Zones

• Initial Roll (0-10% speed): Very slow to detect thermal bow or mechanical issues
• Below First Critical: Moderate rate for thermal warm-up
• Above All Criticals: Can accelerate more quickly to operating speed

Rapid Pass-Through Zones

• Critical Speed Ranges: Accelerate quickly (±15-20% around each critical speed)
• Typical Rate: 2-5× normal acceleration rate
• Purpose: Minimize time at resonance, limit vibration amplitude buildup

Hold Points

• Thermal Soak Speeds: Hold at 30%, 50%, 70% for large turbines
• Duration: 10-30 minutes at each hold
• Purpose: Allow thermal stabilization, reduce thermal gradients
• Vibration Check: Verify vibration acceptable before proceeding

Monitoring and Acceptance Criteria

Real-Time Monitoring

During startup, monitor:

• Overall Vibration Level: Should not exceed alarm limits at any speed
• Bearing Temperatures: Gradual increase acceptable; rapid rise indicates problems
• Speed Tracking: Confirm machine accelerating smoothly
• Phase Angle: Track for unexpected changes indicating mechanical issues

Acceptance Criteria

• Critical Speed Peaks: Should match predictions ±10-15%
• Peak Amplitudes: Should not exceed design limits (typically defined in equipment specifications)
• Steady-State Vibration: Should reach acceptable levels after thermal stabilization
• Repeatability: Successive startups should show consistent behavior

Troubleshooting Abnormal Startup Vibration

High Initial Vibration

Possible Causes:

• Thermal bow from previous operation or shutdown
• Mechanical bow or bent shaft
• Bearing problems (wear, misalignment)
• Looseness or mechanical defects

Vibration Increasing During Warm-Up

Possible Causes:

• Developing thermal bow from asymmetric heating
• Thermal growth affecting alignment
• Bearing clearance changes with temperature
• Thermal expansion closing clearances leading to rubs

Erratic Vibration During Acceleration

Possible Causes:

• Rubbing or intermittent contact
• Loose components settling or shifting
• Coupling problems
• Variable bearing behavior

Documentation and Baseline Data

Initial Commissioning

Establish baseline startup vibration signature:

• Record complete runup data
• Generate Bode plots and waterfall plots
• Document all critical speeds and peak amplitudes
• Archive as reference for future comparison

Periodic Comparison

• Compare current startup to baseline
• Look for changes in critical speed locations (indicates mechanical changes)
• Track changes in peak amplitudes (indicates unbalance or damping changes)
• Monitor for new vibration components not present in baseline

Startup vibration analysis provides a window into machine health that complements steady-state monitoring. Many developing problems reveal themselves most clearly during startups, making startup vibration trending a valuable predictive maintenance tool for critical rotating equipment.

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