Understanding Runup in Rotating Machinery Analysis

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Definition: What is Runup?

Runup (also called startup or acceleration test) is the process of accelerating a rotating machine from rest (or low speed) to its normal operating speed while continuously monitoring vibration and other parameters. In rotor dynamics analysis, a runup test is a diagnostic procedure that records vibration data throughout the acceleration, providing critical information about critical speeds, resonance characteristics, and how the machine behaves during the startup transient.

Runup testing complements coastdown testing and is often performed during routine startups, making it a convenient method for periodic rotor dynamic assessment without requiring special shutdown procedures.

Purpose and Applications

1. Critical Speed Verification

The primary objective of runup testing is identifying and characterizing critical speeds:

• Vibration amplitude peaks as machine accelerates through each critical speed
• Peak magnitude indicates damping level and severity
• Characteristic 180° phase shift confirms resonance
• Identifies all critical speeds between zero and operating speed

2. Startup Procedure Validation

Confirms that startup procedures are appropriate:

• Acceleration rate adequate to pass through critical speeds quickly
• Vibration amplitudes remain within safe limits
• Thermal growth effects during warm-up
• Any speed hold periods are correctly positioned

3. Commissioning and Acceptance Testing

• New equipment first start verification
• Demonstration that design specifications are met
• Baseline data establishment for future comparison
• Validation of rotor dynamic models and predictions

4. Periodic Health Assessment

• Compare current runup to historical baselines
• Detect changes in critical speed locations (indicating mechanical changes)
• Identify increases in vibration amplitude at critical speeds (reduced damping, increased unbalance)
• Early warning of developing problems

Runup Test Procedure

Pre-Test Setup

1. Sensor Installation: Mount accelerometers or velocity transducers at each bearing in horizontal and vertical directions
2. Phase Reference: Install tachometer or keyphasor for speed and phase measurement
3. Data Acquisition System: Configure for continuous high-speed recording throughout startup
4. Safety Systems: Verify all safety systems functional, set vibration trip levels

Test Execution

1. Initial Condition: Machine at rest, all systems ready
2. Start Recording: Begin data acquisition before starting drive
3. Initiate Startup: Follow normal or modified startup procedure
4. Controlled Acceleration: Accelerate through critical speeds at defined rate
5. Monitor Continuously: Watch vibration levels in real-time for safety
6. Reach Operating Speed: Continue to normal operating conditions
7. Stabilize: Allow thermal and mechanical equilibration
8. Stop Recording: Capture complete transient plus steady-state operation

Acceleration Rate Considerations

• Too Fast: Insufficient data points at each speed, may miss critical speeds
• Too Slow: Excessive time at critical speeds, potential for damage; thermal changes during test
• Typical Rate: 100-500 RPM/minute for most industrial equipment
• Critical Speed Zones: May accelerate faster through known critical speeds

Data Analysis Methods

Bode Plot Analysis

Standard presentation format:

• Plot vibration amplitude vs. speed (upper plot)
• Plot phase angle vs. speed (lower plot)
• Critical speeds appear as amplitude peaks with phase transitions
• Compare to acceptance criteria and design predictions

Waterfall/Cascade Plot

• 3D plot showing frequency spectrum evolution with speed
• Clearly shows 1× synchronous component tracking with speed
• Natural frequency resonances appear as horizontal features
• Excellent for identifying sub-synchronous or super-synchronous components

Order Tracking

• Analyze vibration in terms of orders (multiples of running speed) rather than absolute frequency
• 1× component remains at same order throughout runup
• Natural frequencies appear as changing order lines
• Particularly useful for variable-speed equipment

Comparison: Runup vs. Coastdown

Aspect	Runup	Coastdown
Direction	Increasing speed	Decreasing speed
Energy State	Adding energy	Dissipating energy
Temperature	Cold to warm	Warm to cool
Control	Active (can adjust rate)	Passive (natural deceleration)
Duration	Shorter (powered acceleration)	Longer (friction/windage only)
Frequency	Every startup	Every shutdown
Risk	Higher (accelerating into resonance)	Lower (decelerating out of resonance)

When to Use Each Method

• Runup Preferred: When startup is controlled and can be adjusted; when operating temperature data needed; for routine monitoring
• Coastdown Preferred: For safety-critical testing; when slower passage through critical speeds desired; when power disconnection is easier than controlled startup
• Both Methods: Comprehensive assessment comparing hot vs. cold conditions, validating consistency

Special Considerations for Flexible Rotors

For flexible rotors operating above critical speeds:

Multiple Critical Speeds

• Must pass through first, second, and possibly third critical speeds
• Each requires adequate acceleration rate
• Total startup time may be several minutes
• Vibration monitoring at all critical speeds essential

Acceleration Strategy

• Slow Acceleration: Below first critical for thermal preparation
• Rapid Pass-Through: Accelerate quickly through each critical speed zone
• Possible Hold Points: At intermediate speeds for thermal stabilization
• Final Acceleration: To operating speed above all critical speeds

Automated Runup Systems

Modern machinery often includes automated runup sequencing:

• Programmable Acceleration Profiles: Optimized rates for each speed range
• Vibration-Based Control: Automatically adjust rate based on measured vibration
• Temperature Interlocks: Hold acceleration until thermal criteria met
• Safety Shutdowns: Automatic trip if vibration exceeds limits
• Data Logging: Automatic recording and archiving of each startup

Runup testing provides essential empirical data about rotating machinery behavior during the critical startup transient. Regular runup data collection and comparison enables early detection of developing problems, validates startup procedures, and ensures safe passage through critical speed ranges.

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