ID Fan Structural Resonance RCA

Case Study: ID Fan Structural Resonance RCA

Sources:

newdocs/ID Fan.pdf — Field vibration data and diagnosis (Indian Institute for Production Management)
newdocs/Fundamentals_to_RCA_Structural_Resonance_ID_Fan.pptx — Physics-based RCA training (Dibyendu De)

Equipment

Equipment: Hot Zone ID Fan
Motor: 1200 KW
RPM: 994 (VFD-driven, variable speed 700–945 RPM)
Drive: VFD installed 2023, commissioned 2022
Coupling: Metallic disc spacer (flexible disc)
Fan bearings: Spherical roller bearings, oil lubrication
Foundation: Rigid, ground level, concrete floor + steel frame
ISO Class: III (acceptable: 4.5 mm/s, still acceptable: 11.2 mm/s)
Monitoring points: 4 bearings (MNDE, MDE, FDE, FNDE), tri-axial (H, V, A)

Field Observations

Symptoms

Speed-dependent vibration: increases in specific RPM bands
Directional change: axial vs horizontal dominance varies with speed
Alignment and bolt tightening performed, but vibration persists
VFD installation preceded onset of the problem
Motor base bolts unable to tighten properly (damaged threads)
Multiple shims discovered under motor base

Vibration Data (velocity mm/s RMS)

At 945 RPM:

Point	H	V	A
MNDE (1)	2.34	1.99	1.82
MDE (2)	5.26	2.61	1.55
FDE (3)	1.75	1.92	3.04
FNDE (4)	1.83	0.54	1.34

At 700 RPM:

Point	H	V	A
MNDE (1)	1.24	1.33	1.21
MDE (2)	3.64	1.89	1.59
FDE (3)	2.18	2.06	3.74
FNDE (4)	0.98	0.36	0.74

Key: MDE-H at 945 RPM (5.26 mm/s) exceeds marginal threshold. FDE-A consistently elevated (3.04–5.22 mm/s across speeds).

Spectral Analysis

945 RPM: Dominant 1X peak at MDE-H indicates soft foot at motor base
700 RPM: 1X and 2X peaks indicate misalignment. Axial vibration at fan drive end increases with speed reduction.

Phase Analysis

Phase angle 158 deg at 945 RPM, 175 deg at 700 RPM
Phase variation with speed suggests non-synchronous behavior and structural resonance crossing

Root Cause Analysis (Physics-Based)

Why Procedural Fixes Failed

Actions focused on symptoms (alignment, bolts) rather than governing physics
Assumed constant stiffness and rigid boundary conditions
Ignored speed as an independent variable
Corrections could not survive operating conditions

Physics Model

All rotating machines can be locally approximated as a Single Degree of Freedom (SDOF) system:

m x'' + c x' + k x = F(t)

Natural frequency: omega_n = sqrt(k / m)
Frequency ratio: r = Omega / omega_n
Peak response at resonance (r ~ 1)

Key Insight: Structural Resonance

VFD allows excitation across a wide speed range, exposing resonances hidden at fixed speed
Effective stiffness includes base, bolts, shims, grout, and frame
Stacked shims and loose anchors reduced k, introducing nonlinearity
Nonlinear stiffness (k*x + alpha*x^2) generates even harmonics (2X) without impacts
Phase instability across speeds confirms resonance crossing, not just misalignment

True Root Cause

Variable-speed excitation intersected structural natural frequencies. Effective stiffness and boundary conditions were unstable. Soft foot and misalignment were symptoms, not root causes.

Corrective Action Sequence (Order Matters)

Structural correction — Restore base stiffness: remove stacked shims, proper grouting, replace damaged bolts
Verify resonance shift — Speed sweep to confirm natural frequency moved out of operating range
Apply alignment — Only after structural stability is achieved
Validate — Full operating speed range verification
VFD skip bands — If resonance cannot be fully eliminated, program VFD to skip problematic RPM bands

RAPID AI Relevance

This case demonstrates several Module B fault patterns:

Foundation rules (structural resonance, soft foot)
Coupling rules (misalignment symptoms)
AFB rules (bearing point vibration patterns)
Speed-dependent behavior requires trend analysis across operating conditions (Module B.2)
Directional ratio changes (H vs A dominance shift) are key Module B discriminators
The failure of procedural fixes validates the physics-based approach underlying RAPID AI