| name | rotating-equipment-field-issues |
| description | Rotating equipment field troubleshooting — pump cavitation (NPSH deficit, suction conditions), pump seal failures (API 682 seal plans, seal flush plan selection, face loading), mechanical seal face wear patterns, bearing failures (overheating, spalling, false brinelling, electric discharge machining EDM), alignment errors (soft foot, parallel/angular misalignment, thermal growth), coupling failures, vibration analysis (FFT spectra, imbalance 1×, misalignment 2×, bearing defect BPFO/BPFI), compressor surge (stall margin, anti-surge control), critical speed, resonance, oil whirl/whip, gear failures (pitting, scuffing, micropitting), API 610/617/618 requirements. |
| metadata | {"priority":8,"promptSignals":{"phrases":["pump seal","cavitation","bearing failure","alignment","vibration","compressor surge","mechanical seal","rotating equipment","oil whirl","seal failure"],"minScore":2}} |
Rotating Equipment Field Issues — Complete Troubleshooting Guide
Pump Cavitation
NPSH Analysis
NPSHa (Available):
NPSHa = (P_suction − P_vapor) / (ρg) + v_s²/(2g) [m]
Or: NPSHa = (P_atm + ρgh_s) / (ρg) − P_vapor/(ρg) − h_f_suction [h_s = suction head; h_f = friction loss]
NPSHr (Required): from pump curve; must have NPSHa ≥ NPSHr + 0.5–2.0 m safety margin (API 610: 0 m margin by definition; plant practice: 0.6–2 m)
Field symptoms of cavitation:
- Crackling/gravel noise from pump
- Erratic flow and pressure fluctuations
- Vibration increase (spectrum: broadband noise floor rises)
- Impeller pitting (suction side of vanes, especially at leading edge)
- Performance degraded (head and flow drop from rated)
Root causes in field:
- High liquid temperature (T rise → Pv rise → NPSHa drops)
- Suction line restriction (strainer clogged → more friction → NPSHa drops)
- Throttled suction valve (never throttle suction)
- Pump running far right of BEP (flow increase → NPSHr increases)
- Vapor entrainment from upstream flashing, gas breakout
Fixes:
- Lower fluid temperature (subcooling: precool feed)
- Clean strainer (track pressure drop across suction strainer; replace filter element)
- Open suction valve fully (never throttle suction)
- Reduce flow to closer to BEP
- Lower pump (raise suction head); increase suction line diameter (reduce velocity)
- Raise vessel pressure (if applicable)
Mechanical Seal Failures
API 682 Seal Plans
Plan 11 (most common): Recirculation from pump discharge back to seal chamber; pressurizes seal; keeps seal faces clean
Plan 13: Recirculation from seal chamber to suction; used when chamber pressure > suction needed for vent
Plan 21: Cooler on Plan 11 line; for hot services (T > 80°C)
Plan 23: Internal circulation within seal; cooler between seal faces and barrier; best for hot services
Plan 52: External reservoir; unpressurized; for double seals; barrier fluid at lower pressure than process
Plan 53A/B/C: Pressurized dual seal (API 682 Category 2/3); barrier fluid at 1.5–2 bar above seal chamber
Plan 54: External pressurized system with separate pump; for critical services
Seal failure modes:
Face wear (excessive):
Cause: insufficient flush flow (Plan 11 orifice too small → flash across faces for high-vapor-pressure fluid); contamination in seal chamber (solids abrade faces); dry run (no liquid when pump starts)
Diagnosis: face groove pattern (concentric rings → misalignment; radial marks → particles; heat checking → dry run)
Fix: increase flush flow; use Plan 23 with cooler for hot service; install flush filter; add startup delay or low-flow protection
Seal face crystallization:
Salt or scale deposits on seal face → abrasion → leakage; especially in water injection, produced water, or slurry
Prevention: use Plan 32 (external flush with clean compatible fluid) instead of process recirculation
O-ring failure (secondary seal):
Elastomer incompatibility with process fluid → swelling, hardening, extrusion
High temperature: Buna-N (NBR) limited to 120°C; FKM (Viton) to 200°C; PTFE encapsulated to 260°C
Chemical attack: check Fluid Power Engineering compatibility table; incompatibility → seal failure in weeks
Dynamic O-ring wear:
Pusher-type seal: O-ring rides on shaft or sleeve → fretting wear → debris in seal faces
Fix: use bellows seal (no dynamic O-ring) for chemically aggressive service or where fretting is issue
Face materials:
Silicon carbide vs. carbon-graphite: standard for most services; SiC vs. SiC for abrasive; carbon vs. tungsten carbide for low lubricity fluids
Cracked SiC face: thermal shock (cold flush onto hot face after dry run)
Bearing Failures
Failure Mode Analysis
Spalling (fatigue):
Subsurface fatigue crack at Hertzian contact → propagates to surface → flaking
L₁₀ life: 90% reliability design life; actual failure at expected time → correct design, normal fatigue
Premature spalling: overloading, misalignment increasing loads, incorrect preload, contaminated lubricant
Contamination damage:
Hard particles (dirt, metal debris) → dents in raceway → stress concentrations → early fatigue
Diagnosis: pitting marks with raised edges (dents from hard particle indentation)
Prevention: oil filtration ≤ ISO 16/14/11 for precision bearings; seal integrity
False brinelling:
Static vibration with no rotation → fretting at contact ellipses → oxidized debris → Hertzian indent pattern (not rotational)
Occurs: during transport, standby pumps on vibrating structures, pipelines
Prevention: rotate standby equipment periodically (weekly); use correct lubricant (micro-oxidation inhibitors)
Distinguished from true brinelling: debris between dents; no rotation marks
Electric Discharge Machining (EDM) damage:
Stray electric current passes through bearing → arc discharge at rolling element contact → cratering, fluting
Modern problem: VFD-driven motors with high-frequency switching → shaft voltage → EDM in bearings
Diagnosis: fluted raceway (regular circumferential damage pattern); dark lubricant from electrical discharge
Fix: insulated bearings (ceramic-coated OD or ceramic elements); shaft grounding brush; common-mode choke on VFD output
Overheating:
Lubricant starvation (too little or too much — overfilling creates churning → thermal runaway)
Wrong lubricant viscosity (ISO VG 68 when VG 32 needed → high churning at high speed)
Excessive preload → bearing temperature spikes
Monitor: bearing temperature alarm at 80°C; trip at 95°C (or Δ20°C above ambient as alternate trigger)
Oil analysis:
Ferrography (particle shape and size); Fe, Cu, Cr in lube oil indicate component wear
ISO 4406 cleanliness target: ≤ 17/15/12 for turbomachinery bearings
Shaft Alignment
Soft Foot
Soft foot: foot of equipment doesn't contact baseplate fully when hold-down bolts tightened → machine distorts → bearing misloaded → vibration, seal damage
Types: angular soft foot (foot at angle to baseplate), shim-bound (shim stack too thick; foot rocks), spring-related (corroded foot or bolt)
Test: loosen each foot one at a time while monitoring dial indicator on shaft; if dial moves > 0.05 mm → soft foot exists
Fix: shim correction; grind foot; epoxy grout (corrects irregular baseplates); clean corrosion
Parallel and Angular Misalignment
Parallel (offset) misalignment: shaft centerlines parallel but offset by δ [mm]
Angular misalignment: shaft centerlines intersect at angle α [degrees or mm/100mm]
Vibration signature:
Imbalance: dominant 1× (once per revolution) in radial direction; amplitude proportional to imbalance mass × eccentricity
Misalignment: dominant 2× (twice per revolution); also 1×; axial vibration prominent for angular misalignment
Coupling looseness: multiple harmonics (3×, 4×)
Alignment tolerances (API 686):
Soft coupling (elastomeric): parallel ≤ 0.05 mm; angular ≤ 0.1 mm/100 mm
Rigid coupling (disc or diaphragm): parallel ≤ 0.025 mm; angular ≤ 0.025 mm/100 mm
Hot alignment: must account for thermal growth of equipment to operating temperature
Thermal growth correction:
ΔL = α × ΔT × L [α_steel = 12 μm/m·°C; ΔT = T_operating − T_cold; L = centerline height]
Typical pump: 0.1–0.5 mm thermal rise at operating temperature; must pre-offset cold alignment accordingly
Vibration Analysis (FFT Spectrum)
Frequency Signatures
Imbalance: 1× RPM dominant; radial; phase stable; correct by field balancing (ISO 1940 balance quality grades)
Misalignment: 2× RPM dominant (can also be 1×); high axial for angular; coupling forces
Bearing defects:
BPFO (ball pass frequency outer race) = (N/2) × RPM × (1 − d×cosα/D) [N = number of balls; d = ball diameter; D = pitch diameter; α = contact angle]
BPFI (inner race) = (N/2) × RPM × (1 + d×cosα/D)
BSF (ball spin) = (D/2d) × RPM × [1 − (d×cosα/D)²]
Bearing defects: sub-synchronous sidebands around 1×; high-frequency noise floor
Resonance:
Natural frequency excited by running speed or harmonics → large amplitude at resonance; phase shifts 90° at resonance
Check: bump test (impact test) to identify natural frequencies; separate from operating speed by > 20%
Oil whirl/whip (journal bearings):
Oil whirl: sub-synchronous vibration at ~0.47–0.49 × RPM; from hydrodynamic oil film instability; usually benign
Oil whip: whirl locks to rotor natural frequency → destructive; occurs at rotor speed = 2× critical speed
Prevention: tilting pad bearings (stable at all speeds); increase bearing load; reduce oil viscosity
Compressor Surge
Surge Mechanism
At low flow: pressure rise steeply; flow reverses momentarily → backflow → impeller re-ingests gas → oscillating → destructive noise, vibration, heat
Surge point: minimum stable operating point on compressor map
Surge margin:
SM = (Q_surge − Q_operating) / Q_operating × 100% [%]
Design: SM ≥ 10–15% at rated speed; higher for variable-speed or variable-density gas
Anti-surge control (ASC):
Recycle valve: opens when approaching surge limit → recycles discharge to suction → increases flow through compressor → moves away from surge
Surge controller: monitors flow (ΔP across suction measurement) and discharge pressure; opens recycle when operating point too close to surge line
Surge detection:
Rapid pressure reversal (fast DP switch); vibration spike; temperature spike at discharge
Acceleration-based detection: dP/dt > threshold → immediate recycle valve open
Field causes of surge:
- Process flow demand decreases below minimum stable flow
- Discharge pressure rises unexpectedly (downstream valve closes)
- Inlet gas composition changes (heavier gas → different head-flow curve)
- Compressor fouled (dirty impellers → reduced flow at same speed)
- Speed drop (driver trip → speed decreases → moves toward surge)
Standards
| Standard | Scope |
|---|
| API 610 | Centrifugal pumps for petroleum/chemical |
| API 682 | Shaft sealing systems (mechanical seals) |
| API 617 | Centrifugal compressors |
| API 618 | Reciprocating compressors |
| API 670 | Machinery protection systems |
| API 686 | Machinery installation and commissioning |
| ISO 1940-1 | Balance quality grades for rigid rotors |
| ISO 10816 | Vibration evaluation by measurements on non-rotating parts |
| ISO 13709 | Centrifugal pumps (international equiv. API 610) |
Output
Identify equipment and failure type. For each:
Pump cavitation: NPSHa [m] vs. NPSHr [m]; margin [m]; suction conditions; fix: strainer ΔP, flow reduction, temperature
Seal failure: API 682 plan used; chamber pressure [barg]; flush flow [L/min]; face materials; failure mode: wear/crystallization/O-ring; replacement plan
Bearing: failure mode: fatigue/contamination/false brinelling/EDM; lubricant ISO VG; contamination level (ISO 4406); VFD-driven? → insulated bearing required?
Alignment: soft foot check result [mm]; parallel offset [mm]; angular [mm/100mm]; thermal growth correction applied [mm]
Vibration: dominant frequency: 1× (imbalance) / 2× (misalignment) / sub-sync (whirl/surge); BPFO/BPFI calculated [Hz]; severity per ISO 10816
Surge: surge margin [%]; anti-surge valve position [%]; recycle line adequate size?
Applicable API standard + corrective action.
