SOP: Oil Health & Aeration

🧪 OIL HEALTH & AERATION DIAGNOSTICS

Hydraulic System Fluid Chemistry

Understanding foam, aeration, and contamination in HPU systems

⚠️ SAFETY PROTOCOL

STOP. READ THIS FIRST.

  • Pressure Release: Depressurize HPU system before opening any oil circuit (nitrogen accumulator risk).
  • Hot Oil: Operating temperature can reach 55-65°C. Wear protective gloves when handling components.
  • Fire Risk: Hydraulic oil is flammable. Keep ignition sources away during inspection or sampling.
The Foaming Phenomenon

Surface Foam vs. Entrained Air (Aeration)

Two distinct states affecting hydraulic oil, each with different causes and consequences:

Characteristic Surface Foam Entrained Air (Aeration)
Appearance Large bubbles on oil surface
Visible in sight glass		 Milky, cloudy oil everywhere
Microscopic bubbles
Location Reservoir surface only Distributed throughout oil volume
Primary Cause Contamination (water, detergent)
Excessive pump suction velocity		 Low oil level
Suction line leak
Vortex formation
System Effect Level fluctuation
Mostly cosmetic issue		 Reduces Bulk Modulus
Governor becomes "spongy"
Erratic response
Severity Moderate CRITICAL

The Bulk Modulus Problem

What is Bulk Modulus? The "stiffness" of hydraulic oil under pressure. Clean oil has high modulus (fast, precise response). Aerated oil has low modulus (slow, "spongy" feel).

Impact on Governor:

  • Normal Oil: Governor Command → Instant wicket gate movement (< 0.5 sec)
  • Aerated Oil: Governor Command → Delayed, uncertain movement (1-2 sec lag) → Frequency hunting

⚠️ Critical Effect: Aeration is a major cause of "soft" governor response and frequency instability on your 1.3 MW units.

Symptom Mapping & Root Causes

Foaming - Visual & Causes

Symptom: Large bubbles (> 1 mm) on oil surface, forming and bursting rapidly.

Main Causes:

  1. Water Contamination: Even 0.1% water can cause severe foaming.
    Test: Karl Fischer moisture test. Target: < 200 ppm water.
  2. Detergent/Soap Contamination: From improper tank cleaning.
    Action: Drain, flush with clean oil, refill fresh.
  3. Excessive Pump Suction Velocity: Creates surface turbulence.
    Check: Pump inlet velocity should be < 1.2 m/s.
  4. Degraded Anti-Foam Additives: Expired oil (> 5 years).
    Action: Complete oil change.

Diagnostic Procedure:

  1. Observe sight glass while unit running. Note bubble size and persistence.
  2. If large, persistent foam present → Sample oil for water content.
  3. If water > 500 ppm → Drain and refill tank.
  4. If water < 500 ppm but foaming persists → Check pump suction design for vortices.

Aeration - Visual & Causes

Symptom: Milky, cloudy appearance throughout volume. Oil looks "whipped" or "emulsified".

Main Causes:

  1. Low Oil Level: Pump suction pulls surface vortex, ingesting air.
    Check: Maintain oil level at 75-85% of sight glass height.
  2. Suction Line Leak: Air ingress through loose fittings or cracked hose.
    Test: Pressurize suction line with air, spray soapy water to find leaks.
  3. Damaged Shaft Seal: Pump shaft seal admits air on suction stroke.
    Symptom: Oil foams rhythmically with pump stroke.
  4. Vortex Formation at Suction: Improper tank baffling.
    Solution: Install anti-vortex plate at suction inlet.

Diagnostic Procedure:

  1. Check oil level immediately. If low, top up and observe if cloudiness clears in 10-15 min.
  2. If level OK, inspect all suction line connections for tightness.
  3. Perform "Leak Test": Isolate suction line, pressurize to 0.5 bar, soap spray joints.
  4. If no external leaks found → Suspect pump shaft seal. Listen for rhythmic "whining" sound.
Visual Diagnostics - HPU Sight Glass Interpretation

What to Look For at Sight Glass

Normal Oil

  • Color: Clear amber/gold (ISO VG 46 hydraulic oil)
  • Transparency: Can see through glass to opposite side
  • Bubbles: Few or none; present ones should rise and vanish quickly
  • Sediment: No visible sediment at glass bottom

Abnormal Conditions

Visual Appearance Diagnosis Action
Milky white throughout Aeration (entrained air) Check level, look for leaks
Large bubbles only on surface Surface foaming Test for water contamination
Dark brown/black color Oxidation/oil degradation Perform oil analysis, schedule change
Visible metal particles Pump or valve wear Drain oil, inspect pump internals
Water droplets (separate layer) Free water contamination Drain water, find ingress source
Governor Leak Detection - "Drying" Procedure

Identifying Internal Leaks in Governor Valves

Purpose: Determine if oil is leaking internally between pilot valve and main distributing valve, causing slow response.

Procedure Steps:

  1. Isolate System: Close main isolation valve to governor. Release pressure.
  2. Visual Pre-Check: Inspect all external joints for oil weeping. Wipe clean and mark with chalk.
  3. Drying Test:
    • Remove oil from valve housing with compressed air (low pressure, 2-3 bar).
    • Let system sit for 30 minutes.
    • Re-inspect marked areas for fresh oil presence.
  4. Leak Classification:
    • Pilot Valve Leak: Oil appears around pilot piston housing → Replace seals.
    • Main Valve Leak: Oil appears on main valve body → Repair main valve.
Maintenance Schedule & Oil Analysis
  • Daily: Visual check of HPU sight glass for cloudiness or bubbles.
  • Weekly: Check oil level, top up if below 75% of glass.
  • Monthly: Acoustic scan of HPU pump for cavitation sounds.
  • Quarterly: Oil analysis (viscosity, water content, particle count).
  • Annually: Complete oil change if any of following:
    • Water content > 500 ppm
    • Viscosity change > 10% from ISO VG 46
    • Particle contamination > ISO 18/16/13

Oil Sampling Procedure

  1. Run unit for 30 minutes to reach operating temp (50-60°C).
  2. Take sample from middle depth of tank (not surface, not bottom) with clean syringe.
  3. Fill bottle, label with date, unit ID, running hours.
  4. Send to lab for analysis.
← Return to Operations Center