SOP: HPU & Nitrogen Systems

Critical SOP: HPU & Nitrogen Systems

Ref: 1.3 MW Francis Units | Pressure Hygiene & ESD Protocol

⚠️ HIGH PRESSURE DANGER

DANGER: Hydraulic oil at 100+ bar is lethal.

  • De-Energization: Before any HPU disassembly, open manual ball valves to drain Accumulators back to tank. Manometer must show 0 bar.
  • Injection Risk: Never use hands to check for leaks. Pressurized leak can inject oil into bloodstream.
SOP 1: Nitrogen Accumulator Protocol

Accumulators are the "Battery" of the safety system. They allow MIV closing even if power (pumps) fails.

1.1 ESD Energy Verification (Quarterly)

Objective: Ensure sufficient stored energy for Emergency Shutdown (ESD).

  1. Isolation and Drainage: Isolate one accumulator bank. Drain oil side to tank.
  2. Connect Kit: Attach nitrogen charging kit to top bladder valve.
  3. Measure Pre-Charge (P0):
    • P0 should be approx. 90% of Minimum Operating Pressure (Pmin).
    • Example: If MIV needs 80 bar to close, P0 should be ~72 bar.
  4. Pass Criteria: Pressure > 90% of Target.
  5. Fail Criteria: Bladder empty or collapsed. Recharge immediately.
💡 Technical Note: If nitrogen requires frequent recharging, the rubber bladder is likely porous and needs replacement.
SOP 2: Oil Hygiene Standards

Sludge is the enemy of hydraulic precision. "Stiction" in valves causes jerky MIV movement.

2.1 ISO Cleanliness Protocol

  • Oil Type: Use only high-quality L-HL or L-HM hydraulic oil (ISO VG 46/68).
  • Target Cleanliness: maintain ISO 4406 18/16/13 or better.
  • Sampling: Take samples from middle zone of tank (not bottom sludge) annually.

2.2 High Pressure Filter Replacement

  1. Check "Clogging Indicator" on pressure filter housing daily.
  2. Red Flag = Stop & Replace. Do not bypass.
  3. Procedure:
    • Switch to duplex standby filter (if exists) or shutdown HPU.
    • Bleed pressure. Unscrew bowl.
    • Install new cartridge (Do not clean disposable elements).
    • Check O-ring condition.
SOP 3: Hydraulic Hammer & Stroke Time

Dirty oil leads to valve sticking. If valve sticks then releases suddenly, MIV can slam shut.

3.1 Zhukovsky Equation (Water Hammer)

ΔP = ρ · c · Δv
  • ΔP: Pressure Rise ("Hammer").
  • ρ (Rho): Water density.
  • c: Speed of sound in water (~1200 m/s).
  • Δv: Change in velocity (Water stop).

3.2 Link to Hygiene

Scenario: Sludge causes proportional valve to stick. MIV doesn't move 5 seconds, then snaps (closes) in 1 second.

  • Result: $\Delta v$ becomes instantaneous. $\Delta P$ spikes to 50+ bar.
  • Consequence: Penstock rupture or spiral case fracture.
  • Prevention: Clean oil ensures smooth, linear actuation (e.g. controlled 40-second closure).
Checklist
  • Quarterly: Check Nitrogen Pre-Charge.
  • Daily: Check Filter Clogging Indicators.
  • Annually: Lab analysis of L-HM Oil (ISO 4406).
  • Safety: Never work on pressurized accumulators.
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