SOP: MIV & Bypass Logic

🌊 MIV & BYPASS LOGIC PROTOCOL

Pressure Equalization & Physical Sealing

Preventing "wire-drawing" erosion and ensuring reliable sequencing

⚠️ CRITICAL OPERATING RULE

NEVER FORCE MIV OPEN.

  • Differential Pressure Limit: MIV cylinder is NOT designed to open against full penstock pressure. ΔP must be < 2 Bar.
  • Seal Damage: Opening under differential pressure rips the main seal out of its seat.
  • Safety: Hydraulic overload can burst cylinder seals or hoses.
0% Closing Rule - Zero Tolerance

SCADA vs. Physical Reality

Trap: SCADA showing "0% Open" does not guarantee sealing. Limit switch might be moved or misaligned.

Law of Erosion (Wire-Drawing):

  • If MIV is physically open even by 2° (e.g. at 88° instead of 90° in seat), high velocity water jet shoots through the gap.
  • Velocity: > 40 m/s under 150m head.
  • Effect: Cuts through stainless steel seat ring like a laser within weeks.

Verification Procedure:

  1. Visual Check: Look at mechanical weight arm position. Mark the actual "Closed" position on wall/floor.
  2. Leak Check: With MIV and Bypass closed, open Spiral Drain.
    • Flow should stop completely within 5 minutes.
    • If water continues > 5 L/min, MIV seal is passing.
Bypass Interlock Logic - Pressure Handling

Authorization Opening Sequence

PLC MUST enforce the following logic chain before energizing MIV Open Solenoid:

Step Action Condition to Proceed
1 "Start Unit" Command No Faults
2 Open Bypass Valve Bypass Limit Switch = OPEN
3 Wait Spiral Fill Pressure Rise > 2 Bar/sec
4 Pressure Equalization Check (P_penstock - P_spiral) < 0.5 Bar
5 Open MIV Command Equalization CONFIRMED

"Distributor Leakage" Symptom

Scenario: Bypass valve opens, but Spiral pressure never equalizes with Penstock pressure.

Diagnosis: Wicket Gates (Distributor) are leaking water into runner faster than Bypass can fill the spiral.

Action:

  1. DO NOT bridge the interlock.
  2. Perform Wicket Gate seal inspection.
  3. See Distributor Synchronization SOP.

Cylinder & Weld Checklist

Mechanical Integrity Inspection

MIV body takes highest static stress of any component (16 Bar continuous).

Annual Inspection Points:

  1. Hydraulic Cylinder Mounting:
    • Check torque on trunnion bolts.
    • Inspect pin bushings for ovality/play (> 0.5mm play = replace).
  2. Welded Seams (NDT Mandatory):
    • Perform Dye Penetrant test on flange-to-body welds.
    • Critical Zone: Valve body to cylinder trunnion transition (highest stress).
  3. Weight Arm:
    • Inspect safety cage welds.
    • Ensure weight stack is secured and cannot shift.
Troubleshooting Table
Symptom Probable Cause Corrective Action
MIV won't open (Logic Block) Pressure not equalized Check Wicket Gate leakage; Check Bypass filter
MIV creeps closed during operation Cylinder internal bypass Rebuild hydraulic cylinder seals
Loud "Bang" when closing Closing too fast Adjust throttle valve on HPU return line (Target: 60-90 sec)
Hissing water sound when closed Seat erosion (Wire-drawing) CRITICAL: Schedule outage for seat ring grinding/replacement
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