Gas Turbine Control Systems, Monitoring, and Maintenance Essentials

Gas Turbine Control Systems, Monitoring, and Maintenance Essentials

Control System, Instrumentation & Installation

  • Open loop vs closed loop control
    • Open loop: no process measurements; manipulated variable positioned manually or programmatically.
    • Closed loop: measured process variables feed back to adjust the manipulated variable to control a device; majority of combined cycle plants use closed-loop.
  • Closed loop control may include: feedback, feed forward, or both.
  • Modern turbine control architecture:
    • Distributed Control System (DCS) at the plant level.
    • Condition monitoring and optimization integrated with functional level systems (e.g., lubrication).

Life Cycle Costs (Combined Cycle Plant)

  • Initial purchase costs: 7-10\%
  • Maintenance costs: 15-20\%
  • Energy costs: 70-80\%

Condition Monitoring (Prerequisites & Data)

  • Prerequisites for effective Condition Monitoring:
    • Timely failure predictions
    • Simple, reliable, user-friendly, easy to repair system
    • Flexible, cost-effective, economically viable
  • Condition Monitoring data includes: Diagnosis, Combustion analysis, Aero-thermal analysis, Vibration & Mechanical analysis, Corrosion analysis, Trending & prognosis, “What if” analysis, GUI
  • Condition Monitoring involves: mechanical analysis, performance-based analysis, and corrosion monitoring
  • Predictive maintenance outcomes:
    • Reduced under-utilization
    • Decreased downtime and increased equipment availability
    • Reduced maintenance costs

Temperature & Pressure Measurement Devices

  • Temperature: Thermocouples, RTDs, Pyrometers
  • Pressure: Bourdon tube gauges, Liquid manometers, Impact tubes, Pitot–static tubes, Pressure transducers, Barometers

Vibration Measurement & Transducers

  • Vibration transducers translate vibrations into a time-vs- voltage output
  • Three transducer types:
    • Displacement transducer
    • Velocity transducer
    • Acceleration transducer
  • Velocity pickup: magnet in coil; relative motion induces voltage proportional to velocity

Monitoring & Diagnostic Instruments

  • Instrumentation systems include: signal conditioning & amplifying equipment, data transmission cables, data integrity checking, baseline generation & comparison, problem detection, generation of diagnostics/prognoses, plotting/documentation/reporting

Gas Turbine Operation Diagnostics

  • Conditions:
    • Surging: rapid shaft vibration and discharge pressure fluctuations
    • Fouling: decreased pressure ratio and flow rate, higher exit temperature, reduced efficiency
    • Air filter clogging: increased pressure drop across filter, power loss
  • Compressor diagnostics; turbine diagnostics cover additional fault indicators

Turbine Operation & Maintenance (Issues)

  • Common issues include:
    • Discrepancies between actual and designed operating conditions
    • Process defects from non-design conditions
    • Decreased output due to operation away from design
    • Turbine damage indicators (e.g., NGV/Nozzle issues), compressor FOD

Maintenance Types & Philosophies

  • Failure Based Maintenance (FBM)
  • Scheduled/Preventive Maintenance (PM)
  • Predictive Maintenance (PDM)
  • Proactive Maintenance (PAM)
  • Condition Based Maintenance (CBM)
  • Reliability Centered Maintenance (RCM)
  • Total Productive Maintenance (TPM)
  • Strategic maintenance philosophies

Maintenance Strategies by Machinery Criticality

  • Critical machinery: Proactive, Predictive
  • Essential machinery: Preventive, Predictive
  • General-purpose machinery: Breakdown, Predictive (portable equipment)

PDM Techniques

  • Vibration analysis
  • Oil analysis
  • Wear particle analysis
  • Ultrasonics
  • Infrared thermography
  • Performance evaluation

Maintenance Process & O&M Scope

  • Maintenance consists of: Planning, Scheduling, Execution, Inspection, Overhaul, Repair
  • Focus: Turbine Operation and Maintenance

Maintenance Planning & Scheduling (Planning Prior to Action)

  • Planning and scheduling are undertaken before maintenance action
  • Machine performance and condition monitoring data are vital for proper planning

Gas Turbine Overhauls

  • Work-scope planning guide (MTBO) for Gas Turbines
  • Reliability considerations
  • Service Bulletins, Modifications, etc.
  • Hard Time Items/Parts
  • ON Condition concept for modules
  • Cost per hour of operation considerations

Shop Quality, Overhauls & Fleet Factors

  • Shop quality, operational maintenance, power/heat rating, engine work scoping, condition monitoring, fleet management
  • Reducing shutdown rates; improving methods
  • OEM focus on control plans; use of improved statistical methods to identify factors affecting reliability

Background & Theory: Work-Scope Planning

  • Work-scope planning driven by published Work-Scope Planning Guides (WSPG)
  • WSPG thresholds are historical; evolving with fleet experience and technologies
  • Soft time thresholds exist; opportunity to reduce overhaul costs

Rising Costs & Reliability

  • Costs driven by: increasing probability of events and secondary damage; rising material costs
  • Reliability/confidence settings important in operation & maintenance phase

Module Cost Structure & Project Selection

  • $/EFH, /kW-h, Condition Drivers, WSPG Thresholds
  • Extend? Performance limits and maintenance planning influence cost vs performance
  • Reliability considerations in project selection

Capital Cost, Fuel Cost & Economics

  • Capital cost: 350-450\ USD/kw for new plant; 400 MW plant ≈ 140\,MUSD
  • Small CHP: up to 850\ USD/kw
  • Fuel cost is the largest share: ~70\% of total cost
  • Example: improving efficiency can save ~20\,M USD per year

Maintenance Cost & Case Numbers

  • Maintenance cost: 2-2.5\ USD/MWh
  • Example: 400 MW, 8000–8750 hours/year
    • Energy produced: 400\,MW × 8000–8750 h
    • Estimated annual maintenance budget: 6.4-8\,MUSD/year

Best Maintenance Practices Objectives

  • Establish a risk-based maintenance interval
  • Part tracking and parts inspection program
  • Define maintenance scope and QA/QC program
  • Awareness of inspection methods and life assessment
  • Awareness of best industry practice

Maintenance Strategy: Location & Fleet Considerations

  • Location-specific decisions:
    • Cost of unavailability
    • Fleet size/redundancy
    • Geography/infrastructure
    • Installed base and parts availability (OEM vs non-OEM)
    • In-house skill and experience
    • Economic plant life (30+ years)

Gas Turbine Maintenance: Inspect-Only vs Planned Maintenance

  • Inspect-only components: casings, rotor, IGV blades may not require planned maintenance; cleaning/inspection with OEM; R&R as extra work
  • Planned maintenance: standard inspections; components replaced/repaired/upgraded per R&R program; RBM/CBM with OEMs; advanced NDT inspections

Gas Turbine Type Comparison

  • Heavy Industrial (Single Shaft) vs Aero-derivative:
    • Advantages (Heavy Industrial): high reliability, robust for base-load; lower fuel flexibility; longer intervals for some inspections
    • Advantages (Aero-derivative): up to ~140 MW, high fuel flexibility, recoverable heat, high efficiency (>40%), fast maintenance, fast start-up
    • Disadvantages (Heavy Industrial): heavier, larger, lower specific power
    • Disadvantages (Aero-derivative): higher cost, more limited power range (~50-100 MW), shorter inspection intervals for some components

Availability & Inspection Intervals (Plant Availability)

  • Heavy Industrial (typical intervals, hours):
    • Combustion: 8{,}000
    • Hot Gas Path: 24{,}000
    • Major: 48{,}000
  • Availability metrics (days/year): around 343-351\,d/y (progression over time)
  • Aero-derivative: maintenance actions include crank washing, bore-scope, major overhauls; typical availability ~350\,d/y$$

Availability Optimization – Key Objective

  • Prevent unscheduled outages (increase reliability)
  • Extend time between inspections (MTBM)
  • Reduce outage duration while preserving quality and HSE

Industry & Stakeholder Focus

  • Valuing availability; adopting proactive maintenance management
  • Train/retain staff, engage with suppliers and forums
  • Shift toward longer-term agreements to secure spares and services
  • Conclusion: proactive maintenance and strategic planning drive reliability and cost efficiency

End of Webinar Notes