Gas Turbines, Lubrication System, Bearings and Seal Notes

Simple Gas Turbine Components

  • Compressor: Air enters at ambient temperature, compressed to higher pressure and temperature.
  • Combustor: Fuel injected, gases rapidly expand.
  • Exhaust Turbine: Converts energy into work to rotate engine or drive other components.

Gas Turbine Classifications

  • Single Shaft: All components operate at the same speed.
  • Multiple Shaft: Components operate at different speeds; high-pressure rotor rotates the compressor, low pressure operates at variable speeds.

Ground-Based Turbine Types

  • Frame type
  • Heavy duty/industrial type
  • Aircraft derivative
  • Small units
  • Microturbines

Gas Turbine Essential Requirements

  • Ease of installation and maintenance.
  • High reliability and efficiency.
  • Conformance to environmental standards.
  • Flexibility in service and fuel requirements.

Gas Turbine Applications

  • Aircraft propulsion (turbojet, turboprop, turbofan).
  • Power generation.
  • Mechanical drives.
  • Marine propulsion.

Aeroderivative Gas Turbines Advantages

  • Faster startup.
  • High power-to-weight ratio.
  • Low specific cost.
  • Used in standby, electrical load peaking, marine, and emergency applications.

Stator Vanes

  • Stationary component converting rotational energy into static pressure.
  • Prevents flow spiraling by redirecting it parallel to the axis.
  • Stator vanes are fixed, while rotor vanes rotate.

Nozzle Guide Vane

  • Directs high-velocity hot gases from the combustion chamber to turbine blades.
  • Made from nickel-based superalloys due to high temperatures.
  • Essential structural part of gas turbines.

Gas Turbine Lubrication System

  • Lubricated components: bearings and seals.
  • Bearings: journal and thrust type.
  • Journal bearings: support rotating shafts, reducing friction.
  • Thrust bearings: absorb axial loads.

Labyrinth Type Oil Seals

  • Mounted on journal bearing sides; must be lubricated.
  • Purpose: Decrease mechanical losses, prevent wear, reduce friction.
  • Secondary: Remove heat, clean, and transmit friction.

Lubrication System Components

  • Oil reservoir.
  • Pumps (pressure and scavenge).
  • Defaulters.
  • Coolers.
  • Magnetic chip detector.
  • Pressure relief valve.

Lubrication System Operation

  • Oil supplied to bearings to reduce friction.
  • Typical gas turbine rotation speed: 3,600 RPM.
  • Exhaust temperatures exceed 625 degrees Celsius.
  • Operating temperatures near 120 degrees Celsius.

Oil Requirements

  • Correct viscosity.
  • Additives for oxidation, varnish formation, foam stability, filterability, long wear life, metal compatibility, and wave protection.

Combined Cycle Turbine

  • Exhaust heat used to produce steam, which drives a steam turbine.
  • Improves overall system efficiency.
  • Often uses one shared lubrication system; oil must shed water.

Aircraft Lubrication System

  • Components: oil tank, pump, filter, high-pressure oil line, scavenge pumps, scavenge filter, cooler.

Lubrication System Types by Sump

  • Dry Sump: Oil stored outside the gas turbine in a tank (used in aeroderivative gas turbines).
  • Wet Sump: Oil stored in engine cavities (used in aviation gas turbine engines).

Turbine Engine Oil System Types

  • Pressure Relief Systems: Maintain constant pressure.
  • Full Flow Type: Pressure varies with engine speed; provides essential engine protection.
  • Total Loss System: Used in short-duration operations (e.g., target drones).

Components of Full Flow Lubrication System

  • Pressure pump with suction filter
  • Scavenge Pump
  • Pressure Filter to remove small particles.
  • Pressure relief valve (between oil pump and filter) to limit system pressure. *Bypass filters have high capacity and eliminates much smaller particles than the full flow filters.
    • From the pressure pump, oil is passed through the pressure filter, which will then remove any small, particles, which may block the feed jets, pre jets that feeds the oil now into the bearings.

Torque Meter Pump

  • Boosts engine oil pressure higher than normal.
  • Oil falls into compartments and is moved by scavenger pumps.
  • Oil passes through air-cooled oil cooler, heat dissipates into atmosphere.
  • Components: scavenge pumps, pressure filter, pressure relief valve, suction filter, cooler.

Scavenging System

  • Returns oil to storage tank after cooling components.

System components

  • Low oil pressure Indicators
  • Valves, pavers, pressurizing components.
  • Temp temperature and pressure gauges, temperature regulating valves.
    * fittings, valves, plumbing, and the seals.

Pump Types

  • Gear Pumps: High-speed pressure, quiet operation.
  • Girota Pumps: Outer element with one more tooth than the inner element, creating cavities to expand on the inlet (draw in oil) and contract on the outlet (pressurize).

Filters

  • Common filters: cartridge screen and screen disc.
  • Essential characteristics: filter particles up to five-micron size, water resistance, dirt retaining capacity, high flow rate with minimum pressure drop, and resistance to rupture at higher pressures.

Oil Cooler

  • Reduces oil temperature by transmitting heat to another fluid.
  • Oil temperature control via thermostatic devices.

Magnetic Chip Detectors

  • Detect metal chips/shavings in system.
  • Installed in oil filter or sump.
  • Indicate wear; can trigger warning circuit.

Additives

  • Detergents: control deposit formation.
  • Ashless dispersants: suspend harmful products.
  • Oxidation and bearing corrosion inhibitors: antioxidants.
  • Viscosity index improves: improve viscosity-temperature relationship.
  • Four point depressants: gravity flow at low temperatures.
  • Extreme pressure anti wear additives for necessary load carrying capacity prevents scuffing.
  • Foam in inhibitors reduce the foaming tendencies.
  • Emulsifiers reduce the surface tension. The emulsifiers, assisting easy separation from water, mist suppression to reduce formation of oil mist.

Oil Fundamentals

  • Petroleum (mineral oils) or synthetic.
  • Petroleum oil = base fluid from crude oil + additives.
  • Synthetic oils: excellent low-temperature fluidity, low pour point, excellent oxidation stability, high flashpoint, etc., but more costly.

Test Methods for Oil Quality

  • Viscosity, pour point, total acid number, flash and fire point, oxidation test, foaming tendency, rust prevention, error release,gravity, color, work point, copper corrosion, carbon residue.

Contamination

  • Internally generated (wear of components).
  • Externally generated (dirt, water, fuel soot, products of oxidation/nitration/sulfation).

Good Lubricant Properties

  • Good cooling properties.
  • Nonsludging and foam resistant.
  • Correct viscosity.

Oil Film Bearings

  • Fluid film bearing: smooth, low-friction motion between solid surfaces.
  • Lubricant can be liquid, gas, or solid.

Aeroderivative Turbines Lubrication System

  • Two lubricating systems: one for aerogas generator (ball and roller bearings).

Role of Bearings

  • Control rotor position.
  • Balance forces.
  • Provide damping.

Bearing Types

  • Journal bearings: support radial loads.
  • Thrust bearings: support axial loads.

Bearing Material Selection

  • Material hardness, resistance to high temperature, higher fatigue strength, anti wear, anticorrosion properties.
  • Carbon, graphite, ceramics, and even plastics are used in different bearings. *Principal types of steel used in anti friction bearings, through hardened steels for heavy duty and in case the hardened steels with complexity of the design is involved.
    • M 50 Steel : premium melted alloy refined using vacuum induction melting and vacuum arc remelting, exhibiting excellent resistance to multi actual stress as in softening at high service temperature as well as in good resistance to oxidation.

Babbitt Journal Bearings

  • Lining bonded with Babbitt (low melting point soft alloy).
  • Soft alloy wears instead of journal/shaft in case of failure.
  • Excellent oil-lubricated bearing surface; good compatibility.
  • Disadvantage: lower fatigue strength at high temperatures.
  • Maximum design temperature should be below 149 degrees Celsius.

Factors Influencing Bearing Design

  • Load acting on bearings, shaft speed range, lubrication system type, contamination level, oil temperature, lubricant viscosity.

Fluid Film Bearings vs. Rolling Element Bearings

  • Key difference: expected life.
  • Rolling element bearings life is based on operating conditions.
  • Fluid film bearings can operate for decades when properly designed and maintained.

Operating Principle of Fluid Film Bearing

  • Stationary and rotary surfaces separated by lubricant film.

Modes of Lubrication

  • Hydrodynamic, hydrostatic, squeeze film, and hybrid.

Hydrodynamic Lubrication

  • Surfaces completely separated by unbroken lubricant film.
  • Motion causes film to thicken and pressurize.

Hydrostatic Lubrication

  • High-pressure fluid film created by external source (pump).
  • Maintains fluid film under high load and low speed conditions.

Squeeze Film

  • Separation based on viscous fluid between approaching surfaces cannot be squeezed out instantaneously.

Hybrid Mode

  • Combination of modes (e.g., hydrodynamic and hydrostatic).

Stribeck Curve

  • Non-linear function of contact load, lubricant viscosity, and lubricant entrainment speed.
  • Lambda Ration: parameter used to distinguish lubrication regimes.

Tilting Journal Bearings

  • Self-acting bearings; support rotors.
  • Higher load carrying capacity; accommodate misalignment.

Types of Seals

  • Gaskets, O rings, packing, noncontact type, non friction, and then contact type friction.

Noncontact clearance Type

  • There's is natural contact between the rotating and the stationary parts. So that's clear and sealed, reliable, and extensive application in high speed turbines.

Mechanical Contact Seals

  • Are used in high degree sealing applications, like your main shaft bearing houses or accessory drive shafts or shafts exits from the accessory gear case, and variable state of vane bearings in the compressor case.

Leakage

  • The engine schematic showing locations of various Liberum seals.

Ring Seals

  • Which the bores from small clearances around the shaft, floating and fixed based on whether they are fixed or free with respect to the stationary routing. The floating seals is classified into rigid type floating and segmented type. Then, medium contact between the rotor and stationary, again, makes these seals ideal for high speed rotating machinery.

Seal Assembly

  • Basic units: seat (mating member completing the precision face) and seal head (housing and end face member and the spring assembly).

Magnetic Seal

  • Simple mechanical seal variant where face loading action is by magnetic action.
    *It's magnetic ring fitted in the equipment housing, and it highly reliable, light, and compact.