20 Lubrication

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Text of 20 Lubrication

Lubrication of rotating equipment

Oil characteristics and more

Ronald Bakker Shell Global Solutions

Shell Global Solutions International B.V., 2010. All rights reserved.

Contents Introduction Basic turbine lubrication system Turbine lubrication requirements What is a turbine oil & what properties does it need? How do we test and evaluate these properties? Oil and oil system cleanliness Field cases Oil Condition Monitoring Questions

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1

Common Turbine Lubrication System Features

Oil tank Oil pump Oil cooler Oil supply/return pipes Plain journal bearings Thrust bearing Turbine speed control system & valves Filters Gearbox in geared systems

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Typical Gas Turbine Lubrication Systemcooling air

compressorair out

Power turbine

Generator

oil demister

IGV

duplex filter lube oil pump hydraulic oil pump

MOOG valves servo protection filters

extra by pass filter

Oil reservoir Either combined for bearing and control system or separated (2 tanks: 1x bearing oil & 1x hydraulic oil for control system)

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2

Industrial Steam Turbine - working

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Industrial Gas Turbine

Simplified turbine schematic3. Combustion chamber 4. Expansion turbine 6. Exhaust

5. Turbine outer casing

2. Compressor

1. Air intakeSiemens SGT6-6000G Gas Turbine 300 MW to 500 MW for Combined Cycle Applications

Picture courtesy of Siemens web site

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3

Industrial Gas Turbine - working

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Schematic of Steam Turbine Lube System

Main lube filter Main pump HP Thrust bearing Main lube oil tank Vacuum extract pump IP

To bearings Hydrogen sealing LP LP Jacking oil feed LP Generator and exciter Stator water cooling

Purifier Stages (HP, IP, LP) BearingsLube oil purifier

Control system not shown

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4

Turbine Components Requiring Lubrication Journal bearings (Hydrodynamic) Used to support the weight of the turbine rotors. A journal bearing consists of two half-cylinders that enclose the shaft and are internally lined with Babbitt, a metal alloy usually consisting of tin, copper and antimony

Thrust bearings (Hydrodynamic) Axially locate the turbine rotors. A thrust bearing is made up of a series of Babbitt lined pads that run against a locating disk attached to the turbine rotor

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Turbine Components Requiring Lubrication

Pocket type bearing (jacking hole in centre)

3 wedge bearing (jacking hole off centre)

Tilting pad thrust bearing Tilting pad journal bearing

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5

Typical Turbine and Bearing Oil Flow

SWF File

Hydrodynamic Shell Global Solutions International B.V., 2008. All rights rights reserved. Shell Global Solutions International B.V., 2010. All reserved.

Hydrodynamic Oil Wedge Principle At rest, metal-to-metal contact, no oil film layer is present.JOURNAL

As the journal begins to rotate, it tends to climb up the bearing and onto a layer of oil. This reduces friction and allows the journal to slide. Increase in rotational speed drawns oil into the wedge-shaped clearance space, and fluid pressure is developed between the journal and bearing. At full journal speed, the converging wedge exists under the journal, and a minimum film thickness exists to one side of the bearing: Hydrodynamic lubricationPRESSURE FORCES DIAGRAM

BEARING

PRESSURE ZONE

HIGH PRESSURES ZONES

Any Deposit will reduce running clearance and increase bearing temperature.

Picture courtesy of PALL

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6

Turbine Components Requiring Lubrication (cont.) Jacking Oil System: During turning, high pressure jacking oil is used to increase oil film thickness. Also to float the shaft before starting rotation from rest During start-up and shut down the rotor must be rotated slowly (barred) to avoid uneven heating or cooling which would distort or bow the shaft & to prevent them settling in the bearings, due to weight resting on one spot A barring mechanism or turning gear is used to do this

Jacking oil creates hydrostatic lubrication during turning

SWF File

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Steam Turbine - Control system

Steam turbines use a control system to operate the steam valves This is to control the turbine operational speed (governor) Hydraulic fluid is used to power this system High hydraulic pressures (possible leaks) Steam pipes are above auto-ignition temperature of mineral oils ( 200C

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Key property #2: Air Separation and Foaming

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18

Aeration and Foaming

Foaming Oil surface

> 1 mm dia. air bubbles Rise rapidly to surface Burst or produce foam

Aeration Oil Reservoir

10-3 to 1 mm air bubbles Entrained in oil Slow to rise to surface

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Aeration and foaming

Oil condition invariably diagnosed as foaming problem Majority of cases actually caused by entrained air & poor ARV Addition of silicone anti-foamer seriously worsens aeration Silicones cannot be easily removed once added

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19

Effect of silicone anti-foam agents on ARV

Air content (% vol)

10 8 6 4 2 0 0 2 4 6 8 10

Anti-foamzero

0.2 ppm 2.0 ppm

12

Time (minutes)

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Effect of oil reservoir design on deaeration

d

d

y

Oil volume = constant Residence time = constant Bubble rise time (DEAERATION) @ depth d, dx

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20

Dangers of Excessive Aeration and Foam Loss of oil pressure Reduced oil flow Increased oil compressibility Failure to reach minimum pressure Poor response in high pressure servos Local oxidation of oil Adiabatic compression of air bubbles Highly loaded areas e.g. thrust bearings Possible blackening of white metal bearings Filter blocking Excessive pressure drop associated with no apparent contamination

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Visible Foam in Oil Reservoir

Breaks in foam layer

Excessive Foaming

Acceptable Foaming Shell Global Solutions International B.V., 2008. All rights rights reserved. Shell Global Solutions International B.V., 2010. All reserved.

21

Principle Causes of Excessive Aeration Inadequately specified new oil Presence of silicone anti-foam agents System design and/or operation Excessive oil circulation rate Vertical section in oil return line Oil cascading down from excessive height Highly aerated oil fed too close to suction strainer Air leaks in pump suction system Oil pressure too high Vacuum entrainment on high speed plain bearings

Excessive build up of oxidation products Basic metal salts and/or greases

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Aeration and foaming: equipment and operation Excessive oil circulation rate Vertical section in oil return line Oil cascading down from excessive height Highly aerated oil fed too close to suction strainer Air leaks in pump suction system Oil pressure too high Vacuum entrainment on high speed plain bearings

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