Overview of Machinery Condition Monitoring and Diagnostics with Case Study

Roengchai Chumai

Technical Leader, GE Oil & Gas

Agenda

• Introduction

•Machinery Protection System

•Machinery Management System

•Common Machine Malfunctions

•Case Study

•Q & A

MACHINERY CONDITION MONITORING CONCEPT

Introduction

The first machine monitoring systems employed were human senses. Over time and through experience, a person would gain skill to sense when a failure was occurring.

Unfortunately, this experience was often gained through observation of an actual machine failure.

Machinery Protection System

Systems that shutdown a machine or return it to a safe or nondestructive mode of operation without human intervention.

Machinery Protection System

Critical Assets

• Gas Turbines

• Compressors

• Main line Pumps

• Recip Compressors

Online - continuous monitoring & protection

Basic Trends and Status only

Portable Data Collector Vibration Monitor

Vibration

Card

Machine Control System

Machinery Management System

Use of systems that provide data and information that is interpreted and applied by people to correctly operate, maintain, and assess the condition of their machinery.

Machinery Management requires

information!

Machinery Management System

Critical Assets

• Gas Turbines

• Steam Turbines

• Generators

• BFW Pumps

Essential Assets

• Fans

• Pumps

• Blowers

• Etc…

Online – periodic (scanning) monitoring

SCANNING Monitor

BOP Assets

Offline – portable (walk-around) + thermography, third

party devices & lube oil analysis

OFFLINE Portable

Data Collector

Switch/Hub

Online - continuous monitoring & protection

ONLINE Monitor

Machine Data

• Direct measurement

–Vibration and Position

–Rotor Speed

–Bearing Temperature

• Indirect Measurement

–Process data e.g. pressure, flow, molecular weight, MW, MVAR, etc.

–Performance data

Vibration Measurements

• Overall Vibration Amplitude

• Position

• Frequency

• nX Filtered Vibration Amplitude & Phase

• Shape or Form

You must have access to this information to properly manage your machine.

Steady State

Transient

Slow Roll

Stopped

A.

B.

C.

D.

Constant Machine Speed Process or Load Change

Start Up Shut Down

:

:

Machine Operating Modes

COMMON MACHINE MALFUNCTIONS

Machine Malfunction Identification

“When vibration is viewed as a ratio of forces to stiffness, the perspective changes and the focus becomes ‘What has changed in the machine, the forces acting on it or its stiffness? ‘rather than simply‘ more vibration is

bad, less vibration is good.’”

Stiffness Dynamic

Forces ResponseVibration

Mass

StiffnessFrequency Natural

Shaft Bow

Unbalance

Misalignment

Misalignment

Causes of Misalignment

1. Thermal growth

2. Foundation problems - include cracked grouting, a loose soleplate, and loose anchor

bolts

3. Soft Foot – the case is wrap because one or more machine feet are not coplanar after

tightening hold down bolts cause from inadequate shimming or by an excessive number

of shims

4. Piping strain

5. Improper alignment

Misalignment

Effects

1. Rub - cause the rotor to wipe bearings and seals

2. Shortened Bearing Life - by rub and

temperature

3. Damaged Seals/Packing - deflect shafts and

push seals and packing outside design limits

4. Cracked Shafts - deflect beyond the design limits

can create high reversal stresses

5. Shortened Coupling Life - by producing

extreme heat in elastomer couplings

Rubbing

Causes of Rub

1. Radial Load

2. High Vibration

3. Axial Thrust

4. Locked Seals

Rubbing

Rubbing

Shaft Crack

Shaft cracks begin in regions of high local stress. Shafts are subjected to large-scale stresses due to static or dynamic bending and torsional twisting, static radial loads, or residual stresses from heat treatment, welding, or machining operations.

Shaft Crack

Shaft Crack

Shaft Crack

Fluid Induced Instability

When a rotor operates in fluid-induced instability, it is actually operating in a stable limit cycle of high vibration. But the rotor is unstable in the sense that it is operating outside desired operational limits.

Fluid Induced Instability

Many Other Malfunctions

• Surging & Rotating Stall in Compressor

• Pump Cavitation

• Super synchronous instability in Pump

• Combustion Rumble in Gas Turbine

• Thermal Sensitivity in Generator

• Crack/Broken Rotor Bar in Motor

• Rough Load Zone in Hydraulic Turbine

• Synchronous Instability (Morton Effect)

• Torsional-Lateral Coupled Vibration

• Structure/Resonance Induced Vibration

• etc.

MACHINERY DIAGNOSTICS CASE STUDY

• The unit was run at normal operating condition.

• Fluctuating vibration amplitudes observed at Brg#3 & #4 of IP turbine

• Contacted machinery diagnostic services engineer to site for vibration analysis

Case Study#1-Steam Turbine-Generator Set

Machine Information

• Steam turbine-generator set, 762 MW in coal fired power plant

• Well instrumented with X-Y probes each bearing plus a Keyphasor®

• Problem noted at Brg#3 & #4 of IP Turbine

Direct & 1X Trend Plots – Brg#3

50 → 105 microns, pp 105 → 50 microns, pp

50 → 120 microns, pp 120 → 50 microns, pp

Trend Plots – Brg#3 & #4

Polar Plots – Brg#3

Polar Plots of Brg#3 & #4’s

Waterfall Plot of Brg#3’s Vibration

1X Vibration

Waterfall Plot of Brg#4’s Vibration

1X Vibration

Plus Orbit Shapes vs. Trend

Obstructed Rotor Movement shown in Orbit Shape

Conclusions & Recommendations

• High vibration amplitude at brg#3 was due to increasing 1X vibration

• Synchronous (1X) vibration changed in amplitude and phase at brg#3 & #4

• Obstructed orbit shape was observed at brg#3

• It was suspected rubbing around brg#3 (e.g. oil deflection) and recommended to inspect at the next opportunity available.

Inspection Results

Coke Deposits

Rubbing Marks

Case Study#2 – Hydro turbine-Generator Set

• Francis runner type hydro turbine & reversible pump

• Rated output at 145 MW at turbine mode

• XY probes installed at all three guide bearings

• Online machine condition monitoring system installed

• It is reported excessive vibration at turbine guide bearing when reducing load before shutdown

Turbine Speed & Vibration Trend vs. Spectrum

Turbine Speed

1X Phase

Overall Amplitude

1X Amplitude

0.35X

Overall vibration amplitude of turbine increased due to 0.35X component.

Unstable Orbit Shape w/ Predominant 0.35X

368 – 513 um, pp@08.30.02 hrs.

382 – 565 um, pp@08.30.14 hrs.

Turbine bearing clearance is 500 um

Be aware of bearing worn out!

Spectrum of Unstable Vibration

102 um, pp@1X

232 um, pp@0.35X

Conclusions & Recommendations

• Unstable vibration occurred in short period when reducing load before shutdown

• Predominantly at subsynchronous vibration at 0.25X to 0.35X with unstable orbit shape

• Above characteristics are recognized as “Rough Load Zone” or Spiral Vortex in draft tube

• Recommendations

• Avoid operation at 25% to 70% wicket gate opened

• Apply air injection at draft tube, if above load range cannot be avoided

Vortex Interaction – Rough Load Zone

Zone of flow separation and vortex formation

Spiral vortex filaments

Comment! Or Question?