Mischievous Magnetism in Rotating Machinery - Paul Nippes (MPS)

”Safeguarding Machinery Worldwide “www.gaussbusters.com

Paul I. Nippes, P.E.Beth Galano

Magnetic Products and Services, Inc.Holmdel, NJ 07733

www.gaussbusters.com

Copyright 2005

Mischievous MagnetismIn Rotating Machinery


“Stray Magnetism” denotes “lost” or misplaced magnetism that may, or may not, cause troubleMischievous Magnetism (MM) is trouble-causingSources, paths and effects consist of:

asymmetry in magnetic patterns of electrical machinesresidual magnetism circular magnetism in ferrite components

Damaging effects to rotating machineryElectrostatic current effects are not addressed

Paper Content


Magnetism is friendly when usefully-employed for energy conversion or instrumentationMischievous magnetism is destructive, and/or impedes proper performance of rotating machinery

character and strength of MM can vary considerably direct measuring of MM is impossible when the machine is operating

Many rotating machinery problems, caused by MM, go un-detected or un-corrected

“This is a four-month bearing”Magnetism may also be blamed for mysterious occurrences Proving or disproving effects of MM can be very challenging

Mischievous Magnetism (MM)


Sources and Effects of MM


Electromagnetic Sources and their Effects in Electrical Machines

A1 Uneven Air Gap Axial Voltage Bearing Through-Currents

A2

A3

A4

A5

A6

A7

A8

Shorted Stator Laminations Axial Voltage Bearing Through-Currents

Shorted Rotor Winding Turns Axial Magnetism Homopolar Bearing Currents

Rotor Winding Ground Faults Axial Magnetism Homopolar Bearing Currents

Stator Winding Circuit Imbalance Axial Voltage Bearing Through- Currents

Shorted Stator Winding Turns Axial Voltage Bearing Through Currents

Electrical System Unbalance Axial Voltage Bearing Through Currents

Excitation Power Switching Capacitive Coupled Bearing Through Currents

Cross Reference Codes to Figures Aa and Ab


Figure Aa


Boiler Feed Pump from a large Generator Unit


Boiler Feed Pump from a large Generator Unit


Figure Ab


Example of Through Magnetism generating Eddy Currents


Example of Through Magnetism generating Eddy Currents –currents discharge at outside edges passing through the babbitt


Residual Magnetism in TG Components

B1 Axially Oriented Homopolar Bearing Currents

B2 Radially Oriented Axially at Bearing

Homopolar Bearing Currents

B3 Circular Encircles the Shaft centerline internally

Homopolar Effects

B4 Localized/concentrated in pole pairs

Random Local Currents in:Journal BearingJournalThrust BearingThrust CollarCasing, frame or housing

Cross Reference Codes to Figure B


Figure B


Electric Current Erosion of Thrust Bearing Padsfrom a Natural Gas Compressor

Spark Tracks on the Active Pad Face

Pad Steel Backing Erosionon Two Adjacent Pads


special case because it has complete invisibility

accidental direct through-current injection

generator rotor winding fault to ground, passing current axially through the rotor shaft

intentional direct through-current injection

common practice called “head shots” - passing direct current axially through the shaft, inducing circular magnetism adequate for magnetic particle indication* of surface axial cracks.

*Admittedly, MPI is the most efficient and revealing methodfor finding and evaluating such cracks, however magnetism remains,unless completely removed following testing via Down-Cycle Demagnetizing.

What is Circular Magnetism?



Large Turbine-generator showing Strange VibrationIndicated Are Responses On One Unit

when Shaft Grounding Brush was Lifted and then Restored

27/05/20057:29:09 AM

28/05/20057:29:09 AM

27/05/20057:29:09 PM

0.000

0.048

0.096

0.144

0.192

0.240

GEN BRG6 VIBR (MyInSQL:U1GV005) GEN BRG6 VIBR (MyInSQL:U1GV005) CyclicCyclic(28/05/2005 7:06:52 AM) 0.028 mm(28/05/2005 7:06:52 AM) 0.028 mm

0.011 mm (0 days, 15:27:36)0.011 mm (0 days, 15:27:36)(27/05/2005 3:39:17 PM) 0.039 mm(27/05/2005 3:39:17 PM) 0.039 mm 0.011 mm (0 days, 15:27:36)0.011 mm (0 days, 15:27:36)

No. Server TagName Minimum Maximum Units Description:1 MyInSQL U1GV005 0.00000 0.24000 mm GEN BRG6 VIBR2 MyInSQL U1GV004 0.00000 0.24000 mm GEN BRG5 VIBR3 MyInSQL U1TE003 -35.00000 15.00000 mm LP TURB DIFF EXPN4 MyInSQL U1DT059 0.000 50.000 C AMBIENT TEMP (BOILER BASEMENT)5 MyInSQL U1GK002 0.000 100.000 C GEN BRG6 DRN OIL TEMP6 MyInSQL U1TP011 0.000 20.000 kPa COND ABS PRES (MEASURED)7 MyInSQL U1BT608 100.000 600.000 C RH OUT AVG TEMP8 MyInSQL U1GC012 -100.000 400.000 MW 1 UNIT MW9 MyInSQL U1TZ001 2.00000 2.10000 mm TURB ROTOR POSN10 MyInSQL U1GL609 0.000 140.000 kPa SEAL OIL/H2 DIFFERNTIAL PRES11 MyInSQL U1TS001 0.000 3500.000 RPM TURB SPEED12 MyInSQL U1GT006 0.000 100.000 C C H2 CLR H2 IN TEMP

Brush

Up Down

27/05/20057:33:20 AM

28/05/20057:33:20 AM

27/05/20057:33:20 PM

0.000

0.060

0.120

0.180

0.240

0.300

GEN BRG5 VIBR (MyInSQL:U1GV004) GEN BRG5 VIBR (MyInSQL:U1GV004) CyclicCyclic(27/05/2005 8:40:10 AM) 0.082 mm(27/05/2005 8:40:10 AM) 0.082 mm

-0.054 mm (0 days, 22:38:59)-0.054 mm (0 days, 22:38:59)(28/05/2005 7:19:10 AM) 0.028 mm(28/05/2005 7:19:10 AM) 0.028 mm -0.054 mm (0 days, 22:38:59)-0.054 mm (0 days, 22:38:59)

No. Server TagName Minimum Maximum Units Description:1 MyInSQL U1GV004 0.00000 0.30000 mm GEN BRG5 VIBR2 MyInSQL U1GV005 0.00000 0.30000 mm GEN BRG6 VIBR3 MyInSQL U1TV001 0.00000 0.30000 mm TURB BRG1 VIBR4 MyInSQL U1TV002 0.00000 0.30000 mm TURB BRG2 VIBR5 MyInSQL U1TV003 0.00000 0.30000 mm TURB BRG3 VIBR6 MyInSQL U1TV004 0.00000 0.30000 mm TURB BRG4 VIBR7 MyInSQL U1TP011 0.000 20.000 kPa COND ABS PRES (MEASURED)

Brush

Up Down


HIGH VIBRATION IN DUAL SHAFT GROUNDING BRUSHES.NORMAL VIBRATION WHEN ONE GROUNDING BRUSH IS LIFTED


ATESTFOR

CIRCULARMAGNETISM


Magnetism Fundamentals


1. Shut down unit (or, at least, partially disassemble) to get access for magnetizing coils and measuring instruments

2. Auto-Down-cycle a component in the primary, or possibly all three axes, to determine the presence and orientation of residual magnetism that it may possess.

The programmed sequence of auto-down-cycling isthe key to evaluating residual magnetism.

MPS REMOVAL OF ANY MAGNETISM


Effective Down-cycling


If Auto-Down Cycling :a) Reduces MM; Repeat down-cycling to minimize.

b) Makes small change in MM; Repeat down-cycling with repositioned coil and at reduced power levels.

c) Increases axial magnetism notably; Indicates reorientation of magnetism from cross-wise or circular mode to an axial mode. Continue down-cycling as axial magnetism increases and then drops to the lowest value possible. Reduce further according to item a) above.



Unusual Effects Related to MM

1) Vibration and Position ProbesInaccurate physical vibration indications:

due to saturation of the shaft probe track by magnetism saturated steel has the appearance as air gap to the probe

Magnetic bases placed onto a probe track show as sharp pulses on the vibration orbit.


2) EfficiencyCirculating eddy currents

consume considerable energy that goes into the product as heatprimarily in turbines and compressors Evidence of this shows as melted steel puddles at contact faces and partingsmay never be indicated by unit monitors or instruments


3) Damage often requiring shutdown

Journal, anti-friction and thrust bearings suffer from pitting, sparking and arcing

Melted steel voids and puddles requiring component replacement or machining


bearinghousing

3 padbacksides

face of bottom pad

Turbine Bearing Housing and Pads Followinga Severe Crash Due to Residual Magnetism


FLUTING OF TAPERED ROLLER BEARINGCAUSED BY ELECTRICAL CURRENTS


Shaft Current EffectsTHRUST COLLAR BEARING

FAILED

EROSIONFLUTING


Stray current melting at flange parting indicating high losses and reduced unit efficiency from high residual magnetism


Mischievous Magnetism can cause:considerable damagereduced efficiencyunit down time

To prevent:investigative and establish a prevention program and follow it. Trend with Shaft Condition Monitors in addition to traditional unit monitors

confirm early warning and indications

Summary and Conclusions

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Mischievous Magnetism in Rotating Machinery - Paul Nippes (MPS)

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