“If it ain’t broke, don’t fix it”

Ray’s Big 5 CM wins – stories where

condition monitoring paid off big time!

Vibration analysis- the main CM

technique

Diagnosing causes of vibration

Amplitude

Direction (H, V, A)

Frequency

Phase of 1X vibration

…and how these vary with operating conditions (speed, load, etc.)

…use diagnosis charts to find likely cause/s. (Even an iPhone App)

REDUCTION AT

SOURCE

Balancing

Balance magnetic forces

(motors)

Fix clearances or

looseness

Reduce aerodynamic

effects REDUCTION OF RESPONSE

Change natural frequency of structure:

Add (or reduce) mass

Change stiffness

Increase damping

Detune with dynamic vibration absorber

ISOLATION

Isolate the source

Isolate affected equipment

m

kf

Control of vibration

23/12/2015

Hydraulic power

pack: motor

bearing failures

Vibration analysed

to get spectrum

Vibration

velocity –

log scale

Motor: 2970

r/min

5 piston

swash plate

pump

Peak vibration 45

mm/s rms !!!

at 2970 r/min

Vibration at

10X from

piston

strokes:

unchanged

Vibration at

2X from

misalignment

: unchanged

After motor-pump

base beam stiffened

Overall vibration

only 5 mm/s rms,

with 1.5 mm/s rms

at 2970 r/min

The 5MW fan that was different

Motor 24t, 4m

above ground

level

4 fans: but only this one had very high

vibration @ 1X (738 r/min, or 12.3 Hz)

Bearing structure stiffened with

vertical tie bolts. Vibration reduced

80%

But, vibration again increased – motor

was moved 2.5mm axially to get rotor

on its magnetic centre

So, mass was added….

Again, vibration was just acceptable

Masses up to 5t added to de-tune system

…

Soil checked: OK

(possible that

underground

water affected

foundation

stiffness)

Bearing sliding

supports

modified.

Vibration

increased

steadily- then

bearing failed.

Major off-line

investigation

Modal analysis

23/12/2015

Bearing vibration only 2.3 mm/s.

Fan has run OK since.

Steam turbines

– still the

mainstay of

power

production 350MW

23MW

500MW

Overall condition indicator:

Valves Wide Open test

• Control valves WIDE open

• Steady conditions

• May need to reduce inlet steam pressure

• Test readings: key temperatures, pressures, MW (or panel instruments if proven)

• No special test flow measurements

• MW output adjusted for

variations from rated values.

Test data TEST A Correctn factor

TEST B

Correctn factor

Generator Output MW 355.8 349.7

Steam Pressure - Main kPa 12155 1.02285 12255 1.02053

Steam Temperature - Main °C 529.5 0.99832 526.7 0.99773

Steam Temperature - Reheat °C 525.8 1.0101 539.5 0.99873

Reheater Pressure Drop % 6.76 0.99814 6.03 0.99633

Condenser Pressure - kPa 9.34 1.01225 12.44 1.03615

Generator Power Factor 0.923 1.00012 0.945 1.00064

Steam Temp. Cont. Spray - Main kg/s 6.5 0.99889 24.6 0.99584

Steam Temp. Control Spray - Reheater kg/s

0 1 0 1

Final Feedwater Temperature °C 234.9 1.0005 230.5 0.98957

Combined correction factor 1.04741

Corrected VWO Output MW 372.7

Unit had a record run of 6

months continuous on line

service….

Test data TEST A Correctn factor

TEST B

Correctn factor

Generator Output MW 355.8 349.7

Steam Pressure - Main kPa 12155 1.02285 12255 1.02053

Steam Temperature - Main °C 529.5 0.99832 526.7 0.99773

Steam Temperature - Reheat °C 525.8 1.0101 539.5 0.99873

Reheater Pressure Drop % 6.76 0.99814 6.03 0.99633

Condenser Pressure - kPa 9.34 1.01225 12.44 1.03615

Generator Power Factor 0.923 1.00012 0.945 1.00064

Steam Temp. Cont. Spray - Main kg/s 6.5 0.99889 24.6 0.99584

Steam Temp. Control Spray - Reheater kg/s

0 1 0 1

Final Feedwater Temperature °C 234.9 1.0005 230.5 0.98957

Combined correction factor 1.04741 1.03521

Corrected VWO Output MW 372.7 362

Enthalpy

Drop

Efficiency

P1T1

P2

T2

T3P3

Expansion line

Enthalpy

Entropy kJ/kg °K

kJ/kg

Isentropicenthalpy

drop

Actual

h drop Ideal

h drop

Actual h drop

Ideal h drop

Usually 85–

90%.

Lowers with

damage or

blade deposits.

Enthalpy, entropy from temperature and pressure.

-4

-3

-2

-1

0

1

2

3

4

5

6

0 1000 2000 3000 4000 5000 6000 7000 8000

VWO

IP effy

IP PR

% Deviation in CM parameters with hours in service

350MW reheat unit – 3 casings

Blading stages: 8 HP, 6 IP, 6 LP.

Steam forced

cool run @

7400h

Pressure

Ratio

Inlet/Outlet

also handy

IP Blading

500MW

HP, IP, 2 x LP

Other use of enthalpy/entropy plot

Symptom: burnt paint on an LP

hood Temperature well above usual 40 degC = internal leakage of hot steam into exhaust space

Likely cause: failure of outer bellows in steam inlet piping.

Pieces of bellows found inside condenser, so temporary repair

Outer bellows

leak - into LP

hood

P1 T1

P2

T 3

P3

Expansion line

Enthalpy

Entropy kJ/kg K

kJ/kg

Isentropic

enthalpy

drop

Saturated steam zone

A

T2

Temperature usually at

saturation - if greater, steam

is superheated, has

bypassed blading.

THEN …..255 °C

steam inlet noticed

in LP2 feedwater

heater piping - 95 °C

is usual

Expansion

bellows

failures.

914mm diameter,

two sets in each

of 4 pipes.

Another type 500MW unit. VWO Output from

DCS trend close to special tests

VWO from plant

instruments

VWO using special

test instruments

R

Inlet strainer and blading

blockage –turbine

troubles detectable

by performance analysis

Massive turbine vibration

HP

P

LP

Generator 3000 r/min Gearbox and Exciter

Journal bearings

Coupling

120MW steam

turbine

generator, 17

years’ service

Generator

was balanced

in situ: novel

method.

On return to

service,

vibration went

off scale!

Generator balancing had been done - with boiler off

line!

Coupling disconnected

Exciter wired to run as motor

Rope wrapped around rotor, to

overhead crane to

overcome initial inertia

When rolling,

exciter (i.e. “motor”) switched on, raised

rotor to 3000 r/min

for balancing

runs

Jacking oil supplied

at bottom to lift

rotor, reduce start-

up friction

Pump is stopped

when machine

> 600 r/min.

Local pressure gauges

now read the oil film

“wedge” pressure –

proportional to load

Vibration on return to service ..!!!

All OK...until

speed reached

about 2950

r/min Generator

vibration

increased

suddenly, got so

great that turbine

was shut down Starting up

tried again but

problem

remained

Vibration

analysis

instruments

installed

0 20 50Vibration frequency Hz

Vibrationvelocity

Vibrationincreasing @19.5Hz

19.5 Hz (1190 r/min) is the First Critical

Speed of the generator rotor…

Vibration transducers installed, analyser on PEAK HOLD, turbine started up

The turbine is synchronised and loaded OK

Vibration started when

Auxiliary Oil Pump was

stopped (Main Oil Pump

on the rotor line takes

over at near 3000 r/min).

19.5Hz vibration still

present, could be

increased and

decreased by varying

cooling water flow to

oil coolers.

Aux pump was left

in service, and the

turbine loaded

without high

vibration.

Pump was

stopped with no

effect (phew!).

The facts

• Bearing stability? - Available references searched

– Cause: “Resonant Whirl”

– Solution: “modify bearing”

BUT…. “Turbine has been in service for 17 years with no problem!”

Vibration at

frequency of

1st Critical

Speed, while

rotating > 2X

this speed

A trigger -

sudden drop in

pressure of oil

supplied to

bearings when

Auxiliary Oil

Pump stopped

(125kPa to

70kPa)

Changing

oil

temperature

(i.e. its

viscosity)

altered

vibration

ESDU

66023 Engineering Sciences

Data Unit (IMechE, UK)

2

/

d

c

Nbd

WW d

e

Symbol Description Details Data

W Load on the

bearing

Rotor mass is 32072 kg.

With a semi-flexible

coupling, it should be

evenly shared between

the two bearings.

….confirmed: wedge

pressure readings

similar at 200 kPa

157313N

2

/

d

c

Nbd

WW d

e

Symbol Description Details Data

e Dynamic

viscosity of oil

in the bearing

Heavy grade turbine oil

(VG 68).

Oil draining from the

bearing is close to the

average temperature in the

bearing. Control Room

instruments used.

Temperatures varied

between 40°C and 71°C.

64 × 10-3

N.s/m² @

40°C;

13× 10-3

N.s/m² @

71°C

2

/

d

c

Nbd

WW d

e

Symbol Description Details Data

N Rotational

speed

Full speed was used

50rev/s

b Length of

bearing

Measured on bearing

0.392m

2

/

d

c

Nbd

WW d

e

Symbol Description Details Data

d Diameter of

bearing

From plant data –

the journal diameter

(b/d = 1.03)

0.381m

cd Diametral

clearance –

bearing to

journal

Measured at the

maintenance

outage.

0.406 mm

to

0.686 mm

2

/

d

c

Nbd

WW d

e

Sommerfeld Number Load parameter W’

0.1

1.0

10

0.1 0.5 0.9

Eccentricity ratio

Oil 71°C

Oil 40°C

Increased risk of half-frequency whirl

Original operation

Operation when bearings modified

Recommended area

Lines of increasing constant b/d

Bearing too short

ESDU66023

plot

2

/

d

c

Nbd

WW d

e

Operating

points

calculated,

plotted

Bearing instability can occur!

Recommended: shorten bearing - to same b/d ratio as newer turbine of same make.

Differences found between spare parts, plant drawings!

Adjacent twin machine found to have short bearings!

• Bearings shortened -

machine returned to

service

• Vibration problem solved

• …. 1st Critical Speed

was higher - 1320 r/min.

World’s most

common machine

(after motors)

Use 25% of

world’s total

motor-driven

electricity,

….or about 6.5%

of global

electricity

production!!

Pumps

Erosion of

impeller

Sealing

rings

Ring section diffuser

pump

Internal

leakage

Increased clearance

increases recirculation

Erosion at

sealing rings

Pump internal wear

Head

H

Flow Q

Internal leakage

recirculation

H-Q with wear

Pump internal wear

0

5

10

15

20

0 500 1000 1500

Days in service

% R

ed

ucti

on

in

he

ad

230kW Cooling water pump

degradation

Increasing internal

leakage reduces

Head at chosen

datum flow

Close to linear for 4500kW pump, too

y = -0.155x2 + 0.4907x - 0.1388

-12

-10

-8

-6

-4

-2

0

2

0 1 2 3 4 5 6 7 8 9 10

% r

ed

uc

tio

n in

He

ad

@ d

atu

m

flo

w

Time: years since overhaul

Boiler Feed Pump wear trend

Effect of increased internal wear in

relates to Specific Speed:

Using data at Best Efficiency Point:

N = Rotation speed, r/min

Q = flow per impeller eye, m³/h

H = head per stage, m

(Number resulting is close that from

US units)

75.0H

QNNs

0

2

4

6

8

10

12

14

16

18

20

0 1000 2000 3000 4000 5000

Specific Speed (US units)

% Increase in

power

Clearances worn to 2X

design

Clearances worn to

1.5X design

Head-Flow method for CM

At around normal duty point is enough.

Checks condition of pump AND its system.

Repeatable pressure and flow measurement needed,

and speed for variable speed pumps.

Optimum time for overhaul - on energy

saving basis (1)

1 Pump wear causes

drop in plant production

2 Pump duty is

intermittent to meet

demand

• Overhaul readily

justified

• Wear means extra

service time and extra

energy

Optimum time for overhaul - on energy

saving basis (2)

3 Pump wear does not affect plant production, at least initially. Constant speed, output controlled by throttling – monitor control valve position

4 Pump wear does not affect plant production, at least initially. Output controlled by varying speed –monitor pump speed

•Same basic method applies...

An example-

Boiler Feed

Pump in PS

Pump overhaul cost - $50 000

Cost of power

3c/kWh

Pump runs for

90% of time on

average

Tested at 24

months since last

overhaul

Extra power

used = 2300

– 2150 =

150kW

÷ motor efficiency to get extra power consumed by motor/pump combined…

= 167kW

Extra power cost: (720h is average month) =

167 × 0.03 × 0.90 × 720

kW $ % h

= $3246/month

The average cost rate of deterioration:

$ 3246 ÷ 24 =

$135 /month/month

The optimum time for overhaul:

= 27.2 months

C

OT

2

Total cost curve

often fairly flat

around the

optimum

The method does not apply to all

pumps…..

Small pumps may cost more to test than overhaul, energy costs too small to justify work?

Pumps of Specific Speed above 2000 have flat or declining Power-Flow curve, so increased leakage does not use more power

Conclusions

Condition

monitoring is an

exciting activity

with big benefits

CM is much

more than

vibration analysis

Performance

analysis adds the

energy-saving

dimension -

USE IT

raybeebemcm@gmail.com

FedUni programs in Maintenance and Reliability Engineering: on-line distance learning (open to all: conditions apply) (were

Monash Uni)

Happy Monitoring !