TDBFP Gear-Coupling Misalignment & Diagnosis

TDBFP Gear-Coupling

Misalignment & Diagnosis

by

Sh. T. K. Rao & Sh. L.K. Nayak

NTPC, Talcher-Kaniha

TDBFP - Criticality

TDBFPs are major Turbine Auxiliary vital to Power generation and at

the same time they help in improving overall cycle efficiency.

Unavailability of TDBFPs leads to running of MDBFP thereby

increases Auxiliary Power Consumption.

Availability & reliability of TDBFPs play a major role in Power plant

performance, and assumes much higher significance under PAT

regime.

TDBFP - Basic Configuration

TDBFPs of TSTPS:

Stage-I:

Single stage booster pump & 4-stage main pump (Make:

Worthinton Pumps, Italy) & single flow 6-stage nozzle governed

Turbine (Make: Ansaldo, Italy).

Pump capacity: 1038 m3/hr @ 227.5 KSC at 5680 RPM.

Stage-II:

Single stage booster pump & 4+1-stage main pump (Make:

BHEL) & single flow 16-stage nozzle governed Turbine (Make:

BHEL).

Pump capacity: 895 m3/hr @ 222.5 KSC at 5465 RPM.

TDBFP Machine-Train Configuration

TDBFP - Alignment Facts

Complexities of TDBFP Alignment

Precision alignment of coupled shafts within acceptable tolerances is

necessary to avoid undue stress & premature failure of components.

Alignment of multi-shaft machine is a cumbersome task, and gets

further complicated where different sub-equipments have significantly

different thermal growth from cold-to-hot running conditions like TDBFP.

Manufacturers recommend cold-offsets to be provided during alignment

to take care of thermal growth issues.

Gear-couplings have also been provided to accommodate thermal

expansion & some amount of misalignment to minimise undue stress

on shafts, bearings, seals, etc.

Early detection of misalignment can help in reducing component

failures due to undue stress thereby increase equipment availability &

reliability.

Misalignment Detection

Misalignment Characteristics

Misalignment disguises itself well on machinery and what we observe

are secondary effects of misalignment as it slowly damages machine

components.

Common effects of misalignment are stressed components, elevated

temperature, premature bearing, seal, coupling or shaft failures,

excessive leakage at seals, etc.

Tools & techniques commonly used for detecting any coupling &

alignment problems during running of the equipment is monitoring of

parameters like vibration, temperature, etc.

Misalignment Detection...

Misalignment Detection through Vibration

Amount of misalignment cannot be directly reflected by vibration

because of the followings:

Misalignment exerts static forces on machinery components,

whereas Vibration is the system response to dynamic forces on

machinery.

Vibration sensors measure Motion, and not Force.

Modified dynamic behaviour of machine components under stress

due to misalignment is measured through vibration.

Amount of vibratory motion is dependent on mass, stiffness &

damping, and change in any of these characteristics can change

vibration amplitude without any change in force.

The vibration amplitude cannot solely & accurately reflect the

misalignment condition & its severity. In fact, the vibration amplitude

increases at times after correcting misalignment condition.


Additional Online Tools for Detecting Misalignment

When machine components are stressed due to misalignment, they will

definitely generate heat during motion, and that can be detected

through thermal measurement.

Temperature change due to misalignment is also found to be varying

almost linearly with change in alignment.

Temperature monitoring or thermography of coupling & inboard

components can be used to detect misalignment.

Vibration Characteristics of Gear-Coupling Misalignment & Wear

Misalignment can usually be detected by following vibration symptoms:

High 1X or 2X vibration components

Comparatively higher axial vibration levels

Out of phase vibrations across the coupling


Vibration Characteristics of Gear-Coupling Misalignment & Wear...

These symptoms can appear during misalignment, but not always.

Vibration spectral pattern due to misalignment will frequently be

different depending on type of coupling, shaft & support bearing.

Vibration signatures from misalignment of equipments with flexible

couplings have components at other multiples of running speed like 3X,

4X, 6X etc, but the phase angle information makes sense only if the

predominant vibration component is 1X.

Gear-couplings, being flexible, do absorb some amount of

misalignment by stressing themselves. So, the actual misalignment

severity is not reflected in the shaft or bearing vibrations.

Under this suppressed misalignment condition, the couplings may wear

out faster due to increased stress & friction. A worn-out gear-coupling

may then slip above certain load/speed leading to rub & rotor instability

which is reflected in vibration misleading the diagnosis of real cause.

TDBFP- Case Study

CASE BACKGROUND:

One of our TDBFPs was running within normal vibration range, the

maximum pedestal vibration (Table-1) of around 20 microns & 5 mm/s and

turbine shaft vibration of around 30-45µ. When the equipment was brought

back after one shutdown, the shaft vibration was found to have increased

to around alarm value of 50-55µ, and it was fluctuating at higher speed or

load, sometimes crossing the 70µ tripping limit.

Table-1 Pedestal Vibration

H V A Shaft Vibration

Remarks

BPNDE 13/2.8 12/2.4 10/1.1 - Pedestal vibn:

µ pk-pk / mm/s pk

Shaft vibn:

X / Y (µ pk-pk)

BPDE 17/4.4 12/4.7 10/2.2 -

GBOB 11/2.9 14/2.8 16/3.5 13/11

GBIB 11/3.5 9/1.7 10/2.2 19/30

TFJB 9/0.7 8/1.0 8/1.2 31/45

TRJB 13/1.2 12/0.7 10/1.6 32/26

MPDE 20/4.0 18/4.0 13/3.2 -

MPNDE 15/2.1 20/5.1 6/4.4 -

APPROACH

Setting & monitoring multi-parameter trend of online vibration with speed, flow,

steam pressure, temperature, bearing temperature, control valve position, etc.

Pedestal vibration measurement & trending with past data.

Collection of additional shaft vibration data by hooking up multi-channel FFT

analyser to online vibration monitoring system.

TDBFP- Case Study…

FINDINGS & ANALYSIS

No abnormality was observed with process parameters and no

correlations could be established between vibration increase & change

in operating parameters except that of speed.

Pedestal vibration values were around the earlier reading without

registering any significant change.

Online parameter trend


FINDINGS & ANALYSIS…

Vibration spectra of turbine front & rear collected at different instances

contained 0.38X, 1X & 2X components.

1X vibration component was predominant & almost stable, but sub-

synchronous 0.38X one was found to be fluctuating thereby changing

overall value observed in the trend.

Turbine FJB Vibration Spectra Turbine RJB Vibration Spectra

Indicates fluid induced rotor instability or rub with change in system

stiffness due to change in clearance or alignment state.


FINDINGS & ANALYSIS…

Overall vibration orbit plots at

turbine front (left) & rear(right)

journal bearings at two different

instances indicate random

unstable behaviour at both ends

& constrained motion at front

end.

Turbine Vibration Orbits

PROGNOSIS

Recommended for changing lube

oil pressure, temperature & flow to

observe its effect on unstable

vibration behaviour, but no fruitful

result was observed.

Fluid induced instability condition

was not established.

TDBFP- Case Study… PROGNOSIS……

Full spectrum-plot was studied to

gain further insight.

Significant reverse

components were observed

indicating rub.

Recommended to check further

coupling, alignment & bearings.

Cold-alignment reading was

observed to be slightly out,

turbine front end bearing had

minor scratch marks, and

gear-coupling had worn out.

Full Spectrum

Worn-out coupling


REMEDIAL ACTION

Worn-out coupling was replaced.

Bearing babbit was polished off scratch marks & refitted.

Realignment was done.

On box up & starting, the equipment could be satisfactorily

operated with maximum vibration in the range of 35-40µ with

predominant 1X vibration only without any fluctuation.

Post-maintenance vibration trend


POST-ANALYSIS & LEARNINGS

Comparing analysed & diagnosed problem with actual findings, following

significant learnings have emerged:

Presence of sub-synchronous component was attributed to fluid induced

rotor instability, but on seeing the worn-out coupling, actual source of it

could be explained

Because of worn-out teeth, gear-coupling might be slipping at

higher load & speed thereby generating rub & instability

symptoms.

Since, the direction of precision was not clearly evident from

key-phasor dots in chaotic orbit, rub condition was not clearly

visualised.

To further substantiate the findings, when full-spectrum plot

was studied, significant reverse components were observed at

turbine front end with worn-out coupling confirming the nature

of precision as that of rub.

Therefore, full-spectrum study should be employed along with

study of orbits & half spectrum to increase the probability of

correct diagnosis.


POST-ANALYSIS & LEARNINGS…

Increased 1X component was thought of occurring due to increased

clearance & minor misalignment. Although as-found cold-alignment

readings corroborated this, the cause of gear-coupling wear is perplexing.

Gear-coupling can absorb some amount of misalignment by

stressing itself, and to avoid wear due to friction under stressed

condition, it is lubricated.

It is quite possible that the lubricant might have been

leaked/squeezed out due to misalignment, thereby creating

friction & wear.

Such situation can arise due to unforeseen cold-to-hot

movement of the machine leading to misalignment in actual

running condition.

Hence, it is prudent to measure & compensate for actual

thermal growth by using some means like laser-detector

system.


POST-ANALYSIS & LEARNINGS…

It is not always possible to detect the severity of misalignment through

vibration because

Some misalignment can be absorbed by flexible couplings like

gear-coupling, and vibration reflects motion under dynamic

force and not static force due to misalignment.

We should employ other technologies like IR Thermography to

complement vibration for misalignment detection.

There is no easy or inexpensive ways to determine in certainty if the

rotating machinery is misaligned when it is running because

Misalignment disguises itself well in rotating machinery with

flexible couplings.

Therefore, it is better to actually check & verify alignment

values and gear-coupling lubrication at certain intervals or

suitable opportunity.

Conclusion TDBFP reliability & availability can be improved by:

1. Use of Full-spectrum along with orbit & other vibration plots to correctly

diagnose rub & fluid induced instability problems and addressing them.

2. Use of IR Thermography to complement vibration in detecting misalignment

conditions.

3. Employing tools like laser-detector system in determining for compensating

actual cold-to-hot movements to ensure running condition alignment within

acceptable tolerances.

4. Actual offline checking & rectification of alignment & lubrication condition of

gear-couplings at suitable opportunity to ensure their healthiness and avoid

premature component failures due to misalignment.

5. Additional investigation should be done to find & attend root cause of

significant shift in alignment, if any.

Documents

TDBFP Gear-Coupling Misalignment & Diagnosis