simpson - boiler feed pump turbine case study

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Boiler Feed Pump Turbine Subsynchronous Vibration Case Study

Jeffrey Simpson, P.E. LG&E and KU Energy LLC

[email protected]

ABSTRACT

This paper presents the case study of a boiler feed pump turbine subsynchronous vibration problem that

resulted in numerous shutdowns, production limits and an outage of a large steam turbine generating

unit. Diagnosing the problem proved to be challenging due to assumptions established from the

machine history and other informational factors. The machine had been retrofitted with an advanced

vibration monitoring and diagnostics system that provided detailed vibration data capture prior, during

and after events. This paper describes the events, diagnostics tools, root cause process and pitfalls

surrounding this unusual subsynchronous vibration problem.

INTRODUCTION

A single GE Type DRV631 19,000 HP turbine driven boiler feed pump (TDBFP), Figure 1, pumps water to

the boiler to produce 2400 PSIG, 1005 F steam that drives a GE Type G2 525 MW steam turbine

generating (T/G) unit commissioned for service in 1982. Loss of the TDBFP will result limiting the

generating limit to 160 MW (365 MW loss) due to having operate with a motor driven start up pump.

Both the TDBFP and T/G are instrumented with GE Bently 3500 vibration protection circuitry and GE

Bently System 1 continuous data collection/diagnostics software. Each bearing is equipped with X and Y

orthogonal proximity probes and bearing metal temperature thermocouples.

On November 13, 2012 while operating at 4,768 RPM (79.5 Hz), the TDBFP turbine protection circuitry

tripped the machine due to high inboard bearing vibration, followed by the main generating unit

tripping from loss of feedwater. Before resolving the problem nine (9) days later, the TDBFP turbine had

tripped a total of five (5) times on high inboard bearing vibration, was unavailable and resulted in nearly

61,000 MWH of lost generation. Logical root cause analysis efforts can sometimes lead down the wrong

path requiring reevaluation and extra time to solve a problem.

MACHINE HISTORY

In 1996 this TDBFP turbine experienced a 1/2X operating speed vibration problem which caused a series

of random trips and outages. At that time the unit did not have a continuous data collection and

diagnostics system to analyze the vibration, only overall amplitude was available. Delaware Analysis

Services (then CJ Analytical) was contracted to instrument, analyze and diagnose the cause of vibration

trips. Upon discovering that the vibration was at 1/2X operating speed, Delaware recognized it to be a

known problem with this type of TDBFP turbine inboard bearing caused by a subsynchronous resonance

as a result of the rotor operating at 2X first critical and triggering the rotor first critical to excite the

bearing pedestal natural frequency that happens to be at that same frequency1. The solution was a

redesign of the elliptical sleeve bearing to a pressure dam type sleeve bearing by simply re-pouring

and machining a recess in the upper half of the bearing to provide downward oil force on the journal,

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Figure 2. This increased the first critical above the pedestal natural frequency and eliminated the

subsynchronous high vibration occurrences.

Figure #1 GE Type DRV631 19,000 HP Turbine Driven Boiler Feed Pump

The TDBFP turbine was last overhauled in 2006 where the pressure dam bearing was re-installed after

being reconditioned to proper clearances. The 1/2X vibration phenomenon was also experienced on two

other company TDBFP turbines, also corrected with pressure dam bearings. Any change to stiffness or

bearing load such as might be caused by bearing profile, wiped bearing, looseness, clearance/fit, cracked

pedestal, alignment, steam loading, etc. can lower the first critical frequency back down to where

interaction with the pedestal is possible. Both the TDBFP and T/G are retrofitted with GE Bently 3500

vibration protection circuitry and GE Bently System 1 continuous data collection/diagnostics software.

DISCUSSION

Analysis of the November 2012 TDBFP turbine vibration trip by LG&E KU Generation Engineering using

the Bently System 1 discovered a sub-synchronous vibration of 7.67 mils P-P at near 1/2X of 4,768 RPM

operating speed that had exceeded the trip limit of 5 mils, Figure 3. A trend plot of the inboard bearing X

and Y proximity probes 1X operating speed and overall (direct) vibration demonstrated the 1/2X

vibration went from normal to trip level in just a few seconds on both X and Y probes, lessening the

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prospect of a probe or instrument related issue, Figure 4. A different view of the event in Figure 5 shows

a waterfall plot of the 1/2X vibration disappearing within a couple seconds of the machine tripping. At

the time of the trip the unit and pump were at approximately 90% load and slowly increasing to meet

demand. There was no sign of oil whirl/whip as the orbit showed no signs of distress as can be seen by

the large round subsynchronous orbit, Figure 6. Bearing lube oil supply temperature was within

acceptable limits at 111F and no significant change in bearing metal temperature was discovered. The

pump bearing vibration also increased but at much lower amplitude levels. A review of the control

system data showed the BFP turbine bearing overall vibration amplitude and variability had step

increased in June 2012, about one week after coming back on from a maintenance outage and five

months before the November 1/2X trip, Figure 7.

Figure #2 Pressure Dam Sleeve Bearing

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Figure #3 BFPT Inboard Bearing 1/2X Vibration

Figure #4 100 Sec Trend of Inboard Direct and 1X Vibration, 1st Trip

1/2X

1X

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Figure #5 Inboard Bearing Waterfall Plot, 1st Trip

Figure #6 Inboard Unfiltered Orbit, 1st Trip

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Figure #7 Trend of Inboard Overall Vibration, March Aug 2012

The TDBFP and main T/G unit were returned to service several hours after the first trip to meet load

demand and determine if the vibration trip was a one-off event. Approximately 23 hours later the

question was answered by a second trip of the BFP turbine and main T/G unit, again due to BFP turbine

inboard bearing 1/2X high vibration at nearly the same operating speed. Based on the 1/2X vibration

history of this machine, a problem with the inboard pressure dam bearing was suspected. Although the

proximity probe gap voltages, Figure 8, did not indicate a wiped bearing an inspection was performed to

check dimensions and fits, all of which were found acceptable. As a precaution the bearing was

refurbished due to light scoring. During the period of the TDBFP outage the main generating unit was

operated at reduced load using a motor driven start-up pump.

Figure #8 Trend of Inboard Bearing Gap Voltages Prior to 1/2X Trip

Mil lCreek.U4.4F3358.PV



2.1633

mils


Mil lCreek.U4.4TPSM21AI.PV

Mil lCreek.U4.SV.UNITLOAD.001H.avg.AvgValue

BFPT BRG 2X VIBRATION

8/6/2012 2:30:51 AM3/8/2012 1:01:48 AM 151.06 days

MC4

0

1

2

3

4

5

6

7

8

9

10

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Shaft alignment of the grease lubricated double engagement gear spacer coupling, Figure 9, was

checked using a LUDECA laser alignment system. The TDBFP was returned to service with a refurbished

inboard pressure dam bearing. A live vibration spectrum was displayed in System 1 as the pump speed

was increased with main unit load. After only three hours of service the 1/2X vibration appeared at an

operating speed of 4,527 RPM so the pump was shut down before tripping. The TDBFP was rolled up

one more time to verify if the problem repeated at the same speed. This time the 1/2X vibration began

at a lower speed of 4,223 RPM indicating the speed at which the 1/2X occurred was decreasing, Figure

10. This was different than the problem experienced in 1996 where the subsynchronous vibration

repeated at relatively the same rotor operating speed.

Figure #9 Double Engagement Gear Spacer Coupling

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Figure #10 1/2X Occurring at Lower Speeds for Subsequent Events

Refurbishing the bearing made no difference and the problem was potentially worsening. The bearing

appeared to be unloaded as the orbit in Figure 6 is circular. Referencing back to the control system data

in Figure 7, signs of a problem conceivably began five months earlier, about a week after the June 2012

maintenance outage. Engineering began to evaluate what conditions could result in unloading a bearing

and if there was work performed during the outage that could be related to the problem. Steam

influences can lift turbine rotors but the inboard bearing is at the opposite end of the steam nozzle and

the machine was not known to have low pressure nozzle issues. Steam supply control valve linkages

were visually in good condition. A review of the June outage work revealed that the TDBFP turbine

steam exhaust expansion joint located directly below the inboard bearing end was replaced. The rubber

joint visually appeared to be stretched very tight so there was concern the new joint could be pulling

down on the frame, lowering the inboard bearing support thus unloading the bearing. A review of the

shaft alignment job performed after the refurbished bearing showed that the alignment data did not

repeat and several vertical moves were made. Due to the lack of repeatable data the vertical position

was restored to as-found by reinstalling the original shims. Laser alignment systems allow for alignment

of flexible couplings without uncoupling the machine provided there is access to mount the laser

brackets on the coupling hubs at each end. An observation was made that the turbine end coupling hub

was not accessible due to the tight spacing between coupling gear housing and the turbine frame, Figure

9. During alignment the turbine end laser bracket was incorrectly mounted on the coupling gear housing

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instead of the hub resulting in erroneous and inconsistent alignment data. The coupling spacer would

need to be removed to properly check alignment.

During removal of the coupling spacer an unexpected finding was made. A small amount of water

drained out from the coupling as the spacer was removed and the gear teeth were found to have no

grease, corroded and heavily contaminated with debris, Figure 11. This could be the source of the 1/2X

vibration if the coupling were locking up causing a change to the rotor dynamics and unloading the

bearing. It was obvious the coupling had to be cleaned and re-greased but this unearthed yet another

problem of not sufficient clearance to slide back the coupling covers to access the gear teeth, Figure 12.

The only way to gain access to the gear teeth and the small 5/32 space on the backside of the teeth

would be to provide enough clearance to slide the coupling covers out of the way which would require

removing the entire turbine rotor and partial disassembly of the pump. There was no way that proper

routine lubrication PMs had ever been performed or ever could be performed on the coupling other

than during machine overhauls. The coupling was painstakingly cleaned out using wire and other small

implements to fish out debris, followed by re-greasing through the ends of the gear teeth.

Figure 11 Turbine End Coupling Half As-Found

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The TDBFP was returned to service seven days after the first incident. Watching the live data as the

speed increased there was no sign of the 1/2X until it suddenly appeared again at 5,015 RPM, but 15%

higher than last observed and 5% higher than the first trip which was an explainable improvement

because as load increases the friction between the coupling gear teeth also increases. The machine was

taken out of service. This time a more aggressive effort was made to clean out the coupling, again using

wire and other small implements to meticulously fish out debris but also repeatedly flushing a solvent

through the gear teeth in hopes of removing as much debris as possible. Additionally the grease type

previously used was found not to be for couplings, so Conoco-Phillips Coupling Grease was applied

during the second attempt by forcing grease through the teeth. Not being able to slide the coupling

housing back on the hub and the tight clearances between the hub and housing gear teeth made it

difficult, if not impossible to ensure all friction surface areas were properly greased.

Figure #12 No Access to the Gear Teeth

The TDBFP was again put into service for the fifth and final time since the first vibration trip nine days

prior. Full load was obtained without any signs of the 1/2X running speed vibration. Gear coupling

manufactures recommend biannual greasing and periodic cleaning to ensure reliable coupling

performance. Suspecting the problem would likely return from not being able to perform proper routine

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maintenance, a Goodrich contoured diaphragm non-lubricated coupling will be installed as a permanent

solution at the next planned outage opportunity, Figure 13.

Figure #13 Goodrich Diaphragm Coupling

CONCLUSIONS:

A misapplied coupling design in service since 1982 resulted in a locked up gear coupling when under

load due to lack of lubrication caused by the inability to perform proper routine cleaning and greasing.

This resulted in unloading the bearing causing a fluid induced rotor instability manifested at 1/2X

running speed vibration. Having an online vibration diagnostics system was a valuable asset for

automatic capture of the data and post event analysis. To permanently mitigate the root cause, a non-

lubricated diaphragm coupling will be installed. The past history of the inboard bearing subsynchronous

resonance initially led engineering to assume a repeat of a previous problem. In retrospect, an

inspection of the coupling in conjunction with the bearing refurbishment would have been prudent.

REFERENCES:

Eshleman, Ron; Guy, Kevin; Jackson, Charles. "Auxiliary Turbine Subsynchronous Vibration", Vibration

Institute article, 1985. 1

Bently, Donald. Fundamentals of Rotating Machinery Diagnostics, 2002.

simpson - boiler feed pump turbine case study

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