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Hines Power Block 2Steam Turbine Mechanical Failure

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Agenda

Background

Event Timeline

Root Cause Analysis Data

Vibration History

Operating Conditions

Coupling Bolts

Summary

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Background

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Background

Power Block 1 (2x1) ~500MW

CTGs are Siemens-Westinghouse W501FC+

STG is Westinghouse 2326RT2

Commissioned April 1999

Power Block 2 (2x1) ~530 MW

CTGs are Siemens W501FD2

STG is Siemens HE

Commissioned December 2003

Power Block 3 (2x1) ~530 MW

CTGs are Siemens W501FD2

STG is Siemens HE

Commissioned November 2005

Power Block 4 (2x1) ~510 MW

CTGs are General Electric 7FA+e

STG is GE D11

Commissioned April 2007

Power Block 1

Power Block 2

Power Block 3

Power Block 4

Cooling Pond

(inlet)

Cooling Pond

(outlet)

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Background

PB2 Nameplate Data

Two (2) STG-5000F CTGs

187 MW each

One (1) Siemens HE STG

188 MW

23-stage HP section

11-stage IP section

3-stage LP section

Two (2) NEM three-pressure, unfired HRSGs

HP: ~1800 psig/1040 F ~420 kpph

IP: ~400 psig/600 F ~80 kpph

LP: ~80 psig/500 F ~90 kpph

HP

IP/LP

HP-IP/LP Coupling

IP/LP Interface

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Event Timeline

2003 – Turbine brand new, but 42-inch Titanium L-0 fleet had issues with OEM 1st revision blades (e.g., tip and mid-span snubber losses). Exclusion zone established and time limits in exclusion zone were set.

2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014

Time2014 Expanded next slide

2007 – L-0 blades were replaced with new design (revision 2) on PB2S as a fleet validation unit. After OEM testing, no restrictions on

time in exclusion zone were required. Stress levels in exclusion zone were deemed acceptable.

2011 – Major (50khrs) outage; HP-IP/LP coupling re-assembled without data sheet or proper preload/set screw setting.

Incorrect data sheet for generator coupling supplied: 7 bolts at 33% less-than-full preload (correct setup is 8 bolts).

2012 – High transient vibration on STG during summer runs begin.

2012/13 – After ~ 5yrs, L-0 blades started to show erosion at trailing edge near platform.

2013 – Bearing #3 was raised to add more load to stabilize

bearing from high vibrations. Little success.

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Event Timeline (cont’d)

Jan ‘14 Feb’14 Mar ‘14 April ‘14 May ‘14 Jun ‘14 July ‘14 Aug‘14 Sept ‘14 Oct ‘14

Time

• 2014 Mar-April – added a balance weight that changed slope of vibration increase ~5x.

• 2014 May 24 – Condenser black plant event, caused STG speed >360 rpm and vacuum >8 in HgA. Most likely cause for an existing erosion pit to grow from

~ 0.025" to 0.126" on L-0 blade and arrest. Crack was dormant with exclusion zone running stresses.

• 2014 June – OEM issued the first release of a detailed coupling assembly procedure as a result of other fleet coupling failures. These units were without

axial thrust loads like PB2 design.

• 2014 July 7 – STG Mechanical Failure Event resulted in all coupling bolts failing in ductile tension overload (not fatigue), raised the stress levels on L-0

section that caused multiple blade failures, all instrument pickup mounts failed, damaged control valve, stop valve, and damaged multiple stationary devices.

Duke RCA identified most likely cause was the failed coupling bolts initiated event, which allowed the L-0 dormant crack(s) to propagate.

• Collateral Damage – Cold slug of Hot Reheat steam or condensate caused a water hammer event due to failed non-return valve on Cold Reheat

line. As a result, water made it in the HP Turbine where it flashed and stretched the HP bonnet bolts and caused them to fail in tensile overload.

Water hammer event also caused turbine movement and cracked Bearing #2 housing, which allowed about ~1,300 gals of turbine lube oil (~20%

capacity) to be released and then catch fire.

• 2014 Aug – PB3S L-0’s were inspected with acceptable erosion on trailing edge.

• 2014 Oct – PB3S was found with loose coupling bolts on HP-IP/LP coupling after experiencing high vibration post L-0 inspection. OEM increased the

coupling bolt preload spec by as much as 40% for PB3S.

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Event Timeline (cont’d)

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Vibration History

20062004 2008 2010 2012

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L-0 Erosion History

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Condenser Incident

>8” HgA and >360 rpm ~ 450 sec

5/24/15

May 24, 2014: Loss of auxiliary power caused loss of cooling water and condenser back pressure

increased during steam turbine tripThis event likely started a crack (or multiple cracks) at erosion pit areas on L-0 blade(s) that were then left

dormant with normal back pressure limits and stresses

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Summary of Coupling Bolt Stretch Info

New spec .009-.010”

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HP-IP/LP Coupling Bolts (as found)

Close up of #1

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HP-IP/LP Coupling Bolts (tensile failure)

Failed PB2 New Bolt

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Hines PB2S Rotor Model

HP-IP/LP

CouplingHP-Generator

Coupling

HP-IP/LP Coupling carries shear, torque (LP&IP), axial (thrust), and minimal bending loading (fair faced

alignment). Loss of the coupling bolts would force IP/LP rotor to apply loads on stationary components to

support itself.

IPLP HP

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L-0 Blade Failure Scenario

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Summary

Trailing edge erosion was occurring on L-0 blades since installation in 2007; equipment was being inspected by a combination of OEM and Duke NDE resources on a regular basis.

During 2011 Major Outage, HP-IP coupling bolts were not properly secured and/or possibly not adequately preloaded, which allowed loss of tension over time and subsequent changes in vibration.

A 5-24-14 condenser back pressure vs speed event initiated a fatigue crack from an erosion pit and left a 3.2mm dormant crack on the trailing edge of a L-0 blade.

On 7-7-14, the unequally loaded HP-IP coupling bolts finally failed from excessive axial tension as fewer tight bolts were left to share the loading from thrust bearing unbalance.

As a consequence of the coupling bolt failure, the L-0 blades experienced very high vibratory loading, causing fatigue failure.

Other pre-existing conditions with a damaged non return valve caused water induction event in the HP turbine and HP stop valve.

The resulting oil leaks led to local fires that caused additional damage.

The Good News: the unit was rebuilt and returned to service on June 19, 2015.