Steam turbines: some case studies in condition monitoring

Ray Beebe DipMechEng MEngSc FIEAust CPEng(ret)

(28 years in power generation, then 24

years as academic)

Long experience on investigations on

pumps, steam turbines, boilers etc. +

initiatives in condition monitoring

Keen to share – ran training in-house,

wrote handbook

Ray Beebe

Professional engineer/

middle manager

Power Stations etc.

Australia and UK

Monash University

1992 >>

Led Maintenance and

Reliability programs 15

years (postgraduate,

study online)

Conference

papers/speaking – 7

countries

Articles in magazines

worldwide

3 books

“Retired” 2010: now

part-time @Federation

University

RAY’S OPINIONS ON USE OF CM

Asset Condition Monitoring – using data of performance, vibration, inspection, etc. to support maintenance decisions

Turbines are critical plant, well justify application of CM.

Open a turbine only if compelling technical and economic reasons. (High temperature parts get heat affected, may be difficult to dissemble).

OEM advice: consider

Several CM techniques: Vibration analysis and performance analysis most common

Today about performance analysis: three main parameters:

Valves Wide

Open output

Stage

Pressures

and Ratios Expansion

line and

Enthalpy-

Drop

Efficiency

Inlet Control Valves WIDE open, checked

Steady conditions for hour

May need reduced inlet pressure

Test readings (or DCS if proven) of key

temperatures, pressures, MW. No special test

flow measurements

MW are corrected with OEM factors for any

variations from rated terminal conditions.

Valves Wide Open Output: indicator of overall

condition

Parameters:

Corrected stage

pressures (to

standard inlet

conditions)

Pressure ratios,

inlet/outlet

better – not

dependent only

on turbine inlet

pressure value.

…are closely

linear with

flow/load (NOT

Last Stage).

Can use ratios to

detect blockage in

section

Stage

Pressures…

0 Steam flow /Load 100%

EFFECT OF BLADE DEPOSITS, DAMAGE SHOWS

Pressure ratio, inlet to outlet, increases

Effect of

restriction in

section

0 Steam flow /Load 100%

ENTHALPY- ENTROPY CHART (MOLLIER)

A-B-C-D:

Expansion Line

for typical

reheat turbine

E – F:

Expansion Line

for typical non-

reheat turbine

Saturated

(wet) steam

area

(Courtesy Engineering toolbox)

Usually 85–90%.

Lowers with damage or

blade deposits.

Enthalpy, Entropy found from temperature and pressure. Enthalpy Drop Efficiency = Actual Enthalpy Drop Ideal Enthalpy Drop

Enthalpy Drop Efficiency: useful in superheated

zone only

Enthalpy – Entropy diagram -

part of “Mollier” Chart

CASE STUDY A: 200MW (BLADE DEPOSITS)

Usual load 210MW. Steam control valves 90% open

VWO when new: 216MW

Later, operator noticed control valves 100% open

So, VWO test run: 210MW (-6MW )

Planned full Heat Rate test for more diagnostics, but piping

isolations delayed test for 3 months:

VWO was now 216MW

Unit had 11 shutdowns in this time !

Likely problem: deposits on blading,

removed by blade washing effect during

startup/shutdown

CASE STUDY B: BLADE

DAMAGE (200MW)

194

196

198

200

202

204

206

208

210

212

0 10000 20000 30000 40000

VWO

80

81

82

83

84

85

86

87

0 10000 20000 30000 40000

HP Effy

5.9

6

6.1

6.2

6.3

6.4

6.5

6.6

6.7

0 10000 20000 30000 40000

HP Press Ratio

Service hours

METAL PIECE CARRIED THROUGH, BENT STATIONARY BLADES

CASE STUDY C: BLADE DEPOSITS (350MW)

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

After continuous service for periods of 6 months. Why a 10MW drop!

Correction Factors from OEM

Corrected

VWO

Output

Steam

Forced

Cool run

here

-4

-3

-2

-1

0

1

2

3

4

5

6

0 1000 2000 3000 4000 5000 6000 7000 8000

% V

ariation f

rom

new

Pressure

Ratio IP

turbine

Enthalpy

Drop

Efficiency IP

turbine

Service hours

IP Blading

DEPOSITS ON IP BLADING

CASE STUDY D: OTHER

USES OF

STAGE PRESSURES VS

FLOW/OUTPUT

Air leakage into turbine so high all air pumps had to run to start up. At 45% load, air leakage problem ceased….

Pressure at Stage 4 extraction becomes positive at that load. Leaking flange found in piping from gland to feedheater line.

If a measured stage temperature (e.g. at “A”) is higher than expected (at “B”), steam may be bypassing blades to mix with stage extraction flow: joint leak?

CASE STUDY E: USES OF

EXPANSION LINE PLOT

(500MW)

Entry to LP

casings is

below

centreline

Burnt paint seen

on 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

saturation - if above,

then steam is

superheated, has

bypassed blading.

255 °C temperature

noticed at extraction to

LP2 feedwater heater

95 °C is usual temperature

(read in Control Room).

Inner

bellows

Expansion

bellows in LP

inlet pipe had

failed.

914mm

diameter,

two sets in each

of 4 pipes.

CASE STUDY F:

500MW

Initial test: Acceptance

High accuracy tests run every 2 years

Slight performance decline evident, but not unusual

After Test 4, operators noticed greater control

valves opening needed, so DCS tests run weekly

Clearly, degradation somewhere.

VWO RESULTS

Plant has DCS

Little drift proven in key instruments

Array of screen-based displays

VWO accurate tests set up, and data taken into

historian (Uniformance PHD)

VWO found both ways:

VWO Output using DCS trend close to special

tests

VWO from plant

instruments

(DCS)

VWO using special

test instruments

R

Boiler

desuperheater had

collapsed - metal

carried into turbine

ENTHALPY DROP EFFICIENCY UP TO AFTER REPAIR

(TURBINE INLET TO HP BLADING OUTLET)

81

82

83

84

85

86

87

88

89

90

31-Jan-93 15-Jun-94 28-Oct-95 11-Mar-97 24-Jul-98 06-Dec-99 19-Apr-01 01-Sep-02

HP

en

thalp

y d

rop

eff

icie

ncy %

DCS data

gave

inconsistent

point

Accurate

tests

CORRECTED PRESSURES UP TO AFTER REPAIR: AFTER

CONTROL VALVES AND FIRST STAGE

12000

12500

13000

13500

14000

14500

15000

15500

16000

16500

11-Jan-93 8-Oct-95 4-Jul-98 30-Mar-01

Date

Co

rre

cte

d p

ress

ure

s kP

a(a

bs)

Corr FSP

Corr PAGVs

ENTHALPY DROP EFFICIENCY: UP TO AFTER REPAIRS

81

82

83

84

85

86

87

88

89

90

91

31-Jan-93 15-Jun-94 28-Oct-95 11-Mar-

97

24-Jul-98 6-Dec-99 19-Apr-

01

1-Sep-02 14-Jan-04

Excessive

scatter in

DCS

results for

this

parameter

from DCS

to be

useful for

CM

INFORMATION FROM CONDITION MONITORING CAN HELP THE

MAINTENANCE DECISION – TO OPEN A TURBINE ONLY WHEN THERE IS

A TECHNICALLY AND ECONOMICALLY COMPELLING REASON.

SAME METHODS CAN BE USED TO INVESTIGATE OBSERVED

PERFORMANCE SHORTFALLS.

SPECIAL TEST INSTRUMENTATION NEEDED, BUT WORTH TRYING

USE OF DCS.

OTHER CM METHODS NEEDED: VIBRATION ANALYSIS, NDT

INSPECTION, MAYBE OIL CHECKS.

CONCLUSIONS

FURTHER DETAILS?

Ask via email: raybeebemcm@gmail.com

Papers available – Google “Ray Beebe” or email

THANK YOU – I hope it was useful to you

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