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CAN

STEAM

TURBINE

Training related to ST

performance

13 July 2017

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INTRODUCTION - GOALS

▪ Steam turbine

Main principles

Technology

Focus on performances

Efficiency

• ST Terminal conditions

• Where losses could come from?

▪ Degradations

Illustrations on main degradation process

How to monitor?

How to prevent or maintain?

Comments when there are in the specific case of Can steam turbine

EDF-CIT / steam turbine / 13/07/2017

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PART 1

PRINCIPLES

PERFORMANCES


| 4EDF-CIT / steam turbine / 13/07/2017

STEAM TURBINE PRINCIPLES

ST IN THE WATER/STEAM CYCLE

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STEAM TURBINE PRINCIPLES

▪ To convert thermal power into

mechanical power

• Efficiency : nearby 90%

• Mechanical losses : at

bearing (by friction) less than

0.5%

• Generator loss: nearby 1%



6

THERMODYNAMIC CONVERSION IN A SINGLE

STAGE

The thermodynamic 1st principle application to a steam turbine

single stage (static part 0→1 + wheel 1 →2) is : ( )02

2

=+=+ qwv

ddh t

Consequently, the energy conversion is : 02

2

00

2

22

22ttt hh

vh

vhw −=

+−

+=

This formula could be figured as followed in the Mollier (H versus S) diagram:

h

s

t : mechanical

work

v²2 /2 : remaining

velocity

v²0 /20

1

2

1is

2'is

Losses in the static

part

Losses in the wheel

P0

P1

P2

dP=0

2is

Available enthalpy

drop

v²1 /2


LOSSES IN THE STEAM PATH


FLOW CALCULATION IN A MULTISTAGE

TURBINE

An empirical formula, very simple and often used, is the Stodola relationship giving the mass

flow function of inlet and exhaust pressure for a given ST rotation speed

i

ei

T

PPStM

22 −=

St : Stodola’s coefficient which depends on rotation speed and

steam turbine geometry.

Pi, Pe : inlet pressure, exhaust pressure

Ti : steam inlet temperature

Generally, Pe >> Ps, the steam turbine mass flow is therefore nearly proportional to inlet pressure

PePs

M

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STEAM TURBINE TECHNOLOGY

▪ Steam turbine supplied by Alstom

Impulse type

Combined HP / IP turbine

LP turbine, single flow

Last LP stage blades : fir-tree root, with snubber

▪ Some dates

Commissioned in 2006

End of 2016 : 77000 OH, 90000 EOH

LP turbine maintenance in 2015

1st major overhaul for HIP turbine in 2018


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STEAM TURBINE PERFORMANCE

▪ Steam turbine manufacturer designs steam turbine consistent with the chosen

water steam cycle conditions

Normal conditions such as high inlet temperature (540°C at Can) or pressure (172bar at

Can)…

Exceptional conditions such as a large range of condenser pressure (from 50mbar to

250 mbar) (nominal at 85 mbar)

Then, the OEM can calculate the performance (heat rate)

• VWO (105%) 7973

• Nominal load (100%) : 7976 kJ/kWh

• 85% load 8033

• 60% load 8244

• 30% load 8971

• 100% summer load 8676 (condenser at 250 mbar)

▪ The steam turbine performance can be checked following a performance test

procedure (such as ASME PTC6) which needs specific instrumentation, notably a

flow nozzle for accurate main flow measurement

Unit 1 June 2006 : 7932 measured against 7994 kJ/kWh guaranteed (compliant)


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IMPACT OF THE LAST STAGE BLADE CHOICE

▪ Example

Blade length approx 800mm 800mm 1100mm

Exhaust area approx 6 m² 2 x 6m² 10m²

Output at 60 mbar -4% +1.25% ref

Output at 200 mbar +3% 0 ref

Output at 250 mbar +4% ref impossible


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IMPACT OF THE TERMINAL CONDITIONS

▪ Correction curves…. Can

HP Pressure -0.20%HR pour +10 bar

HP Temperature -0.30%HR when +10 °C

IP Pressure (reheat pressure drop) +0.08%HR when +1%reheat pressure drop

IP Temperature -0.27%HR when +10 °C

Condenser pressure +0.56%HR when +10mbar



summer

nominal

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▪ Order of magnitude



With time, steam turbine performance is

deteriorating :

- Operating Conditions : notably steam

quality

- SPE or FOD

-Vibration behavior

-…

“A survey of 31 steam path audits

indicated average section efficiency

deterioration was 12.5% for the HP, 9.1%

for the IP, and 3% for the LP.

Most of the total loss in these turbines

was caused by either leakage (about

50% of the total) or surface roughness

(about 36% of the total).”

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ENTHALPY DROP MEASUREMENT SEE INFORMATION SYSTEM (CNK 00 M OVE --- ER 101B) - PAGE 21

▪ HP Efficiency :

Hinlet – Hexhaust

---------------------------

Hinlet- Hexhaust,isentropic


Expected :

Design 86.4%

Perf test 83%

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PART 2

DETERIORATION


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STEAM TURBINE DAMAGES AFFECTING THE

STEAM PATH COMPONENTS

▪ Solid particle erosion at HP and IP

▪ Foreign object damages

▪ Deposits

▪ Creep and creep fatigue : high temperature

▪ Corrosion fatigue (mainly LP blades)

▪ Fatigue

▪ Stress corrosion : LP at disk and blade root

▪ General Corrosion : where carbon steel

▪ Pitting corrosion

▪ Fretting

▪ Moisture effects :

Erosion (mainly in LP section) … max acceptable 12%

▪ Water induction

▪ Vibration

▪ Rubbing


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SOLID PARTICLE EROSION (SPE) AT HP AND IP

FOREIGN OBJECT DAMAGES (FOD)

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SPE

▪ SPE is caused by oxide scales exfoliates from boiler

▪ Among other effects, SPE causes :

Damages of blades and diaphragms

Increase in surface roughness

Deterioration of seal strips (increase of clearance)

Then could involve significant reduction in turbine efficiency

▪ How to monitor

1st wheel pressure versus flow monitoring

Enthalpy drop test

Boroscopic inspection

▪ How to prevent

Bypass operation at start-up

Boiler chemical cleaning

Fine mesh at main valves (only for short period…)


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DEPOSITS

▪ Unit 1 HIP rotor in 2015


Red color

-> probably iron oxides

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CAN FEEDBACK

▪ Unit 1: In 2007, the unit 1 was no more able to reach its full capacity. Indeed, to

accept the steam flow rate, the pressure at turbine inlet has notably increase,

showing that the steam turbine swallowing capacity had reduced.

▪ Decision was taken to open the HP/IP module: deposits were observed in large

quantity in the steam path (up to 2mm thick on the blades)

Mainly on stages HP 3 to 5 then decreasing along the others HP stages

Mainly on stage IP 6 then decreasing along the others HP stages

▪ Seals strips were also found deteriorated.

▪ The main suspected root cause was operation with bad steam quality during

commissioning and first months of operation before COD.

▪ The taken measures included:

Steam path cleaning (on damages seen on blades)

Gland packing replacement

And also several upgrade in operation procedure (notably of boiler one)

▪EDF-CIT / steam turbine / 13/07/2017

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DEPOSITS

▪ Chemical compounds are soluble in superheated steam. Their solubility decreases

as the steam expands trough the turbine

▪ Among others effects, deposits causes

Reduction in flow capacity

Reduction of turbine efficiency

▪ Others chemical compounds (Sodium…)

Stress corrosion, pitting corrosion…

▪ How to monitor

Steam quality monitoring

1st wheel pressure versus flow monitoring

Enthalpy drop test

Boroscopic inspection

▪ How to prevent

To keep a good water/steam chemistry, below required limits


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STEAM QUALITY

▪ Required at CAN (ST operating manual)



STEAM QUALITY

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RECOMMENDATIONS

▪ Monitoring

Steam path pollution is a concern since the commissioning: we recommend to perform a

performance monitoring.

At least, an annual “enthalpy drop test” for HP and IP turbine is simple to implement and could give a

could view on performance deterioration

▪ Steam quality

Steam quality is very important to keep, especially at start-up

After shutdown period, don’t hesitate to operate on bypass for a while

▪ Boiler cleanliness

To be checked (see with boiler specialist)

Separator to be inspected (to control that there is no bypass of the separator and that priming is very

low)

▪ Unit 2 HIP turbine cleaning

To plan HIP major maintenance as soon as possible (planned in 2018 according OEM maintenance

program)


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MAIN VALVES DYSFUNCTION

▪ Currently, a IP stop valve of the unit 1 shows a dysfunction (closing slowing down

for the last 8% of the closing). A hydraulic problem is suspected, still on-going.

▪ These valves are very important for the safety (stop valves) and the availability of

the plant.

▪ Our recommendations:

▪ To solve the pending issues as soon as possible

▪ To continue regular test and maintenance based upon OEM recommendation. We

recommend not to change these test and maintenance periodicity without a full

study (FMEA with feedback from the plant)


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MOISTURE EFFECTS

▪ Erosion is caused by water droplet present in the

last LP stages

▪ Detached pieces of eroded blades could impact

others LP blades

or condenser tubes

▪ Erosion could locally weaken blade, wire or snubber

resistance and increase risk of blade cracking


Leading

edge

Condenser

tubes

burst

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FATIGUE IN LP BLADES

▪ Fatigue in LP turbine blade is one of the most common causes of steam turbine

catastrophic failures (with fire and overspeed)


Stresses at blade feet are

large for these long blade

Many scenario for failure are

possible

For instance, cracking could

be initiated by low cycle

fatigue and propagated by

high cycle fatigue

Initiation could be also from

stress corrosion, erosion,

pitting corrosion…

Unsteady flow, turbulent

wakes, flutter are at the

origin of strong excitation

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OUR RECOMMENDATION

▪ These LP turbines are mainly in good shape.

Our recommendations:

▪ To continue regular maintenance based upon OEM recommendation. Considering

summer conditions in Can, the LP blades should be strongly “shaked” and it is

critical to inspect them regularly.

▪


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LP BLADE LIMITATION IN SUMMER ▪ During summer time, the condenser pressure reaches frequently in the afternoon the

alarm limit. Then, load needs to be reduced (see example hereafter) to recover margin

on the condenser pressure. We could highlight that such margin recovery is very

limited due to the curve shape (sliding and not constant) of the alarm limit.


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OUR RECOMMENDATION

In addition of the power capacity decrease, there is also a strong efficiency reduction.

Our recommendations:

▪ Main cooling system

To limit the condenser pressure increase, we recommend preserving the cooling system

performance (cooling tower performance). That means regular monitoring of the fouling, regular

cleaning….

If possible, improvement of cooling system: Can Power plant has a project to add a new small

cooling tower. Providing there is no issue with circulating pumps operation (no vibration with this new

operating point), this project will bring useful margin for the summer operation of the LP turbine.

▪ Steam turbine:

The limitation (alarm threshold) comes from the last stage blade. The manufacturer could provide

alternative LP blades with higher limits but with bad consequences on all over the year performance

and above all, these alternative blades have larger roots which should need the LP rotor

replacement.

A monitoring of any air ingress and if any, a research of its origin and then repair

▪


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LONG TERM DETERIORATION

Stress monitoring : This system, available is not currently used.

Our recommendations

▪ The power plant, becoming older, has this system available to regularly check its

lifetime consumption related to its normal or exceptional operation procedure.

▪ That may help

To evaluate lifetime consumption related to creep and fatigue damages of high

temperature component

To identify which operation case is responsible for lifetime consumption

To assess the impact of a new kind of operation (change in cold start-up number…) or

others


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VIBRATIONS / RUBBING▪ Vibrations

They depends on

• Shafts line alignment… in case of house bearing level change, coupling shift

• Bearings behavior… in case of oil quality change or internal clearance

modification…

• Unbalance… in case of light rubbing (thermal), mechanical change

▪ Rubbing in either axial or radial directions depending on the cause

Can be caused by

• Differential expansion

• Sliding feet or keys seizing

• Creep (high temperature parts)

• Overheating by windage

Rubbing manifests vibrations

▪ On a performance point of view, vibrations and rubbing could deteriorate

interstage sealing as well as shaft end gland packing

Increasing clearances and so leakage losses

▪ How to monitor : vibration monitoring + analyser to have diagnostic


Light rubbing due

to blade creep


GLAND STEAM SYSTEM

We could suggest performing flow

measurement at gland packing exhaust : That

will help in the assessment of leakage from

valves stem and from gland packing and so

far to appreciate their degradation.

Another simple criteria for this assessment is

to follow at which load the auxiliary steam

supply valve closed (when the gland steam

system is becoming self sufficient) : Higher the

load, worst the leakages from stems and HP

packing are.

teşekkürMERCI


LOSSES IN BLADES

1. AERODYNAMIC LOSSES

There are several types of losses:

1. Losses due the wake (trailing edge of the blade),

2. Losses by incidence (related to the mismatch of the velocity triangle compared to blade angles),

3. Losses in the boundary layers, which depend on the blades surface status (roughness),

4. Losses due to secondary flows at top and root of the blades s (Pintrados > P extrados),

5. Losses due to aerodynamic shocks, in case of flow locally supersonic (long blades),

6. Losses by remaining velocity at the turbine exhaust, which could be reduced with a diffuser

4 : secondary flows 5 : losses by aerodynamics shocks


LOSSES IN THE BLADES

2. MOISTURE

The wet steam expansion creates water droplet by condensation. These droplets are centrifuged in

the wheel. By the way, this water flow : :

❑ doesn’t produce any work (i.e efficiency loss)),

❑ results in an erosion phenomenon of the blades and inner casing.

We could consider that 1% wetness in the steam decreases the stage efficiency by 1% (Bauman’s

rule) . Moreover, other steam expansion shall not go beyond 10-12% wetness (or 14-15% for half speed

turbine).


LOSSES IN THE BLADES

3. LOSSES BY LEAKAGE

These losses are related to sealing between :

1. the fixed blades (nozzles, diaphragms) and shaft,

2. the moving blades shroud and the inner casing of the steam turbine.

These losses are proportional to the shaft diameter or inner casing diameter .

To reduce theses leakages, the manufacturer installs sealing which reduce the leakage flow. Moreover, it

could be beneficial to reduce the shaft diameter or to avoid small blades which have a relative loss larger .

Labyrinth seals at moving

blades

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