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4 A B B R e v i e w 5 / 1 9 9 6

conomic power generation calls for

power plants that exhibit long lifetimes at

high levels of availability. In recent years,

the power industry has experienced fre-

quent cases of deficient OEM design,

often with the result that the required

availability was no longer being achieved

after a number of years. A case in point

is the low-pressure steam turbine.

Numerous reports have appeared that

deal with specific low-pressure (LP) tur-

bine problems. While the problems have

essentially concerned the strength of the

rotors, disks and blades [1, 2], the dy-

namic design of the shaft line has also

been a cause of failure [3]. In addition,

steam expansion in zones where the

moisture level is high can lead to water

droplet impingement and loss of material

from blades and blade carriers. Expen-

sive repair work may have to be carried

out as a result.

Many plant operators tackle such

problems by installing an updated ver-

sion of the original design. Often, this is

available from the original equipment

manufacturer (OEM) and forms part of

his spare parts business. There are also

operators, however, who prefer com-

pletely different technical solutions, de-

veloped and installed by other com-

panies. In such cases, the operators

benefit from the substantial progress that

has been made in the area of aero-

dynamics in recent years. Besides solv-

ing the original mechanical problems,

these solutions also improve the effi-

ciency, allowing payback of the cost of

modification in just a few years. A recent

market trend which is especially evident

in Europe is towards LP turbine retrofits

which are driven entirely by the desire to

improve the efficiency.

Examples are given in the following

which highlight possible retrofit solutions

to problems caused by stress corrosion

cracking, torsional vibration and erosion.

In all of the cases described, it will be

seen that advantage can be taken of

proven technologies and that predomi-

nantly long-term-tested standard com-

ponents can be used. Thorough investi-

gation of the original weaknesses in the

OEM design of older LP turbines has

made it possible to match the retrofit

solution exactly to the conditions exist-

ing in the plant, and hence totally elimin-

ate the problem area.

Stress corrosion cracking

Stress corrosion cracking of steam tur-

bine rotors in both nuclear and fossil-

fired power plants has been receiving

considerable attention for many years

because of the large number of cracks

which have been detected in the course

of inspections and, more dramatically,

because of occasional rotor bursts [1].

Early surveys showed that the problem

existed worldwide and involved all manu-

facturers of LP rotors with shrunk-on

disks. Specifically, an analysis of keyway

inspections in one manufacturer’s tur-

bines showed that 96 percent of the

boiling water reactors’ and 36 percent of

the pressurized water reactors’ LP tur-

bines showed signs of stress corrosion

cracking. Moreover, the signs were not

limited to the keyway and shrink-fit

areas, being also found in the disk rim

and blade attachment area . A de-

tailed analysis of the cracks observed

showed that cracking rates were highest

for materials with a high yield strength.

Depending on the temperature at the

crack location, the apparent propagation

rate can as high as 25 mm (one inch) per

year. It was estimated that safe oper-

ation would only be possible providing

inspections are carried out at unrealisti-

cally short intervals. A plant operator’s

recent publication [4] states that the life-

time of such LP rotor disks can be as

short as 10,000 hours, and that propa-

gation rates even higher than those pre-

dicted by laboratory tests occur.

gives an overview of the observed

crack depths known from the literature

2

1

L P S T E A M T U R B I N E S

Edwin Krämer

Hans Huber

Dr. Brendon Scarlin

ABB Power Generation

Low-pressure
steam turbine
retrofits

Page 5

8 A B B R e v i e w 5 / 1 9 9 6

shields containing cobalt in BWR power

plants are eliminated from the outset

when the leading edges of the blades

are induction-hardened.

Surface erosion/corrosion

These phenomena can lead to a major

loss of material on certain components,

especially at high moisture levels in the

medium temperature range (eg, in LP

turbines without a reheat stage). Gen-

erally, higher alloyed materials and,

where possible, steam with a higher

oxygen content and pH value, have a

positive effect since, under such con-

ditions, an erosion/corrosion-resistant

magnetite film forms on the surfaces. At

the same time, the turbine should be

designed to ensure a homogeneous,

low-velocity, streamlined flow, without

any local excesses in the steam flow

rates. Because of this, the risk of erosion

varies for the different components:

A moderate risk of corrosion/erosion

exists on the crossover pipes, inside the

extraction components and on the back

of the blade carriers. This does not gen-

erally endanger the machine parts them-

selves. The chief negative effect is that

the iron content in the steam cycle in-

creases (particularly in BWR plants). Use

of low-alloy steels or spray coatings will

avoid or even eliminate the removal of

material through corrosion/erosion.

A strong risk of corrosion/erosion

exists in certain areas on the inside of

the blade carrier in cases where high

steam flow rates and moisture exist in a

certain, sensitive temperature range.

Local damage can be minimized with the

help of erosion-resistant spray coatings

or rings made of 12 % Cr steel. The best

results are achieved when components

such as the blade carrier are manufac-

tured from 12 % Cr cast steel.

Wire drawing

Wire drawing occurs at connecting

points, but only in cases of leakage in

the sealing gaps in the wet steam area

and with high pressure gradients. Risk is

greatest at the horizontal connecting

flange and the blade carrier suspension

points. This type of erosion is more ag-

gressive than any other and can cause

massive local cavitation in a very short

time. Besides having a negative effect on

the turbine efficiency, it also impairs the

strength and operational reliability of the

component.

The standard ABB design has proved

to be an outstanding success for LP tur-

bine retrofits for reasons that include the

following:

• It is free of distortion, does not hinder
expansion and ensures a tight seal,

thereby allowing low-alloy steels to be

used.

• Elastically supported sealing rings
made of erosion-resistant material are

used at locations where relative dis-

placement occurs.

• Protection rings made of erosion-
resistant material are used at fixed

blade carrier suspension points and

other similarly endangered sites.

Higher efficiencies improve

cost-effectiveness

Many power supply utilities are com-

mitted to maintaining a high technical

standard for their plants. One approach

is to improve the operating efficiency of

older turbogenerators. In connection

with this, it should be noted that the

plants often exhibit good availability and,

unlike the examples discussed pre-

viously, have no mechanical deficiencies

that represent a potential risk.

A German utility had its entire fleet of

300-MW and 600-MW turbines investi-

gated in order to determine their poten-

tial for improvement. The turbines were

about 20 years old and had run up more

than 130,000 hours of operation. The

results of the study indicated that, quite

apart from alterations to the power plant

process (heat extraction, gas turbine

topping cycles), redesign of the LP tur-

Effect of induction hardening on the erosion/corrosion resistance
of blading steel

n Number of water droplet impingements Red Untreated
L Volume loss Green Induction hardened

Water droplet size 0.2 mm
Impingement velocity 300 m/s

5

3

mm3

2

1

0
105 106 107 108

n

L

L P S T E A M T U R B I N E S

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96-05-402-06 EPS

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96-05-402-8a EPS

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