Uprating Through Condenser Reconstruction - Balcke-Dürr ...

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Uprating Through Condenser Reconstruction - Balcke-Dürr ...

Special publication

a 98e

Uprating Through

Condenser Reconstruction

J. Scheurlen, R. Scharf, W. Schulz

VGB Kraftwerkstechnik 02/1992

English Issue


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Uprating Through Condenser Reconstruction

Uprating Through Condenser Reconstruction

J. Scheurlen, R. Scharf and W. Schulz

Abstract

Uprating Through

Condenser Reconstruction

Due to necessary modifications to the feed

water chemistry at Stade nuclear power plant,

the turbine condensers fitted with brass tubes

have been converted to use austenitic cooling

water tubes. At the same time, a completely

new design of tube bundle and air

cooling system has been installed. A significant

improvement in heat transfer and considerably

more effective air extraction and also

increased net output by up to 4 MW have

been attained using the new condenser.

Kurzfassung

Authors

Leistungserhöhung durch

Kondensatorumbau

Durch die notwendige Änderung der Speisewasserchemie

im Kernkraftwerk Stade wurden

die mit Messingrohren ausgerüsteten

Turbinenkondensatoren auf austenitische

Kühlwasserrohre umgerüstet. Zugleich wurde

eine völlig neue Konstruktion der Rohrbündel

und des Luftkühlersystems eingesetzt. Mit

dem neuen Kondensator wird eine deutlich

bessere Wärmeübertragung und eine erheblich

wirkungsvollere Luftabsaugung sowie eine

um bis zu 4 MW gesteigerte Nettoleistung

erreicht.

Dipl.-Ing. J. Scheurlen

Dr.-Ing. R. Scharf

PreussenElektra AG

Hanover/Germany

Dipl.-Ing. W. Schulz

Balcke-Dürr AG

Ratingen/Germany

In 1987, the operator of the Stade nuclear

power plant (KKS) faced the task of adjusting

the feedwater chemistry to higher ammonia

(NH3) contents in order to create more favourable

conditions for steam generator operation.

This adjustment made it necessary to replace

all the cupriferous materials in the steam circuit

as it is a well-known fact that copper and

ammonia are not compatible. The most extensive

individual measure in connection with

this material change involved the turbine condensers

which have a total of 43,000 brass

tubes (total length 570 km) and yellow metal

tubesheets. Difficulties with ammonia corrosion

occurred even with the relatively low

NH3 contents of the previous mode of operation

and this led to repairs having to be carried

out. So-called air pockets, in which the

non-condensable gases including an increased

concentration of NH3 occurred, formed due to

the unfavourable design of the tube fields.

Various proposals were investigated as a preliminary

step to the required reconstruction

measures. The comparison in Ta b l e 1 shows

that the concept involving completely prefabricated

bundle modules (variant 6) could be

implemented within an extremely short period,

thus offering decisive cost advantages.

The reconstruction times required for all variants

involving retubing on the jobsite were

considerably longer than the time required for

manufacture in the workshop. Moreover, the

poorer conductivity of the copper-free tube

material would have meant a definite reduction

in the turbo generator output if the tube

layout remained unchanged. Operating experience

with turbo generators which have been

reconstructed in this way confirmed this [1].

It is possible to avoid such undesirable loss of

output when carrying out the reconstruction

on the jobsite using a more up-to-date tube

Table 1. Concepts for the reconstruction of the condenser in the KKS.

layout, but only at the expense of significantly

longer inspection times. The concept involving

condensers completely prefabricated in

the workshop (variant 5) also proved to be uneconomic.

Design

The thermodynamic design data were determined

after checking how the turbo generator

is used:

Cooling water inlet temperature 9.8 °C

Cooling water mass flow 4 x 7,425 kg/s

Heat flow to be discharged 1,214 MW

The external dimensions of the bundle modules

were to a great extent determined by the

existing condenser shell. A height limitation

due to the concrete crossbeam of the power

house made the situation more difficult.

The optimum was found to be a cooling surface

area of 37,820 m2 for the entire condenser

with a tube outer diameter of 22 mm, a

condenser pressure of 34.7 mbar thus being

guaranteed.

Titanium and stainless steel were considered

as tube material.

The advantages of titanium are:

− somewhat better thermal conductivity

− unsusceptibility to standstill corrosion

The advantages of stainless steel are:

− greater admissible spacing of the supporting

walls,

− lower price

As no corrosion had occurred on the stainless

steel tubes (material no 1.4439) used for re-

1 2 3 4 5 6

Measure Retubing and new tubesheets Prefabricated

condensers

(Condenser

modules)

Tube layout Unchanged Modernised

Manufacture Jobsite Workshop

Prefabricated

tube

internals

(Bundle

modules)

Outages x calendar days 2 x 81 1 x 97 2 x 100 1 x 122 2 x 50 1 x 39

Turboset output Decrease Increase

Cost factor 5.4 4.1 7.0 5.1 3.8 1

3


Uprating Through Condenser Reconstruction

Figure 1. Bundle cross-section with flow pattern.

1 steam pass 7 condensate collecting tray

2 air pass 8 flat steel crossbeams

3 horizontal tube field 9 longitudinal beams

4 air cooler 10 screw bolts

5 supporting wall 11 connecting tubes

6 air cooler cover

pair of the existing condensers, this material

was also selected for the new condensers.

4

The Tube Bundle

F i g u r e 1 shows a cross-section of the tube

bundle. Noticeable features are the very simple

overall shape and also the simplicity of the

details. This shape permits an ideal flow. The

velocity in the steam passes is virtually constant

all the way down. The pressure loss due

to friction is minimal. The velocity decreases

in the lower section of the passes up to below

the horizontal tube field. The transformation

of the kinetic energy of the steam into static

enthalpy is highly desirable in this section as

the temperature and pressure increase caused

by this improves condensation. More steam

condenses on the cooling water inlet side than

on the outlet side due to the greater temperature

difference between the steam and the

cooling water. In order not to impede this effect,

large recesses are provided in the supporting

walls to balance the flow in a longitudinal

direction.

The bundle arms become thicker as they go

downwards to the beginning of the air pass

(internal steam pass) and in this way provide

the necessary pressure gradient in the direction

of the air cooler. All paths leading there

are very short. The somewhat subcooled condensate

accumulating in the bundle rains onto

the two collecting trays, flows from there

out of the bundle in counterflow to the steam

and is heated on the way. The collecting trays

also prevent the external section in the lower

tube field from being flooded. The heavily

subcooled condensate enriched with air which

flows out of the air cooler is heated and deaerated

by the steam flowing upwards.

The Bundle Module Support Structure

A prerequisite for the design was that the rigidity

and stability required to transport and

install the prefabricated modules be achieved

using as little material as possible and without

disturbing the flow. Furthermore the connections

between the condenser casing and

the module should be easy to carry out. F i g -

u r e s 1 and 2 show how the requirements

could be met. Four flat steel beams (to which

the crane girder is secured to relocate the

modules), the two air cooler covers of a reinforced

construction and the two lower longitudinal

beams serve as reinforcement in the longitudinal

direction. Only the two longitudinal

beams are located in the steam flow but in areas

with a relatively low velocity. Vacuum

forces from the condenser side wall are transferred

to the supporting walls of the module

via weld-in joint tubes or via screw bolts in

inaccessible areas. The spacing of the supporting

walls ensures that no inadmissible

tube vibrations occur even under the most unfavourable

operating conditions.

Figure 2. Bundle module – support structure.

Figure 3. Welding the tubes to the

clad tubesheet.

Manufacture and Erection

Especially stringent tightness requirements

are placed on nuclear power plant condensers.

A great deal of importance was attached to

quality assurance when manufacturing the

stainless steel tubes. The longitudinal joints

were welded using fully automatic equipment

and subjected to an eddy current examination.

All tubes were also checked for tightness during

a subsequent helium test.

The quality of the explosion clad/roll bond

clad tubesheets, the base material of which is

boiler plate and the cladding material stainless

steel, material no. 1.4439, exceeds the

requirements set out in the AD standards. The

boreholes in the tubesheets and the supporting

walls were carried out with numerically controlled

boring mills within precisely specified

tolerances.


The four bundle modules were constructed in

the Balcke-Dürr AG workshops in 1989. Optically

aligned auxiliary equipment was used

for the precision construction of the support

structure. The condenser tubes could be easily

inserted through the tubesheets and the 17

supporting walls. All tubes are rolled into

the tubesheets. An automated TIG pulsation

welding method was used to make the weld

joint between the tubes and the tubesheet

( F i g u r e 3 ) . The forces occurring between

the tube and the tubesheet are discharged via

the rolled joints. The purpose of the weld is

solely to ensure that the tube/tubesheet joint is

absolutely tight. The 100 % roll-in and dye

penetrant examinations carried out on these

welds together with the random X-ray examination

using microfocus equipment rule out

any discontinuities.

Workshop prefabrication of complete bundle

modules ready for installation offers two significant

advantages:

− the plant is at a standstill for less time and

− manufacture is carried out under more favourable

conditions due to the existing

infrastructure being used.

With respect to the transportation of the modules,

there was basically a choice between

Figure 4. Dismantling of the existing condenser fill.

combined boat and road transport or just road

transport. Preference was given to road transport

so as not to jeopardize the delivery date

should the waterways freeze. Each module

measuring approx. 4,400 mm x 4,500 mm and

13,300 mm and weighing approx. 80 tons

could be transported to the power station in

24 hours without any difficulties.

In the plans for the replacement of the four

condenser halves, the new bundle modules

were scheduled to arrive before the intended

power station shutdown so that interim storage

was necessary.

Preliminary work such as fortification of the

entry roads to the interim storage areas, setting

up the crane facilities, the installation of

the positioning rails and the equipping of the

site workshop could be carried out before the

modules were supplied. When the power station

was shut down for inspection in 1990,

only the remaining work in the area between

Uprating Through Condenser Reconstruction

the power house wall and the condensers had

still to be carried out. The cooling water hoods

were removed and stored in the power house

during the reconstruction work. It was then

possible to detach all the internals from the

existing condensers and to remove them

through the openings in the power house wall

( F i g u r e 4 ) . The positioning rails required

to move the bundles in a longitudinal and

transverse direction were then installed in the

condenser casing. Within three days the four

bundle modules were collected from the interim

storage area and, with the help of a flat

carriage and a crane, placed on the positioning

rails (Figure 5), drawn in by winches and

moved in a transverse direction. After having

been precisely aligned, the bundle modules

were secured in the casing via various welded

and bolted joints and the cooling water hoods

screwed on again.

The times provided for the reconstruction of

the two part condensers in the 1990 inspec-

Figure 5. Placing a module on the positioning rails.

DP in MW

4

3

2

1

0

* At 100 % reactor output

At 100 % cooling water mass flow

0 5 10 15 20 25

q W1 in °C

Figure 6. Calculated increase in output as a result of the reconstruction

of the condensers.

∆P = change in the electric output

ϑ W1 = cooling water inlet temperature

5


Uprating Through Condenser Reconstruction

tion schedule were fully complied with. The

reconstruction could be completed ready for

service within 39 days with the work being

carried out in two shifts.

6

Measurements on the Condensers

The measuring equipment required to determine

the heat transfer coefficient was installed

and tested a year before the reconstruction of

the condenser. Long-term stability was required

in addition to a great degree of precision

because this equipment is subsequently

to be used to monitor the condenser. Therefore

the temperature measurement as per the

VGB Directive [2] was selected instead of the

exhaust steam pressure measurement which is

problematic when applied for this purpose.

The temperature measurement is carried out

using a specially developed probe which is insensitive

to changes in the direction and the

velocity of the flow. Each condenser is provided

with four probes, two each on the cooling

water inlet side and outlet side, located

approx. 0.5 m above the tubing.

Ten pressure measuring points mounted on

the periphery of each condenser half were

used to measure the condenser pressure for

the acceptance tests.

The temperatures of the four cooling water

trains are measured in the two ball return lines

to the collecting trays of the condenser tube

cleaning system on the outlet side where the

formation of strands is to be expected. This

simple solution has proved very effective. It

is, however, necessary for the recirculation

pumps to be running during the measurement.

The cooling water inlet temperatures are

measured once in each train immediately before

the condenser.

p in mbar

q S – q G in K

54

52

50

48

46

44

42

40

38

36

15

10

5

0

m

·

L = 9.4 kg/h

Resistance thermometers Pt 100 with 1/3 DIN

tolerance and four-wire connection, a diameter

of 6 mm and uniform length of 300 mm

are used as measuring probes at all measuring

points. The thermometers are not in thermometer

wells but in the medium flow. Clamped

joints with teflon rings are used to secure and

seal the thermometers.

Acceptance Measurements

Measurements were carried out soon after the

power generating unit had been recommissioned

to prove the guaranteed performance

values.

The following results were obtained:

− The condenser pressure was 1.2 mbar below

the guaranteed value.

− The condensate subcooling was 0 to 0.5 K

(guaranteed value < 1 K).

m

·

L = 47.7 kg/h

I II III IV I II III IV

Figure 7. Curves of the measured axial pressure and subcooling.

m˙ L = leakage air mass flow

p = pressure

ϑ s - ϑ G = mixture subcooling

(q S – q G) or (q G – q W1) in K x O 2 in mg/kg k

1.0

0.9

0.8

6

4

2

0

18

16

14

12

10

8

6

4

2

0

− The O2

content in the condenser was approx.

1 µg/kg (guaranteed value < 10 µg/

kg).

The new condensers achieve an increased net

output in particular during the summer when

the cooling water inlet temperatures are high

(Figure 6).

Test Measurements

One bundle was equipped with test instruments

to check the effective functioning

of the condenser. The arrangement of the

measuring points and the position of the four

measuring levels are shown at the top of F i g -

u r e 7 . Below that is an example of the pressure

and mixture subcooling values measured

at two different leakage air mass flows in a

part condenser. If the corresponding pressure

curves of the two leakage air flows are com-

q S – q G

q G – q W1

0 10 20 30 40 50 60

m

·

L in kg/h

Figure 8. Parameters for changed leakage air mass flow.

� = efficient evacuation pump

+ = defective evacuation pump

k = relative heat transfer coefficient

x . O 2 = oxygen content of the condensate

ϑ s - ϑ G = mixture subcooling

ϑ s = saturation temperature

ϑ G = mixture temperature

ϑ G - ϑ W1 = mixture excess temperature

ϑ W1 = cooling water inlet temperature


pared, only minimal differences are found up

to the air cooler inlet; only the pressure level

of the greater leakage air flow is approx. 2

mbar higher. The pressure drops in the air

cooler are still approximately the same at

measuring levels IV and III, ample steam still

flows and condenses, the partial pressure of

the air is low.

That means that no undesirable air accumulations

occur here. There is therefore a proper

axial air flow to the suction point on the cooling

water inlet side.

The pressure drop of the greater leakage air

flow does not fall to almost zero until this air

flow reaches measuring levels II and I: only a

little steam still flows and the partial pressure

of the air is high. As required, the air accumulates

before the suction point.

The changes can be seen even more clearly in

the subcooling curve. At measuring levels IV

and III the mixture subcooling in only approx. 1

K. It then rises steeply to over 13 K at measuring

level I. Not only is this unusually high value surprising

but even more so the fact that the steam/

air mixture is cooled down almost to the cooling

water inlet temperature. The mixture subcooling

and therefore the extractable air mass flow

almost reach the theoretical maximum. The

most important parameters from two series of

measurements are shown in F i g u r e 8 as a

function of the leakage air mass flow.

One series of measurements was carried out using

an evacuation pump running at low capacity

and the other series with an evacuation pump

operating at normal capacity. The evacuation

pump used in each case extracted air from both

part condensers as it was not possible to separate

the suction line. During the measurements,

the part condenser fitted with instruments was

charged with changed air mass flows (design

value for the evacuation system according to

VGB Recommendation [3]: 10.8 kg/h).

It is very obvious that the drop in the K value

and the O2 increase occur simultaneously but

only when the steam/air mixture in the air

cooler has almost reached its lowest temperature.

The excess temperature diagram shows

particularly clearly when this lowest possible

temperature is reached.

The generally extremely low O2 content in the

condensate and its slow increase as a function

of the air mass flow rate are particularly noticeable

during air extraction as per the design

values.

Uprating Through Condenser Reconstruction

The series of measurements with the evacuation

pump running at a low capacity yielded

condensate subcooling values of between 0.2

and – 0.1 K at all leakage air mass flows. The

condensate subcooling was not measured during

the series of measurements with pumps

functioning normally.

The high efficiency of the bundle permits a

drastic reduction in the size of the evacuation

pumps due to the higher air content in the

mixture or it enables large air ingresses to be

controlled without impairing the condenser

vacuum.

References

[1] GKN-1: Nennleistung reduziert. Atomwirtschaft,

Nachrichten des Monats, Februar 1991,

S. 49.

[2] VGB-Richtlinie „Bestimmung des Wärmedurchgangskoeffizienten

von wassergekühlten

Oberflächenkondensatoren“. VGB-KRAFT-

WERKSTECHNIK GmbH, Essen (in Vorbereitung).

[3] VGB-Empfehlung für Auslegung und Betrieb

von Vakuumpumpen bei Dampfturbinen-

Kondensatoren. VGB-R 126 L. Ausgabe 1986.

VGB-KRAFTWERKSTECHNIK GmbH,

Essen.

7


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