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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