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Conversion of a Belgium CCGT plant into an open cycle GT VGB PowerTech 4 l 2016
Authors
Kurzfassung
Vilvoorde – Kurzfristiger Umbau
zum offenen GT Betrieb – Ein
strategischer Schritt
Aufgrund des am Strommarkt gestiegen Anteils
regenerativer, aber fluktuierend liefernder Er-
zeugungskapazitäten, leidet Belgien wegen der
gleichzeitigen wirtschaftlich bedingten Stillle-
gung einiger konventioneller Kraftwerke ins-
besondere im Winterhalbjahr an Versorgungs-
engpässen. Um diese Situation zu entschärfen,
trat Anfang 2014 der „Energieplan Wathelet“
der Belgischen Regierung in Kraft, welcher
u.a. vorübergehend stillgelegte Gaskraftwerke
verpflichtete, an einer Auktion für Reserve­
kapazitäten teilzunehmen. Die Anforderungen
an Verfügbarkeit und Leistungsbereitstellung
dieser Reserven entsprachen dabei denen eines
Spitzenlastkraftwerks.
Uniper (vormals E.ON) entschied sich, sein vor-
läufig stillgelegtes GuD-Kraftwerk (V94.3A) in
Vilvoorde als Reserve bereitzustellen. Aufgrund
der vorgenannten Anforderungen musste das
Kraftwerk jedoch vom Kombibetrieb in den of-
fenen Gasturbinenbetrieb umgebaut werden.
Hierfür war der Einbau eines Bypass-Kamins
zur Umfahrung des Kessels erforderlich. Der
Kamin musste zudem den lokalen Anforderun-
gen insbesondere hinsichtlich der Akustik genü-
gen. Aufgrund der Brisanz der Engpässe war
der Umbau bis zu Beginn der Winterperiode
2014/2015 durchzuführen. Dabei blieben für
Herstellung und Montage des Bypass-Kamins
lediglich drei Monate bis zur Sicherung der Be-
triebsbereitschaft. Der vorliegende Artikel be-
schreibt die Hintergründe, das Umbaukonzept
sowie den Umgang mit dem Herstellungs- und
Montageprozess unter dem Einfluss außeror-
dentlich knapper Terminbedingungen. l
Vilvoorde – Fast track conversion to
open cycle GT – A strategic move
Ödön Majoros and Jürgen Neumann
Dipl.-Ing. Univ. Ödön Majoros
Projectmanager
Uniper Technologies GmbH
Gelsenkirchen, Germany
Dipl.-Ing. Jürgen Neumann
MBA, Projectmanager
G+H Schallschutz GmbH
Ludwigshafen, Germany
Introduction
The Belgian electricity market is currently
facing significant structural shortfalls on
the security of supply due to the increas-
ing share of intermittent power supply and
the decreasing wholesale prices, which
forced several conventional power plants
to shut-down or be mothballed. In order
to improve the situation, the Belgium Gov-
ernment’s so-called “Energieplan Wathe-
let” was put into force at the beginning of
2014. Amongst others, it obliged existing
but mothballed gas-fired power plants to
participate in an auction for reserve capac-
ity, starting with the winter 2014/2015.
The requirements on availability and pow-
er supply on the reserves were those of a
peak load power plant.
Uniper (formerly E.ON) decided to pro-
vide its mothballed CCGT (V94.3A) in
Vilvoorde for strategic reserve. Due to
the requirements, the plant needed to be
converted from combined-cycle into open-
cycle mode. The retrofit of a bypass stack
to disconnect the boiler was an essential
part of this conversion. The stack needed
to fulfil local demand especially concern-
ing acoustics. Due to the imminent short-
falls the conversion was required to be fin-
ished before the next winter. Finally there
were only three months left for fabrication
and installation until required operability
of the plant. This paper describes the back-
ground, the concept for conversion and the
handling of the fabrication and installation
processes under the influence of the very
restrictive timeline.
The Vilvoorde power plant
The history of the Vilvoorde power sta-
tion (formerly Verbrande-Brug) goes back
to the late 1950s and early 1960s. The
steam power plant located in the vicinity
of Brussels Airport (Belgium) was origi-
nally built and owned by Engie (formerly
GDF Suez) and comprised three coal-fired
Units (3 x 125 MWe). Unit 1 and Unit 2 en-
tered commercial operation in 1959 and
1961, respectively. Unit 3 followed in 1965.
Following a grid accident, the steam tur-
bine of Unit 3 was destroyed in 1982 and
replaced in 1986 by a new steam turbine
(Brown Boveri design, 140 MWe).
In the late 1990s, Unit 1 and Unit 2 were
demolished, whereas Unit 3 was converted
into a combined-cycle gas turbine (CCGT)
power plant. During the conversion, the
old coal-fired boiler was removed, the ex-
isting steam turbine was refurbished, rel-
evant infrastructure (e.g. condenser, cool-
ing tower etc.) was overhauled and a new
gas turbine (V94.3A) with associated heat
recovery steam generator (HRSG) was
installed. The repowered Unit 3 entered
commercial operation in 2001, with an in-
creased net power output of about 385 MW
(GT: about 263 MW, ST: about 122 MW)
and an increased net electric efficiency of
54 %.
The CCGT layout has the typical V94.3A
axial arrangement with air-filter and gen-
erator upflow the GT and exhaust diffusor
and HRSG downflow, all arranged in line.
The HRSG is of a vertical type. The CCGT
was not equipped with a bypass stack or
diverter (Figure 1).
In 2009, Engie and Uniper agreed to swap
about 1,700 MW of conventional power
generation capacity, including the transfer
of ownership on the Vilvoorde power sta-
tion, which kept on operating until mid-
2013. However, since the market condi-
tions for gas-fired combined-cycle power
stations have changed dramatically due to
the increasing share of renewable energies
and decreasing wholesale prices, it was
decided to mothball this highly efficient
plant until an economic operation would
become feasible again.
As Vilvoorde was not the only shut-off
plant, the Belgian electricity market is cur-
rently facing significant structural short-
falls on the security of supply due to limit-
ed back-up capacity for power generation.
This will probably get worse until 2017. In
order to improve the situation, the Belgium
government approved the so-called “Ener-
gieplan Wathelet” in July 2013, which was
enforced in March 2014 by Belgium’s sen-
ior council. This plan includes three pillars:
–– Lifetime extension of one nuclear plant
(Tihange 1),
–– Incentives to install new gas-fired power
plants via subsidies and
–– Obligation for existing but mothballed
gas-fired power plants to remain opera-
tional as strategic reserve during three
successive winters, starting with the
winter of 2014.
In order to reveal the lowest cost for this
strategic reserve, the plant owners were
obliged to participate in an auction for re-
serve capacity with a tendering deadline in

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VGB PowerTech 4 l 2016 Conversion of a Belgium CCGT plant into an open cycle GT
July 2014. Uniper decided to comply with
the new law to proactively support secur-
ing Belgium’s electricity supply. Thus, ne-
gotiations with the Belgium Government,
with the Commission for the Regulation of
Electricity and Gas (CREG) and with the
transmission system operator (Elia) were
started.
Amongst others, the framework of Bel-
gium’s Strategic Reserve stipulates that the
plant operator would be given a 5 hour pre-
notification before start-up. It furthermore
requests fast load ramps in the order of 10
to 15 MW/min and time from start-up to
maximum output of about 2 hours. In ad-
dition, the plant operator needs to ensure
a high availability during the winter peri-
ods (defined as from November 1 to March
31) but with the prospect of maximum 350
operating hours per year. These are clearly
the requirements set out for a peak load
plant. To comply with them, Uniper was
forced to convert Vilvoorde into an open
cycle gas turbine (OCGT) power plant,
which is ideal to provide the required per-
formances of a strategic reserve plant.
The “Belgium Energy Reserve” project was
initiated, with the aim to modify the GT’s
exhaust gas system by retrofitting a bypass
stack and partly re-activating and modify-
ing the relevant infrastructure. The steam
turbine remained mothballed.
The project was kicked-off in mid July
2014, once CREG and Elia had agreed
with this basic concept and had granted
an extra month for the conversion, mean-
ing that Vilvoorde would have to be ready
for operation on December 1, 2014. Thus,
the plant operator was facing the situation
that he needed to convert and re-commis-
sion his mothballed power plant within
less than five months. The main challenges
in achieving this ambitious goal will be
discussed in the following, both from the
plant owner’s and the main supplier’s per-
spective.
Plant owner’s perspective –
Reasons for and benefit
of the retrofit
As noted above, the main technical re-
quirements and boundary conditions for
Vilvoorde’s participation in the strategic
reserve are:
–– Quick start-up/fast ramp rates,
–– Highly reliable operation,
–– Very few operating hours per year and
–– Competitive electricity cost for peak
power supply.
Given the anticipated operating map, it
is obvious that the Vilvoorde plant would
certainly experience cold starts and may
as well experience warm and hot starts.
It is well known that the start-up time of a
combined-cycle power plant is restricted by
the load ramp of the HRSG. For a current
Siemens F-class CCGT, this would typically
mean 2 to 3 hours (cold), 1 to 1.5 hours
(warm) and 30 to 60 minutes (hot) start-
up time respectively [1]. At the same time,
the typical load ramp would be in the range
of 5 %/min. For plant specific reasons at
Vilvoorde, however, the start-up from cold
to maximum power takes about 9 hours,
with the 15 min step response being lim-
ited to 3 %/min.
Modern OCGT plants on the other hand
typically reach maximum output within
less than one hour (cold, warm, hot) and
achieve load ramps of about 15 %/min [2],
while Vilvoorde requires about two hours
and can reach ramp rates in the order of
10 to 15 MW/min. Thus, it is clear that an
open cycle configuration offers higher flex-
ibility for peak power supply. The down-
sides of this configuration are lower elec-
trical efficiency and reduced power output.
The second reason for the conversion was
the need for very high availability during
the winter period. Given the age of the
steam turbine (almost 30 years) and some
other important systems, a reliable CCGT
operation would have required significant
investment and time for refurbishing those
parts of the power plant. By converting to
an open cycle configuration, the number
of systems that could potentially fail de-
creased, and thus, the risk for unplanned
unavailability was reduced.
And last but not least, one main reason
was that the technical feasibility of the
conversion had been shown in a study [3]
conducted in 2013 by Uniper Technologies
GmbH, the engineering service provider of
the Uniper group. This study had conclud-
ed that Vilvoorde was probably one of the
most viable sites for this type of conversion.
This is mainly due to the excellent acces-
sibility, along with a suitable plant layout
to be described later on. Furthermore, the
study presented a sound cost estimate, pro-
viding sufficient confidence that the con-
version could be executed with a reason-
able investment. However, the study also
concluded that such a conversion would
typically require about 9 to 12 months from
project start to completion. Thus, the only
viable option to comply with the demand
of the strategic reserve posed an enormous
challenge for the conversion time to the
power plant owner.
Plant owner’s perspective –
Challenges and solutions
From the plant owner’s perspective, the
following challenges needed to be over-
come within very limited time:
–– Setting up a lean and mean proect man-
agement structure and management
processes.
(I.e. ensuring clear responsibilities and
avoiding time-consuming decision-mak-
ing processes.)
–– Puttingtogetheraneffectiveprojectteam.
(I.e. sourcing the needed experts and en-
suring their full commitment to the job.)
–– Parallelising as much work as possible,
even steps that are usually executed in
sequences. (E.g. designing new founda-
tions parallel to tendering new exhaust
gas system etc.)
–– Preparing sound and complete tech-
nical specifications for the main lots.
(E.g. for new exhaust gas system/addi-
tional foundations/emission measure-
ment system etc.)
–– Preparing and obtaining the relevant
permits in time. (I.e. building and envi-
ronmental permit, required to start con-
struction work 10 m above ground.)
Fig. 1. Plant layout before conversion.

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Conversion of a Belgium CCGT plant into an open cycle GT VGB PowerTech 4 l 2016
–– Finding and selecting suitable vendors/
partners for the main lots on very short
notice. (E.g. procure GT exhaust gas sys-
tem, from specification to contract, in
less than 4 weeks.)
–– Getting all the work done required by de-
mothballing. (E.g. turbine and generator
inspections, modification of the cooling
system etc.)
To highlight the enormous challenge of
this project with respect to the timeline,
the main project milestones are summa-
rised in Tabl e 1.
To provide some insight, two cases are
shown in the following, which contrib-
uted to the overall success of the project.
However, these are just two pieces among
a dozen others.
New foundation
A major challenge was the fact that the pre-
sent foundation of the existing exhaust gas
system was not designed for carrying ad-
ditional loads. Hence there was a need for
construction of a new two-part foundation
beside the diffuser upstream of the HRSG
in such way that the new bypass stack
would be properly supported. In a regular
project, this foundation would have been
designed after the final dimensions and
loads had been determined and confirmed
by the stack supplier, which is a typical task
in the supplier’s engineering phase. In this
project, the foundation needed to be de-
signed and prepared in parallel to the pro-
curement of the new stack in order to gain
time for the stack installation. Therefore,
the new foundation with 14 vibration-free
drilled piles (length 11 m, diameter about
400 mm) on each side of the diffuser was
designed with sufficient safety margin to
support a stack of 55 m height and 10 m
diameter (Figure 2).
The stack finally built was indeed some-
what lower and smaller, so the foundation
was slightly overdesigned but more impor-
tantly ready in due time. The pile system
was chosen to minimise negative vibration
effects on the nearby GT and the diffuser’s
foundation.
The piling works started just 2 weeks after
contracting the new bypass stack, once the
stack supplier had confirmed their loads
were not exceeding the foundation design-
er assumptions. The piling work, which
was completed in just one week, was per-
formed in sequence, starting with the west
side before moving to the east side. Once
the piles were finalised on one side, the ex-
cavation and foundation works started im-
mediately. Due to this seamless transition,
the two foundations were even completed
ahead of schedule.
Cooling water system
In combined-cycle configuration, the power
plant has a cooling load of about 225 MW
(thermal), which is discharged via a natu-
ral draught cooling tower. The conversion
to open cycle configuration results in a
significantly reduced cooling requirement
of only about 5 MW. They derive from the
generator and the oil coolers of the gas tur-
bine package only. Hence a modification
of the cooling system became necessary.
Given the plant’s limited operating hours,
the most efficient way was seen in the re-
use of the existing infrastructure with slight
modifications. For winter operation, it is
advised not to use the evaporative part of
the cooling tower at such a low heat load.
The risk of icing of the internals and con-
sequentially mechanical damage is most
likely. It was decided to operate the cooling
tower in bypass mode and to use the cooling
water’s heat capacity in the basin (approx.
3,150 m3) and the concrete header (approx.
1,350 m3) only. With a maximum design
temperature for the GT intercooling system
of 28 °C and assuming an initial cooling wa-
ter temperature of 8 °C, this would result in
an available operation time of 22 hours at
full load. After that, make-up cooling water
from the nearby canal would be required.
In the original layout, however, the loca-
tions of the cold cooling water extraction
from the basin and warm cooling water
return to the basin were very close to each
other. In order to avoid a “thermal short-
circuit”, the cooling water return pipe was
therefore extended by about 40 m to en-
sure proper cooling (Figure 3).
An additional advantage of the chosen
cooling concept was the significant noise
reduction by changing from the cooling
tower draught into bypass operation mode.
A fact that helped keeping low the overall
noise emissions from the plant.
Fig. 2. Piling works for the new foundations.
Tab- 1- Project milestones-
No Milestone Date
1 Gate 2 decision to proceed with project 01-07-2014
2 Submit offer for auction of strategic reserve capacity 04-07-2014
3 Technical kick-off 17-07-2014
4 Start of bypass stack tender 04-08-2014
5 Submit environmental and building permits 05-08-2014
6 Place order for bypass stack 22-08-2014
7 Environmental and building permits granted 11-/22-09-2014
8 Foundation for new bypass stack ready/start erection bypass stack 15-10-2014
9 GT and alternator reboot works finished 31-10-2014
10 Cooling water system modification finished 31-10-2014
11 General de-mothballing activities finished 31-10-2014
12 Bypass stack ready for hot commissioning 21-11-2014
13 Hot commissioning completed 29-11-2014
14 Start participation in strategic reserve 01-12-2014

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VGB PowerTech 4 l 2016 Conversion of a Belgium CCGT plant into an open cycle GT
Supplier’s approach
From the supplier’s perspective, the task
was clear: Install a new bypass exhaust
gas system in less than half the time usu-
ally required. Typically, the cycle time is
7 to 8 months from receipt of order to read-
iness for operation. In this case, only three
months were granted.
Stacks for gas turbines are meanwhile
highly standardised regarding their design
principles and duct sizes. However, local
regulations for emissions like plume spread
and noise limits, which influence stack
height and silencer design, as well as the
vital need to cope with the prevailing en-
vironmental loads – mainly wind and seis-
mic loads – lead to large varieties of stack
layouts even for identical gas turbines. Fur-
thermore, diverging engineering standards
for different locations (like Eurocode vs.
ANSI) and the very specific decision if and
where to put platforms to reach emission
control measurements, again influencing
on the structural design, add up to count-
less varieties. When considering all this, it
is clear that there is no “one-fits-all-pur-
poses” stack available. Nearly each stack
is uniquely designed and custom-made for
the specific project and it is obvious that
there are no stacks produced in advance to
be stocked. The variety would simply be too
large and there is no economic benefit. In
addition, the stack’s cycle period of three-
quarters of a year is usually not the limiting
factor on a new-build project. Hence, the
stack for Vilvoorde also needed to be cus-
tomised and fabricated especially for this
project before installation could start.
Process
Stack customisation and fabrication is a
job process that defines a generally known
product, which is then uniquely designed
and built. Customisation is done in terms
of height, acoustics and structural design.
It is essential, as for all job processes, that
there is a proper product routing and flow
of information in order to obtain a good
performance [4]. Thus, the mission was to
get the processes right to have any chance
of meeting the deadline. A closer look on
the sub-processes, their sequence and du-
ration evolves a clearer picture of the chal-
lenge and shall reveal some ideas of where
time savings may be possible (Ta b l e 2.
The processes depicted in Table 2 sum up
to 31 weeks or 7.2 months in total. Thus,
the main question was how to speed-up
this process such that the stack could be
ready in time. In general, there are the
following approaches, which will be dis-
cussed later on:
–– Local vs. overseas fabrication,
–– Parallelisation of work sequences,
–– Increase manpower,
–– Apply shift work,
–– Multiple fabrication facilities,
–– Skip process.
Leveraging the time-line
A basic approach to optimise the overall
schedule is to minimise transport time.
There were approximately 50 truckloads of
bulk material to be loaded and unloaded in
total for this project. Overseas fabrication,
which is a usual step towards cost effec-
tiveness, would imply maritime transport
plus trucking to and from the port. Alone
this transport would require minimum
6 weeks, which did not seem promising
here. Going for local fabrication, the trans-
port time can be effectively reduced. How-
ever, this requires the place of fabrication
to be within one or two days travel distance
from the place of installation.
Depending on the project, additional time
may be gained with parallel execution of
sub-processes, which are usually handled
in sequence. The overall process can be cat-
egorised in a software stage, including de-
sign and drawings and in a hardware phase
with fabrication and installation.
Fabrication of hardware is definitely need-
ed as explained above. However, fabrica-
tion cannot start without the software
being finished first and no installation
without prior fabrication. Thus, the option
of parallelisation seems to be of limited vi-
ability in this case.
Next alternative and a typical advice when
timelines become tight, is “increase man-
power” or “go for shift work”. A standard
approach would be to look for doubling
workforce or to run at least two shifts per
day to make it in half the time. Both the
software as well as the hardware stage
would in general be potential candidates
for this approach.
Unlike for line processes doubling the
workforce was not that easy here, as work-
ers and staff needed to have a high level of
training and understanding for the prod-
uct. This understanding is an essential part
of a job process. Thus, it was hardly imagi-
nable to ramp up the workforce with highly
trained externals for just this one job and
set them free after it was done. Moreover
doubling the workforce would have re-
quired to also double the workspace, ma-
chinery and tools in order to perform the
work, which was not feasible either.
Working in double shifts in order to use
space and equipment at its best was also
not seen as a promising solution. The an-
ticipated design and fabrication works are
widely complex and require to be executed
in specific sequences. This again is an inte-
gral part of a job process. These complex
and sequenced work could not have been
passed from one worker to the other with-
out frictions and decrease in efficiency.
This along with the commonly known
limitations of shift works like lower overall
performance and negative implications on
safety and quality [5] resulted that the shift
options did not get pursued further. Thus,
neither doubling workforce nor shift work
could be applied here.
Another option was to involve two or more
workshops to work on different parts in
parallel instead of one shop only. This
seemed promising, but would have re-
quired a high level of coordination, ensur-
ing a proper flow of information. Another
downside is that misfits on the interfaces
will only be found on site during installa-
tion, where rectification is very inefficient.
In addition, if only one workshop fails in
this cooperation, all effort of the others
would have become useless.
As the hardware stage could obviously
not be skipped, the question was raised
if cutting the software phase would be a
suitable approach. However, how can one
produce workshop drawings without engi-
neering?
Although each stack is unique, there was a
small chance to have the suitable produc-
tion documents already in hand from pre-
vious jobs. A closer look on the key design
data revealed: The gas turbine is a Siemens
V94.3A, which determines mass flow and
exhaust temperature. The stack height
Tab. 2. Sub-processes and their leadtimes.
Engineering and design 2 weeks
Drawings for workshop
fabrication and installation
8 weeks
Procurement 2 weeks
Fabrication 10 weeks
Transport 2 weeks
Installation 7 weeks
Fig. 3. Interface of new and existing cooling
water pipe.
Fig. 4. Dispersion of the exhaust plume on two
V94.2 open cycle stacks.

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Conversion of a Belgium CCGT plant into an open cycle GT VGB PowerTech 4 l 2016
was specified with 55 m, the acoustic per-
formance needed to comply with the local
VLAREM II [6] regulation and the design
were to be made according to current Eu-
rocode. The GT is a common heavy duty
machine with a large number of installa-
tions worldwide. There was no doubt that
there will be some drawings available in
the archive. The stack height on the other
side was pretty tall for hot GT stacks typi-
cally built. As the emissions of GTs are rela-
tively low and the stack exit velocities are
fairly high compared with other fossil-fired
plants, stack heights for these applications
usually range between 30 and 35 m. The
specified 55 m decrease the chances to find
a readily available design.
The Vilvoorde plant is located in an indus-
trial area with the noise emission measure-
ment point in the nearby residential neigh-
bourhood. According to the VLAAREM
II, a maximum 40 dB (A) sound pressure
level was allowed at this area. A calculation
showed that this results in an allowable
noise emission of 95 dB (A) sound power
level at the stack exit. With an emission
sound power level of more than 140 dB (A)
at the gas turbine exit in mind, this looks as
a challenge itself.
And last but not least, the stack should be
designed for the environmental loads in
Belgium which are fairly moderate. How-
ever, the structural design was requested to
be made according to the latest Eurocode
which, as it is relatively new, would finally
sort out quite a high number of potential
candidates from the archive.
While checking the inventory it became
clear that there were no blueprints for a
V94.3A stack with height of 55 m avail-
able. The tallest ones were in the high
forties. The plant owner was consulted to
understand the requirement of 55 m. The
reasons named were the adjacent HRSG
stack with 55 m height and, in about 30
m distance to the potential stack location,
the existing building of the formerly coal-
fired boiler house with 80 m height. The
customer raised concerns that the exhaust
plume and heat released through the new
stack could have adverse impacts on the
buildings and equipment. Based on [7]
dealing with plume vertical velocity and
spread and own empirical data of exhaust
plumes on open cycle stacks, confidence
could be gained that a stack height of
45 m is sufficient to save the surroundings
(Figure 4).
After agreeing on a lower stack height,
there was still trouble to find a blueprint
for a corresponding V94.3A stack. Espe-
cially the VLAREM II requirements caused
concerns. Luckily it became obvious that a
stack just designed for another but some-
what larger turbine could match the re-
quired height and acoustic and could easily
deal with the mass flow.
Fortunately this one crossed the line just at
the right time to provide the required data
to enable process shortcut without whom
the project would otherwise not have been
finished on time.
Exhaust system
before conversion
New bypass stack
with blanking plate
Location of optional
blanking plate
for reconversion
into CC
Flow
Blanking plate
for OC operation
Fig. 5. Layout after conversion.
Plant layout and
conversion planning
In order to keep the overall footprint small,
the HRSG was placed as close as possible
towards the GT. Hence there was sim-
ply not enough clearance upstream the
HRSG inlet to install a standard elbow or a
switchable damper system. The HRSG was
furthermore not to be removed during the
conversion in order to allow for a possible
later re-conversion to combined cycle. It re-
mained preserved for expected future use.
Luckily there was a short rectangular si-
lencer duct installed between diffusor exit
and HRSG inlet. This is quite common for
V94.3A installations built during the turn
of the millennium. Those silencers work as
a first sound barrier before the flue gases
enter the boiler. Diffusor and silencer duct
were all internally insulated (“cold cas-
ing”) and additionally entirely enclosed for
acoustical reasons. The future stack should
also be of a cold casing design. The inter-
nal liner was agreed to be made of 1.4512
(AISI 409) which is a proven material for
exhaust systems on gas-fired plants.
The diffusor geometry was not to be modi-
fied as it is regarded as a physical part of
the GT. It became obvious that the use of
the rectangular silencer duct was the only
chance to divert the exhaust gases before
entering the HRSG. Although this duct
was too short to install an aerodynami-
cally optimised elbow, it was decided to
tie-in at that location, since the pressure
loss was regarded to be moderate. The top
section of the silencer duct was opened
and a vertical blanking plate was installed
at its back to divert gases upwards and
shut off the HRSG. Although this blanking
plate is a fix installation it is designed to
be removed once combined cycle process
should become economical again. In that
case, the bypass stack would be taken out
of service by inserting a horizontal blank-
ing plate just above the rectangular duct
(Figure 5).
As size and location of the duct opening
was unique, an adapter piece was neces-
sary to connect the silencer duct with the
already designed stack. This adapter piece
as well as the blanking plate needed to be
engineered specifically for this project.
The new foundations were due to the site
requirements outside the standard pitch
of the support structure. This asked to en-
gineer a modified support as well, which
meant another setback for the intended
software process shortcut.
Conversion
All processes started right at the time of
receiving the order. Procurement and fab-
rication of the main stack parts started
immediately and as the foundation points
were meanwhile agreed on, the modified
support structure went into engineering.

6. VGB DIGITAL
VGBPowerTech-Autorenexemplar-©2015
41
VGB PowerTech 4 l 2016 Conversion of a Belgium CCGT plant into an open cycle GT
It was essential to provide the foundation
loads within very short time in order to en-
able the customer to start with piling and
foundation works immediately. In parallel
the design of the adapter piece and blank-
ing plate was pushed.
The next steps were to install an opening in
the roof of the acoustic enclosure and the
modification of its substructure to allow
the stack to penetrate through the roof.
Then silencer duct top was then opened
(Fi g u r e 6) and the inner wall prepared
for installing the counter bearings for the
blanking plate.
Meanwhile, the first components left fab-
rication from the workshop which was
within 1,000 km driving distance from the
site. Just 5 weeks from receiving the order,
the first parts arrived on site ready for pre-
assembly.
In order to save installation time and effort,
the stack was designed for a flange bolted
site assembly. This requires additional
work in the workshop, but allows quicker
site assembly and installation compared
with welded joint design. Furthermore, it
allows supplying the individual stack com-
ponents as large as possible, without the
need for time-consuming oversize loads.
The internal insulation system allowed
pre-insulation of large-size areas already
in the workshop. This also speeded up the
installation process. After bolting the duct
parts, the internal insulation just needs to
be closed at the erection joints to complete
the assembly.
All parts were delivered to site using stand-
ard trucks, with an average shipping time
from shop to site of 2 days per load. Proper
pre-planning and regular communication
between workshop and site facilitated
just-in-time deliveries to site. Everything
came in to site on time, order and quantity
needed. This enabled continuous assembly
Fig. 6. Cut-out in silencer duct top.
Noise Control for Power Generation
G+H Schallschutz GmbH
Buergermeister-Gruenzweig-Straße 1 | 67059 Ludwigshafen | Germany | Tel. +49(0)621 502-554 | Fax +49(0)621 502-593 | info@guh-schallschutz.de
Gas turbines are the key engines for the
power generation and desalination plants
all over the world. The silencing equipment,
air filtration and exhaust gas systems for
heavy duty gas turbines are provided by
G+H Schallschutz GmbH.
G+H Schallschutz GmbH is a leading
global specialist for acoustics and
auxiliary equipment for gas turbine
power plants. Many customers
worldwide are already relying on our
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G+H-Anz-VGB 04_2016_01.indd 2 14.04.16 11:44

7. VGB DIGITAL
VGBPowerTech-Autorenexemplar-©2015
42
Conversion of a Belgium CCGT plant into an open cycle GT VGB PowerTech 4 l 2016
works and avoided wasting resources for
either idle waiting time or additional ef-
fort to place and remove equipment to and
from interim storages on site.
Sub-assemblies were made as far as possi-
ble on ground in order to minimise works
on height. This facilitates and speeds up
the installation process. Nonetheless it re-
quires the availability of large lifting capac-
ities, which were available in this project.
By following this principle, the entire stack
– except for its support structure and the
silencer baffles (Fi g u re 7) – was erected
in 4 lifts only.
With creative and pragmatic solutions,
well-structured processes, which aim at
high levels of prefabrication, fast shipment
and just-in-time delivery, skilled personnel
and a large portion of luck, this demanding
project was completed successfully. It may
look simple to pay attention to all these
odds and ends. However, it was this focus
on the process, which enabled to deliver a
new exhaust system of almost 400 tonnes
within only 3 months and to re-commis-
sion the power plant few days before the
deadline (Figure 8).
Summary
The security of electrical power supply is
a factor which is often regarded as a given
good. With the current market transitions
towards an increased share of fluctuating
energy production, the wide lack of finan-
cial incentives for dispatchability and the
quickly changing political frameworks, this
good is rather decreasing than enhancing. Fig. 8. Plant after conversion.
It is expected that this situation will con-
tinue to present an increasing challenge for
energy companies and their suppliers, and
ultimately our society.
The present article showed that with
creative and pragmatic solutions and
well-structured processes a combined cy-
cle power plant can be converted into an
open cycle power plant within 5 months,
if the boundary conditions are favourable.
This allowed the timely participation in
Belgium’s Strategic reserve and with an
availability of 100 % during the winter
2014/2015, Vilvoorde made a valuable
contribution, which it will continue to do
in the forthcoming winters.
However, the price for the higher flexibil-
ity was lowering the plant’s electrical effi-
ciency. As there is a clear trend that con-
ventional power plants need to be operated
in a more flexible way [8], such and other
conversions that diminish valuable exergy
may become more frequent in the future.
This might be desirable from an economic
and security-of-supply point of view. From
a resource point of view, though, exergy
maximisation should always be the goal.
This aspect seems to be often forgotten in
today’s discussions about the energy sup-
ply system of the future and it raises the
question, how the ideal energy system of
the future should look like.
References
[1] Schuhbauer, Ch.: Bewertung von Kohlekraft-
werken und Verbesserung ihrer Dynamik in
Hinblick auf die zukünftigen Anforderungen,
p. 6(2012).
[2] Clipstone, J. et al.: Feasibility assessment of
conversion from CCGT to OCGT. Uniper Inter-
nal (2013).
[3] Brown, St.et al.: Strategic Operations Man-
agement, p 77 (2000).
[4] Monk, T.H.: Maintaining safety and high per-
formance on shiftwork (1993).
[5] Vlaams Reglement betreffende de Milieu-
vergunning, Titel II, 1 Juni 1995.
[6] Schloss, A. et al.: Plume vertical velocity as-
sessment of a proposed gasfired power station
at Russel City Energy Center (2007).
[7] Wiese, L. et al.: Flexibility requirements for
fossil-fired power plants to support the growth
of the share of renewable energies, VGB Power
Tech 7/2013. l
Fig. 7. Installation of baffles.

8. International Journal for Electricity and Heat Generation
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