International Conference Power Plants 2012

Society of Thermal Engineers of Serbia Oct. 30th - Nov. 2nd, 2012 - Zlatibor, Serbia

http://e2012.drustvo-termicara.com/english/

Abstract

In the Balkan and Central Europe, there is a large fleet of older oil & coal fired power plants

with a potential for repowering into modern combined cycle power plants. This paper will

review the specific HRSG design features required for such repowering projects. Compared

to green field projects, repowering a steam turbine with a new train of gas turbine and HRSG

includes some special challenges. The HRSGs have to be tailor made to suit the existing site

constraints. We will explain how a Vertical type HRSG can be adapted and modelled to

match a limited foot print. In addition we will present the specificities of the design of HRSGs

when the Gas Turbine fires heavy fuel oil, as mandatory in some projects. CMI has recently

completed such a large repowering projects, for instance Senoko Singapore; Dunamenti

Hungary, Shoiaba/SEC/ Saoudi Arabia, … These experiences will be the support throughout

this paper to explain these specific repowering challenges.

Fig.1 CMI is a HRSG specialist and offers both Horizontal and Vertical designs

Background of Senoko Project

In December 2004, Senoko Power of Singapore has completed an innovative plant

repowering project. Three old large coal fired boilers had been demolished to be replaced by

3 gas turbines of 250 MW each, coupled with a Heat Recovery Steam Generator (HRSG)

based on state-of-the- art 3 pressures plus reheat. Produced steam is used in the former old

steam turbine and condenser. This is the repowering concept of an old steam turbine, which

has typically a longer life span than the boielrs. This paper reviews all of the specific

characteristics of the HRSG for repowering as used for Senoko project but also at Dunamenti

a 2010-2011 repowering project in Hungary.

Compared to a greenfield plant, where standard ‘reference plant’ concepts can apply,

repowering projects must always be specific to suit existing plant layout and steam turbine

operating conditions. This was especially true at the Senoko plant. Firstly, the new HRSGs

had to match the very limited foot print of the old conventional boilers, where dimensions of

the new HRSG had to be adapted. Then, the HRSG design was selected as Vertical type.

Secondly, modular construction including highly prefabricated heat exchangers were

required to match the 18 months stoppage. Finally, the Senoko facility is a very congested

site with no room for large cranes. Consequently, hydraulic lifting jacks have been used

instead for boiler erection of heavy items. A good deal of existing civil structures and cooling

systems were re-used. After the first GT26 block, having the equivalent capacity of more

than two of the smaller steam sets, has come into operation, it was possible to shut down

the remaining two units for repowering. The second repowering step was then performed in

a narrow 18 month window. Thanks to this ‘two-steps’repowering, the interruption time was

minimal and the plant returned to service as scheduled at the end of 2004. Alstom was the

project leader, and they in turn awarded the HRSG to CMI. The choice of CMI, as a specialist

in the Vertical HRSG, was significant. The HRSGs are Vertical natural circulation with three

pressure levels plus reheat : HP, 322 t/h at 128.9 barA, 568°C; IP, 27.9 t/h at 41.5 barA,

320°C; LP 17.8 t/h saturated at 5.4 barA, 237°C and RH, 332 t/h at 39.4 bar 568°C.

Site limited available area

At Senoko, there were constraints imposed by the space occupied by the original fired boiler

which had to contain the gas turbine, the HRSG and the connections to the steam turbine. So,

available area per HRSG was limited to only 30.6 meters in length by 28.1 meters in width.

HRSG with all auxiliaries (feedwater tank, feedwater pumps, sampling, dosing skid,…) had to

be included in such a reduced area. Inside the building, a cargo lift was also to be installed.

All piping had to remain within this perimeter boundary because of the surrounding

enclosure, and some limits of supply and even Alstom’s equipment was located inside the

HRSG enclosure creating major risks of interfaces. Last but not least, the GT outlet extended

about 1 meter into the boiler enclosure, reducing even further the available area for HRSG.

Therefore, it was challenging to install such a large HRSG in such a reduced area. At the early

engineering stage, CMI exchanged with Alstom 3D model of HRSG including auxiliaries.

Alstom then consolidated its 3D plant model with PDMS software. Detailed three dimension

plant modelling prevented clashes at the engineering stage without adverse construction

problem (Fig.2).

Fig.2 HRSG modeling in 3 dimensions

Vertical HRSG flexibility

Standard Vertical HRSG for such GT class ‘F’ combined cycle typically measures an overall

length of about 35 meters. This length is based on 20.4 meters tubes long which are the

longest tubes for Vertical HRSG used by CMI without any other limitations. Such standard

design applies for greenfield site or when there is no space limitation. Even though this

standard boiler is already quite short, this was not short enough to match the Senoko

repowering site. Unlike horizontal HRSG, boiler tube length impacts directly the boiler

length; this is a specific feature of Vertical HRSG. CMI took advantage of this lateral and

vertical space flexibility arrangement offered by the Vertical design (Fig.3).

GT

Overall boiler length 35 meters

Typical tubes length 20.4 m

Typical

heigth

9 m

Fig. 3 Vertical HRSG overall length based on longest used boiler tubes

This feature is very useful in case of repowering. As often, the boiler width was not such an

issue in comparison with boiler length. For Senoko, the longest possible tube was 18.4

meters and the boiler casing was enlarged in proportion. While making such casing

adjustment to available space, the driving criterion is to keep the gas pressure drop

unchanged. In other words, gas velocity and gas path cross section shall also remain

unchanged (Fig.4). It is important to remember here that HRSG gas pressure drop is

proportional to the square of gas velocity, meaning that gas pressure drop (guaranteed value

by HRSG supplier) is very sensitive to available gas flow cross section.

Gas cross sectionGas cross

section

unchanged

Longest

tube

20.4 m

Reduced

tube/boiler

length

FlexibleSlightly

enlarged

boiler

width

Fig. 4 Arrangement flexibility offered with shorter tubes.

As a consequence of this wider casing, path had been divided in 3 wide sections instead of 2

as usual for the standard solution. There were 3 heat exchanger modules side by side over 4

levels for a total of 12 modules (Fig.5). The modules were completely shop prefabricated and

hydrostatically tested in workshop. The largest module weighted 145 tonnes with overall

dimensions 23.6 m * 3.9 m * 2.9m high. They were transported from harbour to site on

hydraulic trailers.

Heating

surface

modules

flexible

arrangement

Standard

'greenfield'

arrangement

Senoko

enlarged

casing width

Fig. 5 Heat exchanger modules arranged in 2 or 3 modules in width

This way, it became possible to fit the HRSG on this site. In case of available length is even

shorter, a last option would be by means of entering flue gas on the large casing face instead

of the standard small face (Fig. 4). This is a specific feature of the Vertical HRSG as the first

Gas flow

heating surface is about 9 meters high (Fig.3), and this allows sufficient space for the inlet

duct arrangement underneath. This option is applicable up to middle size HRSG and is useful

in case of repowering plant where, for instance, the existing steel structure shall be

recovered. But, this option was not requested at Senoko.

Boiler modular construction

Heat exchangers modules were factory assembled under strict CMI quality control. As per

CMI standard design, each module was made of parallel serpentine tubes mounted on tube

support plates and connected to headers at each ends. As a result of this module prefabrication

design, only header to header welds had to be carried out at site. An important constraint at

Senoko site was to ensure that there would be enough access around the operating unit for the

repowering of the second and third units. The hydraulic trailers carrying the modules had to

manoeuvre around the operating unit of phase 1 (Fig. 6). To perform so precision movement,

trailers selected were self-propelled rather than tractor pulling. This allowed more flexibility

in those manoeuvres because of the reduced convoy length, and its large angle orientation

wheels. Also, those advantages offered by the self-propelled trailer were required for the final

positioning of modules inside boiler frame due to limited space left as explained hereafter.

Those hydraulic trailers proved to be very accurate in this exercise with a positioning

tolerance of only 1 or 2 mm. Taking into account maximum trailer turning radius, a cinematic

study of movement sequence had been performed by CMI during engineering.

Module lifting with jacks

Senoko site precluded the use of a large crane. In case of prefabricated modules for

Horizontal HRSG, large cranes are typically needed to tilt and lift them. Instead for Senoko,

module lifting was performed with 28 hydraulic jacks installed on top of steel structure. This

module erection procedure is standard for CMI Vertical HRSG. The first module was brought

to position in the frame and its 7 tube support plates were attached to those jacks through

suspension cables and plates. The hydraulic trailer was then lowered, transferring module

weight carried to those cables. The process was repeated for the other two modules to

complete this first level. This completed level was then jacked up sufficiently for the next

level of modules to be placed underneath it similarly, and the process was repeated until all 4

levels of modules had been suspended (Fig. 8). It is important to note that modules remained

always in horizontal position; module tilting operation is not required for Vertical HRSG

erection. The complete assembly was then jacked up to its final position in the steel structure.

At that stage, the complete pressure parts weight amounting to 1450 tonnes was still hang on

those 28 jacks. By introducing pins in suspensions plates and lowering jacks, weight was

Fig. 6 Boiler modules on hydraulic trailor had to manoeuvre on congested site

transferred to the 28 final suspensions points. Hydraulic jacks were released, dismantled and

reinstalled on the next unit. Then, adjacent header ends were welded together; no other

internal pressure part welding was needed except for a few tube/tube welds between level 1

and level 2. From modules arrival harbour up to final suspension of the 12 modules on to the

boiler frame, it took only 6 days without using crane, nor even scaffolding. This is a

remarkable short time also due to this work repeat.

Fig. 7 Second level of module ready to be lifted Fig. 8 Completion of 12 modules suspended

Typically, hydraulic jacks are better synchronized in jacking up than lowering loads.

Lowering load is a feature that is not normally needed by CMI module erection procedure

described hereabove. However, at Senoko, space was so limited (Fig. 10) that this special

feature was required. Indeed, when the erection of each module is in a gradually reducing

area, the problem is to get the third module exactly aligned relative to the 2 aside modules

already suspended. Hydraulic jacking system had been selected to get the capability to lift

and to lower the complete load in a synchronized way. Synchronization between jacks is very

critical during the lowering down because of the risk of unbalanced pressure between jacks

and consequently uneven load distribution on the steel structure. Considering the 1450 tonnes

hanging on this jacking system, uncontrolled load distribution between the 28 jacks would

have been unacceptable. To give clearance to the trailer to manoeuvre for the third module to

be positioned exactly underneath, the whole assembly was jacked up of several meters and

then relowered at the initial elevation to install suspensions plates. Hydraulic jacking system

proved to be very accurate in positioning with a maximum tolerance of only one of two

millimetres (Fig.9).

Fig. 9 Module erection Fig. 10 Self propelled trailer manoeuvres in limited space

Indoor HRSG

Indoor HRSG was specified indoor by Senoko Power. Not only is the Singapore climate

characterized by temperatures in the low to mid 30’s and high relative humidity for much of

the year, but the power plant is located on the coast and buildings provide protection against

airborne salt. The main purpose was weather protection, but enclosure was also used for noise

abatement in terms of far field acoustical emissions. It is to be noted here that boiler stack

was also equipped with a flap damper for inside weather protection, and boiler bottling up to

keep it warm during outage. Compared to Horizontal HRSG, stack damper is a standard

feature of CMI Vertical HRSG as the stack is centred on HRSG. For enclosure support, the

main HRSG steel structure was easily extended to a secondary frame wrapped and attached

all around. Senoko Power had specified to CMI architects some colours and aesthetic criteria

(Fig. 11). For instance, all building side walls were extended 2 meters above roof platforms in

order to hide top equipment such as silencers and louvers. Those louvers were installed for the

natural ventilation of building.

Dunamenti, Hungary

Fig. 11 Senoko repowering HRSG buildings phase 1 (CCP3) on left, and phase 2

(CCP4&5) on rigth

Fig. 12 Dunamentia G3 (GDF-SUEZ – ELECTRABEL) BUDAPEST, Hungary

After the G2 repowering project in 1995, CMI supplied a HRSG for the G3 project that was

commissioned in 2011.

SHOIABA HRSG project summarized:

• 10 vertical HRSGs, outdoor

• Flue gas by-pass stack (as option)

• Crude oil firing with sootblowers

• Hot casing (external rockwoll insulation)

• 2P (LP steam for the external deaerating FWT)

• Natural circulation

• Site conditions: 9 to 50°C

• ASME design

• HRSG prefabricated in 4 pressure part modules

• Number of tubes: 3456 tubes of 17 meters

• Stack

Fig. 13 Shoiaba/SEC/ Saoudi Arabia, General Arrangement Drawing

Specificities of a HRSG designed for continuous fuel oil firing in the Gas Turbine:

• Heavy duty gas turbines operate typically on natural gas, without risk of fouling and

typical dew point at 60°C

Both Horizontal or Vertical HRSG are suitable such purpose

• Crude oil has high sulfur content with Acid Dew Point around 145°C

• High sulfur content dictates the proper HRSG selection because all metallic surfaces

must remain above ADP to prevent internal corrosion

• Temperature of condensate water entering finned tubes must be controlled to remain

above ADP to avoid acid formation on tubes

It limits the heat recovery in the back end of the HRSG

• Ducting metal must remain above ADP which is not feasible with internal insulation.

HRSG must be externally insulated with hot ducting walls at gas temperature (no

condensation occurrence)

For HRSGs operated on continuous crude oil, heat exchangers are designed as follow:

Finned tubes with maximum ~160 fins per meter

Solid fins prefered

Staggered or inline tubes arrangement

On line cleaning Sootblowers inside tube banks

Off line Water washing system capability and drains

Limitation of tube rows per bank for efficient cleaning

Good accessibility of pressure parts for inspection

Fig. 14 CMI has designed many HRSGs behind gas turbines firing light and heavy fuel oil.

Conclusions

There is real potential for repowering of old steam turbines in the Balkans. In a lot of old

conventional plants, fired boilers have exhausted their useful life before its steam turbine.

Over the years, much has been said about repowering, but very little has been done so far.

Repowering of those old steam sets into efficient combined cycle with new GTs and HRSG is

a cost effective solution. Today, available gas turbines can provide the exhaust energy for

steam turbines of 120 -150 MW of which there are many examples in Europe dating back

from the 1970’s. These units could be repowered so as to increase power supply with a

significant improvement in operating efficiency, flexibility and emissions. CMI has

completed the Senoko and Dunamenti plants, which are a very successful example of such

repowering. At Senoko, the Vertical HRSG has been proofed to be very accommodating for

those the specific repowering constraints, which always require a tailor made design to suit

limited space. In addition CMI Vertical HRSGs are uniquely fit behind Gas Turbine firing

fuel oils.

To be presented by:

Pascal Fontaine, Product Manager, CMI Liege Belgium.

Xavier d’Hubert, Business Development Mgr East & Central Europe.

CMI Groupe Avenue Greiner, 1

4100 SERAING (Liège) BELGIUM