COMBINED CYCLE
Pego experience confirms BENSON
as proven HRSG technology
Following improvements carried
out in the wake of HP evaporator
cracking problems encountered
at Hamm-Uentrop and Herdecke
some three years ago, a good
body of reliable performance has
now been amassed with BENSON
HRSGs at combined cycle plants.
Extensive recent monitoring and
analysis at the Pego plant in
Portugal, for example, reported
here, confirms that the BENSON
HRSGs are operating reliably, and
are suitable for flexible operation,
including fast starts.
Figure 1. Pego plant and HRSG (HRSG photo: NEM)
Jan Brückner and Gerhard Schlund, Siemens, Germany
www.modernpowersystems.com
The 17 BENSON HRSGs currently in
commercial operation have amassed more
than 175 000 operational hours.
The Pego BENSON HRSGs
The latest BENSON HRSGs to enter
commercial service are the two at ElecGas’s
Pego combined cycle power plant in Portugal
(see Figure 1). These can be considered “state
of the art”, having benefited from the
operating experience of the BENSON fleet
to date. They can be taken as representative
of BENSON HRSG technology. Their
evaporators
have
been
extensively
instrumented and their behaviour investigated
in depth. The results are summarised here.
Figure 2. Schematic diagram of BENSON
HRSG evaporator, as installed at Pego
Eva 2
outlet
Eva 1
outlet
Separator
Evaporator 1
Exhaust flow
direction
Evaporator 2
I
n Modern Power Systems, October 2008,
pp 33-37, Siemens reported on measures
taken to deal with cracks and leaks in the
HP evaporators of the horizontal-exhaustflow BENSON HRSGs installed at the
Hamm-Uentrop and Herdecke combined
cycle plants. As described in that article, the
cracking problem occurred in the inlet section
of Evaporator 2 and the cause was greater than
expected temperature fluctuations, in turn due
to dynamic instabilities in Evaporator 1,
which is connected in series to Evaporator 2
(Evaporator 1 and Evaporator 2 comprising
the HP evaporator section of the HRSG).
The corrective measures – installation of
throttling orifices at the inlet of Evaporator 1,
installation of T-pieces to improve
water/steam distribution at the inlet to
Evaporator 2 and addition of expansion loops
in the Evaporator 2 tubes – were installed in
June/July 2008 and subsequently the HRSGs
have run without any new cracks or leaks.
Since then several new BENSON HRSGs
have been brought into commercial operation.
Table 1 lists all commercially operating
installations to date. The HRSGs at Cottam
were fitted with expansion loops from the
outset, while all units entering service after
Herdecke and Hamm-Uentrop have been
equipped with the same features that were
installed on those units, ie orifices, T-pieces
and expansion loops. None of them have
experienced cracks or leaks.
Downcomer
Blow
down
Orifice
Expansion
loop
T-piece
Eva 2 inlet Eva 1 inlet
Star distributor
June 2011 Modern Power Systems
21
COMBINED CYCLE
The Pego HRSGs are installed downstream
of Siemens SGT5-4000F gas turbines in a
single shaft arrangement. They were supplied
by NEM under licence to Siemens (owner of
the BENSON boiler technology), and received
their PAC (provisional acceptance certificate)
in March 2011.
The HRSG of Pego unit 30 has been fitted
with equipment to perform wall temperature
measurements in the area of the HP
evaporator. These measurements are being
carried out in order to demonstrate that the
evaporators are operating in a dynamically
stable way and that the design values for the
temperature spread in Evaporator 2 are not
exceeded. Similar investigations have been
done at Irsching 5 and Sloe Centrale.
The basic components of the BENSON HP
evaporator, as employed at Pego, are shown
in Figure 2. The operating principle can be
summarised in a few sentences. The feedwater
enters the evaporator through the inlet lines of
Evaporator 1 (Eva 1 inlet) and is distributed
to the heating surface tubes of Evaporator 1.
The distribution of the mass flow to the
individual harps or tube rows is determined by
the tube parameters, the level of heating and
the extent of throttling due to orifices at the
Evaporator 1 inlet.
The mixture of water and steam enters the
downcomer through the outlet lines from
Evaporator 1 (Eva 1 outlet) and is distributed
uniformly to the inlet headers of Evaporator
2 through the star distributors and T-pieces
which are installed at the end of the connecting
lines from the star distributors to increase the
number of inlets to the Evaporator 2 inlet
headers. Depending on the load and operating
mode, superheated steam or wet steam exits
Table 1. BENSON HRSGs in commercial operation
PAC
Project/
Country
CCPP
supplier
HRSG
supplier
1999
Cottam
UK
Siemens
2007
Herdecke
Germany
Siemens
Ansaldo
2007
Hamm-Uentrop
Germany
Siemens
2010
Langage
UK
2010
Deutsche Babcock SGT5-4000F
1
SGT5-4000F
multi shaft 1:1
~ 400 MW
1
Ansaldo
SGT5-4000F
single shaft
~ 800 MW
2
Alstom
Alstom
GT26
multi shaft 2:1
~ 885 MW
2
Lingen
Germany
Alstom
Alstom
GT26
multi shaft 2:1
~ 850 MW
2
2010
Irsching 5
Germany
Siemens
STF
SGT5-4000F
multi shaft 2:1
~ 820 MW
2
2009
Sloe Centrale
Netherlands
Siemens
CMI
SGT5-4000F
single shaft
~ 870 MW
2
2010
Severn Power
UK
Siemens
CMI
SGT5-4000F
single shaft
~ 870 MW
2
2011
Malzenice
Slovakia
Siemens
STF
SGT5-4000F
single shaft
~ 400 MW
1
2011
Pego
Portugal
Siemens
NEM
SGT5-4000F
single shaft
~ 870 MW
2
downstream of Evaporator 2 (Eva 2 outlet),
is routed to a common manifold and then
flows to the separator.
It is well established, and confirmed by a
wide range of measurements, that, in the
BENSON HRSG, the water–steam mixture is
not distributed uniformly to the single tubes
inside the inlet headers of Evaporator 2. This
leads to different steam qualities at the inlet of
the individual tubes, resulting in different wall
Average wall temperature (ºC)
Outlet wall temperature (ºC) (fluid temp.)
Tube mass flow (kg/s)
0.20
0.30
0.40
0.50
0.60
0.70
Steam content at tube inlet (kg/kg)
0.80
0.90
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1.00
Resulting mass flow per tube (kg/s)
Tube wall temperature (ºC)
Overall average wall temperature over the tube height of all tubes of one header: 366°C +60/-21K
0.10
Configuration/ No of
power
HRSGs
single shaft
~ 390 MW
Figure 3. Calculated temperature profile diagram for Pego showing tube wall temperatures
(average and outlet) plotted against steam quality (steam content) at the tube inlet of
Evaporator 2e (hottest row of Evaporator 2). 100% gas turbine load, BENSON mode
500
480
460
440
420
400
380
360
340
320
300
280
260
0.00
Gas turbine
temperatures. See Figure 3, which shows an
example from Pego.
Accordingly, as already noted, expansion
loops are installed at the inlet of each
individual tube of Evaporator 2 to
compensate for different tube expansions
resulting from the temperature spread caused
by different average wall temperatures
among the single tubes of one harp. The
expansion
loops
are
designed
to
accommodate a temperature spread of ±50
K, being the maximum difference between
the average wall temperature (averaged over
tube height) of a single tube and all tubes of
one header.
The Pego evaporators were designed based
on the following assumptions:
• The temperature spread of the average wall
temperatures will not exceed ±50 K.
• The evaporator will be dynamically stable
at each load point.
• Evaporator 1 will not be superheated at the
outlet.
• The Evaporator 2 risers of each row will
exhibit balanced temperatures.
Thermocouples were installed at the upper end
of the Pego Evaporator 2 heating surface
tubes, as shown in Figure 4 in order to confirm
these assumptions.
Figure 5. Location of thermocouples at Pego
for wall temperature measurements on
Evaporator 1 and 2 risers
22
Modern Power Systems June 2011
126
Tube number
84
85
42
43
1
Flue
gas
20
18
17
16
Rows
Figure 4. Location of thermocouples at Pego for outlet wall
temperature measurements on Evaporator 2
www.modernpowersystems.com
COMBINED CYCLE
Figure 6. Overview of the relation between the calculated, measured and “averaged over height”
wall temperatures and how the “average over height” is calculated
Calculated wall temperatures over
the entire tube length/height
Outlet wall
temperature
Tcalc n
Tcalc 1 + Tcalc 2 + Tcalc + Tcalc n
n
Measured wall temp. at tube outlet
Calculated wall temp. Profile over height
Average calculated wall remp. Over the height
Tcalc ...
ø Twall calc
Tube length/height
The thermocouples measuring the wall
surface temperature were insulated in order to
eliminate the influence of the gas side
temperature.
Figure 5 shows thermocouples installed on
all risers connected to the outlet harps of
Evaporator 1 and Evaporator 2.
The measurement system has been
operating since the commissioning of Pego.
Saturated conditions must exist in
Evaporator 1 at all loads and this is confirmed
by the measurements taken in Evaporator 1.
To confirm that the limit of a maximum
+50/-50 K temperature spread between the
average wall temperature of single tubes and
the average wall temperatures of the entire
harp is not exceeded the average wall
temperature over the height of each tube
during operation needs to be known. The tube
wall temperature can only be measured at the
upper end of the tubes just below the upper
cavity. These measured outlet wall
temperatures therefore need to be converted
into an average wall temperature, as shown in
Figure 6. This was done using thermodynamic
calculations based on the boundary
conditions valid for the load cases under
investigation (see Figure 3 and Table 2).
Tcalc 1
Flue gas
Tcalc 0
Temperature
The results
Fast start tests
During commissioning of the Pego combined
cycle power plant, intensive investigations
www.modernpowersystems.com
375
350
0
325
300
0
275
250
0
225
200
0
175
150
0
125
100
0
75
50
0
25
0
0
600
1200
1800
2400
3000
3600 4200
Time (sec)
4800
5400
6000
TURBINE SPEED (Hz)
CALC GT LOAD (MW)
GT EXH GAS MASS FLOW (kg/s)
HP DS/HTR2 OUT PRES (bar)
HP STM FLOW (kg/s)
CALCULATED ST LOAD (MW)
HPS D/STR S/HTR3
2OF3 T (ºC)
EG temp at
flue gas stack 9ºC)
EG temp at HRSG inlet (ºC)
7
750
7
700
6
650
6
600
5
550
5
500
4
450
4
400
3
350
3
300
2
250
2
200
1
150
1
100
5
50
0
7200
6600
All temperatures/ EG mass flow
Others
Figure 7. DCS data from Pego for 100% GT load
EG = exhaust gas
Figure 8. Spread of tube outlet wall temperatures, row 16, harp 22, 100% GT load
Harp 26
Harp 27
Harp 22
Harp 23
Harp 24
20
18
17
16
500
50
450
40
400
30
350
20
300
10
250
0
200
-10
150
-20
100
-30
50
-40
0
0
600
1200
Row16 Harp22
Average temperature
1800
2200
3000
Row16 Harp22
Minimum temperature
3600 4200
Time (sec)
4800
Row16 Harp22
Maximum temperature
5400
6000
Row16 Harp22
Max. minus
deviation
6600
-50
7200
Max. deviation from average temperature (K)
Rows
Harp 25
Flue
gas
Wall temperature (ºC)
An overview of the measurement data from
Pego is given below, along with the results of
analysis performed for representative load
points.
Analysis has been performed at the load
points listed below to confirm that the
evaporator is operating correctly. The choice
of points is designed to encompass the full
range of normal evaporator operation:
• 34% GT load (inlet guide vane);
• 60% GT part load;
• 100% GT full load;
• 48% GT part load;
• Cold start;
• Advanced FACY (Fast Cycling) warm
start; and
• Shut down.
The first step of the measurement analysis is
to display the measured outlet wall
temperatures together with selected DCS
(distributed control system) data describing
the load point and the status of the evaporator.
By way of example, the DCS data for the
100% GT load case are shown in Figure 7. Two
plots are created for each harp to display the
outlet wall temperatures. The first one shows
average temperatures and the existing
temperature spread. The second one shows all
temperature measurements taken on the harp
over a certain time period. Figure 8 and 9
show, respectively, these two plots for Eva 2e
left harp (row 16/harp 22), at 100% GT load.
It can be seen from this example that at 100%
load the evaporator is dynamically stable and
that the temperature spread is within the
allowable limits, ±50 K.
The same analysis has been done for all the
load cases listed above. In every one of these
cases it has been shown that no instabilities
occur and that the temperature imbalances do
not exceed the ±50 K limit.
Results of the analysis for each of the load
points listed are summarised in Table 2.
Row16 Harp22
Max. plus
deviation
June 2011 Modern Power Systems
23
COMBINED CYCLE
were done on the fastest achievable start-up
times. The monitoring at Pego has shown that
the BENSON design allows maximum gas
turbine ramp rates to be achieved, without any
limit arising from components of the HRSG.
Start-up times of close to 30 minutes from gas
turbine ignition to full load were
demonstrated (see Figure 10) without
sacrificing component life time.
Figure 9. Tube outlet wall temperatures, row 16, harp 22, 100% GT load
Harp 26
Harp 27
Harp 22
Harp 23
Harp 24
20
18
17
16
Rows
Harp 25
Flue
gas
500
450
Wall temperature (ºC)
400
Proven technology
Analysis of the data from the Pego combined
cycle plant is giving clear confirmation that the
evaporators are behaving within design
assumptions:
• The maximum existing temperature spread
of the average wall temperature is
approximately 50 K (+42/-8). 100 K (+50/50) was assumed for the design.
• The measurements have shown that the HP
evaporator is operating at dynamically
stable conditions during each phase of start
up and shut down as well as at each steady
state load.
• In addition to that, the front harps of
Evaporator 1 have always exhibited
saturated temperature at the outlets. In
combination with the dynamically stable
operation this indicates that the orifices
installed at the Evaporator 1 inlet are
working as expected and are designed
correctly.
• Measured values for variables such as
pressure drop across the evaporator, outlet
steam temperatures, and average wall
temperatures, have been successfully
reproduced by calculation.
In addition, it has been shown that combined
cycle plants equipped with BENSON HRSGs
are extremely flexible in terms of start-up times
and the HRSG components do not limit the
ramp rates of the gas turbine.
Siemens has now gained a good body of
operational experience with the BENSON
HRSG and it has firmly established itself as a
MPS
proven technology.
350
300
250
200
150
100
50
0
0
600
1200
1800
2200
3000
3600
4200
Time (sec)
4800
5400
6000
6600
7200
375
350
0
325
300
0
275
250
0
225
200
0
175
150
0
125
0
100
75
0
50
25
0
0
600
1200
1800
2400
3000
3600 4200
Time (sec)
4800
5400
6000
6600
7
750
7
700
6
650
6
600
5
550
5
500
4
450
4
400
3
350
3
300
2
250
2
200
1
150
1
100
5
50
0
7200
All temperatures/ EG mass flow
Others
Figure 10. Fast start test at Pego
GAS TURBINE SPEED (Hz)
CALC GT LOAD (MW)
STEAM TURBINE SPEED (Hz)
CALCULATED ST LOAD (MW)
HP DS/HTR2 OUT PRES (bar)
HP STM FLOW (kg/s)
HP RED STN CTRL-V POSN (%)
EG temp at HRSG inlet (ºC)
HPS D/STR S/HTR3
2OF3 T (ºC)
EG temp at
flue gas stack (ºC)
GT EXH GAS MASS FLOW (kg/s)
EG = exhaust gas
Table 2. Summary of measured data and results of analysis
Measurements and DCS data during commissioning
Date
(all 2010)
06/10
04:00-06:00
Plant load
100% GT
GT
power
m-flow
HP
Evap
(kg/s)
HP evap
mode
(MW)
Pressure
HP evap
(separator)
(bar)
277
131
77
BENSON
Maximal temperature spreads
∆TS/x
Attemp.
in
operation
HP
Eva,
row/harp
Measured
outlet
wall temp.
(K)
Calculated
average
wall temp
(K)
No
2a middle,
20/26
65 (+40/-25)
61 (+40/-21)
2c middle,
18/26
55 (+34/-21)
26 (+18/-9)
(K)
11.5
- ditto01/10
00:00-02:00
60% GT
160
97
53
BENSON
15
No
2a right,
20/27
70 (+55/-15)
50 (+42/-8)
26/10
19:30-21:30
34% GT (IGV)
95
76
45
Level
0.91
No
2d left,
17/22
60 (+45/-15)
26 (+23/-3)
19/11
04:30-06:30
48% GT
132
87
50
BENSON
11
Yes
2c middle,
18/26
65 (+55/-10)
47 (+42/-5)
13/09
07:40-11:40
Cold start
2d left,
r17/h20
80 (+45/-35)
30 (+23/-5)
08/10
12:00-14:00
Warm start/
Advanced FACY
2a right,
20/27
75 (+55/-20)
30 (+24/-6)
13/09
20:00-24:00
Shut down
2e left,
16/22
90 (+50/-10)
-
24
Modern Power Systems June 2011
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