HRSG design | DrumPlusTM
a faster way to switch from simple to combined cycle
The combination of NEM Energy’s DrumPlusTM HRSG design and a diverter damper has shown the
ability to switch-over from simple cycle to combined cycle at 100% gas turbine load for an F-class gas turbine, without the need for GT ramp down. This enables CCGT plant operators to respond more quickly to grid demands, as well as saving money and reducing emissions
G. Hariharan, P. Witte, P. Rop, S. Ruijgrok NEM Energy, The Netherlands
Figure 1. Lifetime-critical components in a conventional drum type, horizontal- gas-path HRSG
To accommodate the fluctuating nature of renewables, combined cycle power plants are moving away from continuous base load operation to cycling operation involving a higher number of starts/stops, shorter start-up times and longer part-load operation. In addition to operational flexibility, combined cycle plants have also been evolving in terms of overall plant efficiency thanks to advancements in gas turbines, which have been scaling in size and operating at much higher temperatures than before. The elevated exhaust temperature and flow translates to higher pressure, flow, and temperature for the bottoming cycle. For the HRSG, this means thicker and larger pressure part components due to
higher design temperature and pressure. But thicker pressure part components militate against flexibility as they experience larger thermal stress during a stress cycle.
As indicated by the arrows in Figure 1, the pressure part components critical to lifetime under cycling conditions are the hottest headers in the HP and RH system as well as the HP drum, where the water–steam separation occurs. The HP drum is the component with the largest wall thickness and therefore most critical for lifetime. One of the strategies to resolve the challenge of a thick drum is to eliminate the drum and adopt a once-through HRSG (OTSG) design. In such a system, the outlet of the evaporator system always remains dry with suitable feedwater control. But due to the absence of a circulation system and blowdown to remove contaminants, a once- through system requires installation of a proper condensate polishing plant.
The other approach to reduce drum fatigue is to retain the drum while minimising the size and thereby reducing the thickness. The NEM patented DrumPlusTM
design follows the
Separator bottles
Knockout drum
latter concept and works in a similar way to a convention natural circulation drum. In the DrumPlusTM
design the HP drum is
Bypass stack with diverter damper
replaced by a ‘knockout’ drum for primary water– steam separation and ‘separator bottles’, which are a group of small vessels for secondary water– steam separation. In Figure 2 the separator bottles and knockout drum are indicated in the general arrangement of a horizontal-exhaust-gas-flow three-pressure and reheat system. The knockout drum diameter and thickness is minimised compared to a conventional drum, resulting in reduced thermal stress. For an operating pressure of 160 bar, the typical design values for a conventional and DrumPlusTM
HRSG are:
Conventional drum: 2000 mm inner diameter with 140 mm wall thickness. DrumPlusTM
drum: 1300 mm inner diameter with 90 mm wall thickness.
Thanks to the reduced drum wall thickness, DrumPlusTM
can support an unrestricted start-up of the gas turbine. The El Segundo FlexPlantTM
10, in Figure 2. DrumPlusTM horizontal-gas-path HRSG, with diverter damper 22 | May 2025|
www.modernpowersystems.com
California (Figure 3), is an example of a DrumPlusTM installation, in commercial operation since 2013. The facility’s two SGT6-5000F gas turbines can
+ diverter damper:
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