Heat recovery steam generators |
How fast starts impact HRSGs & what can be done about it
The growing need for combined cycle plants to be able to start quickly and get power to the grid rapidly has significant implications for heat recovery steam generators. The good news is that new-build HRSGs can be designed and existing HRSGs retrofitted to enable unrestricted gas turbine start-up to full load
Vignesh Bala VP, Operations – HRSG Services, Vogt Power International
The operating profile of combined cycle plants has changed significantly over the last couple of decades. Combined cycle plants were generally designed in the late 1990s and early 2000s for base load operation. These plants were replacing the aging coal fleet and providing base loaded power. However, with the proliferation of renewable energy sources and a growing demand for power, the future will be shaped by an “all of the above” approach towards power generation (nuclear, energy storage, wind, solar, hydro and gas). Under this approach, power generation will no longer be restricted to one or two sources but will rather be comprised of a diverse mix of technologies, with a component of energy storage.
For this generation mix to work seamlessly together, a fast-reacting, dispatchable source of generation is required. Combined cycle plants are now being relied upon to be this quick acting, reliable source of power and hence are being asked to cycle repeatedly. The ability to start quickly and get power on the grid has become very important for combined cycle plants. New plants are being designed with this need in mind and a number of existing plants are contemplating retrofitting their heat recovery steam generators to have this fast start capability.
Figure 1. Installation of new Vogt Power International HP drum at existing unit (photo: Vogt Power International)
m Figure 2.
Schematic of a single row
superheater in a steam side parallel
arrangement. This design allows a
smaller header OD to be
employed and hence reduced thickness. m = manifold ch = common header
HP SH1
HP SH2
ch
Conventional start vs fast start During a conventional start-up, the gas turbine is started up but is sometimes held at a lower load to allow time for components to warm up and for the temperature in the HRSG to increase in a controlled manner. Depending on the configuration of the plant it can take two hours or longer to get to full load. In contrast, in a combined cycle plant optimised for fast start, the gas turbine is started as if it was a simple cycle machine and ramped up without any restriction, to full load. The HRSG and steam turbine are decoupled from the gas turbine and do not place any limitations on the gas turbine start-up. This can mean that the power from the gas turbine is available much more quickly, and this provides flexibility to the grid. However, achieving this fast start capability means that the HRSG components need to be able to accept high exhaust gas flows and rapid changes in exhaust gas temperature and need to be appropriately designed.
18 | March 2025|
www.modernpowersystems.com
Drum thickness considerations The HP drum is the thickest component in an HRSG and during rapid start-up events, the temperature difference between the outer shell and inner part of the drum is very large. This temperature gradient across the thickness of the drum creates thermal fatigue stresses that over several cycles manifest themselves as cracks and
failures.For a fast start application, the thickness of the drums needs to be minimised as much as possible. One way to do this is to use materials with high yield strength, such as SA-299B or SA-302B. Further, the OD of the drum itself, and consequently its thickness, can also be reduced by considering lower drum hold up times (defined as the time taken for the water level in the drum to drop from the normal water level to the low level cutoff) of up to 90 seconds.A growing number of existing plants are planning and implementing drum replacements with the new drums designed for fast start applications (see Figure 1).
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