Modern heat recovery steam generators (HRSGs) are being reengineered to go from cold iron to full output in minutes, not hours. The playbook blends flexible construction, high‑strength alloys, and automated sequencing so startups can be accelerated without compromising equipment life (Power Magazine).
One number captures the shift: GE’s latest combined cycle reaches full power from a cold start in under 30 minutes (Power Magazine). Older HRSGs routinely took 45–120+ minutes to get there (Power Magazine).
That speed now underwrites grid flexibility. Indonesia’s 2025–2035 plan calls for ~71 GW of new capacity—about 60% gas/coal and 40% renewables—while its grid code (Reg. 20/2020) requires all >5 MW plants to participate in AGC (Automatic Generation Control) (Reuters; ESDM). The HRSG’s job: deliver steam on-command, survive frequent cycles, and meet life‑cycle targets.
Flexible pressure-part architecture
Fast-start HRSGs make thermal shock survivable by design. Coils and piping are intentionally flexible, with only one header spring‑supported so other headers can float; tube “harps” build in bends that absorb row‑to‑row temperature differentials (Power Magazine). In high‑temperature sections—superheater, reheater, evaporator—designers use sliding supports and flexible riser manifolds, and top‑support the hot parts so they can expand freely (Power Magazine).
Thin, small‑diameter pressure parts keep thermal gradients down. Vogt Power’s fast‑start approach minimizes HP (high‑pressure) drum outside diameter and wall thickness—using high‑strength alloys like SA‑299B or SA‑302B—and targets HP drum walls on the order of ~1.25 in or less (Modern Power Systems; Modern Power Systems; see also “Drum thickness considerations”: Modern Power Systems). Superheater headers trend smaller by shifting to single‑row, parallel circuits; thin single‑row headers can eliminate tube bends at welds and permit faster temperature‑change rates (123dok). Plants with older, thick‑walled drums/headers often retrofit to these slimmer designs to arrest cycling cracks and failures (see Figure 1; Modern Power Systems).
Condensate management becomes a design feature. Separate drain manifolds and fast‑drain systems prevent quenching during starts; ALSTOM’s arrangement puts a large lower drain manifold below the superheater header to collect condensate during hot restarts, stopping water from backing up into hot tubes (123dok). Steam circuits are fully drainable and ventable to handle abrupt shutdown/start‑fill cycles (Power Magazine).
To reduce localized stress, designers upsize headers, downcomers, and drain piping. A horizontal three‑pressure HRSG at El Carmen (Mexico) used oversized superheater/reheater headers and downcomers, plus full‑penetration tube‑to‑header welds, to ride through fast transients without cracking (Power Magazine).
Heat retention matters. Plants add stack dampers or slide‑gates to block natural‑draft cooling, and insulate stacks and cold‑end ducting so the next start is closer to a warm start (Power Engineering). Some facilities also use dampers to maintain >500 psia in the HP drum during outages—far less expensive than a purge (Modern Power Systems).
Spray attemperators (water‑spray coolers) in final superheater stages give precise steam temperature control as the gas turbine ramps; some designs bias one stage toward the coil back end for low‑flow stability (Modern Power Systems).
(For a visual of flexible assemblies and weld approaches, see “in Figure 3 are the … fast transient operations without the”: Power Magazine.)
Alloy choices for rapid cycling
Fast‑start operation elevates thermal fatigue, so metallurgy shifts to higher strength, higher chromium steels. Where classic HRSG drums/headers used 2¼Cr–1Mo (e.g., ASME SA‑299, SA‑302), designers now deploy 9–12% Cr alloys so thin sections still meet pressure and stress limits. Vogt, for example, targets Grade 92 (9Cr‑1Mo‑V‑Nb) in superheater headers—especially in light of lowered allowable stresses for Grade 91—and specifies high‑yield SA‑302B for HP evaporator drums (Modern Power Systems; Modern Power Systems). In practice, hot sections broadly use modified 9Cr steels (P91/P92) to balance pressure capability with thermal‑stress resistance.
Header geometry doubles as metallurgy. A 2007 ALSTOM study showed that small‑diameter, thin‑wall, single‑row headers eliminate a tube bend at the weld and “permit more rapid rates of temperature change” than thick, multi‑row headers—with step changes in thickness protecting the weld area (123dok).
Corrosion control follows the temperature gradients. High‑chrome (martensitic stainless) tubes are specified in flow‑accelerated corrosion hot spots, and designers keep economizer flue gas above the dew point during transients to avoid condensation corrosion (Power Magazine).
Beyond the HRSG, steam‑path components see upgrades too: HP/IP turbine casings and valves adopt higher‑grade materials and welded designs, enabling thinner walls and lower thermal inertia during starts (Power Engineering).
Welds are fully penetrated and stress‑minimized to prevent cycling cracks at transitions (Power Magazine). Insulation is heavy on hot sections; mounts use hangers, springs, flexible flanges, and expansion joints to avoid anchoring heat and imposing frame stress.
Corrosion mitigation remains a chemistry task as well. Plants apply standard boiler treatment programs (boiler chemicals) alongside mechanical design choices. Dissolved oxygen is typically addressed with targeted agents (oxygen scavengers) to protect pressure parts.
The combined effect—thin‑wall, high‑strength alloys plus flexible assemblies—lets HRSGs tolerate 25–45 °C (45–80 °F) row‑to‑row differentials without breaching fatigue limits; spares for critical tubes/headers are often specified with extra margin (123dok; Power Magazine).
Startup logic and automated sequencing
Fast‑start capability is as much software as steel. The DCS (distributed control system) executes automated sequences that bring the HRSG up in parallel with the gas turbine. From the very first fire, a portion of GT exhaust is routed into the HRSG while the rest bypasses to stack—accelerating section warm‑up without slowing the turbine (Power Magazine).
Bypass dampers are modulated automatically using drum ramp‑rate feedback and an optimized PID (proportional–integral–derivative) loop, so drum pressure/temperature follow a target curve while operators avoid manual juggling (Power Magazine). Conventional holds are minimized; the instruction is to lift gas‑turbine holds as soon as the HRSG allows, keeping IGVs (inlet guide vanes) wide open at the highest permitted gas temperature (Power Magazine).
Terminal attemperation decouples heat input from the steam turbine’s limits. Final‑stage spray attemperators let the HRSG heat the steam path while delivering exactly the inlet steam temperature the turbine can accept, preventing rotor stresses from exceeding allowable limits (Power Engineering). DCS sequences handle valve openings, pump starts, and spray activation; that can include starting critical pumps such as dosing pumps when chemistry control is interlocked with starts (Power Engineering).
Purges are pulled forward. Many plants implement NFPA purge credits so HRSG fuel/duct‑burner purging is performed during shutdown, with triple‑block‑and‑bleed valves and pressure monitoring in fuel/ammonia lines. That allows skipping a standalone purge at startup and trims minutes from the timeline (Modern Power Systems).
Auxiliary steam and pre‑warming strategies convert cold starts into semi‑warm ones. Plants may maintain condenser vacuum or use sparging steam from auxiliary boilers or steam accumulators; a cold start consumes roughly seven times the fatigue life of a warm start, so keeping the drum hot saves life even if it adds some fuel (Modern Power Systems). Stack dampers and HP‑drain tests to catch leaking drains are part of this choreography (Modern Power Systems; Modern Power Systems).
Protection stays active. Advanced CCGTs (combined‑cycle gas turbines) tie rotor‑stress monitors into the DCS, allowing automatic throttling—or even a turbine trip—if thermal or rotational stresses approach limits (Power Engineering). Shutdowns are similarly choreographed, with controlled feedwater ramp‑downs, damper adjustments, and safe venting to avoid abrupt cooldowns.
Quantified gains and trade‑offs
In modeling and practice, the combination of hardware and control logic removes tens of minutes. One plant with 45–60 minute cold starts was modeled for faster ramps; the study showed a ~30–50% reduction without materially shrinking component cycle life (Power Magazine). Field experience shows cold starts to full load in 15–30 minutes are routine for fast‑start configurations (Power Magazine).
That speed pays in markets with 5‑minute dispatch: synchronizing in under 10 minutes opens non‑spinning reserve revenues unavailable to slower peers (Power Engineering). The business case is strengthened as renewables rise; conventional units struggle to earn capacity value unless they can cycle (Power Magazine; Power Engineering).
There are costs. Capital rises for flexible internals, enriched attemperation/duct burners, stack dampers, auxiliary steam, and advanced controls, and O&M increases for washes and instrumentation (Power Engineering; Power Engineering). Owners accept a modest per‑start life usage to gain availability and revenue, and mitigate that hit by keeping drums warm between cycles (Modern Power Systems).
In Indonesia’s gas‑plus‑renewables build‑out, these design/control packages are seen as essential (Reuters). The industry consensus from data‑backed studies and plant reports: thoughtful fast‑start HRSGs—advanced materials, flexible construction, and smart controls—can dramatically shorten startups (often by tens of minutes) while keeping fatigue and emissions within limits (Power Magazine; Power Magazine; Modern Power Systems).
Sources: Power Magazine; Modern Power Systems; Power Engineering; Modern Power Systems; Power Magazine; Power Magazine; Modern Power Systems; Power Magazine; Power Magazine; Reuters; Power Magazine; Power Magazine; 123dok; Power Engineering; Modern Power Systems; Modern Power Systems; 123dok; Power Magazine; Power Magazine; Modern Power Systems; Modern Power Systems; 123dok; Power Engineering; Power Engineering; Power Engineering; Power Engineering; Power Magazine; Power Magazine; ESDM.
