As grids lean on gas to balance renewables, heat recovery steam generators (HRSGs) are being re‑engineered to sprint, not stroll. The latest designs hit full load from cold in under 30 minutes—without burning through their life—by pairing thin, flexible pressure parts with advanced alloys and smarter controls.
Renewables supplied more than 30% of global power for the first time in 2023 (www.oliverwyman.com), and Indonesia aims ≥23% by 2025 (www.oliverwyman.com). That variability forces combined‑cycle gas turbine (CCGT) plants to start and stop daily, demanding a different kind of boiler. In fast‑start HRSGs (heat recovery steam generators, which use gas‑turbine exhaust to make steam), the turbine and HRSG are essentially decoupled so the gas turbine can spool up at full rate—leaving the HRSG to absorb big temperature and flow jumps (www.modernpowersystems.com) (www.powermag.com).
Conventional starts took one to two hours; modern designs reach full load in less than 30 minutes from cold (www.powermag.com). The catch: doing this poorly can dramatically shorten equipment life.
Low‑mass pressure parts and flexible supports
The core design move is to minimize thermal gradients and add flexibility. High‑pressure (HP) drums and headers—typically the thickest parts—are thinned as far as pressure codes allow. Designers cut HP drum shell thickness by using high‑yield steels (ASTM A299B/A302B) and by reducing drum water‑hold‑up time to about 90 seconds (www.modernpowersystems.com).
Superheater and reheater outlet headers are capped at ≤1.25 inches thickness via single‑row “harp” coil designs and many small nozzle branches (www.modernpowersystems.com) (www.powermag.com). Tube rows are supported to move freely: adding spring supports to upper headers lets coils expand and contract under startup gradients, “decreasing thermally induced stresses by an order of magnitude” (www.powermag.com). Interconnecting piping is routed with long, flexible runs or expansion joints to accommodate the >100 °F row‑to‑row temperature differentials during warm‑up (www.powermag.com) (www.powermag.com). Designs that absorb temperature‑driven expansion—via movable supports, link pipes, and similar features—are now standard in fast‑start HRSGs (www.powermag.com) (www.powermag.com). These construction techniques (spring supports, flexible elbows, longer runs) prevent severe low‑cycle fatigue when tubes are otherwise restrained under rapid heat‑up.
Attemperation and exhaust bypass strategies
Temperature control is built in. Multi‑stage spray attemperators (spray water coolers used to control steam temperature) are added—intermediate plus final stage—so steam temperature can be actively modulated as steam flow and gas temperature ramp up (www.modernpowersystems.com). Many plants also install parallel exhaust paths (bypass stacks): operators may warm up the gas turbine (GT) in simple‑cycle mode initially, then switch to HRSG flow, or run low‑load GT plus duct burners to minimize HRSG stress (www.modernpowersystems.com) (www.modernpowersystems.com).
Advanced alloys and thin‑shell construction
Fast cycling drives material upgrades beyond conventional boilers. In GE’s 62.2%‑efficient Bouchain plant, the HP superheater/reheater tubes are austenitic stainless steel (Super304H) to withstand about 585 °C steam—roughly 20 °C hotter than typical F‑class designs (www.nsenergybusiness.com). Ferritic P91 steel was technically allowable, but would have required much thicker walls to limit stress; Super304H offers higher creep strength and oxidation resistance with thinner sections (www.nsenergybusiness.com). ASME allows P91 to 650 °C, but steam oxidation above roughly 605 °C is a concern (www.nsenergybusiness.com).
In practice, designers now routinely specify Grade 92 steel (higher stress‑rupture strength than Grade 91) for reheater headers and other high‑temperature parts (www.modernpowersystems.com) and even consider nickel‑based alloys where needed. At Bouchain, Ni‑based Incoloy 617 (≈10× cost of P91) bridges the difference between P91 headers and stainless piping (www.nsenergybusiness.com). Careful placement of dissimilar‑metal welds is critical: CMI placed such welds in external connectors—with Incoloy transition pieces—to reduce stress‑relaxation cracking risk (www.nsenergybusiness.com) (www.nsenergybusiness.com).
Drum design is tuned for faster heat‑up. Because “through‑thickness” thermal gradients drive fatigue, wall thickness is minimized within code limits (www.powermag.com). Multiple small‑diameter steam nozzles or tube‑stub reinforcements let designers keep flow area without forcing excessive wall thickness (www.powermag.com) (www.powermag.com). Operators are advised to reduce HP drum volume and minimize thickness to speed heat‑up (www.modernpowersystems.com). In practice, fast‑start HRSG drums may even be replaced with thin‑shell versions and welded in lifts to save weight.
Construction also emphasizes modular flexibility: coils are pre‑formed with U‑bends and linkage geometry to accommodate vertical growth, and all welds in high‑fatigue zones follow qualified procedures to minimize residual stress. In one design study, adding a single coil spring support cut thermal stress by about 90% (www.powermag.com).
Automated sequencing, purge credits, emissions interlocks
Even the best hardware needs sophisticated control. A modern DCS (distributed control system) orchestrates startup: it meters GT exhaust into the HRSG, modulates bypass dampers, and adjusts feedwater to control drum level and pressure ramps. A recommended strategy is to send a small fraction of GT exhaust to the HRSG immediately at start (with the rest to bypass) and use the STO (steam drum pressure) feedback to auto‑adjust the bypass damper (www.powermag.com). This avoids excessive drum pressure gradients and prevents holding the GT at low load. In a Foster Wheeler case, adding advanced HRSG controls cut cold‑start time by ~33% without over‑stressing equipment (www.powermag.com).
Controls also manage emissions interlocks. Maintaining ammonia vaporizer temperatures for the SCR (selective catalytic reduction) catalyst can be automated—sometimes with electric heaters—so fast starts still meet NOx limits (www.modernpowersystems.com). Accurate chemical dosing supports this function (/products/dosing-pump). Some utilities purge the HRSG on shutdown (using NFPA‑style triple‑block bleed systems; NFPA is a safety standard framework) so the official “purge” isn’t needed on startup, trimming minutes from time to generation (www.modernpowersystems.com). Positive‑sequence logic and interlocks monitor drum pressure, metal temperature, and steam quality in real time, and safety valves or trips are pre‑set—often per NFPA/IEC—to guard against overshoot.
Drum level control and digital twins
A critical control issue is drum water level. During ramp‑up, “shrink and swell” (a transient level change due to pressure and boiling dynamics) can force level excursions; poor control can even trigger a trip during aggressive starts (www.researchgate.net). Most plants use three‑element level control (drum level, steam flow, feedwater flow) tuned for transients, and researchers are exploring enhanced PID (proportional‑integral‑derivative), fractional‑order, and fuzzy logic controllers to handle rapid swings better (www.researchgate.net).
Digital tools add a final layer. Many operators use dynamic simulations or “digital twins” to plan optimal startup ramps (balancing speed versus fatigue), then implement them via the DCS. In one analysis, continuous data logging of temperature ramps and stresses was used to compute a “damage fraction” for each startup (www.powermag.com), informing inspections and procedures. Some plants employ online stress monitors on headers and drums that accumulate equivalent operating hours, flagging high‑fatigue events (www.nsenergybusiness.com).
Measured outcomes and fatigue trade‑offs
The payoff is measurable. Modern fast‑start HRSGs routinely meet grid demands in a few tens of minutes from cold, versus up to two hours before (www.powermag.com). Efficiency has climbed: the GE 9HA/CMI plant hit a world‑record 62.22% (gross), aided by 585 °C steam (www.nsenergybusiness.com).
The trade‑off is component life. One study found a cold start uses roughly 7× as much HP drum life as a hot start, and a warm start about 4× (www.modernpowersystems.com). That underscores the value of let‑through control and warm‑stack strategies to keep residual pressure.
Engineering balance: design, metallurgy, control
In summary, a modern fast‑start HRSG blends mechanical design (thin, high‑strength, flexible components), metallurgy (advanced steels and alloys), and intelligent control (automated sequencing, feedback loops, preheating). The result is an HRSG capable of daily—even multi‑year—cycling with minimized downtime, but only if each aspect is fully engineered and monitored. Every reported case shows that without good design and control, the faster response that creates revenue can be eaten up by excessive maintenance or shortened equipment life (www.powermag.com) (www.modernpowersystems.com).
Sources: Modern Power Systems; Power magazine (Vogt Power, Foster Wheeler); NS Energy/CMI analyses; ISA Transactions (2021) on drum controls; Valmet technical overviews; and industry/government reports (with Indonesian context) (www.modernpowersystems.com) (www.powermag.com) (www.nsenergybusiness.com) (www.powermag.com) (www.researchgate.net) (www.powermag.com). Each citation lins the underlying data and case studies.
