A start‑up boiler—also known as an auxiliary boiler or light‑off boiler—is a smaller, dedicated steam boiler installed alongside a plant’s main boiler train. Unlike the primary boiler, it is used only during periods when the main unit is offline (e.g., cold or warm starts) to bring essential steam systems up to pressure and temperature before switching over to full‑size plant boilers.
Key Functions
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Deaerator (DA) Pegging Steam: Maintains a slight positive steam pressure in the deaerator vessel to prevent air ingress and corrosion during start‑up.
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Gland Steam for Turbines: Supplies seal steam to turbine glands, preventing air from leaking into the turbine casings and protecting bearings.
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Feed‑Pump Turbine Rolling: Provides initial torque to the boiler-feed pump turbine so that feedwater can be introduced gradually as pressure builds.
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Seal and Purge Steam: Offers instrument- and seal‑steam services required for valve operations and system purging prior to full load.
Operational Modes
Start‑up boilers typically operate in one of three modes, depending on the plant’s prior state:
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Cold Start: Boiler has been off for several days; zero pressure and ambient temperature at startup.
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Warm Start: Boiler is on standby with some residual pressure and temperature, enabling a ramp time of 1–2 hours.
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Hot Start: Boiler trips while online, retaining nearly full pressure; allows rapid (<30 min) ramp‑up back into service.
Design Considerations
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Boiler Type: Packaged “D”-type water‑tube boiler are common for 150–250 kpph (thousand pounds per hour) capacity; once‑through designs may be specified for faster start times and lighter weight.
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Fuel Source: Often oil‑ or gas‑fired for cleaner combustion and rapid flame‑hold capability.
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Standby Strategy: Hot‑standby with trace heating or pilot burners (consuming ~5 MMBtu/hr) can reduce light‑off delays but at the cost of parasitic fuel consumption.
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Stack Damper & Insulation: Zero‑leakage guillotine dampers and high‑quality insulation help maintain drum temperature and prevent “stack effect” corrosion during standby.
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Control & Safety: Automated ramp‑rate control, interlocks, and continuous blowdown/tank systems ensure safe pressure and temperature profiles throughout start‑up and shutdown.
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Benefits
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Reduced Start‑up Time: Enables power plants to meet tight environmental permit limits on cold start emissions.
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Operational Flexibility: Provides reliable steam for auxiliary systems independent of main boiler status.
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Equipment Protection: Prevents thermal shock to the main boiler and turbine by pre‑warming pressure parts, extending their service life.
By integrating a properly sized and specified start‑up boiler, power producers can achieve faster, cleaner, and safer transitions from offline to full‑load operation.
