HRSG Q-T Profile Diagram (Temperature vs Cumulative Heat Duty)Red: Flue Gas Cooling Curve | Blue: Water/Steam Heating & Evaporation
Evaporator & Drum States
Drum Saturation Temp (T_sat):592.5 °F (311.4 °C)
Gas Leaving Evaporator:612.5 °F (Pinch Bound)
Water Leaving Economizer:580.5 °F (Subcooled)
Pinch Point Margin:20.0 °F (Optimal Economic)
Heat Section Breakdown
Superheater Duty (Q_sh):46.5 MMBtu/h
Evaporator Duty (Q_eva):204.8 MMBtu/h
Economizer Duty (Q_eco):147.6 MMBtu/h
Gas Enthalpy Drop:806 °F Temp Drop
Hydraulics & Steaming Safety
Economizer Steaming Risk:SAFE (+12°F Subcooled)
Gas-Side Backpressure (est):11.5 in w.c. (28.6 mbar)
Gas Turbine Power Penalty:-2.8 MW (Backpressure)
Net Cycle Power Gain:+25.7 MWe Net
5 Fatal Traps & Engineering Pitfalls in HRSG Sizing
1. The Economizer Steaming Vapor-Lock Catastrophe
Designing with an approach temperature difference smaller than 8°F to 10°F (4.5 to 5.5°C) in an attempt to preheat feedwater higher invites severe operational danger. During gas turbine low-load turndown or rapid load ramping, gas turbine exhaust temperature climbs while feedwater flow rate drops. Water boils inside the economizer tubes, generating steam voids ("steaming economizer"). This causes vapor-lock, starving the tubes of cooling liquid, instigating violent steam-collapse water hammer, and rupturing tube hairpin return bends.
2. Ultra-Tight Pinch Point Capital & Draft Loss Trap
Specifying an excessively tight pinch point (e.g. (< 10^circ ext{F} / 5.5^circ ext{C})) yields diminishing returns that destroy plant economics. As the pinch point approaches zero, the Log-Mean Temperature Difference (LMTD) collapses, requiring an exponential surge in finned tube surface area. The dense additional tube rows increase gas turbine exhaust backpressure beyond 18 to 22 in w.c., derating gas turbine base load by 4 to 6 MW, completely wiping out the modest steam turbine power gain.
Combined cycle plants operating in cyclic peaking or daily start-stop (DSS) service suffer severe thermal stress across thick HP steam drums (often 4 to 6 inches of carbon steel plate). Rapid startup without warm-keeping systems induces inner-to-outer wall temperature gradients exceeding 100°F (55°C). Over hundreds of cycles, cyclic hoop and through-wall plastic strain creates fatigue cracking at downcomer and riser nozzle bore crotches, leading to mandatory multi-million-dollar drum replacements.
4. Cold-End Acid Condensation Under Supplemental Duct Firing
Using supplemental duct burners with fuels containing sulfur increases flue gas water vapor and sulfur dioxide concentrations. As exhaust gases traverse through the cold-end low-pressure economizer, the outer tube fin temperature drops below the sulfuric acid dew point (240°F to 275°F). Carbon steel spiral fins dissolve into iron sulfate sludge within months, choking the gas flow passages and forcing emergency plant outages.
Oversizing interstage desuperheater (attemperator) spray valves causes spray water droplet impingement against hot steam pipe walls. Oversaturated water droplets do not fully atomize within the liner, pooling on the bottom of the pipe and quenching high-pressure superheater headers. Severe thermal shock creates thermal fatigue craze-cracking and shears internal thermal liners loose, sending metal shrapnel directly into steam turbine stop valves.
What is the Pinch Point in an HRSG and why does it dictate steam production?+
The Pinch Point in a Heat Recovery Steam Generator is the minimum temperature difference between the cooled gas turbine exhaust gas leaving the evaporator and the saturation temperature of the boiling water inside the evaporator drum: Delta T_pinch = T_gas,eva,out - T_sat. Because heat can only transfer from hot gas to cold water (Second Law of Thermodynamics), this temperature difference represents a thermodynamic bottleneck. A smaller pinch point (e.g. 15 to 20 deg F vs 40 deg F) extracts more heat and increases steam production, but requires an exponentially larger evaporator surface area and raises gas-side draft loss.
What is the Approach Temperature Difference and why must it stay positive?+
The approach temperature difference is the difference between the evaporator saturation temperature and the temperature of the feedwater leaving the economizer: Delta T_approach = T_sat - T_water,eco,out. It is typically designed between 10 deg F and 15 deg F (5 to 8 deg C). A positive approach ensures water remains subcooled inside the economizer tubes. If the approach drops below 5 deg F or becomes negative during low-load turndown, steam bubbles form inside the economizer tubes ("steaming in the economizer"), causing vapor lock, flow instability, and severe water hammer that ruptures tube bends.
How does gas turbine exhaust backpressure affect combined cycle plant performance?+
The tube banks, fins, ductwork, and catalytic selective catalytic reduction (SCR) beds of an HRSG create flow resistance against the gas turbine exhaust. For every 4 inches of water column (in w.c.) or 10 mbar of additional backpressure imposed on the gas turbine exhaust flange, the gas turbine power output decreases by approximately 1.0% to 1.2% and its heat rate worsens by 0.35% to 0.5%. Designing HRSGs with excessively tight tube spacing or undersized casing cross-sections destroys more gas turbine output than the extra steam generated can recover.
What is a Q-T (Heat vs Temperature) diagram in HRSG design?+
A Q-T diagram plots gas temperature and water/steam temperature along the vertical axis against cumulative heat transferred along the horizontal axis. It visually displays the temperature profiles across the superheater, evaporator, and economizer, showing the physical temperature gap at the pinch point and approach point. It allows thermal engineers to verify that temperature cross-overs (impossible heat transfer where water would be hotter than gas) do not occur at any point in the cycle.
What causes Flow-Accelerated Corrosion (FAC) in low-pressure HRSG evaporator sections?+
Flow-Accelerated Corrosion (FAC) occurs primarily in low-pressure (LP) economizers and evaporators operating between 250 deg F and 350 deg F (120 to 175 deg C). The high turbulence of two-phase steam-water mixtures entering headers washes away the protective magnetite (Fe3O4) oxide film on carbon steel tube walls. Operating with reducing all-volatile treatment (AVT-R) or low dissolved oxygen accelerates this dissolution, causing rapid tube thinning and catastrophic rupture. Specifying 1.25% or 2.25% chromium alloy (P11/P22) in LP circuits eliminates FAC.