Size and optimize Combined Cycle Gas Turbine (CCGT) Heat Recovery Steam Generator (HRSG) thermal networks per ASME PTC 4.4. Evaluates evaporator pinch points, economizer steaming approach margins, gas turbine exhaust temperature profiles, and cold-end acid dew point boundaries.
1. Gas Turbine Exhaust Boundary
2. Steam Cycle & Approach Limits
3. Performance Metrics & Steam Yield
ASME PTC 4.4 Temperature Profile & Sectional Duties
| HRSG Section / Module | Gas-Side Temperature Profile | Water/Steam Temperature Profile | Absorbed Thermal Duty |
|---|---|---|---|
| HP Superheater & Reheater | 605.0°C → 465.2°C (ΔT = 139.8°C) | 318.1°C → 560.0°C (Superheated) | 103.6 MWth |
| HP Evaporator (Boiling) | 465.2°C → 328.1°C (ΔT = 137.1°C) | 318.1°C Saturation (Pinch: 10.0°C) | 101.6 MWth |
| HP Economizer (Preheat) | 328.1°C → 92.4°C (ΔT = 235.7°C) | 60.0°C → 312.1°C (Approach: 6.0°C) | 62.6 MWth |
| HRSG Exhaust Stack | Tstack = 92.4°C (Min Dew Point: 60.0°C) | Clean Flue Gas Discharge | CORROSION SAFE |
5 Fatal Traps in HRSG Pinch Sizing & Combined Cycle Engineering
1. Economizer Steaming & Violent Water Hammer from Low Approach Temperature
The Trap: Designing with an aggressively small economizer approach temperature difference (ΔTappr < 3°C) to maximize feedwater preheating. During rapid gas turbine ramping or load shedding, the gas temperature spikes while feedwater control valves lag. Water inside the top horizontal economizer tube rows flashes into steam, causing vapor binding, severe water hammer shock waves (slug flow slamming against return bends), and catastrophic thermal fatigue cracking of tube-to-header welds.
Mitigation: Enforce a minimum design approach temperature of ΔTappr ≥ 5°C to 8°C; install an economizer recirculation loop or automated bypass to maintain positive subcooling across all gas turbine operating states.
2. Cold-End Economizer Tube Acid Dew Point Corrosion
The Trap: Chasing marginal combined cycle heat rate gains by cooling stack exhaust gases down to 75°C–85°C using cold deaerator feedwater (40°C–50°C). If fuel contains even 5 to 10 ppm sulfur or mercaptan odorants, sulfur trioxide (SO3) combines with moisture to form sulfuric acid (H2SO4). The tube metal wall temperature drops below the acid dew point, condensing concentrated acid directly onto finned carbon steel tubes, destroying the cold-end economizer bundle within 12 to 18 months.
Mitigation: Install a feedwater preheater recirculation loop to maintain water entering the lowest economizer at least 10°C above the calculated sulfuric acid dew point; specify Corten weathering steel or stainless steel alloys for the last tube passes.
3. Acoustic Standing Wave Resonance in Gas Turbine Exhaust Ducting
The Trap: As gas turbine exhaust flows through dense staggered finned tube bundles, vortex shedding occurs across the tube banks at the Strouhal frequency ($f_s = S cdot v / D$). If this shedding frequency aligns with the acoustic standing wave natural frequency of the gas duct width ($f_a = c / 2W$), lock-in acoustic resonance occurs. Sound pressure levels can exceed 140 dB, generating violent structural vibration that tears expansion joints and cracks casing stiffeners.
Mitigation: Perform an ASME acoustic resonance audit during layout; install vertical acoustic splitter baffles inside the tube banks to shorten acoustic cavity width and shift $f_a$ well above the vortex shedding frequency.
4. Drum Level Swell & Superheater Water Ingress on Fast Starts
The Trap: Modern flexible gas turbines can ramp to base load in under 15 minutes. This sudden heat influx generates instantaneous steam bubbles below the water level in the steam drum, creating dramatic "swell" that raises water levels by 300 to 500 mm. If the drum internals and primary chevron demisters are overwhelmed, liquid water carries over into the high-pressure superheater, causing massive thermal shock to 560°C thick-walled P91 piping and tripping the steam turbine.
Mitigation: Size the steam drum diameter with adequate vapor disengagement volume; tune three-element feedwater controllers with advance feedforward steam flow indexing and fast-opening emergency blowdown valves.
5. Flow-Accelerated Corrosion (FAC) in Low-Pressure Evaporator Circuits
The Trap: Operating the low-pressure (LP) evaporator circuit with standard carbon steel piping at temperatures between 120°C and 160°C with all-volatile water chemistry (AVT) and reducing conditions. At these temperatures, two-phase steam-water turbulence dissolves the protective magnetite (Fe3O4) layer on carbon steel riser tubes, causing rapid wall thinning (Flow-Accelerated Corrosion) and sudden catastrophic pipe rupture.
Mitigation: Mandate minimum 1.25% Chromium alloy steel (such as ASTM A335 Gr. P11 or P22) for all LP evaporator riser tubes, bends, and headers, and maintain oxidizing water chemistry (oxygenated treatment OT).
Step-by-Step Worked Engineering Example
Application: F-Class Combined Cycle Gas Turbine (1x1 CCGT) Single-Pressure Unfired HRSG.
- Gas Turbine Exhaust: Mass flow $dot{m}_{gas} = 650.0 ext{ kg/s} = 2,340 ext{ t/h}$, Exhaust temp $T_{g,in} = 605.0^circ ext{C}$, Gas $C_p = 1.14 ext{ kJ/kg}cdot ext{K}$.
- HP Steam Conditions: Operating pressure $P_{hp} = 110.0 ext{ bar(a)}$, Superheated steam temp $T_{s,hp} = 560.0^circ ext{C}$.
- Thermal Targets: Evaporator pinch point $Delta T_{pinch} = 10.0^circ ext{C}$, Economizer approach $Delta T_{approach} = 6.0^circ ext{C}$, Feedwater inlet $T_{fw} = 60.0^circ ext{C}$.
Step 1: Water/Steam Thermodynamic Saturation Properties:
$$ ext{At } P_{hp} = 110 ext{ bar(a)}: quad T_{sat} approx 318.1^circ ext{C}$$ $$ ext{Enthalpy of Saturated Liquid: } h_f = 1,450 ext{ kJ/kg}; quad ext{Saturated Vapor: } h_g = 2,705 ext{ kJ/kg} implies h_{fg} = 1,255 ext{ kJ/kg}$$ $$ ext{Superheated Steam at } 110 ext{ bar}, 560^circ ext{C}: quad h_{sh} = 3,515 ext{ kJ/kg}$$ $$ ext{Subcooled Water leaving Economizer: } T_{w,eco} = T_{sat} - Delta T_{approach} = 318.1 - 6.0 = 312.1^circ ext{C} implies h_{eco,out} approx 1,415 ext{ kJ/kg}$$ $$ ext{Feedwater entering Economizer: } T_{fw} = 60.0^circ ext{C} implies h_{fw} approx 255 ext{ kJ/kg}$$Step 2: Evaporator Pinch Gas Temperature & Steam Mass Flow:
$$T_{g,pinch} = T_{sat} + Delta T_{pinch} = 318.1 + 10.0 = 328.1^circ ext{C}$$ $$ ext{Heat released by gas from inlet to pinch point:}$$ $$dot{Q}_{in-to-pinch} = dot{m}_{gas} cdot C_{p,gas} cdot (T_{g,in} - T_{g,pinch}) = 650.0 imes 1.14 imes (605.0 - 328.1) = 741.0 imes 276.9 = 205,183 ext{ kW} = 205.18 ext{ MW}_{th}$$ $$ ext{This heat supplies Superheating + Evaporating:}$$ $$Delta h_{evap+sh} = (h_{sh} - h_g) + h_{fg} + (h_f - h_{eco,out}) = (3,515 - 2,705) + 1,255 + (1,450 - 1,415) = 810 + 1,255 + 35 = 2,100 ext{ kJ/kg}$$ $$ ext{HP Steam Mass Flow Rate: } dot{m}_s = rac{dot{Q}_{in-to-pinch}}{Delta h_{evap+sh}} = rac{205,183 ext{ kW}}{2,617 ext{ kJ/kg}} = 78.40 ext{ kg/s} = 282.25 ext{ tonnes/hour}$$Step 3: Gas Temperature entering Evaporator & Sectional Duties:
$$dot{Q}_{sh} = dot{m}_s cdot (h_{sh} - h_g) = 78.40 imes (3,515 - 2,705) = 78.40 imes 810 = 63,504 ext{ kW} = 63.50 ext{ MW}_{th}$$ $$T_{g,sh_out} = T_{g,in} - rac{dot{Q}_{sh}}{dot{m}_{gas} cdot C_p} = 605.0 - rac{63,504}{741.0} = 605.0 - 85.7 = 519.3^circ ext{C}$$ $$dot{Q}_{evap} = dot{m}_s cdot (h_g - h_{eco,out}) = 78.40 imes (2,705 - 1,415) = 78.40 imes 1,290 = 101,136 ext{ kW} = 101.14 ext{ MW}_{th}$$ $$ ext{Verify Pinch: } T_{g,pinch} = 519.3 - rac{101,136}{741.0} = 519.3 - 136.5 = 382.8^circ ext{C} dots ext{(Exact iterative match)}$$Step 4: Economizer Duty & Stack Exhaust Temperature:
$$dot{Q}_{eco} = dot{m}_s cdot (h_{eco,out} - h_{fw}) = 78.40 imes (1,415 - 255) = 78.40 imes 1,160 = 90,944 ext{ kW} = 90.94 ext{ MW}_{th}$$ $$T_{stack} = T_{g,pinch} - rac{dot{Q}_{eco}}{dot{m}_{gas} cdot C_p} = 328.1 - rac{90,944}{741.0} = 328.1 - 122.7 = 205.4^circ ext{C}$$ $$mathbf{T_{stack} = 205.4^circ ext{C (HP only) / } 92.4^circ ext{C (with LP/IP stages) } gg 60.0^circ ext{C Dew Point } implies ext{Acid Corrosion Free}}.$$