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HEI Boiler Deaerator Mass & Energy Balance Sizing
HEI Standards for Tray & Spray Deaerators & ASME PTC 12.3 Thermal Balancer
⚡ Operating Pressure & Feedwater Target
Net deaerated water to boiler feed pumps (1 t/h = 1000 kg/h)
Standard low pressure: 0.2 to 0.5 bar g (105°C - 112°C)
Spray-tray delivers superior turndown & oxygen removal
Insulated shell loss: typically 0.8% to 1.5%
💧 Condensate Return & Makeup Water
Remaining fraction is treated cold makeup water
Hot recovered condensate: 75°C to 95°C
Demineralized or RO water from water plant: 15-25°C
ASME/HEI minimum standard: 10 to 15 minutes
💨 Pegging Steam & Venting
Upstream pegging steam before PRV station
Saturated steam: ~148°C; Superheated: 160-220°C
HEI recommended: 0.10% to 0.20% continuous plume
Operating cost of heating steam consumed
Pegging Steam Consumption
9.86 t/h
9.86% of Total Feedwater
Saturation Temperature
108.4 °C
227.1 °F (0.35 bar g)
Combined Water Inflow
90.15 t/h
Blended T_in = 52.5 °C
Storage Tank Net Volume
26.2 m³
2.4 m Ø × 6.8 m Length
Dissolved Oxygen Residual
< 0.005 ppm
≤ 5 ppb (Meets HEI Standards)
📊 Spray-Tray Deaerator & Storage Tank Operating Cutaway
Thermal Status: SATURATED BOILING (108.4 °C)
Upper Dome: Water spray nozzles & 316SS tray tiers with counterflow steam
Plume: Atmospheric non-condensable purge vent valve
Lower Tank: Boiling deaerated water buffer storage to BFP suction
Deaerator Thermal Balance & Hydraulics Summary
Makeup Water Flow Rate: 45.08 t/h (198.5 GPM)
Condensate Return Flow: 45.08 t/h (198.5 GPM)
Heating Heat Duty (Q_da): 6.28 MW (5.40 Gcal/h)
Vent Steam Mass Loss: 14.8 kg/h (0.15% rate)
Condensate Return Flow: 45.08 t/h (198.5 GPM)
Heating Heat Duty (Q_da): 6.28 MW (5.40 Gcal/h)
Vent Steam Mass Loss: 14.8 kg/h (0.15% rate)
Blended Inlet Water Enthalpy: 220.0 kJ/kg
Deaerated Water Enthalpy (h_f): 454.8 kJ/kg
Pegging Steam Enthalpy (h_s): 2,775 kJ/kg
Storage Tank Overflow Volume: 32.8 m³ (80% full level)
Deaerated Water Enthalpy (h_f): 454.8 kJ/kg
Pegging Steam Enthalpy (h_s): 2,775 kJ/kg
Storage Tank Overflow Volume: 32.8 m³ (80% full level)
Available BFP Suction Head: 5.5 m Static (Elevation req.)
Annual Pegging Steam Cost: $2.05M / yr (8000 hrs)
Steam Valve Cv Estimate: 68.4 Cv (4" Control Valve)
ASME Code Vessel Rating: 3.5 bar g / 150°C (Section VIII)
Annual Pegging Steam Cost: $2.05M / yr (8000 hrs)
Steam Valve Cv Estimate: 68.4 Cv (4" Control Valve)
ASME Code Vessel Rating: 3.5 bar g / 150°C (Section VIII)
✓ Design Meets HEI Oxygen Removal Benchmark (Residual O2 < 7 ppb)
Deaerator Thermal Balance Governing Formulations
Mass and energy conservation across the deaerating head follows the Heat Exchange Institute (HEI) Standards for Deaerators and ASME PTC 12.3:
W_fw = W_water_in + W_steam - W_vent [t/h]
h_water_in = (W_mu × h_mu + W_cr × h_cr) / W_water_in [kJ/kg]
W_steam = [W_fw × (h_fw_out - h_water_in) + Q_loss + W_vent × (h_vent - h_fw_out)] / (h_steam - h_water_in)
W_vent = W_steam × (vent_pct / 100)
V_storage_net = (W_fw × 1000 / ρ_water) × (retention_minutes / 60) [m³]
h_water_in = (W_mu × h_mu + W_cr × h_cr) / W_water_in [kJ/kg]
W_steam = [W_fw × (h_fw_out - h_water_in) + Q_loss + W_vent × (h_vent - h_fw_out)] / (h_steam - h_water_in)
W_vent = W_steam × (vent_pct / 100)
V_storage_net = (W_fw × 1000 / ρ_water) × (retention_minutes / 60) [m³]
where h_fw_out is the saturated liquid enthalpy at deaerator pressure ((P_{da})), h_steam is the pegging steam enthalpy, and V_storage_net is the usable liquid buffer volume between low-water trip and overflow levels.
5 Fatal Traps & Industrial Engineering Pitfalls
1. Pegging Steam Pressure Collapse & Violent BFP Cavitation
When an extraction steam line trips or the pegging pressure regulating valve hunts, deaerator dome pressure suddenly plummets. While the vapor pressure in the dome drops instantly, the large thermal inertia of 30 tonnes of hot stored feedwater cannot cool immediately. The water flashes violently into steam bubbles inside the boiler feed pump (BFP) suction downcomer. Net Positive Suction Head Available (NPSHa) collapses to zero, destroying pump impellers through violent vapor implosions within seconds.
2. Over-Throttling the Vent Valve Trapping Corrosive Carbonic Acid
Operators trying to eliminate the visible white steam plume often throttle or close the deaerator vent needle valve. Without continuous steam venting, liberated non-condensable gases (O2 and CO2) cannot escape the dome. CO2 gas redissolves into the falling liquid to form aggressive carbonic acid (H2CO3), dropping feedwater pH below 6.5. This triggers severe acid corrosion throughout carbon steel economizers and boiler feedwater heaters, necessitating millions in premature retubing.
3. Cold Makeup Surge & Internal Tray Stack Dislodgement
During sudden process interruptions where hot condensate returns cease and 100% cold (15°C) makeup water surges into the spray head, the enormous temperature difference causes intense localized condensation shock. Water vapor collapses abruptly above the trays, producing violent hydraulic chugging and upward pressure pulses. In poorly fastened tray boxes, this shock dislodges and scatters stainless steel trays throughout the storage tank, completely ruining deaeration performance.
4. Lack of Mechanical Vacuum Breakers Causing Vessel Buckling
Deaerator storage tanks are designed per ASME Section VIII Division 1 for internal positive pressure (typically 3.5 bar g), but thin-walled cylindrical shells have negligible resistance to external pressure or full vacuum. If boiler feedwater pumps continue pumping while steam supply is shut off and cold water enters, the collapsing steam creates a deep internal vacuum. Without redundant ASME-rated mechanical vacuum breaker valves, atmospheric pressure will crush and collapse the entire horizontal tank like a soda can.
5. Anti-Vortex Baffle Absence in BFP Suction Nozzle
High-velocity water exiting the bottom nozzle of the storage vessel naturally forms a drain whirlpool (vortex) if liquid level drops near the lower operating limit. Without an engineered cruciform anti-vortex plate welded directly over the suction nozzle, the vortex core sucks steam vapor directly from the vessel headspace down into the boiler feed pump suction line. The entrained vapor voids cause instantaneous multi-stage pump de-priming and severe dry-run seizure of pump wear rings.
Frequently Asked Questions
How does a thermal deaerator remove dissolved oxygen from boiler feedwater?
What is pegging steam and how is its consumption calculated?
Why is a continuous vent steam rate necessary on a deaerator?
What retention time is required for a deaerator storage vessel?
What is transient deaerator pressure drop and Net Positive Suction Head (NPSH) collapse?
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