Overview

Feedwater tank sizing and selection depends on several key factors:

• Total operational BHP
• Operational steam pressure
• Desired water storage time at full load
• Condensate return rate

The following guide will walk through

1. Sizing and selection of a feedwater system to accommodate 3+1 LX-300SG-C Miura boilers
2. How condensate return changes those calculations
3. How a pressurized deaerator system changes the considerations

Atmospheric Feedwater System

Key Equipment Data

• Boiler Model: LX-300SG-C
• Rated Output: 300 BHP per boiler
• Steam Generation Rate:

300 BHP×34.5 lb/hr*BHP =10,350 lb/hr per boiler

• Total for 3 Operational Units:

10,350×3=31,050 lb/hr steam 

Convert Steam Load to Feedwater Flow

• Water requirement ≈ Steam generation (condensate return may reduce makeup, but size for 100% makeup unless specified).
• Convert to lb/hr to GPH:

31,050 lb/hr÷8.34 lb/gal=3,720 GPH

• Convert to GPM:

3,720÷60=62 GPM (for 3 boilers)

Feedwater Pump Guidelines

• Rule: Each boiler requires its own dedicated pump, sized at ~150% of its steaming rate to ensure recovery under load swings.
• Per Boiler Pump Calculation:

10,350 lb/hr→10,350/8.34=1,241 gal/hr→1,241/60=20.7 GPM

• Apply 1.5 safety factor:

20.7×1.5=31 GPM per pump

Summary:

• Each pump: 31 GPM @ NPSHa > NPSHr
• Total apparent GPM: 93 GPM (but since each boiler has its own pump array, tank sizing based on sum for short term surge)

Feedwater Tank Sizing

• Miura Guideline: 20–30 min of storage at full evaporation.
• Use 20 min:

62 GPM×20=1,240 gallons
 

So, a tank in the range of 1,200–1,500 gal is standard.

Pressure Head

• Pumps must overcome:
  • Elevation to DA/tank
  • Boiler operating pressure (100-150 PSIG typical LX series)
  • Include margin for valves and piping loss (8 PSI for DDCV and 29 PSI for MI check valve)
• Typical: Feed pumps for Miura operate against 180–220 PSIG discharge.

Recommended Layout

• One atmospheric feedwater tank or surge tank feeding 3+1 water pumps.
• Standard approach:
  • (3+1) individual feedwater pumps (one per boiler with redundancy)
  • (1) tank sized 1,200–1,500 gallons or vendor standard size
  • Pump controls tied to Miura’s BP controller (on-demand feed)

Optional Considerations

• Will you return condensate? If yes, actual makeup drops and chemistry changes.
• Include N+1 redundancy? If feeding is critical, consider standby pump pair.
• Water treatment integration: Softener/RO sized to same GPM capacity (~60–70 GPM).

Preliminary Specification Summary

Impact of Condensate Return

Makeup Water Requirement

• Original full steam load:

31,050 lb/hr steam≈62 GPM (previous calculation)

• 75% returns via condensate recovery:
  Only 25% makeup water needed:

62×0.25=15.5 GPM for makeup

Key changes:

• Water softener/RO sized closer to 15–20 GPM, not 62 GPM.
• Chemical load drops substantially, but feedwater quality still critical because condensate must be treated and monitored (risk of contamination or oil ingress).

Feedwater Tank Sizing

• Even with condensate return, tank must store system surge and handle transitions (startup, upset, pump operation).
• For 75% return:
  • Operating feed demand is still whole steam rate (62 GPM) because boilers draw full evaporation rate; the source (return + makeup) determines tank level.
  • Recommended: Keep 15–20 min net storage after surge assumption (condensate arrival may lag).
  • Tank size stays similar (~1,000–1,200-gal minimum), not slashed to 25%.
    Reason: Protect against startup dry-out and condensate delay.

Pump Sizing Logic

Individual Pump Capacity

• Each boiler still demands full-rate feed at firing (≈ 20.7 GPM x 150% = 31 GPM per pump, same as before).
• Condensate return does not reduce instantaneous pump flow—only makeup rate changes.

N+1 Redundancy

• If you have 3 boilers → 3 duty pumps, N+1 means 4 pumps total.
• Control:
  • Miura BP controller can manage sequencing
  • All 4 pumps sized same (31 GPM @ 180–220 PSIG)
• Electrical & piping must allow any combination of 3 operating pumps with one standby.

Tank & Control Adjustments

• With high condensate return:
  • Include provisions for condensate contamination monitoring (oil seal, conductivity alarms).
  • Maintain overflow to drain and surge absorption (condensate can arrive in slugs).
• For N+1:
  • Add extra suction line port and discharge manifold isolation valves for standby pump.

Updated Specification Summary

Note

Condensate return does not let you downsize pump or tank significantly because:

• Pumps serve on-demand fill at evaporation rate.
• Tank buffers startup and surge, not just steady state.
• N+1 means you size pumps as if none is redundant during failure.

Pressurized Deaerator Tank System

Which Recommendations Stay the Same

• Pump Redundancy (N+1): Still 3 duty + 1 standby, sized at 31 GPM @ discharge pressure per boiler. Each boiler still sees intermittent, full-load demand. Condensate does not reduce the instantaneous rate.
• Condensate Handling: 75% return still significantly reduces makeup water treatment flow (15–20 GPM), and you still need monitoring for contamination and slug flow.

Key Differences with Pressurized DA

DA Pressure Raises Pump Suction Head

• A pressurized DA typically operates at 5–8 PSIG (often 225-228°F saturated water).
• This improves NPSHa to your feed pumps compared with an atmospheric tank but raises the water temperature, which affects:
  • Pump material compatibility (high-temperature seals)
  • Requires pumps to be designed for hot condensate conditions (no flashing).

Pump Discharge Pressure

• Atmospheric tank: Pumps overcome full boiler operating pressure (~180–220 PSIG).
• Pressurized DA: Still true - pumps feed boiler drum at same pressure, so discharge head ≈ boiler pressure + system losses, unchanged from atmospheric case.

Tank & Surge Volume

• DA storage is smaller relative to atmospheric tanks because:
  • Pressurized systems typically sized for 10 minutes net storage of steam output, not 20–30 minutes.
  • With 31,050 lb/hr (~62 GPM total), 10 minutes = ~620 gallons effective storage.
• You may find DA vessels in 750–1,000-gallon ranges for this duty, instead of 1,200–1,500 gallons typical of atmospheric.

Why smaller?

• Systems with DA generally have closer water/steam control and high-pressure returns from condensate receivers, which shortens surge response time.
• But we still recommend slightly larger storage if condensate timing is uncertain.

Vent & Overflow

• Pressurized DA still needs:
  • Continuous venting to remove oxygen and gases.
  • Overflow return precautions—especially if condensate slugs arrive.

Condensate Integration

• With a pressurized DA, returning condensate usually enters through a deaeration section (spray or tray system) to remove gases before mixing with makeup water.
• Condensate return pumps may need to overcome DA operating pressure (≥7 PSIG) plus line losses. This is a design/sizing check often missed.

Updated Specification Summary for Pressurized DA

Summary Decision Logic

• Pump sizing logic does not change.
• DA tank volume can be smaller than atmospheric, but oversizing is a good safety margin with large or delayed condensate recovery.
• Extra focus on hot water materials, NPSH, venting, and condensate pump integration.