Drain Flash Tank (DFT) — handles boiler startup/low-load drains from economizer, superheater, reheater, and main steam lines; provides pressure let-down and steam–water separation, with optional heat recovery.

Intermittent Blowdown Flash Tank (IBFT) — receives pulsed bottom blowdown from the drum/headers; focuses on flash separation and safe cooling to meet site discharge limits.

Continuous Blowdown Flash Tank (CBFT) — receives steady, small-flow surface blowdown; emphasizes stable heat recovery (flash steam → deaerator/LP header; concentrated brine → blowdown heat-recovery exchanger).

Common synonyms: Expansion tank/vessel; Blowdown tank; CBD/IBD flash tank.

Quick engineering comparison

Abbrev.: DA=Deaerator; BDHR=Blowdown Heat-Recovery Exchanger; LIC/LV=Level controller/valve; AIT/AIC=Conductivity indicator/controller; PT/PSV=Pressure transmitter/safety valve.

Operating principle & sizing cues

Flashing & separation

Hot pressurized water enters a vessel at lower pressure $P_{tank}$, producing flash steam. Approximate flash quality:

where $h_1$ is inlet specific enthalpy, $h_f$ saturated liquid enthalpy at $P_{tank}$, and $h_{fg}$ latent heat at $P_{tank}$.
Use $x$ to size steam outlet, silencer, and DA capacity; the liquid leg sizes the bottom outlet/heat-recovery path.

Vessel pressure & volume

• Tank pressure:

  • DFT/IBFT: balance safe venting with any heat-recovery target.

  • CBFT: match DA pressure (e.g., ~0.10–0.30 MPa(g)) to maximize recovery.

• Effective volume: cover peak incoming pulse (IBFT) or short-term surge (DFT/CBFT) plus margin to secure separation residence time.

Noise & temperature compliance

• Top vent to atmosphere → vent silencer and velocity check.

• Bottom effluent temperature per site code (often ≤60 °C): IBFT/CBFT typically include TIC→cooling water control valve.

P&ID essentials

DFT (Startup drains)

• Inlet manifold: ECO/SH/RH/MSL branches with isolation + check valves; internal distributor/deflector to reduce carry-over.

• Instruments: LG/LT→LIC→LV for level; PT for pressure; PSV on top head.

• Connections: Flash steam → DA/LP header or silenced vent; bottoms → condensate recovery; maintenance blowdown/drain (BDV) provided.

IBFT (Intermittent blowdown)

• Inlet: From drum bottom blowdown valve; recommend tangential entry/spray distributor and wear-resistant liners.

• Cooling: TIC→quench water CV to meet discharge temperature.

• Others: Same safety/instrument scheme as DFT, with greater focus on erosion and thermal-shock mitigation.

CBFT (Continuous blowdown)

• Upstream control: Boiler water conductivity loop — AIT(Cond)→AIC→CV-BD modulates CBD rate.

• Heat-recovery chain: Flash steam → DA; bottoms → BDHR → cooler → compliant discharge or reuse.

• Level loop: LIC→LV to avoid entrainment and steam short-circuiting.

Materials & fabrication

• Shell & heads: SA-516 Gr.70 (or equivalent high-quality carbon steel).

• Nozzle necks: SA-106 Gr.B; Flanges: SA-105.

• Internals / wear parts: 304 or 410 SS as service requires; optional cladding/overlay (e.g., Alloy 625) at high-erosion zones.

• Good wording for specs:
  “Vessel fabricated from SA-516 Gr.70; nozzle necks SA-106 Gr.B; flanges SA-105; internals 304 SS.”

• Additional good practice: corrosion allowance sized for service, qualified weld procedures (GTAW/SMAW common), NDE per code (e.g., RT/UT for butt joints, PT/MT for fillets), and proper supports/nozzle load checks for site piping.