Reference ID: MET-A350 | Process Engineering Reference Sheets Calculation Guide
Introduction & Context
Flash distillation drums (also called flash tanks or flash vessels) are simple phase-separation devices used throughout the process industries to separate a saturated liquid stream into equilibrium vapor and liquid phases. Correct drum sizing is critical: undersizing leads to liquid carry-over and downstream equipment damage, while oversizing increases cost and plot space. The key sizing parameter is the drum diameter, which is governed by the maximum allowable vapor velocity. The calculation below is the classical “vapor-velocity” method recommended in most engineering handbooks (e.g., API 12J, GPSA, Sinnott & Towler) for horizontal or vertical flash drums operating at near-atmospheric pressure.
Methodology & Formulas
Convert volumetric flow to consistent units
\[ Q_{\text{v}} \; [\mathrm{m^{3}/s}] = \frac{Q_{\text{v}} \; [\mathrm{m^{3}/h}]}{3600} \]
Apply the vapor-velocity sizing equation
The cross-sectional area required for vapor flow is obtained from the continuity equation:
\[ A = \frac{Q_{\text{v}}}{U_{\text{max}}} \]
For a circular cross-section, \( A = \frac{\pi D^{2}}{4} \). Solving for diameter:
\[ D = \sqrt{\frac{4 Q_{\text{v}}}{\pi U_{\text{max}}}} \]
where
\( Q_{\text{v}} \) is the vapor volumetric flow rate (m3/s)
\( U_{\text{max}} \) is the maximum superficial vapor velocity (m/s)
\( D \) is the internal drum diameter (m)
Check velocity regime
The recommended velocity range for 1-bar steam (≈ atmospheric flash drums) is:
Regime
Minimum \( U_{\text{max}} \) (m/s)
Maximum \( U_{\text{max}} \) (m/s)
Atmospheric flash drums
0.15
1.0
Values outside this band trigger a warning but do not halt the calculation.
Check geometry ratio
For horizontal drums the length-to-diameter ratio is typically constrained to:
Parameter
Minimum
Maximum
\( \frac{L}{D} \)
2
5
Ratios outside this range are flagged for review.
Use a vertical drum and size it for 150–300 µm droplet removal. A rule-of-thumb is to set the vapour velocity v = k √((ρL – ρV)/ρV) with k ≈ 0.06–0.10 m s⁻¹ for 95 % flood. The minimum cross-section area A = QV / v, then diameter D = √(4 A / π). Round up to the next 150 mm (6 in.) pipe size.
Normal product surge: 5–10 min of liquid flow based on half-full volume.
Feed upset or downstream pump trip: 15 min is common for refinery flash drums.
Slug flow from upstream: add a dedicated slug catcher or increase hold-up to 20 min.
Choose horizontal when the vapour/liquid ratio is small (<0.3 m³ vap per m³ liq) or when large liquid surge is required (>10 m³). Horizontal drums also fit better under structured packing or tray columns where elevation is limited.
Inlet diverter or half-pipe to kill momentum and spread the feed.
Mesh pad (100–150 mm thick) for 99 % removal of 10 µm droplets at moderate vapour loads.
Vane pack if fouling is expected—easier to clean than mesh.
Perforated calming baffle ahead of the liquid outlet to avoid vortex.
Run a dynamic blow-down model (HYSYS Dynamics, Aspen Depressurisation, or similar) at fire case. Ensure the drum can swallow the two-phase swell without carry-over; typical criterion is vapour velocity ≤50 % of the calculated flooding velocity at maximum blow-down rate. If the drum is too small, either increase diameter or add an external blow-down drum.
Worked Example – Flash Drum Sizing for a De-ethanizer Overhead
A small on-shore gas plant needs to install a new flash drum to separate liquid condensate from the de-ethanizer overhead vapor. The drum will operate at 12 bar and 45 °C. The volumetric flow rate of vapor entering the drum has been measured at 500 m³ h⁻¹. Size a vertical flash drum that will keep entrainment below 0.1 kg liquid per kg vapor.
Volumetric vapor flow, Qv = 500 m³ h⁻¹
Maximum allowable vapor velocity, umax = 0.5 m s⁻¹ (mid-range of API RP 521 guidelines)
Convert volumetric flow to SI units:
Qv = 500 m³ h⁻¹ × 0.000278 m³ s⁻¹ per m³ h⁻¹ = 0.139 m³ s⁻¹
Calculate the minimum cross-sectional area required to keep vapor velocity at umax:
Amin = Qv / umax = 0.139 m³ s⁻¹ / 0.5 m s⁻¹ = 0.278 m²
Relate area to drum diameter:
A = \( \frac{\pi D^2}{4} \) → D = \( \sqrt{\frac{4 A}{\pi}} \)
D = \( \sqrt{\frac{4 \times 0.278}{3.142}} \) = 0.595 m
Choose a standard pipe size; the nearest commercially available head is 0.610 m (24 in). Adopt D = 0.610 m.
Determine tangent-to-tangent length using the selected aspect ratio:
L = (L/D) × D = 3 × 0.610 m = 1.83 m
Verify vapor velocity with the chosen diameter:
A = \( \frac{\pi (0.610)^2}{4} \) = 0.292 m²
u = Qv / A = 0.139 / 0.292 = 0.476 m s⁻¹ < 0.5 m s⁻¹ → OK
Final Answer: Use a vertical flash drum with an internal diameter of 0.610 m and a tangent-to-tangent length of 1.83 m. These dimensions keep the vapor velocity at 0.476 m s⁻¹, comfortably below the 0.5 m s⁻¹ entrainment limit.
"Un projet n'est jamais trop grand s'il est bien conçu."— André Citroën
"La difficulté attire l'homme de caractère, car c'est en l'étreignant qu'il se réalise."— Charles de Gaulle
