Arnold-Stewart Settling + Retention per API 12J & ASME VIII
Stokes' law settling, oil viscosity correlations, water-droplet design size, ASME VIII shell, oil-side gap closure.
Stokes settling velocity (water droplet through oil):
dm in µm, μo in cP, ΔSG = SGw − SGo.
Settling constraint (horizontal, Arnold-Stewart):
Retention constraint (volume, horizontal half-full):
Shell thickness (ASME VIII Div 1, UG-27):
Retention time by API gravity:
• > 35° API: 5 min oil / 3 min water
• 25 – 35° API: 10 / 5
• 15 – 25° API: 20 / 10
• < 15° API: 30 / 10
Standard diameters (in): 24, 30, 36, 42, 48, 54, 60, 72, 84, 96, 120
When to use FWKO:
• High water cut (>50%) at the inlet
• Upstream of a heater treater or production separator
• Gas rate is small (≤ ~0.5 MMSCFD) — otherwise use a 3-phase separator
Orientation:
• Horizontal: long settling path, easy access — preferred for high BWPD
• Vertical: small footprint, simpler internals — preferred for very low GOR + high gas
Viscosity warning: If oil μ > 50 cP at operating T, consider a heater treater instead of a FWKO; the settling time grows linearly with μ.
L/D: Keep horizontal L/D between 2.5 and 5. Above 5 → split into parallel vessels or switch to vertical.
A Free Water Knockout (FWKO) is an upstream pressure vessel that separates free (unemulsified) water from a wellhead stream before it reaches a heater treater or production separator. It operates at wellhead pressure and is sized primarily for water-from-oil settling, not for gas-liquid separation.
FWKO sizing checks two constraints: (1) the Arnold-Stewart water droplet settling constraint d × Leff = 1000·Qo·μo / (dm² · ΔSG), and (2) the retention-time volume constraint d² · Leff = 1.42·(Qo·to + Qw·tw). The controlling constraint sets the vessel diameter; ASME VIII Div 1 UG-27 sets shell thickness.
500 µm is the common design droplet for FWKO water settling. A more conservative 200 µm is used when downstream oil specifications are tight or the inlet is foamy. Heater treaters use smaller design droplets (100–250 µm) because they break tighter emulsions.
A FWKO is preferred when the inlet stream has high water cut (>50%) and the downstream stage (heater treater or separator) is intended to handle only the oil plus small residual water. A single 3-phase separator combines gas/oil/water in one vessel and is preferred when gas flow is significant (>0.5 MMSCFD) or water cut is moderate.