CPI / API Oil-Water Separator — Engineering Fundamentals

1. API gun-barrel vs CPI

An API 421-style gun-barrel separator is a long, shallow open basin sized so the slowest oil droplet you care to catch (typically 150 µm) has time to rise from the bottom to the surface before the water exits the basin. For 10,000 BWPD and 150 µm droplets, the basin footprint is roughly 800 ft² — a sizable concrete tank.

A Corrugated Plate Interceptor (CPI) achieves the same separation in 1/5 to 1/10 of the footprint by inserting a stack of inclined parallel plates. Each plate divides the basin into thin channels; an oil droplet only has to rise the small plate-to-plate spacing before it contacts a plate and slides upward along the underside to the surface. Same droplet, fraction of the travel distance, fraction of the area.

2. Hazen overflow-rate basis

Hazen (1904) proved that the capture efficiency of a gravity sedimentation basin depends only on the overflow rate — the ratio of throughput Q to horizontal projected area A_h, not on basin depth:

For 100% capture of droplets with rise velocity V_t:

This is the design master-equation. Once you know V_t (Stokes) and Q (flow rate), A_h is set. The vessel depth and length:width ratio are aesthetic / sediment-handling choices.

3. Stokes rising velocity

For an oil droplet rising through water under viscous-dominated conditions (Re < 1):

In field units (V_t in ft/s, d_m in µm, μ_w in cP):

The d² dependence is brutal: capturing 30 µm droplets requires 4× the area of 60 µm droplets, and 11× the area of 100 µm droplets. The design droplet is the single most consequential input.

μ_w falls from ~1.3 cP at 60 °F to ~0.4 cP at 150 °F — heated water settles 3× faster. Always size at the realistic operating T, not at lab-bench T.

4. Plate-pack geometry

A CPI plate inclined at angle θ from horizontal has horizontal projection L · W · cos θ. At θ = 45°, projection = 0.707 · L · W. A pack of n plates stacked at vertical spacing s (typically 20 mm) achieves total horizontal area:

The plate count is limited by the vessel depth: n_plates = stack_height / s. For an 8-ft plate at 45°, the diagonal stack height is 8 · sin 45° = 5.66 ft. At 20 mm spacing (0.066 ft), that's 86 plates. So one 8 ft × 4 ft × 45° pack module gives ~975 ft² horizontal projection — equivalent to a 30 ft × 32 ft open basin.

The plates are corrugated (not flat) for two reasons: corrugations stiffen the plate against the static head, and they create longitudinal flow channels that prevent cross-flow eddies. The corrugation pitch (50–100 mm typical) does not affect A_h directly.

5. Reynolds re-entrainment

The whole sedimentation concept assumes laminar flow in the channels — turbulent eddies would re-disperse the oil droplets that have coalesced into the underside-of-plate film. Channel Reynolds:

Industry guidance:

• Re < 500: laminar, no re-entrainment — design target.
• 500 < Re < 2,000: transitional — some re-entrainment, derate effective droplet capture.
• Re > 2,000: turbulent — CPI is not effective; add modules to reduce v_axial.

6. Where CPI sits in the produced-water train

A typical SWD-pad train is FWKO → CPI → hydrocyclone → degassing tank → injection. Refineries use CPI as the first stage in the wastewater plant, ahead of DAF and biological treatment.

7. References

• API Publication 421 — Design and Operation of Oil-Water Separators (Mgmt of Water Discharges).
• Stokes, G.G. (1851). "On the effect of internal friction of fluids on the motion of pendulums." Trans. Camb. Phil. Soc. 9.
• Hazen, A. (1904). "On sedimentation." Trans. ASCE 53, 45.
• Frankiewicz, T. & Browne, M. (2002). "Use of CPI separators in the petroleum industry." SPE Production & Facilities.
• Manning, F.S. & Thompson, R.E. (1995). Oilfield Processing of Petroleum Vol. 2, Ch. 7 — Water Treating.
• Vendor literature: Pielkenrood, Monosep, Cameron, Forum Energy.