Gravity oil-water separators engineered to API Publication 421 — the worldwide reference for primary free-oil removal in petroleum refining, petrochemical and upstream produced-water service. We design, fabricate and commission rectangular API, Corrugated Plate Interceptor (CPI) and Tilted Plate Interceptor (TPI) units, sized from first principles using Stokes’ law droplet-rise theory and verified by CFD simulation.
Primary oil removal upstream of DAF, biological treatment and discharge polishing.
Upstream and offshore separation of dispersed crude from formation water and frac flowback.
Pre-treatment for ethylene, propylene and aromatic plant effluent streams.
Removes free and unstable emulsified oil with droplet diameters > 150 µm.
Gravity Separation of Free Oil from Wastewater
An API separator is a long, shallow rectangular basin (or a compact plate-pack variant) that gives free and weakly-emulsified oil droplets enough quiescent retention time to rise to the water surface, where they are mechanically skimmed off. Settleable solids fall to the floor and are scraped to a sludge hopper. The design is rooted in American Petroleum Institute Publication 421 — Design and Operation of Oil-Water Separators, first issued in 1969 and revised most recently in 1990, which remains the worldwide reference for primary oil removal in petroleum and petrochemical installations.
Removes oil present as discrete droplets > 150 µm. Removal efficiency > 90 % for droplets above the design cut-size.
Sand, scale, coke fines and corrosion products settle to the floor and are scraped to a hopper for periodic withdrawal as bottoms sludge.
Skimmed oil layer is decanted to slop-oil tank for re-blending into the crude charge — converts a disposal requirement into a feedstock recovery.
Removes bulk oil and settleable solids before DAF, biological treatment or filtration — protecting them from fouling and shock loads.
Stokes’ Law & the Rise Velocity of an Oil Droplet
An oil droplet suspended in water rises because its density is lower than that of the surrounding fluid. The terminal rise velocity is governed by Stokes’ law, valid for laminar flow (droplet Reynolds number < 1) which is the regime API separators operate in.
Vt — terminal rise velocity (m/s)g — 9.81 m/s²ρw — water density (kg/m³)ρo — oil density (kg/m³)d — droplet diameter (m)μ — dynamic viscosity of water (Pa·s)
The equation has three engineering consequences that drive every design decision on the page that follows:
API Publication 421 specifies a design droplet diameter of 150 µm as the practical cut-size above which gravity separation is reliable. Sizing the basin to remove this droplet ensures > 90 % removal of free oil under design conditions. Below 150 µm the residence times required become uneconomic and downstream DAF or induced-gas flotation is the correct choice.
From Droplet Rise Velocity to Basin Dimensions
The sizing procedure converts the Stokes rise velocity for the design droplet into a basin geometry by enforcing two simultaneous criteria: every droplet must have enough vertical distance to rise to the surface before it leaves the tank, and horizontal velocity must stay low enough to prevent re-entrainment.
Vh — mean horizontal velocity through the basin (m/s)Vt — design droplet rise velocity (m/s)
Ac — vertical cross-section (m²)Q — peak hydraulic load (m³/s)
D — design liquid depth, typically 0.9–2.4 mW — basin width (single channel)
L — effective channel length (m)F — turbulence & short-circuiting factor (1.20–1.74 per API 421, function of Vh/Vt)
The factor F corrects ideal plug-flow theory for the real hydraulics of a rectangular tank: dead zones, density currents, wind shear on the free surface and short-circuiting between inlet and outlet. API 421 publishes a tabulated curve of F as a function of Vh/Vt; values are typically 1.45–1.65 for well-designed inlet diffusers and 1.74 (the upper bound) for poorly-baffled designs. CFD verification of inlet and outlet hydraulics is now the standard way to demonstrate F ≤ 1.50 and reduce length (and concrete cost) accordingly — see our CFD simulation page.
For a typical refinery duty (oil ρ = 850 kg/m³, water at 20 °C, d = 150 µm, Q = 200 m³/h), Stokes’ law gives Vt ≈ 2.0 × 10−3 m/s. Vh is then capped at 0.015 m/s. The resulting basin is roughly 12–15 m long, 2 m wide and 1.5 m deep with a hydraulic retention time of about 30 minutes — numbers that match published refinery practice.
| Parameter | API 421 Recommendation | Engineering Rationale |
|---|---|---|
| Design droplet | 150 µm | Practical cut-size for gravity separation; smaller droplets need DAF/IGF. |
| Vh cap | 0.015 m/s (54 m/h) | Below this, re-entrainment of risen oil is negligible. |
| Vh/Vt max | 15 | Keeps drop trajectory shallow enough to surface within the tank length. |
| D/W ratio | 0.3 – 0.5 | Wider, shallower basins separate better; deep narrow basins suffer density currents. |
| Depth (D) | 0.9 – 2.4 m | Limits short-circuiting; allows scraper-flight clearance and sludge accumulation. |
| L/W ratio | ≥ 5 | Plug-flow approximation deteriorates below this aspect ratio. |
| Turbulence factor F | 1.20 – 1.74 | Depends on inlet diffuser design and outlet weir geometry; CFD-verified for lower bound. |
| Channels | 2 minimum | Allows one off-line for maintenance without bypassing wastewater. |
| Freeboard | 0.45 – 0.6 m | Accommodates wave action, hydraulic surges and oil-layer thickness. |
All values are extracted from API Publication 421 (1990) and corroborated by published refinery practice. Reynolds & Bauhm sizing sheets apply these criteria in spreadsheet form and back them up with CFD verification of the inlet and weir hydraulics.
What Sits Inside an API Separator and Why
A rectangular API separator is far more than a concrete trough. Each internal item is sized and positioned to manage one specific failure mode — turbulence at the inlet, density currents in the body, oil re-entrainment at the outlet, or sludge build-up on the floor. Skip any of them and you lose 20–40 % of the design removal efficiency.
Vertical-slot or perforated-plate distributor across the full basin cross-section. Dissipates the kinetic energy of the influent pipe so flow enters the separation zone as low-velocity plug flow. Velocity step typically > 10:1 reduction. Critical to achieving F ≤ 1.5.
Half-submerged baffle 1–2 m downstream of the inlet. Forces flow downward then upward, separating slugs of free oil before they enter the main basin. Often takes out 50–70 % of the inbound oil load.
Slotted-pipe, rotating-drum or tube-type skimmer continuously removes the floating oil layer. Slotted-pipe with manual or motorised positioning is the API 421 default; rotating drum gives the driest skim (60–80 % oil) but adds rotating equipment.
Chain-and-flight or travelling-bridge mechanism that scrapes settled solids to a sludge hopper at the inlet end. Flight speed 0.6–1.5 m/min; chain materials chosen for splash-zone corrosion resistance.
Submerged baffle just upstream of the outlet, with the lower edge below the water surface and the upper edge above the oil layer. Holds the surface film in the basin so it cannot escape over the weir.
The treated water passes under the oil-retention baffle and over a level-controlled effluent weir. Weir crest level sets the operating liquid depth and therefore the design hydraulics.
Steep-sided hopper at the inlet end collecting scraped solids. Periodic withdrawal to sludge dewatering; recirculation lines prevent compaction.
API 421 requires covered separators for VOC and odour control. Inert-gas blanketing or activated-carbon vent treatment is standard. Cover materials selected for ATEX classification of the refinery zone.
For winter operation in northern installations; raises feed temperature to keep oil viscosity (and rise velocity) within the design envelope.
Rectangular API, CPI and TPI Compared
API Publication 421 covers all three of the dominant gravity-separator configurations. The choice is driven by footprint, droplet-size spectrum, throughput and the value placed on enhanced separation efficiency.
The original 1960s configuration — a long shallow concrete or steel basin with chain-and-flight scraper and slotted-pipe skimmer. Robust, easy to inspect, tolerant of solids. Footprint is large (typical 15–30 m long for a refinery duty).
Use when: footprint is available, solids loading is high, simplicity matters.
Pack of corrugated parallel plates inclined at 45–60°. Shortens the rise path so droplets reach a collection surface in seconds rather than minutes. Removes droplets down to 60 µm in clean service. Footprint typically 25–35 % of a rectangular API for the same duty.
Use when: footprint is tight, oil droplets are smaller, solids loading is moderate.
Smooth parallel plates inclined at 45–55°. Similar shortening of rise path but easier to clean than CPI. Common in offshore produced-water duty where access is limited and skids must arrive pre-fabricated. Removes droplets down to about 30 µm in controlled service.
Use when: compact skid is required, oil is dispersed, downtime for cleaning is constrained.
| Attribute | Rectangular API | CPI | TPI |
|---|---|---|---|
| Design droplet (free oil) | 150 µm | 60 µm | 30 µm |
| Typical footprint (vs Rect API) | 1.00 | 0.25 – 0.35 | 0.20 – 0.30 |
| Solids tolerance | High (chain scraper) | Medium (plate fouling) | Low (smooth plates) |
| Oil removal efficiency (free oil) | > 90 % | > 95 % | > 95 % |
| Maintenance access | Easy (open top) | Plate-pack removal | Plate-pack removal |
| Typical application | Refinery main effluent | Process drains, refinery branches | Offshore produced water, compact skids |
Knowing the Limits Determines the Downstream Train
API separators are the workhorse of refinery primary treatment, but their physics-based limits dictate everything that comes after them. The design intent is to remove the bulk of the free oil and settleable solids so the downstream process can do its job — not to polish to discharge limits.
Chemical surfactants from desalters, caustic washes and process additives stabilise droplets at 1–20 µm where Stokes’ rise is negligible. API separators pass these straight through — DAF with coagulant addition is required downstream.
Soluble BTEX, phenols and oxygenates are not affected by gravity separation. Removal requires biological oxidation, steam stripping or activated carbon — never the API.
API effluent typically carries 50–150 mg/L oil & grease — comfortably above the 5–15 mg/L modern discharge limits. The separator is always followed by DAF / IGF, biological treatment and often a filter polish.
Hydraulic surges raise Vh above the re-entrainment limit and wash settled solids back into suspension. Always preceded by an equalisation basin sized for 4–12 hours of buffering.
The corollary is positive: by handling the easy 80–90 % of the load cheaply (no power, no chemicals, no operator attention), the API separator lets the more expensive downstream stages operate inside their design envelope. Plants that try to skip the API or undersize it invariably overspend on DAF chemicals and biological recovery instead.
Specifying for 25-Year Service in Hydrocarbon Effluent
An API separator sits in a chemically aggressive environment — sour water, chloride splash, hydrocarbon condensate and constant vapour-phase exposure. Material selection is the single biggest determinant of whole-life efficiency. Refineries typically specify 25-year design life; offshore packages typically 20.
| Component | Default Material | Severe-Service Alternative | Notes |
|---|---|---|---|
| Basin shell (rectangular) | Reinforced concrete with epoxy lining | Carbon steel with glass-flake epoxy | Concrete is cheaper for > 30 m³; steel for compact skids. |
| Plate pack (CPI/TPI) | FRP or stainless 316L | Duplex 2205 or super-duplex | Chloride-rich produced water needs duplex; 316L is fine for refinery main effluent. |
| Scraper chain | Glass-filled polymer (NCS-style) | Stainless 316L with renewable wear shoes | Polymer chain eliminates splash-zone corrosion and reduces maintenance. |
| Scraper flights | FRP or UHMWPE | Stainless 316L | Flights wear at sludge hopper end — replaceable strips are standard. |
| Skimmer pipe | Stainless 316L | Duplex 2205 | Always above water line, so chloride pitting is the dominant risk. |
| Cover plates | FRP or aluminium | Stainless 316L | Aluminium is light and corrosion-resistant; FRP is the cheapest spark-free option. |
| Anchor bolts & fixings | Stainless 316L | Duplex 2205 | Splash-zone corrosion at the waterline is the No. 1 failure mode — never use carbon steel. |
Process Block Diagram — Primary → Secondary → Tertiary
An API separator is the first chemical-free unit operation after equalisation. Everything downstream is sized assuming the API has done its job — if the API under-performs, every successive stage will be in trouble.
4–12 h buffering of refinery effluent. Damps flow, temperature and oil-load spikes before the API.
Free-oil & settleable-solids removal. 90 %+ of the inbound oil load is intercepted here.
Removes emulsified oil and chemical floc. Brings O&G to < 15 mg/L for discharge or < 5 mg/L for reuse.
Sand, dual-media or membrane polishing for final TSS removal before discharge or reuse.
The API separator’s under-appreciated value is the amplification it provides for everything that follows. A refinery DAF designed for 30 mg/L feed runs at half the polymer dose of one fed with 150 mg/L. A biological reactor receiving stable, low-oil feed runs at 95 % uptime instead of stalling once a quarter on an upset. The economic case for a properly-sized API is rarely about its own performance — it is about the downstream Operating expenditure it unlocks.
Industrial Applications
Primary separation for crude unit, FCC, coker, hydrotreater and storage-area drainage. Typically two parallel rectangular APIs sized for 100 % of plant flow.
Refinery WastewaterEthylene quench-water systems, aromatic extraction, polymer-plant runoff. CPI is the common choice for the smaller plot-plans.
PetrochemicalOnshore tank batteries and processing facilities. TPI skids handle the dispersed oil before hydrocyclones, gas flotation or walnut-shell filtration.
Produced WaterSkid-mounted TPI as part of the produced-water package. Compact footprint, motion tolerance and ATEX-rated construction are mandatory.
OffshoreStorm-water and bunded-area drainage with intermittent oil contamination. Rectangular API followed by oil-skimmer holding pond.
Process RunoffElectrical-substation runoff containing transformer oil. CPI separators in vaults beneath the transformer pads — small flow, high oil cleanliness required.
Power GenerationWhen Another Technology Is Right
| Technology | Min. droplet | Footprint | Operating expenditure | Best applied when… |
|---|---|---|---|---|
| Rectangular API | 150 µm | Large | Lowest | Space is available, solids are high, lowest lifetime cost wanted. |
| CPI / TPI | 30 – 60 µm | Small | Low | Footprint or weight constrained; relatively low solids. |
| DAF | 5 µm (with coagulant) | Medium | Medium (chemicals) | Emulsified oil, fine solids, polish to < 15 mg/L O&G. |
| Induced-gas flotation (IGF) | 10 µm | Small | Medium | Hydrocarbon-saturated produced water; no risk of explosive atmosphere from air. |
| Hydrocyclone | 10 µm | Smallest | Low | Upstream produced water, high pressure available, compact skid mandatory. |
| Coalescer | 10 µm | Small | Low | Polish after API, low solids feed, dispersed (not emulsified) oil. |
| Membrane (UF/MBR) | 0.01 µm | Small | High | Reuse-grade water; fully-treated feed; fouling-tolerant designs only. |
A complete refinery train almost always uses the API as the cheap primary stage and then layers DAF and biological treatment on top. The single-technology “super-separator” concept (e.g. an oversize CPI to do everything) usually loses on whole-life efficiency because plate fouling drives Operating expenditure up faster than the saved real estate saves Capital expenditure. Reynolds & Bauhm runs the Operating expenditure/Capital expenditure comparison on every quotation.
Designing to the Right Reference
The worldwide reference. Covers droplet-size selection, Vh/Vt criteria, F-factor tables, oil retention baffles, skimmer types and sludge handling. Reynolds & Bauhm sizing sheets follow API 421 as the parent specification.
Effluent Guidelines for Petroleum Refining — sets the discharge limits the downstream train must achieve. The API alone is never enough; combination with DAF and biology is the BAT path.
Best Available Techniques Reference Document for the Refining of Mineral Oil and Gas. Recognises API + DAF + biological as BAT for refinery wastewater across all EU member states.
Refinery process drainage areas are Zone 1 or Zone 2. Covers, vent treatment and any moving equipment (skimmer drives, scraper motors) are ATEX-certified accordingly.
For US installations, fire and electrical-classification standards drive the cover, vent and instrumentation specification. Reynolds & Bauhm builds to both EU and US classifications.
Offshore produced-water discharge in the North Sea is governed by OSPAR (30 mg/L monthly oil-in-water). MARPOL Annex I governs the same in international waters. TPI plus polishing is the standard path.
From Process Calc to Commissioned Asset
Stokes’-law droplet-rise sizing across the operating envelope (winter / summer, peak / average), API 421 F-factor selection, basin geometry, scraper sizing, skimmer duty.
Inlet diffuser hydraulics, density-current mapping, weir hydraulics — we use CFD to certify the F-factor and reduce basin length where the geometry allows.
Full P&ID, control narrative, valve schedule and instrumentation list. Level, flow and oil-on-water sensors specified per refinery standard.
Carbon-steel or stainless tankage in EN 1090 EXC3 shops; concrete civils designed to EN 1992 with our specialist partners. ATEX-certified mechanical and electrical packages.
Field-erection supervision, witness-testing of scraper and skimmer drives, cold and hot commissioning, operator training and as-built documentation.
Level and oil-layer telemetry routed to the refinery DCS; alarms on hopper level and skimmer-discharge flow. Mass-balance reconciliation in the historian.
Common failure modes and the corrective actions that restore design performance.
Symptom: oil breakthrough despite Vh within spec. Cause: inlet jet, density current or wind-driven circulation. Cure: verify inlet diffuser integrity; add baffles; check freeboard. CFD remodelling recommended.
Symptom: effluent O&G >150 mg/L. Cause: Vh >0.015 m/s or weir level set too low, drawing oil layer into underflow. Cure: reduce flow or raise weir; inspect oil-retention baffle.
Symptom: scraper motor overload or hopper blockages. Cause: high solids loading, infrequent desludging or compacted coke fines. Cure: increase desludge frequency; check flight speed; install grit trap upstream.
Symptom: separation efficiency drops in cold months. Cause: water viscosity increase reduces Vt. Cure: size basin for 5 °C design point; install steam coils or equalisation for temperature blending.
| Flow (m3/h) | Basin Length (m) | Width (m) | Depth (m) | HRT (min) | Channels |
|---|---|---|---|---|---|
| 50 | 8–10 | 1.5 | 1.2 | 25–30 | 2 |
| 100 | 10–12 | 2.0 | 1.5 | 25–30 | 2 |
| 200 | 12–15 | 2.5 | 1.5 | 25–30 | 2 |
| 500 | 18–22 | 3.5 | 1.8 | 30–35 | 2–3 |
| 1,000 | 25–30 | 4.5 | 2.0 | 35–40 | 3 |
| Component | Interval | Action |
|---|---|---|
| Skimmer pipe | Daily | Visual inspection; adjust slot height to oil layer |
| Scraper chain | Weekly | Tension check; lubricate bearings; inspect wear shoes |
| Sludge hopper | Weekly | Withdraw accumulated solids; record volume |
| Plate pack (CPI/TPI) | Monthly | High-pressure wash or chemical clean (degreaser) |
| Inlet diffuser | Quarterly | Remove debris; verify slot dimensions |
| Cover & vent | Annual | Integrity check; replace seals; verify ATEX rating |
Parent specification for gravity oil-water separator design, sizing and operation.
Determination of hydrocarbon oil index — standard analytical method for separator performance testing.
Design of concrete structures — applies to rectangular API basin civil works.
Fire safety and explosive atmosphere classification for hydrocarbon vapour management.
All oil-water separation technologies side-by-side: API, DAF, IGF, hydrocyclone, coalescer.
Read MoreDissolved-air flotation for emulsified oil and fine solids downstream of the API.
Read MoreDAF designs specifically engineered for oil & gas service — ATEX, hydrocarbon-rated.
Read MoreFull refinery wastewater treatment train integrating API, DAF, biology and polishing.
Read MoreProcess-engineering perspective on the oil-separation stage within the refinery WWT.
Read MoreUpstream produced-water trains using TPI plate-pack separators as the primary stage.
Read MoreInclined-plate clarifier sister technology — for solid-liquid (not oil-water) duty.
Read MoreInlet grit removal protecting the API scraper from heavy solids.
Read MoreCFD verification of API inlet, weir hydraulics and density currents.
Read MoreDewatering of API bottoms, slop-oil recovery and disposal routing.
Read MoreReagent selection and reactor design for refinery streams — sister discipline to the API.
Read MoreUpstream equalisation tankage to protect the API from hydraulic and oil-load shocks.
Read MoreFrom process calculation through CFD verification, P&ID, fabrication and commissioning — one engineering team, one contract.
Our expertise spans multiple industries with sector-specific water treatment solutions.