API Separators Explained: Gravity Oil Removal Basics, Sizing Rules and the API-PPI-CPI Comparison
Every refinery wastewater train starts somewhere, and for most of the industry that somewhere is an API separator - the oil separation tank standardized by the American Petroleum Institute. Its principle is the same as a sedimentation tank's: exploit the specific-gravity difference between suspended matter and water, and let physics do the rest. Understanding how it is built and where it tops out is essential context before anyone promises a downstream flotation or filtration unit will fix what gravity could not.
Structure and flow path
Most oil separators are horizontal-flow units. Oily wastewater enters a rectangular basin through a distribution channel and creeps along in the horizontal direction. While it travels, oil globules rise to the surface, where an oil collecting pipe or a chain skimmer pushes the floating layer into the pipe and onward to a dehydration tank. Heavy oil and settled impurities accumulate in a sludge hopper at the tank bottom and leave through the sludge discharge pipe. Construction is typically reinforced concrete, occasionally brick and stone.
On the rectangular plan the tank is divided into 2–4 compartments along the flow direction, each compartment generally no wider than 6 meters to keep water distribution uniform. Effective water depth stays within 2 meters, and tank length runs to more than 4 times the compartment width. Chain-type skimmers and sludge scrapers handle floating oil and bottom sludge, one set of each per compartment. Adding a mid-compartment baffle with skimmers and scrapers on both sides creates a two-stage separator - better oil removal, more equipment and more energy. Inserting inclined plates also lifts efficiency, at higher construction investment. In cold regions a steam pipe under the oil collecting pipe keeps oil from solidifying; covers with insulation steam prevent fire risk, oil vaporization and dust ingress.
Three states of oil - and why only one separates well
Oil in wastewater is lighter than water and exists in three states. Suspended oil forms droplets larger than 0.1 mm that float readily; in the petroleum industry it accounts for roughly 60–80% of the oil content in wastewater. Emulsified oil is dispersed below 0.1 mm and refuses to float, contributing about 10–15%. Dissolved oil, limited by petroleum's minuscule solubility, makes up 0.2–0.5%. A gravity separator captures the first category and mostly waves the other two through - which is exactly why downstream polishing exists.
API, PPI and CPI compared
Two descendants of the plain API basin improve the numbers by adding inclined surfaces. The parallel-plate (PPI) and corrugated-plate (CPI) designs shorten the distance a droplet must rise and give it a surface to coalesce on. On oil removal efficiency the API baseline manages 60~70%, while PPI and CPI both reach 70~80%. On floor area at equal treatment capacity, PPI needs half the API footprint and CPI only a third to a quarter. Installation references follow the atlas of small drainage structures (Jianshe [2001] No. 163).
Installation and operating specs
The separator mounts directly on the channel carrying oily sewage and oil-water flow, sewage outlet aligned with the barred inlet, and connects to other equipment by pipeline. The unit must be set level. Before first use it is filled with tap water and the water level adjustment pipe is set so its top matches the upper edge of the oil overflow groove; the water inlet pipe sits far enough from the debris separation box for easy removal. After feed begins, the adjustment pipe is trimmed again until the oil drain pipe discharges oil only. If the drain sits too low, the cover can be opened and oil removed manually. Standard design rules include:
- Canteen and restaurant sewage must pass an oil removal device before entering the sewer.
- Sewage flow is calculated on the design second flow rate.
- Flow velocity for edible-oil sewage inside the tank must not exceed 0.005 m/s.
- Retention time for edible-oil sewage is 2–10 min.
- For manual oil removal units, oil storage volume is at least 25% of effective tank volume.
- A movable cover plate is fitted, and the water inlet pipe must allow cleaning.
Product variants
Commercially the devices divide into automatic and gravity oil separators, and by placement and material into buried FRP units, above-ground FRP units, stainless steel units and reinforced concrete structures. FRP - fibre-reinforced plastic - dominates buried installations for its corrosion resistance, while concrete remains the choice for large refinery basins.
Where it fits today
An API separator is not a complete treatment process and never was. It is a cheap, robust first stage that removes the easy 60-80% of oil so that flotation, coalescing or dissolved-air stages downstream can focus on the emulsified fraction without choking. Sizing it correctly - depth, compartment width, retention, skimming hardware - decides whether the rest of the train runs smoothly or spends its life compensating for an overloaded front end.