API, TPI or coalescer: which oil water separator suits your effluent?

Most oil water separators work by gravity: oil is lighter than water, so given calm conditions and time it rises and can be skimmed off. API separators, plate separators (TPI and CPI) and coalescers differ in how they give the oil that chance, and so in the droplet sizes, solids loads and flows they suit.

At a glance

• API separator: a long open channel for large flows and heavy solids, usually the first stage in refineries.

• TPI or CPI: inclined plate packs that remove smaller droplets in a compact tank.

• Coalescer: media that merges the finest free droplets, best on clean water with few solids.

• None of them removes emulsified oil; that needs dissolved air flotation.

The three compared

Separator	How it works	Solids	Footprint	Typical use
API separator	A long rectangular channel; oil rises over its length and flight scrapers move oil and sludge	Handles heavy loads	Largest	Refinery and petrochemical effluent, first stage
TPI or CPI	Inclined plates shorten the distance oil has to rise	Moderate, collected in a hopper	Compact	Industrial effluent, alone or after an API separator
Coalescer	Media makes fine droplets merge into larger ones that rise	Low; solids must be removed first	Compact	Polishing, and fuel and oil handling areas

API separators take their name from the American Petroleum Institute, whose Publication 421 covers their design.

How to choose

• How much solids? Heavy sludge or grit points to an API separator, or to screening ahead of a plate separator.

• How fine is the oil? Finer droplets need plate packs or coalescing media.

• How much space? Plate separators and coalescers fit where a long channel will not.

• What outlet limit? Strict limits often need two stages in series.

• Is the oil emulsified? If so, add dissolved air flotation after the gravity stage.

Separators in series

Larger plants often combine them: an API separator for the bulk of the oil and solids, then a plate separator or coalescer, then a DAF system where emulsions are present. Each stage protects the next.

Frequently asked questions

Is a coalescer always better?

No. It removes finer droplets, but its media clogs if the water carries solids. It works best after a plate separator or on clean streams.

Do separators need an oil skimmer?

Every separator needs a way to remove the oil it collects, such as a slotted pipe skimmer, a weir or a belt oil skimmer.

Can you supply the whole train?

Yes. Vens makes API, TPI, CPI and coalescing separators, oil skimmers and DAF systems, so we can propose a complete treatment train.

Read more on how TPI separators work and how coalescers work. Send us your details and we will propose the right separator. Customers in the European Union can also order through our European company, VDT Europa OÜ in Tallinn, Estonia.

Standard sizes: the Vens OWS series of oil water separators, 1 to 25 m³/h.