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CPI oil water separators: how corrugated plates save space
Updated: 3 hours ago
A CPI (corrugated plate interceptor) is a gravity oil water separator with packs of inclined, corrugated plates inside. The plates shorten the distance each oil droplet has to rise before it is caught, so the separator can be much smaller than an open basin treating the same flow. It removes free oil and settleable solids. It does not remove emulsified or dissolved oil.
At a glance
Removes: free oil that rises on its own, and solids that settle.
Principle: gravity, made faster by dividing the flow into many shallow layers between plates.
Power: none needed for the separation itself.
Typical sites: refineries, depots and terminals, workshops and wash bays, power plants, and pre-treatment ahead of an effluent treatment plant.
How a CPI separator works
Wastewater enters an inlet chamber where its velocity is reduced and the flow is spread evenly across the width of the tank. It then passes through plate packs set at a steep angle. In each narrow gap between plates, an oil droplet only has to rise a short distance to reach the underside of the plate above. There it meets other droplets, they merge into larger ones, and the oil travels up along the corrugations to the surface.
Solids do the opposite. They settle onto the upper face of the plate below and slide down into a sludge hopper at the bottom of the tank. The oil layer on the surface is drawn off through a slotted skim pipe or an oil skimmer, and clean water leaves over an outlet weir.
Why plates make the separator smaller
In any gravity separator, what matters is how far a droplet has to rise and how long it stays in the tank. An open basin has a deep column of water, so the slowest droplets need a long time to reach the surface, and the basin must be large to give them that time. Plate packs split the flow into many shallow layers, so each droplet only has to cross one narrow gap. The same duty can therefore be met in a much smaller tank.
CPI, TPI, API or coalescer?
Type | Inside the tank | Strengths | Consider |
|---|---|---|---|
Packs of corrugated plates set at a steep angle | Compact; corrugations guide oil up and solids down | Plates need access for inspection and cleaning | |
Tilted plate packs; layouts vary between designs | Compact; plate packs can be lifted out | Plate spacing must suit the solids load | |
Long open channel without plates | Tolerates heavy solids and sudden slugs of oil | Large footprint | |
Coalescing media that make small droplets merge | Catches smaller droplets than plain gravity | Media can foul if solids are not removed first |
What a CPI separator will not do
Gravity separation only works on oil that is free to rise. Oil held in a stable emulsion by detergents, alkaline cleaners or very fine droplets will pass through. So will dissolved hydrocarbons. If your discharge limit is tight and the water carries emulsions, plan for a further treatment stage, such as chemical treatment followed by flotation, after the separator.
Getting the best from a CPI separator
Screen out coarse solids upstream so they do not settle between the plates.
Feed the separator by gravity or with a low-shear pump. High-speed centrifugal pumps break oil into fine droplets that are harder to separate.
Keep detergents and cleaning chemicals out of the oily drain where you can.
Remove oil and sludge regularly so they do not build up and escape with the outflow.
Inspect the plates and clean them when deposits build up. Plan access so plate packs can be washed or lifted out.
Where the water carries volatile hydrocarbons, consider a VOC monitor to track vapour around the separator.
Checklist: what to send us
Flow rate: normal, peak and any storm flow
Type of oil and its density, and the oil content at the inlet
Solids content and type
Water temperature
The discharge limit you need to meet
Space available, and whether the tank sits above or below ground
Preferred material: coated carbon steel or stainless steel
Frequently asked questions
Does a CPI separator need power?
Not for the separation, which is by gravity. Feed pumps, oil skimmers and sludge pumps, if fitted, need power.
Will a CPI separator meet my discharge limit?
That depends on whether the oil is free or emulsified and on the size of the droplets. Send us your water analysis and the limit, and we will tell you whether a CPI alone will do, or whether a polishing stage is needed.
How often do the plates need cleaning?
It depends on the solids and the type of oil. Inspect them regularly at first, then set a cleaning interval based on what you find.
Can a CPI separator handle rainwater from a yard?
Yes, if it is sized for the peak flow or protected by a bypass for storm flows beyond its design. Tell us the catchment and rainfall basis you use.
See the Vens CPI oil water separator and our other oil water separators. Send us your details and we will come back with a proposal. 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.
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