TPI separators: how tilted plate interceptors work and how they are sized

In an open gravity separator, an oil droplet has to rise through the full depth of water before it can be skimmed off, and a long tank gives it the time to do so. A tilted plate interceptor (TPI) puts packs of closely spaced, inclined plates in the flow. Each droplet now only has to rise the short distance to the plate above it, so the same separation happens in a much smaller tank.

At a glance

• Removes: free (not emulsified) oil and settleable solids.

• How: inclined plates shorten the distance oil has to rise and solids have to fall.

• Why: a much smaller footprint than an open gravity separator for the same flow.

• Not for: emulsified oil, which needs dissolved air flotation.

How a TPI works

• Inlet: the effluent enters a quiet zone that spreads the flow evenly across the plate packs.

• Plate packs: oil droplets rise to the underside of the plate above, merge into larger drops and travel up the plate to the surface.

• Solids: settle on the plates, slide down into the sludge hopper and are drained off.

• Oil removal: oil collects at the surface and is removed by a slotted pipe skimmer or an adjustable weir.

• Outlet: clean water passes under a baffle and leaves over the outlet weir.

What decides the size

Oil droplets rise at a speed set by their size, the difference in density between oil and water, and the viscosity of the water. Size matters most: a droplet half the size rises at about a quarter of the speed. The capacity of a plate separator therefore follows the total plate area seen from above, not the volume of the tank. The design takes into account:

• Peak flow, not just the average

• The smallest oil droplet that must be removed, and the outlet oil limit

• Oil density: heavier oils rise more slowly

• Temperature, because cold water is more viscous

• The solids load, which sets plate spacing and hopper size

Maintenance

Plate packs lift out for cleaning, the sludge hopper is drained regularly, and the oil layer is skimmed continuously or on a timer. Keeping the plates clean keeps the separator at its design capacity.

Checklist: what to send us

• Average and peak flow

• Oil type and density, and the oil content at the inlet

• Suspended solids and their nature (grit, sludge, fibres)

• Temperature and pH

• Required outlet oil content

• Space available, and whether the unit will be above or below ground

Frequently asked questions

Will a TPI remove emulsified oil?

No. Emulsified oil does not rise on its own. A TPI removes the free oil first, and a DAF system after it removes the emulsified oil.

TPI or CPI?

Both use packs of inclined plates. A CPI uses corrugated plates, which help droplets gather at the crests. The choice depends on flow, solids and layout, and we propose the one that suits your effluent.

Can a TPI handle solids?

Yes, settleable solids collect in its hopper. Where there is heavy grit or debris, a screen or grit chamber ahead of it protects the plates.

See our TPI oil water separators, or compare the options in our guide to API, TPI and coalescing separators. Send us your details and we will size a separator for your effluent. 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.