Vent Connection in OWS

Because the oil on top of the water continually vaporizes, and the vents keep the fumes below the flash point concentration.

In the first place,oil should be separated from OWS system before waste water is discharged into the nearest waste water network.Oil separators of different make are available in market. Vent is inevitably required in all OWS system.Vent Pipe size and and the extent of protrusion above grade is a matter of design issue and relevant code should be consulted.

Have you ever tried to remove liquid from a tank without a vent..........you may find it rather difficult.

It is also fitted to prevent over-pressurisation.........of the tank when filling it.

That is apart from previous mentioned reasons..........you will generally find that if the tank contains a flammable liquid, even a small amount, if the vent is over a specified diameter it would probably have to be fitted with an anti-flash gauze/device.