The two main design criteria when considering the size of an oil and water separator are flow rate and spill containment.
For example, at an airport, refuelling occurs anywhere on the apron. Subsequently, a large area of catchment needs to be treated. In sites with large area catchments, flow rate is often the defining factor for choosing a full retention separator. The device needs to be sized to suit these high rates of flow.
Alternatively, at sites such as transformer yards, an oil and water separator may be employed to contain large volumes of oil in the event of a fire or a large spill. In these cases, the full retention separator will need to accommodate the desired on-site spill capacity, and this will be the defining factor for choosing the right size device.
By weighing your on-site priorities, and defining key parameters, you can adequately size a device to suit your site. Other considerations include local device standards, environmental compliance policies and any other legal obligations for the site operator.