The key design attributes guiding your choice when sizing an oil and water separator are flowrate and spill capacity. Which of these criteria you prioritise depends upon the site.
At airports, for instance, refuelling can happen at any point across the apron. As a result, a sizeable area of catchment will need to be treated. When selecting a full retention separator for sites with large area catchments, flow rate is generally the priority consideration. The separator has to be sized to handle the higher flow and typically the spill capacity of the device is more than required.
At other sites, for example, substations, the primary concern when sizing a unit is whether the unit can meet the site’s required spill capacity as the flow capacity required for these sites is generally very small. Thus you would typically end up with more flow capacity than required.
You should also consider environmental compliance, any mandated standards for on-site devices and site operator legal obligations.