According to the NESHAP requirements, the closed-vent system with control device should be operated any time waste is present in the separator except when maintenance or repair cannot be done without shutting down the control device.  Three main types of control devices—enclosed combustion, vapor recovery and flare—should be designed to operate at external detectable VOC emissions of 500 parts-per-million-volume above background levels.

Enclosed Combustion Devices
Vapor Recovery Devices
Flare Devices
Ignition Sources
Vapor Space Purging